EG&G 128A Operating And Service Manual

MODEL

128A

LOCK-IN

AMPLIFIER

SEE SAFETY NOTICE

PRECEDNG SECTION I

;EFORE OPERATING INSTRUMENT

OPERATING

AND

SERVICE

MANUAL

M128A; 9/83·500·MIC

MODEL

128A

LOCK-IN

AMPLIFIER

OPERATING

AND

SERVICE

MANUAL

n

~~

G

BROOKDEAL

ELECTRONICS

~~

.:;;;;;~I:..

PRINCETON

APPL/ED

RESEARCH

Copyright

© 1983 EG&G PRINCETON APPLIED RESEARCH

Printed

In U.S.A.

SHOULD YOUR EQUIPMENT REQUIRE SERVICE

A. Contact the factory

(6091452-2111)oryour

local factory
representative to discuss the problem. In many cases it
will be possible to expedite servicing by localizing the
problem to a particular plug-in

circuit

board.

B. If it is necessary to send any equipment back to the fac-

tory, we need the following information.

(1) Model number and serial number.
(2) Your name (instrument user).

(3) Your address.

(4) Address to

which

instrument should be returned.

(5) Your telephone number and extension.

(6) Symptoms (in detail,

including

control

settings).

(7) Your purchase order number for repair charges (does

not apply to repairs in warranty).

(8) Shipping

instructions

(if you wish to authorize ship-

ment by any method

other

than normal surface

transportation).

C. U.S. CUSTOMERS-·-Ship the equipment being returned

to:
EG&G PRINCETON APPLIED RESEARCH

7 Roszel Road
(Off Alexander Road, East of Route 1)
Princeton, New Jersey

D. CUSTOMERS OUTSIDE OF

U.S.A.-To

avoid delay in
customs clearance of equipment being returned, please
contact the factory or the nearest factory

distributor

for

complete shipping information.

E. Address correspondence to:

EG&G PRINCETON APPLIED RESEARCH
P. O. Box 2565

Princeton, NJ 08540
Phone:

6091452-2111

TELEX: 84 3409

WARRANTY

EG&G PRINCETON APPLIED RESEARCH warrants each in­strument of

its

manufacture to be free from defects in material

and workmanship. Obligations under

this

Warranty shall be

limited

to replacing, repairing or giving credit for the purchase
price, at our option, of any instrument returned, freight
prepaid, to our factory

within

ONE year of delivery to the

original purchaser, provided

prior

authorization for such return

has been given by our authorized representative.
This Warranty shall not apply to any instrument which our in-

spection shall

disclose

to our satisfaction, has become detec­tive or unworkable due to abuse, mishandling, misuse, acci­dent, alteration, negligence, improper installation or

other

causes beyond our control.

Instruments

manufactured by

others, and included in or supplied

with

our equipment, are not

covered by

this

Warranty but carry the original manufacturer's
warranty which is extended to our customers and may be more
restrictive. Certain subassemblies, accessories or

corn-

ponents may be specifically excluded from

this

Warranty, in

which case such exclusions are listed in the

Instruction

Manual supplied

with

each instrument.

We reserve the right to make changes in design at any time
without

incurring any obligation to install same on

units

previously purchased.
THERE ARE NO WARRANTIES WHICH EXTEND BEYOND THE

DESCRIPTION HEREIN. THIS WARRANTY IS IN LIEU OF, AND
EXCLUDES ANY AND ALL OTHER WARRANTIES OR REPRE·
SENTATIONS, EXPRESSED, IMPLIED OR STATUTORY, IN·

CLUDING MERCHANTABILITY AND FITNESS, AS WELL AS
ANY AND ALL OTHER OBLIGATIONS OR LIABILITIES OF
EG&G PRINCETON APPLIED RESEARCH, INCLUDING, BUT
NOT LIMITED TO, SPECIAL OR CONSEQUENTIAL DAMAGES.
NO PERSON, FIRM OR CORPORATION IS AUTHORIZED TO
ASSUME FOR EG&G PRINCETON APPLIED RESEARCH ANY
ADDITIONAL OBLIGATION OR LIABILITY NOT EXPRESSLY
PROVIDED FOR HEREIN EXCEPT IN WRITING DULY EXE­CUTED BY AN OFFICER OF EG&G PRINCETON APPLIED
RESEARCH.

Section

II

III

IV

V

TABLE

OF CONTENTS

CONDENSED

OPERATING

INSTRUCTIONS

CHARACTER

ISTICS

2.1

Introduction

2.2 Specifications

INITIAL

CHECKS

3.1

Introduction

3.2 Equipment Needed

3.3 Procedure

OPERATING

INSTRUCTIONS

4.1

Introduction

4.2 Preliminary Considerations

4.2A

Power Requirements

4.2B Fusing

.....

4.2C Warm-Up Period

4.20

Operating Frequency

4.2E

Grounding...

4.2F Noise

.....

4.3 Operating the Model 128A

4.3A

Introduction..

4.3B Reference Channel

4.3C Signal Channel

4.30

Output

Channel Controls

4.4 Mixer

Function

and Harmonic Sensitivity

4.5 Interface Connector .

4.6 Battery Operation .

4.7 Operation

with

the Internal Reference Oscillator

4.7A

Introduction

4.7B Operation . . . . . . . . . . .

4.7C Installation .

4.8 Operation

with

the Internal Tuned

Amplifier

4.8A

Introduction

4.8B

Operation.........

4.8C Installation .

4.9 More Reference Channel Operating Hints

4.9A

Reference Channel Slewing Rate

4.9B

Phase

Errors

with

Small Reference Signals

ALIGNMENT

.

5.1

Introduction

5.2 Required Equipment

5.3 Preliminary Steps

5.4 Procedure

5.4A

+15 V

Adjust

(R310),

-15

V Check, and +5 V Check

5.4B Reference Board Adjustments

5.4C Signal Board

Adjustment

5.40

Mixer Adjustments

5.4E Other Adjustments

Page

1-1

11-1
11-1
11-1

111-1
111-1
111-1
111-1

IV-l
IV-l
IV-l
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IV-l
IV-l
IV-3
IV-5
IV-5
IV-5

IV-6
IV-8

IV-9
IV-10
IV-10
IV-l0
IV-10
IV-10
IV-ll
IV-12
IV-12
IV-13
IV-15
IV-15
IV-15
IV-15

vi

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V-3
V-3
V-4

VI

VII

Number

1·1
IV·l
IV-2
'V-3
IV-4
IV-5
IV·6
IV·7
IV-8
IV-9
IV-10

rvu

IV·12
IV-13
IV·14
IV·15

vi

Number

IV-l
.V-2

TROUBLESHOOTING

6.1 Introduction

6.2 Equipment Required

6.3 Initial Steps . . . .

6.4 Power Supply Checks

6.5

Reference Checks

6.6 Signal Channel Amplifiers

6.6A Preamplifier

6.6B Intermediate AC Amplifiers

6.6C Final AC Amplifier

6.7 Mixer

6.8 DC Amplifiers .

SCHEMATICS, Table of

FIGURES

Model 128A Lock-In Amplifier .
Ground-Loop Suppression by Ten-Ohm Input Ground
Differential Measurement of "Single-Ended" Signal
Differential Measurement of

"Off-Ground"

Signal

Example of Everything

"Done

Wrong"

Errors Depicted in Figure IV-4 Corrected

Typical Model

l28A

Noise Figure Contours

Pulse Train as Reference Drive

Net Phase

Shift

Between Signal and Reference Channels as a

Function

of Frequency
Amplitude and Phase Characteristics of Hi-Pass Filter
Amplitude and Phase Characteristics of Low-Pass Filter

Examples of

Output

Filter Interactions . . . . .

Mixer

Output

for In-Phase and Quadrature Signals

Internal Oscillator Board Installed . . . . . . .

Tuned Amplifier Installed .

Phase/Amplitude Characteristics of Tuned Amplifier
Model 128A Adjustments and

Testpoints

.....

TABLES

Interface Connector Pin Assignments
Frequency Range as a Function of Capacitors

ii

VI-'
VI-'
VI·'
VI-'
VI·'
VI-'
VI·2
VI-2
VI-2
VI-2
VI-2
VI-2

VII·'

Page

.

1-2

IV-l
IV-'

IV-2
IV-2
IV·3
IV·3
IV·5
IV-6
IV-7

IV-7
IV-8

IV-9
IV·12
IV·13
IV·14

. V·2

Page

IV·l0
IV·l1

SAFETY CONSIDERATIONS

A. INTRODUCTION

The apparatus to

which

this

instruction

manual

applies has been supplied in a safe

condition.

This manual

contains

some

information

and warn-

ings that have to be

followed

by the user to ensure
safe operation and to retain the apparatus in a
safe condition. The described apparatus has been
designed

for

indoor

use.

B. INSPECTION

Newly received apparatus should be

inspected

for

shipping damage. If any is noted,

immediately

notify EG&G PARC and file a claim

with

the car-

rier. The shipping

container

should be saved

for

possible

inspection

by the carrier.

WARNING!
THE PROTECTIVE GROUNDING COULD BE
RENDERED INEFFECTIVE IN DAMAGED AP­PARATUS.

DAMAGED

APPARATUS
SHOULD NOT BE ·OPERATED UNTIL ITS
SAFETY HAS BEEN VERIFIED BY QUALI­FIED SERVICE PERSONNEL. DAMAGED AP­PARATUS WAITING FOR SAFETY VERI­FICATION SHOULD BE TAGGED TO INDI­CATED TO A POTENTIAL USER THAT IT
MAY BE UNSAFE AND THAT IT SHOULD
NOT BE OPERATED.

C. SAFETY MECHANISM

As defined in IEC Publication 348 (Safety Require-

ments

for

Electronic

Measuring

Apparatus), the

Model 128A is Class I apparatus, that is, apparatus
that depends on

connection

to a protective con-

ductor

to earth ground for

equipment

and operator

safety. Before any

other

connection

is made to the
apparatus, the protective earth terminal shall be
connected to a protective conductor. The protec­tive connection is made via the earth ground
prong of the M128A's power cord plug. This plug
shall only be inserted

into

a socket

outlet

provid­ed with the required earth ground contact. The
protective action

must

not be negated by the use

of an extension cord

without

a protective conduc-

tor, or by use of an

"adapter"

that

doesn't

main-

tain earth ground

continuity,

or by any

other

means.
The power cord plug provided is of the type illus-

trated in Figure 1. If

this

plug is

not

compatible

with the available power

SOCkets,

the plug or

power cord should be replaced

with

an approved

type of

compatible

design.

WARNING!
IF IT IS NECESSARY TO REPLACE THE
POWER CORD OR THE POWER CORD
PLUG, THE REPLACEM ENT CORD OR PLUG
MUST HAVE THE SAME POLARITY AS THE
ORIGINAL. OTHERWISE A SAFETY HAZARD
FROM

ELECTRICAL

SHOCK,

WHICH
COULD RESULT IN PERSONAL INJURY OR
DEATH, MIGHT RESULT.

L =

LINEORACTIVE

CONDUCTOR (ALSO CALLEU

"LlV£"OR"HOT")

N =

NEUTRALORIDENTIFIED

CONDUCTOH

E

=EARTH OR SAFETY GROUND

Figure 1. POWER CORD PLUG WITH POl.ARITY INDICATIONS

D. POWER VOLTAGE SELECTION AND

LINE FUSES

Before

plugging

in the

power

cord, make sure that

the

equipment

is set to the voltage of the ac

power

supply.

CAUTION!
THE APPARATUS DESCRIBED IN THIS
MANUAL

MAY BE DAMAGED IF IT IS SET
FOR OPERATION FROM 110 V AC AND
TURNED ON WITH 220 V AC APPLIED TO
THE POWER INPUT CONNECTOR.

A detailed

discussion

of how to

check

and.tt

nec-

essary, change the power-voltage

setting

follows.

The line voltage is selected by means of a rear­panel

switch.

FOR SAFETY, UNPLUG THE
POWER CORD WHEN CHECKING THE LINE
VOLTAGE SETTING OR WHEN CHECKING THE
FUSES. FUSES SHOULD ONLY BE CHANGED BY
QUALIFIED SERVICE PERSONNEL WHO ARE
AWARE OF THE HAZARDS INVOLVED. Depend­ing on the

switch

position,

either

"115"or"230"

(both are printed on the

swltchj-wl!lbevisible

to
the viewer. For operation from a line voltage from
100 V ac to 130 V ac, 50-60 Hz,

"115"

should

show.

SECTION I

CONDENSED OPERATING INSTRUCTIONS

The

following

condensed

operating

instructions

are pro-

vided as an assistance in placing

the

Model

128A

Lock-In

Amplifier

into

operationasquickly

as possible.

Generally

speaking,

these

condensed

instructions

will

allow

good

re-

sults

to be

obtainedinmost

instances.

However,

because

of

the

brevity of

these

instructions,

many

considerations

hav-

ing

importanceinparticular

applications

have

been

fore-

STEP ONE

PRELIMINARY

STEPS;

Check

that

the

rear-panel

115/230Vswitch

is in

the

proper

position.

Remove

the

top

cover

and

verify

that

the

two

Reference

Lock-On

Speed

switches

are

properly

set.

For

operation

below5Hz,

they

shouldbesettoSLOW.

For

operations

above5Hz,

they

should

be

settoFAST.

(Units

are

shipped

with

these

switches

settoFAST.)

Replace

the

cover

and

pluginthe

line

cord.

Turn

the

power

on.

STEP TWO

REFERENCE

CHANNEL:

Check

that

the

Reference

Mode

switchissettothe

proper

position.

With

the

switch

setto"f",

the

detectorisdrivenatthe

frequencyofthe

applied

reference

signal.

Setto"2f",

the

detectorisdrivenattwice

the

frequency

of

the

applied

reference

to

facilitate

second

harmonic

measurements.

Next

connect

the

reference

signal

(100

m V pk -pkorgreater)

to

the

Reference

Input

connector

and

wait

for

the

REF.

UNLOCK

lighttogo

out

before

proceeding.

Usually

this

willbebut

a

few

seconds.

However,atlow

frequencies,

a

longer

timeisrequired.

STEP

THREE

SIGNAL

CHANNEL:Set

the

Input

Selector

to

"A"

(single-ended),

"A-B"

(differential)

or

"-B"

(single-ended),

whichever

is

appropraite.

Set

the

Sensitivity

switch

to

"250

mV".

If

signal

channel

filtering

is

desired,

set

the

HI-PASS

and

LO-PASS

Filter

switches

as

required.

Connect

the

signal

sourcetothe

Model

128A

Input.

1-1

gone.

For

this

reason,

it is advisabletoread

Section

IV,

the

complete

Operating

Instructions,tobe

assured

of achieving

optimum

performance.

NOTE:Aswritten,

these

condensed

instructionsdonot

applytounits

having

either

the

Internal

OscillatororTuned

Amplifier

modifications.Ifthe

unit

in

question

has

eitherorbothofthese

modifications,

the

operatorisreferredtoSubsections

4.7

and

4.8.

STEP

FOUR

OUTPUT

CONTROLS;

Set

the

Zero

Offset

toggle

switchtothe

center

(OFF)

position.

Then

set

theDCPrefilterto100msand

the

Time

Constant

switchto0.3

SEC.

STEP

FIVE

FINAL

ADJUSTMENTS:

Rotate

the

Sensitivity

switch

counterclockwise

until

the

panel

meter

beginstodeflect.

Adjust

the

Phase

Quadrant

switch

and

Phase

dial

for

maximum

meter

indication.

When

the

Sensitivity

and

Phase

adjustments

are

optimally

set,

the

meter

should

indicate

the

maximum

possible

on-scale

reading.Ifthe

Overload

light

comes

on,

increase

the

prefilter

and

time

constant

settings.

Also

try

"bracketing"

the

signal

frequency

with

the

HI

PASS

and

LO

PASS

filters.Ifthese

techniques

don't

help,

rotate

the

Sensitivity

switchasfar

clockwise

as is

required

to

eliminate

the

overload.

Set

the

time

constant

as

requiredtoreduce

the

output

noisetoan

acceptable

level.

STEP SIX

READING:

The

input

signal

levelisread

from

the

panel

meter.Ifthe

input

signal

is a

sinewave,

the

meter

directly

indicates

the

rms

amplitudeofthe

input

signal.

However,

if

the

signal

frequencyisnear

enoughtoa

selectedHIorLOPASS

filter

frequency

to

be

influenced

by

the

filter,

the

filter

attenuation

effects

must

be

taken

into

account.

If

the

input

signalisnot

a

sinewave,

then

the

harmonic

responseofthe

instrument

mustbetaken

into

account

as

well.

1-2

a:

w

u.

-'

Q.

~

~

~

c

0

t

-'

~

co

N

...

-'

W

0

i

...

e

::l

'"

u::

SECTION

II

CHARACTERISTICS

2.1 INTRODUCTION

The

Model

128A

Lock-In

Amplifier

enables

the

accurate

measurementofsignals

contaminatedbybroad-band

noise,

power

line

pickup,

frequency

drift,orother

sources

of

interference. It

does

thisbymeans

of an

extremely

narrow

band

detector

which

has

the

centerofits

passband

locked

to

the

frequencyofthe

signaltobe

measured.

Because

of

the

frequency

lock

and

narrow

bandwidth,

large

improvements

in signal-to-noise

ratio

can

be achieved,

allowing

the

signalofinteresttobe

accurately

measured,

even in

situations

where

it is

completely

masked

by noise.

Signals applied

to

the

input

(single-ended or

differential)

1

routed

through

a series of amplifiers

which

allow

..rll-scale sensitivity ranges

downtoone

microvolt.

Switch

selectable low-pass

and

high-pass filters allow

considerable

noise

reduction

aheadofthe

phase-sensitive

detector.

This

pre-detector

noise

reduction

canbefurther

enhanced

by

making

use of

the

optional

plug-in (internal) selective

amplifier. At

the

phase

sensitive

detector,

the

signal is

compared

with

the

reference

signal derived

from

the

experiment.

Only

those

signal

components

which

are

synchronous

with

the

reference

yieldanetdcdetector

output.

Noise

and

other

non-synchronous

signalsdonot

contributeanetdcoutput,

but

onlyacfluctuations

which

canbereducedtoany

arbitrary

value

accordingtothe

amount

of filtering

selected

with

the

Time

Constant

switch.

This switch allows

time

constants

as large as

100

seconds

to

be

selected,

with

provision for achieving larger

externally

determined

time

constants

if necessary.

Post-detector

dc

amplifiers drive

the

panel

meter

and

signal

output

connectors.

Other

features

include

provision

for

calibrated

zero

suppression of up to10x full scale, a

two-position

dc

prefilter,

and

the

capability

of driving

the

reference

input

the

detector

at

double

the

frequency

of

the

signal

applied to

the

Reference

Input

connector

to

facilitate

second

harmonic

measurements.

An

optional

plug-in

oscillator

(internal)

is available for use in

applications

where

the

experiment

does

not

produceareference

signal itself,

but

is capableofbeing

driven

by a signal

furnishedbythe

Lock-In Amplifier.

With its wide range of capabilities

and

easeofoperation,

the

Model

128A

Lock-In

Amplifier

should

find

extensive

applicationinsituations

where

the

accurate

measurement

of signals is

complicated

by

the

presenceofnoise and

interference.

2.2 SPECIFICATIONS

SIGNAL

CHANNEL

(1) INPUT

TYPE

Single-endedordifferentialasselectedbyfront-panel
switch.

11-'

(2)

INPUT

IMPEDANCE

100

Mr2

shuntedbyno

more

than20pF.

(3)

SENSITIVITY

12 full-scale ranges in

1-2.5-10

sequence

from1J.1V

to

250

mV.

(4) FREClUENCY

RANGE

0.5Hzto

100

kHz.

(5) COMMON MODE

REJECTION

At least

100dBat

1 kHz.

(6) MAXIMUM COMMON MODE

VOLTAGE

3 V

pk-pkto20

kHz;

then

--6

dB/octave

above

20

kHz.

(7)

DETECTOR

BIAS

Internal

network

allows dc bias

current

of

either

polaritytobe

providedatthe

"A"

Inputtofacilitate

operation

with

diode

detectors

which

require

biasing.

(See page VII-3

and

Parts

Location

Diagramonpage

VII-2.)

(8) NOISE

At 1 kHz

the

signal

channel

noise will

not

exceed

10

nV/H

zY2.

(9) LOW PASS

FILTER

Switch

selectable6dB/octave

low-pass filter

which

can be

set

to 3 dB

down

frequenciesof100

Hz,

10

kHz, or MAX

(greater

than

100

kHz).

(10)

HIGH PASS

FILTER

Switch

selectable6dB/octave

high-pass filter

which

can be

setto3 dB

down

frequenciesof50

Hz, 5 Hz,

or MIN

(below

0.5

Hz).

(11)

OVERLOAD

DETECT

Front-panel

indicator

lights if

applied

signal plus noise

is large

enoughtocause

overloadatany

of several

critical

overload

monitor

points.

(12)

GAIN

STABILITY

(13) GAIN

LINEARITY

0.05%.

(14)

OVERALL

GAIN ACCURACY

±2%.

REFERENCE

CHANNEL

The

Model

128A

reference channel

automatically

locks

onto

and

tracksanapplied

reference signal over the

entire

operating

frequency

range of

the

instrument.

As a result,

the

instrumentisimmunetofrequency

and phase

shifts

as

long as

the

reference and signal to be recovered change

together.

(1)

TRACKING

RANGE

5 Hz to

100

kHz (FAST) or

0.5

Hz to

100

kHz

(SLOW) as

determinedbythe

settingoftwo

internal

switches.

Faster

lock-on

time

and slewing

rate

obtained

with

switches

settoFAST

make

this range

preferable

except

when

operating

below 5 Hz.

(2) MODES

Eitheroftwo

modes, f and 2f, can be selected by

means of a front-panel switch. In

the

"f"

position,

the

phase-sensitive

detector

is driven at

the

same

frequencyasthe

applied reference signal. In

the

"2f"

position,

the

phase-sensitive

detector

is driven at twice

the

frequency

of

the applied reference signal

to

facilitate

second

harmonic

measurements.

(3) INPUT IMPEDANCE

10

Mrl

shuntedbyno

more

than20pF.

(4) MINIMUM

REFERENCE

SIGNAL

REQUIREMENT

100

mV pk-pk,

any

waveshape crossing its

mean

only

twice each cycle. Minimum

time

required on

either

sideofthe

meanis100

ns.

Amplitude

excursions

must

be at least50mV on each sideofthe

mean.

Maximum

input

signal is 5 V (pk-to-rnean}. Best phase

accuracy

is

obtained

with

a 1 V rms sinewave.

(5) LOCK-ON TIME

A

function

of internal

switch

setting

as follows.

Selected

Range Lock-On

Time

SLOW (0.5 Hz to

100

kHz)

20

sec. per

octave

FAST (5 Hz to

100

kHz) 2 sec. per octave

(6) PHASE

Calibrated Phase

controls

allow the phase of

the

reference drive to

the

Phase-Sensitive

Detector

to be

set at any angle relative to

the

input

signal.

The

controls

consist of a Phase Dial with a range of

100°

and a Phase

Quadrant

switch

which

provides

incremental phase shifts of

90°.

The

phase

shift

11-2

accuracy of

the

dial is

better

than

0.2

0

over the

entire

frequency

range.

The

resolutionofthe

dial is

better

than

0.10.The

incremental

phase shiffs provided by

the

Quadrant

switch are

accurateto0.2°.

The

overall

phase accuracy of

the

instrument,

including

shifts

in

both

the reference and signal channels, is typically

better

than

50.

(7)

DETECTOR

BIAS

Internal

network

allows de bias

currentofeither

polarity

to be provided at

the

REF.INconnector

to

facilitate

operationinsituations

where

the

reference

signal is

taken

from

diodes requiring biasing. (See page

VII-6 and Parts Location Diagram

on

page VII-5.)

PHASE

SENSITIVE

DETECTOR,DCAMPLIFIER

(1)

OUTPUT

DRIFT

(2)

OVERLOAD

CAPABILITY

1000

times full scale up to a

maximumatthe

input

of

650

mV rms. Overload capability is

definedasthe

ratio, at

the

input

of the Model

128A,

of

the

maximum

pk-pk

non-coherent

signal

which

can be

applied

without

overloading the Model

128Atothe

pk-pk

coherent

signal required to yield full scale

Model

128A

output.

Note

that,

expressed as

the

ratio

of

the

pk-pk

non-coherent

signaltothe

rms value of

the

coherent

signal

required

for full-scale

output,

this

number

can be as

greatas2800.

Maximum

acceptable

signal is a

650

mV rms sinewave.

(3)

NON-COHERENT

REJECTION

50

ppm

maximum.

Non-coherent

rejection is

defined

as

that

offset

which

results

from

applying a

non-coherent

signal having a

pk-pk

amplitude

1000

times

the

pk-pk

amplitude

of

the

coherent

signal

requiredtoobtain

full-scale

output.

Example: With a

non-coherent

signal applied having a

pk-pk

amplitude

1000

times

the

pk-pk

coherent

signal

required

to

obtain

full-scale

output,

there

will

occur

an

offsetatthe

output

caused by

the

non-coherent

input

signal.

The

amplitude

of this

offset

will be no

greater

than:

50 X

10-

6

X

1000

= 50 X

10-3of f.s.

output

=50 mV (f.s. =1 V)

(4) TIME

CONSTANT

Front-panel switch allows selection of 6

dB/octave

filter

time

constants

of 1 ms, 10 ms, and .1 s

through

100

s in 1-3-10 sequence. Also MIN (time

constant

:::=0.7

ms) and EXT, which allows

time

constants

longer

than

100

s to be achieved by means of

external

capacitors. A

separate

front-panel

toggle switch allows

another6dB/octave

filter to be

inserted,

if desired.

This filter has a

time

constantofeither

100

ms or 1 s,

whichever is selected.

(5)

ZERO

OFFSET

A

calibrated,

ten-turn,

Zero

Offset

dial,

with

up to

ten

times full-scale

capability

is provided.

(6) FULL-SCALE

OUTPUT

±1 V.

(7)

OUTPUTS

(a) Panel

meter,

± full scale.

(b)

Front-panel

BNC

connector.

One volt

out

corresponds

to full-scale panel

meter

deflection.

Output

resistance is

600

ohms.

(c) Rear-panel

Recorder

Out

binding posts,

spaced

to

accept

standard

double-banana

connector.

Output

resistance is

600

ohms.

GENERAL

(1) AMBIENT

OPERATING

TEMPERATURE

RANGE

(2)

AUXILIARY

POWER

OUTPUT

±15.5

V regulated dc

at

20 mA is

provided

at

rear-panel

connector.

(3) POWER

REQUIREMENTS

100-130

V ac

or

200-260

V ac,

50-60

Hz. May also be

powered

from

±24 V de

source

(such as

batteries).

Power

consumption:15watts.

(4) SIZE

17-3/4"

W x

3-1/2"

H x

14"D(45cmW x 9 cm H x

36 cm D).

11-3

(5)

WEIGHT

14 Ibs

(6.4

kg).

(6)

MODIFICATIONS

(a) Model

128A/97

Monitor

Modification

Three

rear-panel BNC

connectors

are installed

which

permit

monitoring

of:

(1)

OutputofSignal

Channel

before

demodulation

(please

note

that

the

Signal Channel

monitor

is provided as

part

of

the

Tuned

Amplifier

modification

as well), (2)

Squarewave

output

of Reference

Channel,

and

(3) Full-wave

demodulated

Mixer

output

before

Time

Constant

filter.

(b) Model

128A/98

Tuned

Amplifier Modification

Internal plug-in

board

is available

which

provides

a

tuned

bandpassornotch

characteristic

at a

Q-of-5.

The

frequency

can be

adjusted

over a 3: 1

range by

meansofa rear-panel

adjustment,

and

can be

settoany

frequency

from

1 Hz to

100

kHz by changing

capacitors

mounted

on

component

clips

located

on

the plug-in

circuit

board.

(c) Model

128A/99Internal

Oscillator

Modification

An internal low

distortion

oscillator

is available

which

provides a sinewave

output

adjustable

from

0-to-l0

V pk-pk

at

600

ohms.

The

frequencyisadjustable

over

about

a 3: 1 range by

means of a rear-panel

adjustment,

with

the

actual

frequency

range

spannedbythe

adjustment

being

determined

by a pair of

internal

capacitors

mounted

on

the

oscillator

circuit

board.

Operation

from

about

1 Hz

to

100

kHz is

possible. This

option,inconjunction

with

the

2f

mode

of

operation,

is

particularly

useful for

harmonic

detection

where

the

modulation

frequency

must

be at

one

half

the

detected

frequency.

SECTION

III

INITIAL

CHECKS

3.1 INTRODUCTION

The

following

procedureisprovided

to facilitate initial

performance

checkingofthe

Model 128A. In general, this

procedure

should

be

performed

after

inspecting

the

instrument

for

shipping

damage

(any

notedtobe

reported

to

the

carrier

and

to

Princeton

Applied

Research

Corporation),

but

before

using

the

instrument

for

experimental

measurements.

Should

any

difficulty

be

encountered

in carrying

out

these checks,

contact

the

factoryoroneofits representatives. It

mightbenoted

that

it is

not

the

purpose

of these

checkstodemonstrate

that

the

instrument

meets

all specifications,

but

rather

simply

to

show

that

it is

functioning

normally.

If normal

indications

e

obtained

for

the

functions

checked,

one

may

reasonably

assume

that

those

functions

which

are

not

checked

are

working

properly

as well.

3.2 EQUIPMENT NEEDED

(1) Sinewave Oscillator

to

providea100

mV rms sinewave

at 1 kHz.

NOTE:Ifthe

instrumenttobe

checked

is

equipped

with

the

Tuned

Amplifier

modification,

then

the

oscillator

will have

to

provide a

100

mV rms

sinewave at

the

tuned

frequency.Ifthe

instrument

in

questionisequipped

with

the

Internal Oscillator

modification,noexternal

oscillator will be

required.

Th is applies

whetherornot

the

unitisequipped

with

the

Tuned

Amplifier

modification.

(2) General

purpose

oscilloscope. This

itemisrequired

only in

the

caseofunits

having

the

Tuned

Amplifier

modification.

'3) Suitable cables

for

interconnecting

the

above instru-

ments.

3.3 PROCEDURE

(1) Check

the

positionofthe

rear-panel

115/230

switch.

Be sure

the

number

showinginthe

window

corre-

spondstothe

line voltagetobe used.

(2) With the Power switch

settoOFF,

plug in

the

line

cord.

(3)

Set

the

Model

128A

controls

as follows.

Input

Selector: A

Sensitivity:

100

mV

Filters

HI-PASS: MIN.
LO-PASS: MAX.

Phase

Quadrant

switch:

270°

Phase dial:

90°
Mode: f
Zero

Offset

switch:

OFF

(center

position)

111·1

dial:

1.00

(one

turn

from

fully

counterclockwise

position)

Time

Constant:

.3 SEC.

DC Prefilter:

OUT

Reference

Tracking-Rate

switches

(two

internal

switches);

FAST

(unless

unitisequipped

with

Tuned

Amplifier'

or Internal Oscillator set for

frequency

below

5 Hz, in

which

case

switches

shouldbeset

to SLOW). NOTE:

Instruments

are

normally

shipped

with

these

switches set

to

FAST.

Power.

ON

(4) Applies

onlytounits

not

equipped

with

the

Internal

Oscillator

modification.Ifthe

Model

128A

has this

modification,goto

step

5.

Connect

the

outputofthe

external

oscillator

(set for

100

mV rms sinewave

out

at 1 kHz)toboth

the

"A"

and

Reference

Inputs. If

the

unitinquestion

has

the

Tuned

Amplifier

modification,

set

the

oscillatortothe

tuned

frequency

specified

when

the

unit

was

ordered.

(5) Applies

onlytounits

having

the

Internal Oscillator

modification.

Connectacable

from

the

rear-panel

REF.

OSC.

OUT

connectortothe

front-panel

"A"

Input.

There

is no

needtoconnect

th is signal to

the

Reference

Input

connector

(labeled MONITOR in

units

equipped

with

the

Oscillator

modification).

The

connectiontothe

Reference

Channel of

the

Model

128Aismade

internallyatthe

factory.

The

rear-panel

REF.

AM-

PLITUDE

adjustment

should

be

set

so

that

the

amplitude

of

the

signal at

the

REF.

OSC.

OUT

connectoris100

mV rms. An

accurateacvoltmeter

may be useful

for

setting

this

level (units leave the

factory

set

foranominal

100

mV rrns

out).

(6) Applies

onlytounits

having

the

Tuned

Amplifier

modification.

In

the

case

of

units

not

having

this

modification,

go d irectl ytostep

7.

(a)

Connect

the

oscilloscope to

the

rear-panel SIG.

MON.

connector.

(b)

Vary

the

frequencyofthe

signal applied to

the

"A"

Input

(use

the

rear-panel

REF.

OSC.

FREQ.

ADJ.

controlinunits

equipped

with

an Internal

Oscillator)

for

peak

signal

amplitude

as observed

with

the

oscilloscope.

(7)

Set

the

Phase

Quadrant

switchto0°.

Then

adjust

the

Phase dial

for

"0"

panel

meter

indication.

(8)

Set

the

Phase

Quadrant

switch

backto270°.

The

panel

meter

should

indicate

full scale to

the

right ± a

few

percent

of full scale.

The

accuracyofth is readinq

will

depend

on the

amplitude

accuracy of the signal

applied

to

the

"A"

Input.

(9)

Adjust

the

amplitudeofthe

signal applied to

the

"A"

Input

as required to

obtain

exactly

full-scale panel

meter

indication.

(10)

Set

the

Phase

Quadrant

switch to

180°.

The

panel

meter

should

indicate

"0"

±5%offull scale.

(11)

Set

the

Phase

Quadrant

switchto90°.

The

panel

meter

should

indicate

negative full scale ±5% of full

scale.

(12)

Set

the

Phase

Quadrant

switch to

270°

to restore

the

111-2

positive full-scale panel

meter

indication.

(13)

Set

the

Zero

Offset

toggle switch to "+".

The

panel

meter

indication

should

go to

"0"

±5% of full scale.

(14) Begin

rotating

the

Offset

dial

counterclockwise.

The

panel

meter

indication

should

increase linearly, track-

ing

the

dial setting. When

the

dial is fully

counter-

clockwise,

the

panel

meter

should

indicate full scale

±5%. Reset

the

Zero

Offset toggle switch to

the

center

(OFF)

position.

This

completes

the

initial checks. If

the

indicated results

were

obtained,

one

can be reasonably sure

that

the Model

128Aisfunctioning

normally.

SECTION

IV

OPERATING

INSTRUCTIONS

4.1 INTRODUCTION

Even

though

operation

of

the

Model

128Aisstraight-

forward,

there

are a

number

of

factorsofwhich

the

operator

should

be aware

to

be assured of achieving

optimum

performance

in all

situations.

This

sectionofthe

manual

treats

these

considerationsinsome

detail.

Topics

covered include

grounding,

noise

performance,

harmonic

sensitivity,

operation

in

conjunction

with

the

plug-in

accessories,

and

others.

For

an overall

"quick

look"athow

the

instrumentisoperated,

the

operatorisreferred

to

SectionI,the

condensed

operating

instructions.

4.2

PRELIMINARY

CONSIDERATIONS

4.2A

POWER

REQUIREMENTS

The

Model

128A

requires

100-130

V acor200-260

V ac,

50-60

Hz.

The

power

consumptionis15

watts.

The

unit

may also be

poweredbybatteriesbyapplying

±24 Vtothe

appropriate

pinsofthe

rear-panel octal-

connector

(see

BATTERY

OPERATION,

page

IV-l0).

A rear-panel slide

switch

determines

whether

the

ac

power

circuits

are

connected

for

operation

from

100-130

V or

from

200·260V.For

operation

from

100-130V,"115"

should

showinthe

window.

For

operation

from

200-260V,"230"

should

show.

4.28

FUSING

The

Model

128Aisprotected

by a single fuse

mounted

on

its rear panel. A

slow-blow

1/4

A fuse is used for

operation

from

115

V. A slow-blow

1/10

A fuse is used for

operation

from

230

V.Itoccasionally

happens

thataslow-blow

fuse

fails in

shipment

as a

resultofshock

and

vibration.

Hence,

if

the

fuse is

found

to be bad

when

the

instrument

is

operated

for

the

first

time

it is advisable to

try

and

change

the

fuse. If

normal

operation

follows,

chances

are

there

are

no

other

problems.

However, if

the

replacement

fuse fails,

thereissomething

wrong

which

will have to be

corrected

before

proceeding.

4.2C

WARM-UP

PERIOD

For

most

applications,

five

minutes.

Where it is

desired

to

achieve

the

best

possible gain

and

output

stability,

allow an

hour.

4.20

OPERATING

FREQUENCY

Although

one

can,

in principle,

make

equally

accurate

measurements

at any

frequency

within

the

operating

range

of

the

instrument,

operationissimplest

and

least

subject

to

error

over a range having as its

lower

limit,

perhaps

a few

hundred

Hz,

and

as its

upper,

perhaps

10k

Hz.Atvery low

frequencies,

phase

offsets

occur

which

could

matterifone

is

interestedinthe

absolute

phase of

the

input

signal.

Another

problemoflow

frequency

operationisthatofl/f

noise, including

both

that

which

developsinthe

Model

128A

and

that

which

originatesinthe

experiment

itself

to

degrade

the

signal-to-noise

ratio

ahead

of

the

lock-in

amplifier. Increased

response

and

settling

time

could

be a

IV-1

significant

problemifone

were

operatinginconjunction

with

the

optional

plug-in

tuned

amplifier.

At high fre-

quencies,

radiation

and

associated

pick-up

tend

to

be

bothersome.

Another

high

frequency

problemisthat

of

signal

attenuation

as a resultofthe

input

cable

capacitance.

Th is is especially a

problem

when

working

from

a high

source

resistance.

Other

frequencies

to avoid are60Hz

and

its

lower

order

harmonics.

By avoiding

these

frequencies,

the

operator

assures

that

he will be

measuring

the

signal

of

interest

only,

uninfluenced

by

power

frequency

pick-up,

either

internalorexternal.

4.2E

GROUNDING

In

any

system

processing low-level signals,

proper

grounding

to

minimize

the

effectsofground

loop

currents,

usually

at

the

power

frequency,

is an

important

consideration.Inthe

case of

the

Model

128A,

special design

techniques

have

been

employedtogive a high

degreeofground-loop

signal

rejectioninsingle-ended

applications.

Even so, it will

often

prove advisable to

operate

differentiallv.

even

when

ex-

amining

a single-ended signal

source,

10 achieve

the

greatest

possible

rejection.

Figures IV-l

and

IV-2

illustrate

this

point.

Note

from

Figure IV-l

that

the

signal

source

is

located

inside a

grounded

enclosure

(shield), to

which

signal

source

commonisattachedatone

point.

The

braid of

the

/

'U

,

INDUCED emf

(DISTRIBUTED AROUND

LOOP)

RESISTANCE OF AC GROUND

PATH--J

Figure

IV·l.

GROUND·LOOP

SUPPRESSION

BY

TEN-OHM

INPUT

GROUND

SHIELD

l_

10"

L

INDUCE~.mf

'"

(CJSTRIBUTED AROUND LOOP) J

RESISTANCE OF AC GROUND PATH

Figure

IV-2.

DIFFERENTIAL

MEASUREMENT

OF

"SINGLE-ENDED"

SIGNAL

Figure

IV-4.

EXAMPLEOFEVERYTHING

"DONE

WRONG"

The

reductionofpower

frequency

interferenceisnot

the

only

benefit

to be

derived

from

proper

grounding

and

differential

operation.

A

much

more

serious

source

of

interferenceiscoherent

interferenceatthe

signal

frequency

which

results

when

drive signal

currentisallowedtoflow

through

the

braidofthe

signal

cable.

Figures IV-4

and

IV-5

are

providedtoillustrate

this

problem

and

the

steps

which

can

be

takentoprevent

it.

To

begin

with,

Figure

IVA

shows

the

experiment

with

just

about

everything

possible

"done

wrong".

The

lock-in

amplifierisoperated

single-

ended.

The

ground

connectionsatthe

experiment

are

made

to

the

enclosure,

allowing

currentstoflow

through

it,

and,

in

particular,

the

drive signal

currents

have

the

opportunity

to

flow th rough

the

braidofthe

signal cable.

The

drive

signal, in

additiontoproviding

the

reference

input

signal

to

the

Model

l28A,

can be

presumedtobe driving

other

componentsofthe

system

as well.

Dependingonthe

nature

of

the

experiment,

these

currents

could

range

from

very

small to

quite

large,

perhaps

even

amperesifthe

experiment

involves driving a

low

impedance

coil.

Note

that

the

various

loads

for

the

drive

are

represented

by a single

resistor

returnedtoground

somewhere

on

the

enclosure.

Most-of

this

drive

signal

current

canbepresumedtoflow

through

the

shield

backtothe

drive

signal

source.

However,

a small

but

significant

part

of it will

flow

through

the

parallel

path

consistingofthe

braidofthe

signal cable,

the ten

ohm

resistor,

and

the

braidofthe

reference

signal

cable.

The

voltage

dropofthis

current

across

the

resistanceofthe

signal

cable

brai-J,

even

though

attenuated

by

the

ratio

of

the

ten

ohm

resistortothe

braid

resistance, can

constitute

a

serious

sourceofinterference

at low signal levels,

particular-

ly in

that

this

interferenceiscoherent,

in

phase,

and

directly

addstothe

signal of

interest.

It is

not

hard

to

envision

situations

where

this

interference

signal

could

well

be larger at

the

Inputofthe

lock-in

amplifier

than

the

signal

of

interest

itself.

signal

cableisgrounded

directly

to signal

source

common

as

well,

thereby

assuring

that

no signal

currentsorground-

loop

currents

will

flow

through

the

shield,'adesirable

condition

for

the

best

possible

shielding.

The

Model

l28A

is

operated

single-ended,

using

the

"A"

input.

Note

that

the

"low"

side of

the

amplifier

inputisnot

groundedtothe

chassis

directly

butbywayofa

ten

ohm

resistor.

Further

note

that

the

braidofthe

signal

cableisreturnedtothis

resistor,

and

nottothe chassis. A

ground

loop

generator

is

indicatedasbeing

connected

between

the

chassis of

the

Model

l28A

and

siqnal

source

common.

This

path

would

ordinarily

consistoftheacground

"third

wire",

paralleled

by

the

braids

of

other

cables

connecting

the

system

components.

The

ground

loop

generator

will

cause

currents

at

the

power

frequencytoflow

through

the

braid of

the

signal cable,

through

the

ten

ohm

resistor,

and

back

through

theacwound

pathtocomplete

the

loop.

Because

of

the

ten

ohm

ground

employedinthe

Model

l28A,

these

currents

are

attenuated

over

what

they

would

be if

the

Model

128A

input

were

returned

directlytothe

chassis.

More

importantly,

most

of

the

ground

loop

signal is

dropped

across

the

ten

ohm

resistor

and

little

across

the

braid of

the

signal

cable,

the

ratio

being

the

ten

ohmsofthe

resistor to

the

10 to

20

milliohms

(typical)ofthe

braid

resistance, As far as

the

input

of

the

Model

l28A

is

concerned,

the

ground

loop

signal is

reduced

by this

ratio,

and

the

ground

loop

interferenceisthus

perhapsafactor

of

five

hundredorone

thousand

less

than

wouldbethe

case

without

the

ten

ohmqround.

However, in

some

appl

ications, th is

would

notbeenough.

Figure IV-2

shows

how

this

same

signal

couldbemeasured

operating

the

Model

128A

differentially.Inthis

instance,

the

Model

l28A

Input

Selectorissetto"A-B"

and

two

input

cables

are used,

one

connectedtothe

signal

source

and

the

other

to siqnal

source

common.Atthe

source

end,

the

braid

of

both

cables is

returned

to

signal

source

common.Atthe

lock-in

amplifier

end,

the

ten

ohm

ground

serves to

attenuate

the

wound

loop

currents

and

maintain

a

small

ground

loop

signa!

drop

across

the

braids

the

same

as

in Figure

IV-l.

However,inthe

first

instance,

the

amplifier

"looked"

at

the

potential

difference

between

the

center

conductorofthe

cable

and

the

braid.Inthe

second,

it sees

the

potential

difference

between

the

"A"

Input

and

"8"

Input.

The

ground

loop

siqnal

current

flowinginthe

signal

cable

braid is of no

consequence.

The

very high

common

mode

rejectionofthe

amplifier

assures

that

common

mode

power

frequency

pickup

will

not

be a

problem

either.

However,

when

operating

differentially,

it is

important

to

takealittle

trouble

to assure

that

common

mode

inter-

ference arising in

ground

loopsisjust

that,

that

is,

without

a

significant

differential

component.

This

should

not

prove

a

problemaslonqasboth

signal

cables

follow

the

same

path,

Figure IV-3

shows

the Model

128A

operated

differentially

to

measurean"off

~JI

ound

" siqn al.

The

most

important

consideration

in an

application

of this

type

is to be sure

that

the

common

mode

siqnal

componentisnot

so large as

to

exceed

the

common

mode

input

limitofthe Model

128A.

(See Specs.)

IV-2

DRIVE

~

~:£"

0'"''''''

,.0'"

-1

AS

FUNCTIONOFEXPERIMENTAL

PARAMETEI

L..-

...I

Figure

IV-3.

DIFFERENTIAL

MEASUREMENT

OF

"0

FF-GROUND"

SIGNAL

DRIVE

LOAD

k =

Boltzmann's

constant=1.38x10-

2 3

joules/kelvin

T =

absolute

temperature

in kelvins

R

s

= Resistance in

ohmsofthe

resistive

component

of

the

impedance

across

which

the

voltage

iii

mea-

sured

B =

Bandwidth

over

which

the

measurementismade

Mathematically

expressed,

noise figure

canbestated

as:

NF(dB)

=

2010

9 10 Noise

V.olta~_~~g~!':l_~t

of

Amplifier

That

PortionofNumerator

Attributable

IV-2

to

Source

Thermal

Noise

Figure IV-6.

TYPICAL

MODEL

128A

NOISE

FIGURE

CONTOURS

~~

where

Etis

the

total

noise

referredtothe

input

in volts rrns

and

all

other

terms

are as

defined

previously.

Note

thar

IV-3

10~

Et=V4kTBRX10NF/20

o

11>

N

~

Ul

2

:r

o

~

""

o

z

c(

Ii;

~

0:

""

U

~

~

102::-:~__~~---9:::'-'-_--.-l.-

__

.L,

__

--.J

0.5

10

CENTER fR

Noise figure is

not

constant

but

varies as a

functionofthe

source

resistance,

frequency,

and

temperature.

When

the

lociofall

points

having

the

same

noise figure are

plotted

as

a

functionoffrequency

and

source

resistance

(temperature

fixed),

the

result

is a noise figure

contour.

A full

set

of

contours

completely

specifies

the

noise

characteristics

cf

the

antpti#er

ruler its

working

range. Figure IV-6

contains

a

full

setofcontours

for a

typical

Model

128A.

The

utility

of

these

contours

are, first of all,

that

they

clearly

indicate

the

best

noise

performance

region in

terms

of

operating

frequency

and

source

resistance,

and

secondly,

that

they

allow

onetodirectly

compute

the

total

noise

accompany-

ing

the

signal

(amplifier

noise

and

source

thermal

noise

considered,

other

noise

sources

neglected).

.The

relating

formula

is:

IV-l

4.2F NOISE

Any

electronic

signal processing

system

adds

noise to

that

already

accompanying

the

signal to be

measured,

and

a

Lock-In

Amplifier

is no

exception.

Even

though

the

method

of signal processing

used

in a Lock-In

Amplifier

allows very large

improvements

in signal-to-noise

ratio

to be

achieved,

the

amount

of noise

contributedbythe

Lock-In

Amplifier itself

affects

its

performance

and

limits

the

achievable

improvement.

Figure IV-5

shows

the

steps

which

can be

taken

to

circumvent

this

problem.

Allofthe

drive signal

current

is

returned

directlytothe

drive signal

source

except

for

the

very small

component

(reference

input

resistance of Model

128A

is 10

MS1)

whichisappliedtothe

Model

128A

by

way of

the

Reference

Input.

Second,nocurrent,

whether

drive

current,

reference

current,

or signal

current,isallowed

to

flow

through

the

experiment

shield;

the

shield

contacts

groundatone

point

only.

The

only

coherent

signal

which

can

flow

through

the

parallel

pathofthe

signal cable braid

is a small

portionofthat

allowed by

the

ten

megohm

Reference

Input

resistance.

Furthermore,

the

use of

differ-

ential

operati

on assures

that

even th is small

amountcan

have no

effect.

By using

the

arrangements

indicated,

one

could

operate

with

very large drive

currents

without

concern

that

they

might

contaminate

the

signalofinterest.

If

electrostatic

couplingofthe

drive signaltothe

detector

is

a

problem,

mountingaconducting

material

around

the

signal

source

detector

should

prove

helpful.

The

electro-

static

shield

shouldbeconnectedtothe

systematbut

one

point,

signal

source

common.

Figure IV-5.

ERRORS

DEPICTEDINFIGURE

IV-4

CORRECTED

One

convenient

wayofspecifying

the

noise

performance

of

an

amplifieristo

speakofits

noise

figure,

which

indicates

the

amount

of noise

the

amplifier

adds

to

the

source

thermal

noise.

Source

thermal

noise is used as

the

basis for

comparison

because

it is

completely

predictable,

always

present,

andisthe

least

amount

of noise

which

can

possibly

accompany

any

signal. Its value, in volts rms, is given by

the

following

formula.

where:

En = rms noise voltage

within

the

bandwidthofthe

measurement

IV·3

with

a noise figure of 3 dB,

the

amount

of

noise

contributedbythe

amplifier

is 1.4

times

the

source

thermal

noise.At1.4

times

the

thermal

noise,

the

amplifier

noise

just

beginstobe

noticeable.

At

lower

noise

figures,

the

amplifier

for

all

practical

purposes

mayberegarded

as

noiseless.

Generally

speaking,ifone

can

operate

anywhere

inside

the

3 dB

contour,

amplifier

noise

considerations

may

be

neglected.

As critical as

amplifier

noise is in

certain

applications,

it is

nevertheless

possibletooveremphasize

its general

impor-

tance.

For

example,ifthe

signal

amplitudeissignificantly

higher

than

the

amplifier

noise,

the

subject

becomes

purely

academic.

Similarly,ifpreamplificationisprovided

ahead

of

the

Model

128A,

with

the

result

that

the

amplified

source

noise at

the

inputtothe

Model

128A

is far

greater

than

the

amplifier

noise,

thereislittle

point

in striving to

operate

inside

the

3 dB

contour

of

the

Model

128A.

However,

whereapreamplifierisused,

it is

important

that

these

same

considerations

be

carefully

evaluated

for

the

preamplifier.

In

other

words,

when

using a

preamplifier,

try

to

operate

inside

the

3 dB

contourofthe

preamplifier.

A

quick

check

of

Fiqure iV-G

followed

by a

computation

of

the

total

noise

(Equation

IV-3)

should

give

one

a realistic

idea of the

importanceofamplifier

noise

considerations

to

the

measurementathand.

Where

amplifier

noise is a

consideration,

one

should

try

to

operate

inside

the

3 dB

contourbyappropriately

adjusting

the

operating

frequencyorsource

resistance.

The

choice

of

operating

frequencyisusually

determined

by

the

type

of

sensor

used

andbythe

capabilitiesofthe

chopper,

where

oneisused.

Often,

the ex

perimenter

has

some

control

over

his

choiceoffrequency

and

so can

adjust

thingssothat

he

is

operatingatthe

low

noise

endofthe

frequency

range

available to

him.

The

situation

with regard to

source

resistance

may

be less

flexible

because

the

source

resistanceisusually

determined

solely by

the

typeofsensor

used.

Once

the

commitment

to

a

particular

sensor

has

been

made,

it can be

difficult

to

adapt

the

systemtoanother

one.

Hence, in

applications

where

noise is a

uotcntial

problem,

the

choiceofsensor

shouldbemade

carefully

and

with

full regard for

the

noise

characteristics

of the amnlifier.

Two

situations

deserve special

attention,

the

first being

operation

fromasource

resistance very

much

lower

than

optimum,

and the

second

being

operation

fromasource

resistance very

much

higher

than

optimum.Ineither

case,

there

may be a

temptation

to

"improve"

the

source

resistance

situation

by

the

use of resistors. In

the

case

of

the low

source

resistance,

one

maybetemptedtoconnect

a

resistor in series

with

the

source.Inthe

caseofthe

high

source

resistance,

one

may be

temptedtoconnectaresistor

in parallel

with

the

source.

Unfortunately,

neither

approach

to

the

problem

does

any

good

and,infact,

both

will

result

in

further

degradationofthe

signal-to-noise

ratio

ahead

of

the

lock-in

amplifier.

The

series

resistor

adds

its

thermal

noisetothat

already

accompanying

the

signal.

Although

the

amplifier

showsa"better"

noise figure

than

before,

it is

only

because

the

ampl ifier

noiseisnow

less relative to

the

thermal

noise

of

the

combined

resistances (source

plus

IV-4

series

resistor).

The

signal is no larger (in

fact,itmay

well

be

attenuated),

the

noise is

greater,

and

the

improved

noise

performance

is illusory. Recall

that

noise

figure

only

relates

amplifier

noisetothermal

noise, and

does

not

denote

the

absolute

valueofamplifier

noise.

Connecting

a parallel

resistor

to

lower

a high

source

resistance has a similar

effect.

Even

though

the

thermal

noise

doesgodown,

the

signal

amplitude

goes

down

even

more.

For

example,

if a

source

of

resistanceRwere

paralleledbyanother

resistorofthe

same

value,

the

signal

ampl

itude

wouldgodown

by a

factoroftwo.

However,

the

thermal

noise

would

onlybereducedto.707ofits initial

value

(thermal

noise

varies

directly

with

the

square

root

of

the

resistance),

withanet

degradation

in signal-to-noise

ratio.

In

operating

from

low

source

resistances,

however,

one

can

usually

improve

the

situation

dramatically

by using a

transformer

to

raise

the

source

resistance

seen

by

the

amplifier.

The

improvement

one

obtains

withatransformer

is real

because

the

amplitudeofboth

the

signal and

the

noise is

increasedbythe

turns

ratio.

The

source

resistance is

increased by

the

squareofthe

turns

ratio.

For

example,

if

one

hadaten

ohm

source,

one

could

use a 1:

100

step-up

transformer,inwhich

case

the

amplifier

would

see a

source

resistance of

100

kn.

At

100

kS1

the

amplifier

adds

little

additional

noise. Even

though

the

thermal

noiseofthe

transformer

addstoth at of

the

source,

a very

considerable

improvement

is usually

achieved.

P.A.R.C.

manufactures

a

line of

suitable

signal

transformers,

each

designed

for

optimum

operation

over

a given

frequency

range. Perfor-

mance

information

can be

obtained

from

the

factoryorone

of its

representatives.

In using an

external

transformerinconjunction

with

'the

Model

128A,

single-ended

operationofthe

lock-in

amplifier

is advised.

The

extremely

high

inherent

common

mode

rejectionofthe

transformer

makes

differential

operation

of

the

lock-in

amplifier

unnecessary.

When

working

from

a high

source

resistance,

one

could,

in

principle,

use a

transformerinthe

same

mannertoimprove

noise

performance.

Unfortunately,

practical

transformer

design

considerations

usually

prevent

one

from

doing

so. As

a

result,

the

options

availabletoan

experimenter

working

with

a high

source

resistance device,

such

as a

photo-

multiplier,

are

limited.

Practically

speaking,

the

best

one

candoistomake

the

load

resistor

as large as possible.

The

larger

the

source

resistor,

the

less

the

shunting

effect

it will

have,

and

the

better

the

signal-to-noise

ratioatthe

input

of

the

amplifier

will be.

That

this

is so

becomes

clear

when

one

recalls

that

the

signal

amplitude

varies

directly

with

the

load

resistance, while

the

thermal

noise varies

with

the

square

rootofthe

resistance.

Note

that

the

entire

preceding

discussionofnoise

is based

on

comparing

the

noise

generatedbythe

amplifier

with

the

source

thermal

noise. In

many

situations,

other

types

of

noiseofinterference

may

accompany

the

signal as well and

could

even

dominate

it.

Where

this is

the

case,

the

amplifier

can

only

perform

"better"

than

the

noise figure

contours

indicate

because

the

noise

figures are basedona cornpari-

son of

amplifier

noise

with

the

minimum possible noise

which can

accompany

any

signal,

namely,

the

source

thermal

noise.

4.3 OPE;RATING THE MODEL

128A

4.3A

INliRODUCTION

Operation/of

the

Model

128Aisstraightforward.Inmost

instances,

the

operator

simply

connects

the

reference

signal,

waits

for

the

REF

UNLOCK

light to go

out,

and

then

connects

the

signal to be

measured.

The

Sensitivity

and

Phase

controls

are

then

adjusted

for

maximum

output

without

overload.

Should

overload

occur,

thedcfiltering

(Prefilter and

Time

Constant)

is

increased

and/or

the

sensitivity is

reducedasrequiredtoeliminate

the

overload.

The

readinq can

thenbetaken

.

•n

many

Situations,

achieving a successful

measurement

will

depend

notsomuchoncritically

adjusting

the

Model

128A

controlsason

taking

the

proper

steps

indicated

by

the

preliminary

considerations

discussed in

Subsection

4.2.

Factors

such as

proper

grounding

and

operating

inside

the

3

dB

contour

are

most

important

in

making

low level

measurementsofnoisy

signals.

4.38

REFERENCE

CHANNEL

(see

Subsection

4.9

for

additional

information)

Referende

Signal

Requirements

An outsltanding

featureofthe

Model

128A

is its

unique

reference

channel

circuitry

which

allows ittolock

onto

and

trackawide

range of possible

reference

input

waveforms.

Once

locked

on,

the

reference

remains

locked

on,

even if

the

reference

input

signal

changesinfrequency.

There

are

no

Reference

Channel

controlsofany

kind

which

must

be

adjusted

for

proper

reference

channel

operation.

Once

the

light goes

out,

all

that

remains

is to

adjust

the

Phase

controlssothat

the

Reference

signal

appliedtothe

mixer

is

-t

the

proper

phase

relativetothe

signal to be

measu

red. As

statedinthe'

specifications,

the

only

requirements

on the

reference signal are

that

it swing at least plus

and

minus

50

mV

with

respect

to its

mean,

thatitcross

its

mean

twice

each cycle,

and

thatitremainoneach

sideofthe

mean for

at least

100

ns. Sinewaves,

square

waves, triangle waves,

and

many

others

are all

suitable.

However,

for

best

phase

accuracy,

a 1 V rms sinewave is

recommended.

One

waveshape

whichatfirst

glance

may

seem

to suffice,

but

which

does

not,isthe

very

narrow

low

duty

factor

pulse.

For

example,

suppose

one

intended

to use as a

reference

signal

the

pulse

train

depicted

in Figure IV-7,

that

is, pulses

having an

amplitude

of 1 V, a

durationof1

JiS,

andaperiod

of 1 ms.

The

mean

of this signal

wouldbeabout1mV,

and

the

excursions

relativetothe

mean

wouldbe+999

mV

and

-1

mV.

Thus,

the

individual pulses, even

though

they

exceed

the

minimum

excursion

requirementonone

side by

some

950

mV,

lack

meeting

the

excursion

requirement

on

the

other

side

of

the

mean

by a full

49

mV

(50

mV

required),

with

the

result

that

the

Reference

Channel

will

not

function

properly.

Thus,

when

using pulses as

the

reference

input,

take

care

that

the

durationofthe pulses,

relative

to

the

pulse

period,isgreat

enough

to give a

mean

of at least50mV.Inthe

example

just

cited,

if the pulse

duration

were

increased to

100

JiS

as

shown

in Figure

IV-5

AMPLITUDE : 1 V

J

~

ERIOO

: 1

ml

DURATION:

1

III

MEAN:

I mV

'-

-'-_____

EXCURSIONS:

.!"f~vmv

A. PULSE TRAIN WHICH WILL NOT TRIGGER BECAUSE

EXCURSION CRITERIA NOT

MET

J

~

~

AMPLITUDE

: 1V

PEROO:

1ms

~tti~~~~

~~

liS

EXCURSIONS

:+900

mV

- 100 mV

-~

B PULSE TRAIN WHICH MEETS

All.

CRITERIA, INCLUDING

EXCURSION REQUIREMENTS

Figure

IV-7.

PULSE

TRAINASREFERENCE

DRIVE

IV-7B,

the

mean

would

increase to 100 m V,

and

the

excursions

(900

mV

one

side, 100 mV

the

other)

would

be

more

than

adequate

for

proper

triggerinq.

Even

though

the

Model

128A

can

accept

and

trackawide

range

of

possible

reference

siqnals, it is

nevertheless

important

that

the

reference

signal used be relatively

noise

free. Any

superimposed

noise

can

cause small

zero

crossings

to

occurinthe

region of

the

main

waveform

zero

crossings,

with

the

result

that

the

Reference

Channel

momentarily

"sees"amuch

higher

reference

frequency

than

what

is

really

there.

When

this

happens,

the

reference

"lock"

can

be lost.

Frequently,

moderately

noisy siqnals

canbecleaned

up

sufficiently

for

satisfactory

operationbyinterposing

a

single-section low-pass

filter

between

the

reference

siqnal

source

and

the

ReferenceInput

connectorofthe

Model

128A.

On

later

instruments,

spaceisprovided

for

mounting

the

filter

components

on

the

Reference

printed

circuit

board.

Locationofthe

mounting

holes

is

indicated

on page

VII·5.

To

install

the

filter,

transfer

the wire

which

norrnally

goes

to

quick-disconnect

J419

over

to

quickd

isconnect

J429.

Then

install

the

filter

components,aresistor

for

RX4

and

a

capacitor

for CX 1

(RX4

and CX 1 are de

siqnations

given

on

the

schematic

and

parts-location

diagram

for

these

corn-

ponents).

In

most

instances,

optimum

performance

is

obtained

by

setting

the

filter

corner

frequency

(f

::::

1/4rrRC)tothe

intended

reference

frequency.

Switches
Three

switches

are

associated

with

operation

of the

Reference

Channel.

Oneofthem,

the

f/2f

SWitch, is

located

at

the

front

panel.

The

other

two are

internal.

The

front

panel

switch

determines

whether

the

mixer

will be

driven

at

the

frequency

of the

applied

reference

signal or at twice

the

frequencyofthe

applied

reference

signal.

The

"2f"

position

is

used

for

second

harmonic

studies.

For

normal

operation,

the

switchissetto"f".Toexamine

harmonics

higher

than

the

second,

the

operator

would

havetosupplyareference

signal at

the

frequencyofthe

harmonic

to be

measured.

One

should

check

the

position

of this

switch

before

connecting

the

Reference

signal,

when

the

switch

posi

tion

is

changed

during

operation,

the re"ference

channel

will

unlock,

and

time

will be

lost

III

waitill~]

for it to lock on

5 10

1

5 10 2

5 10 25 10 2

2

5 ° 2

FREOUENC.,.

---.

0

0

""-

'SLOW' REF.

r<.:0OE

'FAST' REF

"....,....",

,MODE

-1\

~

I---

\

0

/'

I

REF SIG IN IV rml SINE WAVE
SIGNAL IN' 100 mV

,ml

SINEWAVE

I I I

I I

,

1 1

1

,I

,I

'"

-2

0

+2"

°

+1

-1

+3

again. In

most

situations,

the

lost

time

wouldbebut

a few

seconds

and

of little

consequence.

However, if

one

were

operatingata low

frequency,itcouldbelengthy.

The

internal

switches

determ

ine

the

lock-on range,

either

.5

Hz -

100

kHz or 5 Hz

100

kHz. With

these

switches

set

to

either

FASTorSLOW,

the

unit

will

lock

onto

reference

signals in

the

frequency

rangeof5 Hzto100

kHz

without

difficulty.

However,

only

when

the

switches

are set

to

SLOW will it lock

onto

reference

signals

which

are

below

5

Hz.

The

switches

shouldbesettoSLOW

only

for

operation

below 5 Hz,

because

the

time

required to achieve

frequency

lock

is longer

with

the

switch

settothis

position

than

when

it is

settoFAST.

Detector

Biasing

There is no

provision

for

detector

biasing at

the

Reference

Inputbymeans

of an

internal

network.

The

networkisnot

provided

but

mustbefurnishedbythe

operator

accord

ing

to

the

impedance

and

voltage

requirements

of his

detector.

The

parts

location

diagram

on page VII-5

shows

where

to

install

the

resistors.

Fi~re

IV-B.

NET

PHASE

SHIFT

BETWEEN

SIGNAL

AND

REFERENCE

CHANNELS

AS A

FUNCTIONOFFREQUENCY

Phase

Controls

A high

resolution

potentiometer

covering

a range

of

0-to-l00

degrees

worksinconjunction

withaQuadrant

switchtodetermine

the

phaseofthe

synchronous

detection

process

with

respecttothe

phase of

the

applied

reference

signal.

The Phase Sensitive

Detector

provides

a dc

output

propor-

tional to

the

amplitudeofthe

input

signal. This

output

varies

with

the

cosineofthe

angle

between

the

reference

and

input

signals. When

the

Phase

controls

are

adjusted

for

maximum

output,

the signal

amplitude

can be read

from

the panel

meter

and

the

phase of

the

signal can be read

from

the

Phase dial.

It may

happen

that

meter

fluctuations

due

to noise will

makeitdifficult

to find the

setting

which

gives

maximum

output.

Where this is the case, it will usually

prove

more

accurate

and

expedienttoadjust

for

the

null

obtained

when

the

reference

phase is set at 90° relative to

the

signal phase.

Once

the

null is achieved,

the

Phase

Quadrant

switch

can

then

be

rotated

the

one

position

necessarytoachieve

maximum

outputsothat

the

amplitude

can

be read

from

the

meter.

The

absolute

accuracy

and

resolution

of the Phase

controls

are

statedinthe

specifications. Figure IV-8

shows

the

typical

net

phase

shift

through

the

unit.

Additional

phase

shifts are

introducedinthe

Signal Channel at

the

frequency

extremes. Even at middle frequencies,

the

HI-PASS

and

LO-PASS filters can have an

effectonthe signal phase. A

phase

calibration

can

easily be

made

at any

frequency

by

connecting

the

input

signal (noiseless)toboth

the Signal

and Reference

Inputs,

followed

by

adjusting

the Phase

controls

for

maximum

output.

The Phase

controls

then

indicate

the

net

phase sh itt in

both

the

Signal and

Reference Channels. This

shift

can

thenbesubtracted

from

any

phase

readings

taken

while

operatingatthe

calibration

frequency.

Reference

Monitor

Connector

An

optional

Reference

Monitor

output

canbeprovided

at a

rear-panel

connector.

The

Reference

Channel

output

is

available at this

connector.

This signal is a

square

wave

taken

from

aheadofthe

Mixer

but

after

the

Phase

Control

circuitry,

and

so is at

the

frequencyofthe

applied

reference

signal

(twice

the

frequencyinthe

case of

"2f"

operation)

andatthe

phase set

with

the Phase

controls.

Standard

TTL

logic levels are

employed.

Logic

"0"=0.2V±0.2Vand

Logic"1"=+3.5V±1

V.

4.3C

SIGNAL

CHANNEL

Introduction

Operationofthe

Signal

Channel

controlsisstraightforward.

The

Input

Selector

is set to

"A",

"A-B",

or

"-B"

as

appropriate,

the

Sensitivity

switchisset

for as

near

full-scale

output

as possible,

and

the

filters are used

to

narrow

the

bandwidth

ahead

of

the

Mixer. In

some

applications,itmay

be desirable to

incorporate

the

optional

tuned

amplifier

into

the

Signal Channel as well. A

further

discussion of

these

topics

follows.

Input

Selector

Switch

With

this

switch

set to

"A",

the

signal

appliedtothe

"A"

Inputisprocessedbythe

instrument.

Signal

appliedtothe

"B"

Inputisdead-ended.

Similarly,

when

the

switchisset

to

"-B",

the

situation

is the same

except

that

the

roles

of

the

"A"

and

"B"

inputs

are reversed.

Another

difference

is

that

the

"B"

Inputis1800out-of-phase

with

respecttothe

"A"

Input.Inother

words,

if a signal

which

yields

positive

output

meter

deflection

when

appliedtothe

-"A"

Input

is

appliedtothe

"B"

Input,

an equal

but

negative reading will

be

obtained.

In

the

"A-B"

position,

the

instrument

operates

differentially,

that

is,

only

the

difference

between

the

signals

appliedtothe

two

inputs

is processed

and

read

out.

As discussed in

Subsection

4.2E,

it is generally

advantageoustooperate

differentially,

even

when

process-

ing signals

from

a single-ended

source.

IV-6

Sensitivity Switch

The

Sensitivity switch

should

be settoprovide as near

full-scale

output

as possible. In very high noise

situations,

it

may be necessary to

operate

with less sensitivity

than

would be

employed

if processing a noise free signal of

the

same

amplitude.

If

overload proves a

problematthe

sensitivity which yields

maximum

on-scale

output

meter

deflection,

there

are a cou pie of th ings

the

operator

can

try

before resorting to lowered sensitivity

operation.

First, he

can increase the

output

filtering using

the

Prefilter and

Time

Constant

switches. With a very low

time

constant,

output

amplifier overload can

occur

when

processing noisy

signals. This

type

of overload

problem

can generally be

resolved by

operating

withatime

constant

settingof.3

SEC or higher,

and

with

the

Prefilter set to

100

mSEC

or 1

SEC, as required. Additionally,

one

can

narrow

the

noise

bandwidth ahead

of

the

Mixer by means of

the

high-pass

and low-pass filters. If

neither

increased

time

constant

nor

the filters brings

the

overload

under

control,

there

still

remains

the

additional

stepofusing the internal

tuned

amplifier in

the

caseofa

unit

equipped

with

this

option.

Assuming

noneofthese

steps

helps

the

overload

problem,

such as

would

happen

if noise of

sufficient

amplitude

to

cause overload is at

the

frequencyofthe

signal being

measured,

there

is no alternative

but

to reduce

the

sensitivity .

Hi-Pass and Low-Pass Filters

The

function

of these filters is to eliminate as

much

interference and noise as possible while having minimal

effectonthe

signal of interest. Under

most

conditions,

by

setting these filters so

that

they

bracket

the

signal fre-

I

v---

~

,._._.

-:

-:

/

V

l

/

,/

-

V'H'

~H'

,---

--7

f.-

/

,~---

V

~~~::;;:~~~~::I~~OSN

.t=t-_=;

.... -.

V

.-

._-.-

.'

,

"

'0

quency

as closely as possible,

the

noise

toleranceofthe

instrument

will be increased and

the

noise fl

uctuations

at

the

output

of the Model

128A

will be

reduced.

However, in

using

the

filters, it is

importanttotake

their

effectonthe

signal of

interest

into

account.

Figures IV-9 and IV-10

show

the

amplitude

and phase characteristics of these filters as a

functionofswitch

setting

and frequency. To find

the

net

effect

in a region

where

both

filters

affect

the

signal,

multiply

the

amplitude

transfer fractions and add the phase

shifts.

Use of

the

MIN.

and

MAX. POSitions simultaneously

provides a flat response curve over

the

full

operating

range

of

the

Model

128A

(see specifications). A flat response is

useful for

operationinsituations

where the signal fre-

quency

changes by large factors during

the

course

of the

measurement.

In

certain

applications,

the

operator

rnight liketohave

"3

dB

down"

frequencies

other

than

those

provided. This can

be

done

by changing

the

value of some internal capacitors.

Each of

the

two

filters has a

separate

capacitor

for each of

the

three

switch positions.

The

capacitors

which

determine

the

MIN

and

MAX response characteristics

should

not

be

changed.

The

other

two

can be.

The

Parts

Location

Diagram on page VII-2 can be used to

identify

these

capacitors.

Schematically,

they

are

shown

on page VII-4. In

the

case of

the

Low Pass filter,

the

relationship

between

capacitance

and

3 dB

down

frequency

is given by:

IV-4

!:[---~.~

""

--+----'-\---cA"--

to-

----

.1 .2

'R(OO(NC,.frtHI

Ptl,lS(IH"IHPQS,11OJ\l
ISUHO(fIH[O

Figure

rv-s,

AMPLITUDE AND PHASE

CHARACTERISTICS

OF HI-PASS FIL

TER

IV·7

Figure

IV·l0.

AMPLITUDE

AND

PHASE

CHARACTERISTICS

OF LOW-PASS

FILTER

Two

separate

dc filters are

provided.

The

first, called

the

DC

PREFILTER,

provides

filtering

time

constantsof100

ms, 1 s,orOUT,inwhich

the

filtering

time

constant

is

negligibly small.

The

second,ormain

TIME

CONSTANT

filter, allows filtering

time

constants

from

1 msto100sto

be

selected,inaddition

to MIN

(time

constant

about

.7 ms)

and

EXT

(time

constant

determinedbyexternal

capacitors

connected

to rear-panel

octal

sockets).

Both

filters are

single-section filters having a 6

dB/octave

rolloff.

The

filters

have an

accumulative

effectasshown

in Figure IV-11.

Filters

The

primary

functionofthe

Output

Channel

is to

act

as a

low-pass

filter

and

eliminate

any ac

components

at

the

output

of

the

Mixer.

Inasmuchasonlydeat

the Mixer

output

represents

the

in-phase

componentofthe

signal

of

interest

(the

ac results from noise), an

improvement

in

signal-to-noise

ratioisobtained.Inprinciple,

the

signal-to-

noise

ratio

can be

improved

to any

arbitrary

degree

simply

by

making

the

filter

time

constant

long

enough.

Practical

considerations,

however,

generally set

the

limittowhat

can

be achieved.

The

improvement

in signal-to-noise

ratio

varies

with

the

square

rootofthe

time

constant.

As a result,

the

measurement

times

rapidly

become

lengthyasthe

time

constant

is increased to

obtain

better

signal-to-noise ratios.

As a practical guide,

the

correct

filtering

time

constant

is

the

one

wh ich

reduces

the

noise to an

"acceptable"

level.

where (for

both

formulas)

C

=

the

capacitance

in Farads,

and

f

J dR

=the

desired

3 dB down frequency in Hz.

For

the

Hi-Pass

filter,

it is:

The

equivalent

noise

bandwidth

of

a single-section 6

dB/octave

filteris1/4TC.

Its rise

time

from

10% to 90%

of

full

amplitudeis2.2

TC (0%to95% is 3 TC). If

both

the

Time

Constant

filter

and

the

Prefilter

are

set

the

same,

the

effectisthe

same

as if

one

had

a single

two-section

filter

with a 12

dB/octave

rolloff.

The

equivalent

noise

band-

widthofthis

filter

wouldbe1/8TC

and

the

10%to90%

rise

time

wouldbe3.3

TC (0%to95% =

4.8

TC). When

both

filters are used

but

with

different

settings,

the

relationship

defining

the

equivalent

noise

bandwidth

and

rise

time

as a

functionoftime

constantismore

complex.

For

all practical

purposes,ifthe

time

constantofone

is a

factorofthreeormore

longer

than

the

other,

the

one

with

the

longer

time

constant

dominates

and

the

single

section

expressions

using

the

longer

time

constant

characterize

the

rise

time

and

equivalent

noise

bandwidth

toagood

approximation.

Nevertheless, even

though

the

prefilter

may

do relatively

littletofurther

reduce

the

equivalent

noise

bandwidthifthe

main

Time

Constant

setting

is longer, its

effectinsmoothingarecording

can be significant.

The

usual

procedure

for

setting

these

filters is to leave

the

Prefilter

settoOUT

andtoadjust

the

main

Time

Constant

filter as

requiredtoreduce

the

noisetoan

acceptable

level.

However, if

the

noise level is

sufficiently

high to cause dc

amplifier

overloading, it will be necessary to

set

the

Prefilterto100msor

perhaps

evento1 s to

stop

the

overload.Inmany

instances,

useofthe

Prefilter will

prove

to be

unnecessary.Itmightbenoted

that

the

prefilter

is

particularly

useful

whenarecorder

is being usedtomonitor

the

output

of

the

instrumentinthat

recorder

"jitter"

is

significantly

reduced.

Adjusting

the

Model

128A's

controls

is generally easier

with

the

Prefilter

OUT.

With

the

Time

Constant

switch

settoEXT,

intermediate

time

constant

values or

time

constants

longer

than

100

seconds

canbeobtainedbyconnectinganexternal

capaci-

tor

between

pins

eight

and

nineofthe

11-pin

socketatthe

rear panel.

The

formula

relating

the

capacitor

value and

time

constant

is: C =

TC/100

J1F.

Any

low-leakage film

capacitors

ratedat50Vor

higher can be used. Do

not

use

electrolyticortantalum

capacitors.

IV-5

o1 1.6

C = 25 X 10

"/fl,//I

FREQUENCY

---

...

01 .016 05 1 .16

4.3D

OUTPUT

CHANNELCONTROLS

Figure

IV-l1.

EXAMPLESOFOUTPUT

FILTER

INTERACTIONS

The

following

example

illustrates

how

the

Zero

Suppress

feature

can be usedtoread signal

amplitude

variations.

Suppose

one

hada70

J1V

signal. Assuming

this

signal

were

measured

on

the

100

J1V

sensitivity range,

the

resulting

meter

indication

would

be 70% of full scale.Toexamine

small

variations

in th is signal,

one

would

first

set

the

polarity

switch

to

"+" (assume initial

meter

indication

were

Offset

The

ten-turn

dial

and

its

associated

polarity

switch

allow

calibrated

offsetsofup to

ten

times

full scaletobe

applied.

Two

applications

for

this

feature

are

that

it allows small

amplitude

variations

in a signal

to

be

expanded

and

exam

ined in

detail,

and

that

it allows a signal

amplitude

to

be read

with

greater

resolution

than

is possible

with

the

panel

meter

alone.

For

example,

suppose

one

hadameter

indicationtothe

right. To read

the

amplitude

with

the

greatest

possible

resolution,

the

polarity

switch

would

be

set

to

"+"

and

the

dial

adjusted

for

"null",atwhich

time

the

signal

amplitude

could

be read

directly

from

the

dial.

5 10.5 1 1.6

1 ...

g

10"

~

2

c(

-t--+---'''--+---+--'''I--+---+---'--~IO-t

-

1

01016

05.1

16

FREOUENCY--

.....

DC PREFILTER

gl

~

l(J't---------t

..

~-

Q.

2

c(

1O-2t-----

f-

1_

..

i -

~,ETrfILTERING

1

~FF~CT

IV-8

to

the

right),

followedbyadjusting

the

dial for null.

The

dial

setting

required

wouldbe0.70

and

the

meter

sensitiv-

ity

wouldbe±100

/lV

with

respecttothe

70 /lV

ambient

level. A

recorder

connectedtothe

output

would

allow

the

amplitude

variations

as a

Junctionofsome

experimental

parameter

to be

recorded.

Becauseofthe

Offset

dial range, ±10

times

full scale,

the

sensitivityofthe

measurement

couldbegreatly

expanded.

In

the

exampleathand,

the

Sensitivity

switch

couldbeset

to 10 /lV.

The

signal

amplitude

(70/lV)

would

be less

than

ten

times

full scale

(100

/lV)

andsowould

fall

within

range

of

the

Offset

dial. If

the

dial

were

adjusted

for

null

(setting

7.00),

the

meter

would

read

±10

/lV full scale

with

respect

to

the70/lV

ambient

signal level.

Outputs

The

outputofthe

instrumentisprovidedatboth

the

front

and rear panels.

These

two

outputs

are in parallel.

The

output

resistance is

600

ohms

and

full scale

output

is ±1 V,

which

correspondsto± full-scale

deflectionofthe

panel

meter.Atthe

front

panel,

the

outputisprovided

at a BNC

connector.Atthe

rear

panel,

it is

appliedtooneofa

pair

of

binding

posts.

The

second

(black)

bindinq

postisground.

These

binding

posts

are

spaced

to

acceptastandard

double-banana

connector.

It is

frequently

more

convenient

to use

the

rear-panel

output

when

using a

strip-chart

recorderasthe

readout

device.

Mixer Monitor

Output

An

optional

Mixer

Monitor

outputisprovidedatthe

rear

panel.

The

signal

appliedtothis

outputistaken

directly

from

the

output

of

the

Mixer

and

before

any filtering.

Figure IV-12

illustrates

the

Mixer

output

corresponding

to

in-phase

and

quadrature

signals

respectively.Ifthe

signal

and

reference

inputstothe

Mixer are

either

in phase or

90°

out-of-phase,

the

signal at

the

outputofthe

Mixer will be as

shown.

For

signals

1800out-of-phase,

the

Mixer

output

will

be

the

inverseofthe

in-phase

output,

and

for signals

270°

out-of-phase,

the

output

will be

the

inverseofthe

90°

output.

Taking

the

maximum

possible

area

which

can be

enclosed by

one

cycle

(one

polarity)

as a

unit

output,

the

output

averaged

overacycle

for any Mixer

input

phase

relationship

will be the

unit

output

times

the

cosineofthe

angle

between

the

input

and

reference

signals.

A-

SIGNAL AND REF IN PHASE

APPLICABLE

ONLY

BELOW

50kHz

AND

~IAJ1ItIl~~:'~~1:~:~

B-SIGNAL

AND REF

90°

OUT OF PHASE

Figure

IV-12.

MIXER

OUTPUT

FOR

IN-PHASE

AND

QUADRATURE

SIGNALS

IV-9

The

cosine

response

dependsonthe

sinusoidal

natureofthe

input

signal.Ifthe

signal

wereasquare

wave

and

the

tuned

amplifier

were

not

used,

the

Mixer

output

would

vary

linearly

with

the

angle

between

the

signal

and

reference

inputs.

Nevertheless,

maximum

output

would

still be at 0°

and

180

0

,

and

zero

output

wouldbeobtainedat900and

270°.

It

mightbementioned

that

the

waveforms

illustrated

in

Figure IV-12

apply

onlyatfrequencies

below50kHz

and

withanoise-free

input

siqnal. At

higher

frequencies,

switching

spikes

become

visible

and

some

Mixer

filtering

effects

become

evident.

Even relatively small

amounts

of

noise

accompanying

the

signal

could

completely

obscure

it

at

the

Mixer

output,

especiallyifone

were

operating

without

the

tuned

amplifier.

4.4

MIXER

FUNCTION

AND

HARMONIC

SENSITIVITY

The

purposeofthe

Mixer (Phase Sensitive

Detector)

is to

convolute

the

input

signal in such a

way

that

the

output

of

the

detectoristhe

sum

and

difference

frequenciesofthe

signal

and

the

reference.Ifthe

input

and

reference

signals

are at

the

same

frequency,

and

they

mustbefor

normal

lock-in

amplifier

operation,

oneofthe

output

frequencies

of

the

detector

will be zero,

that

is, de, Noiseorother

interferenceisnot

normally

at

exactly

the

same

frequency

as

the

input

signal

andsodoes

not

produce

zero

frequency

at

the

Mixer

output.

Thedcoutputisproportionaltothe

.~

amplitudeofthe

in-phase

componen

t of

the

input

signal.

This de is passed by

the

low

pass filter(s)

which

follow

the

Mixer, while

theaccomponents,

representing

the

input

noise

and

interference,

are

shunted

to

qround.

Thus

the

signal-to-noise

ratioatthe

output

connectors

and

front-

panel

meterismuch

improved

()Vl~1

what

it is at

the

input

of

the

instrument.

The

reference

inputtothe

detector

is a

square

wave having

a

fundamentalofsome

fixed

amplitude,

and

odd

harmonics

of lesser

amplitude.

From

FOIII

ier analysis,

the

amplitude

of

the

third

harmonicis1/3

the

fundamental

amplitude,

the

amplitudeofthe

fifth

is 1

hi,

the

seventhis1/7,

etc.

Because

the

detector

demodulates

with

respect

to

all

components

of the

reference

input,

any

odd

harmonic

componentsofthe

input

signal

contributetothe

output

as

well.

The

responseofthe

detectortoodd

harmonics

is in

the

same

proportionasthe

amplitudeofthe

harmonic

in

the

applied

reference.Inother

words,

the

Mixer's

third

harmonic

sensitivityis1/3

its

fundamental

sensitivity,

its

fifth

harmonic

sensitivityis1/5,

its

seventh

is 1/7,

etc.

In

the

caseofa

square

wave

input,ifthe

unit

does

not

have a

tuned

amplifier,

andifthe

Signal

Channel

fiIters are

set

to a

frequency

far

from

the

principal

harmonicsofthe

input

signal,

thena180

mV

pk-pk

square

wave will

yield

full-scale

output

on

the

100

mV

sensitivity

range. If

the

unit

has a

tuned

amplifier

set

to the

fundamental

frequency

so

that

only

the

fundamental

reaches

the

Mixer,

the

input

square

wave

must

haveanamplitudeof220

mV

pk-pk

to

yield

full-scale

outputonthe

100

mV range

(the

rms

value

of

the

fundamental

frequency

cornponent

ofa220

mV

pk-pk

square

wave is

100

mV).

In

principle,

the

instrument

does

not

respondtoeven

harmonics

at all

because

the

reference

input,

beingasquare

wave,

does

not

contain

any

even

harmonics.

However,

very

slight

deviations

from

perfect

symmetry

in

the

applied

reference

square

wave

result

in

some

even

harmonic

response,

the

worst

case

being

about

1%.

Note

that

the

overall

instrument

responsetoharmonics

is

not

necessarily as high as

indicated

in

the

preceding

paragraphs.

Phase

and

pre-mixer

attenuation

effects

cannot

be

ignored.

With

either

the

tuned

amplifier

or the hi-

or

low-pass filters in use,

input

harmonics

will be

both

attenuated

and

phase

shifted.

Inasmuchasthe

reference

inputtothe

Mixer is a

square

wave,

the

harmonics

(all

odd)

are at

the

same

phaseasthe

fundamental.

As

mentioned

previously,

the

even

harmonic

responseisquite

small,

and

is

seldom

of

any

consequence,

particularlyifthe

tuned

amplifierisused.

Thereisone

application

where

one

hastobe

concerned

with

the

"subharmonic

response"ofthe

Mixer,

and

that

is

when

measuring

second

harmonics

using

the2fmode.

When

the

instrumentisoperatinginthis

mode,

it has a

funda-

mental

response

(the

fundamental

can be

thoughtofas a

subharrnonicof

the

signal of

interest,

the

second

harmonic)

of

about

oneortwo

percent.

Hence,

for

accurate

second

harmonic

measurements,

it is essential

that

the

fundamental

be

attenuated

aheadofthe

Mixer.

One

convenient

way

of

doing

this is to use

the

optional

tuned

amplifier.

By

operatinginthe

Notch

mode

with

the

Notch

tunedtothe

frequencyofthe

fundamental,

the

fundamentalisreduced

to a negligible level

with

only

1% loss in

the

amplitude

of

the

second

harmonic.Itfollows

that

the

Tuned

Amplifier

can

also be used to

good

advantage

when

measuring

harmonics

higher

than

the

second.

4.5

INTER

FACE CONNECTOR

An

ll-pin

Interface

connectorisprovidedatthe

rear

panel.

Table

IV-l

indicates

the

functionofeach

pin.

Observing

the

connector

from

outside

the

instrument,

the

pins are

counted

counterclockwise

from

the

key.

Pin

Function

1 , Chassis

Ground

2 +

15.5VOUT

(load

limit=20

mA)

3................................

15.5VOUT

(load

limit=20

mA)

4 No

connection

5..................

24 V IN

(battery

operation,

300

mA

req'd)

6 No

connection

7 +24 V IN

(battery

operation,

300

mA

req'd)

8 '"

External

Time

Constant

Capacitor

9

External

Time

Constant

Capacitor

10 No

connection

11 No

connection

Table

IV-l.

INTERFACE

CONNECTOR

PIN

ASSIGNMENTS

4.6

BATTERY

OPERATION

Battery

operation

of the Model

128A

Lock-In

Amplifier

may be necessary

wherenoac

power

is available, or as a last

resort

where

power

line

interference

is a

problem.

Battery

IV-10

operationofthe

Model

128Aisstraightforward

because

the

pointstowhich

one

must

gain access are

providedatthe

rear-panel

l1-pin

socket.

Two

batteries

are

required,

one

to

supply

+24V(300

mAl

and

the

othertosupply

-24

V

(300

mA).

The

+24Vsource

shouldbeconnected

to pin 7.

The

-24Vsource

shouldbeconnectedtopin 5.

Ground

for

bothisat

pin 1. It is

generallyagood

ideatofuse

the

battery

lines

externaltothe

instrument,

andtoprovide

an

ON/OFFswitch

as well.

The

front-panel

ON/OFFswitch

does

not

control

the

instrument's

power

when

it is

operated

from

batteries.

Nevertheless,

the

battery

drainisminimized

if

the

front-panel

power

switchiskeptinthe

OFF

position

so

thatnocurrent

flows

through

the

switch's

built-in

light.

Keep

tile line

disconnected

during

battery

operation.

4.7 OPERATION WITH THE

INTERNAL

REFERENCE

OSCILLATOR

4.7A

INTRODUCTION

In

some

applications,

the

experimental

apparatus

does

not

generateasuitable

reference

output,

but

is itself

capable

of

being

driven

by an

external

signal

source.Tofacilitate

use

of

the

Model

128A

in such

applications,

an

internal

reference

oscillator

modificationisprovided.

With this

modification

installed,

a signalofvariable

frequency

anc

amplitudeisgenerated

by a sinewave

oscillator

inside the

Model

128A.

The

outputofthis

oscillator,inaddition

tc

being

provided

at a rear-panel

connector

for

easy

routing

tc

the

experiment,

can be

applied

internallytothe

Referencr

Channelsothat

it is

directly

drivenbythe

same

signal a

drives

the

experiment.

It is also

providedatthe

front-pane

Reference

INPUT

connector

at an

impedanceof10 krl fo

monitoring

purposes

only.

The

Phase

controls

remain

full'

functional,

allowing

the

internal

reference

signal

and

th

information

signal

from

the

experiment

to be

brouqht

int

phaseatthe

mixer.

4.78

OPERATION

Operationofthe

internal

oscillatorisstraightforward.

Tw

openingsinthe

rear

panelofthe

Model

128A

give access 1

the

adjustments

which

set

the

amplitude

and

frequenc

The

frequency

adjustment

allows

the

frequency

to t

varied

over

about

a 3: 1 range,

with

the

actual

ran:

spanned

dependingonthe

valueoftwo

capacitors

mount.

on

the

oscillator

board.

Table

IV-2

indicates

the

Irequen.

rangeofthe

adjustment

as a

functionofthe

valueofthe

capacitors.

The

capacitors

can be easily

changed;

they

a

held

by

spring-loaded

special

clips

which

release t'

component

lead

when

pressed

downwards.

The

capacltc

should

be low-leakage

types

matchedtowithin

5% Myl

polystyrene,

polycarbonate,

teflon,

and

other

film

capa

tors

ratedat50

V or

better

are all

suitable.

Do

not

l

electrolyticortantalum

capacitors.

Because

the

oscilla

board

plugs

into

the

main

board,

one,

must

remove

the t

coverofthe

Model

128Atochange

the

capacitors.

In

setting

the

oscillator

frequency

adjustment,

it is imp

tant

that

the

control

setting

notbechanged

too

rapidly

the

adjustmentisturned

quickly,

the

oscillator

will s

oscillating,

and

the

operator

will have

to

wait

sev

seconds

for

normal

operation

to be

restored.

Note

that

amplitude

adjustments

affect

only

the

amplitude

of

Approx. Freq. Range

0.53

Hz to

1.58

Hz

1.06 Hz to

3.2

Hz

2.7 Hz

to

7.9

Hz

5.3Hzto

15.8

Hz

10.6 Hz to 32 Hz

27 Hz

to

79 Hz

53 Hz

to

158

Hz

106 Hz

to

320

Hz

270Hzto

790

Hz

530Hzto

1.5 kHz

1 kHz to

3.2

kHz

2.7 kHz to 7.9 kHz

5.3

kHz to 15 kHz
10 kHz to 32 kHz
27 kHz

to

79 kHz

44.5

kHzto130

kHz

Capacitor

Value

10

JJF
5 JJF
2 JJF
1 JJF

500

nF

200

nF

100

nF

50nF
20nF

10 nF

5 nF

2 nF

1 nF

500

pF

200

pF

120

pF

P.A.R.C. EDP

#'5

for Osc. (5%

match)

1521-0193
1521-0105
1521-0207
1521-0066

·1521-0061
1521-0208
1521-0186
1521-0184
1521-0209
1521-0182
1521-0023
1521-0211
1501-0032
1501-0004
1501-0068
1501-0034

P.A.R.C. EDP #'5

for

Tuned

Amp.

(1%

match)
1560-0008
1560-0009
1560-0010
1560-0011
1560-0012
1560-0013
1560-0014
1560-0015
1560-0016
1560-0017
1560-0018
1560-0019

1560-0020

1560-0021
1560-0022
1560-0023

Table

IV-2.

FREQUENCY

RANGE

AS A

FUNCTIONOFCAPACITORS

signal providedatthe

rear-panel Ref. Osc.

Out

connector

(impedance

600

ohms).

The

amplitude

can be adjusted

from 0 V

to

10 V

pk-pk,

The

amplitudeofthe

signal

supplied

to

the

Model

128A

Reference channel

does

not

change. Neither

does

that

suppliedtothe

Ref. In connec-

tor, which acts as a

monitor

point

when

the

unitisoperated

in

conjunction

with

the

internal oscillator.

The

monitor

signal is a

constant

1 V rrns and its source resistance is 10

kn.

The

operator

is

not

advisedtouse this

monitor

signal

as

the reference drive for his

experiment.

The only

other

operating

considerationisthatoftransfer-

ring the Model

128A

from

external

reference

operation

to

internal reference

operation

or vice versa. Internal wires

fitted with

quick-disconnect

contacts

determine

whether
the Reference channel is driven by the internal oscillator
accessory or by an externally derived reference signal
applied to

the

front-panel jack. To gain accesstothese

wires, it is necessary to first remove

the

instrument's

cover,

which is secured by

four

screws. When

the

cover is

removed, locate the

three

adjacent

board

contacts,

J418,

J419,

and

J420.

For

operationinthe

Internal reference

mode,

the

pink wire

from

the front-panel Ref. In

connector

is

connectedtoJ418.

The

white/orange

wire from

the

Internal Oscillator is

connectedtoJ419.

J420isnot

used.

For

operationinthe

external

mode,

the

pink wire

from

the

front-panel

connectorisconnectedtoJ419.

Thewhite/

orange wire is

connectedtoJ420,

and

J418isnot

used.

Also,

the

two

frequency

determining

capacitors should be

removed

when

operating

with an external reference source.

4.7C INSTALLATION
When a Model

128Aisordered

with the Internal Oscillator

modification,

the

instrument

is shipped with the oscillator

installed and

the

operator

need only

concern

himself with

operating considerations.

Should

he desire

to

operate

without

the

oscillator, he has

onlytotransfer

a few wires as

described in

the

preceding paragraph.

IV-11

If

the

Internal Oscillator is

ordered

separately,

the

installa-

tion

is generally made by the

customer.

Three items are

supplied,

the

oscillator

board

itself, a BNC

connector

with

two

wires

attached,

and a metal

"tag"

bearing

the

word

"MONITOR".

The

oscillator circuit

board

is securedtothe

Model

128Abystandoffs

which plug

into

openings in

the

"-

Model

128A

Reference board as shown in Figure IV-13. -

This figure also shows

the

location of

the

various inter-

connecting

wires, all of which are

terminated

in quick-

disconnect

contactssothat

the

installation can be com-

pleted

in a

matterofminutes

with no special

tools

or

soldering required.

*

The

oscillator

adjustments

are ordi-

narily preset at

the

factory for 100 mV rrns

outatthe

frequency specified by the

customer.

The

appropriate

frequency

range capacitors are factory inserted.

The

follow-

ing

procedure

can be used to

make

the installation.

(1) Remove the

top

and

bottom

covers of

the

Model

128A.

The

top

cover is secured by four screws,

two

on

each side. The

bottom

is secured by

ten

screws.

(2)

Mount

the

BNC

connector

in the REF. OSC.

OUT

openinginthe

rear panel (it will be necessary to first

push

out

the

plug, which can

then

be discarded). Be

sure

to

use

the

insulating bushings supplied so

that

there isnocontact

between

the

shell of

the

connector

and chassis ground. After the

connector

is securely

mounted,

twist

the

two

wires together,

moderately

tight, over

their

entire

length.

(3) Note

the

pink wire which

extends

from

the

Ref. In

connector

to

J419

near the

frontofthe

Ref. Bd.

Remove this wire from

J419

and

allow it to hang free.

·Current

production

units

useadifferent

kindofquick-disconnect

pin

than

was

used

previously.

If a

current-production

oscillator

boardistobeinstalled

in an

older

,~nit,

the

quick-disconnect

terminalsatthe

endofthe

involved

leads

will

havetobe

cut

off

and

new

ones

installed.

The

new

terminals

are

supplied

with

the

oscillator

board.

Figure

IV·13.

INTERNAL

OSCILLATOR

BOARD

INSTALLED

REF.

OSC. FREQ. ADJ.

BLACK }

REAR

~NEL

WHITE/GREEN

CONNECTOR

FREQUENCY

RANGE

CAPACITORS

""----,..-(MATCHED

TO

5%)

...---WHITE/ORANGE

(J419)

Note

from

Figure

IV·14,aphotograph

of the Model

12

with

the

Tuned

Amplifier

installed,

that

there are

trim-adjustments

and

two

switchesonthe

Tuned

Ampl

4.8

OPERATION

WITH THE INTERNAL

TUNED

AMPLIFIER

4.8A

INTRODUCTION

In

some

applications

it is

desirabletonarrow

the n

bandwidth

aheadofthe

mixer,

or to

notch

out

a partie

frequency

componentofthe

input

signal. These operati

are

made

possible

if

the

Model

128A

is operated

conjunction

with

the

Model

128A/98

Accessory

Tu

Amplifier.

With

this

accessory

installed,

bandpass or no

operationata Qof5 is

possible

from

1 Hz to

100

kHz.

(10)

Connect

the

black

and

white/green

twisted

leads If

the

rear-panel

connector)

to

the

Oscillator bo

quick-disconnect

contactsasshown

in Figure

IV."

Th is

completes

the

installation.

The

top

and

bottom

CD

may

nowbereinstalled.

To

check

the

oscillator,

sim

turn

on

the

power

and

monitor

the

oscillator

output

an

oscilloscope.

IV·12

J405
J406

J404

REF AMP. ADJ.

red

.

yellow

.

black

.

(9) Press

the

Oscillator

hoard

down

untilitsnaps

into

place.

(6)

Connect

the

led,

yellow,

and

black

wiresasfollows.

(8)

Locate

the

white/orange

wire

from

the

oscillator

and

connect

it to J1119. Dress

this

wire

along

the

Ref. Bd.

(7)

Connect

the

pink

wire

from

the

Ref. In

connector

to

J418.

(5)

Position

tho

oscillator

boardasshowninFigure

IV-13

but

do

1I0t

suap

it in to

place

yet.

Dress

the

red,

yellow,

and

black

wires

outtothe

right.

and

the

longer

whi

te/oranqe

wire

towards

the

front

of

the

instrument.

(4) fie

move

the

ncfclcnce

INPUT

connector

from

the

front

panel.

Then

locate

the

"Monitor"

tag

over

the

panel

opening

(11)(1

remount

the

connectorsothat

the

tag is

secured

in

place

by

the

connector

mounting

flange.

circuit board. In

addition,

there

are

the

two

capacitors,

4.8B

OPE

RATION

mounted on special

spring-loaded

terminals,

which

deter-

The

first

stepisto

set

the

Tuned

Amplifiertothe

intended

mine the rangeofthe

frequency

adjustment.

The

trim-

operating

frequency.

An

appropriate

procedure

follows.

adjustment accessible

throughanopeninginthe

rear

panel

NOTE:

Each

Tuned

Amplifierispresetatthe

factorytothe

sets the

center

frequencyofthe

Tuned

Amplifier.

That

frequency

specifiedbythe

customer.

which is accessible

from

the

sideofthe

Model

128A

sets

the Q and

amplitude

response.

These

two

adjustments

(1)

Remove

the

top

coverofthe

Model

128A.

This

cover

interact to

some

degree.

The

two

switches

determine

the

is

securedbyfour

screws,

twooneach

side.

tuned amplifier

function.

Oneofthem

allows

the

operator

to select

either

Tuned

Amplifier

operationorFlat

opera-

(2)

Set

the

Model

128A

controls

as follows.

tion. The

other

gives

the

choiceofBandpassorNotch

Sensitivity:

250

mV

operation. These

latter

two

functions

have relevance

only

Input:

A

when the first

switchisset

to SE

LECTIVE.Ifit is

set

to

Low Pass: MAX

FLAT, the

tuned

amplifier

circuitryisbypassed

and

the

Hi Pass: MIN

Model

128A

operates

exactly

as if

the

Tuned

Amplifier

had

Reference

Mode: f

never been installed. Phase

switch:

270°

The frequency range

of

the

rear-panel

adjustment

as a dial:

90.0°

functionofthe

valueofthe

two

replaceable

capacitors

is

Time

Constant:

.3 SEC.

the same as

for

the

Internal

Reference

Oscillator.

Also,

the

Zero

Offset:

OFF

(dial

setting

irrelevant)

restrictions as to

the

typesofcapacitors

which

canbeused

DC

Prefilter:

0 UT

are the same as

outlinedinSubsection

4.7B.

There

is,

however,

one

difference,

namely

that

the

capacitors

used

in (3)

Select

and

install

the

two

capacitors

which

set

the

the Tuned

Amplifier

mustbematched

to 1%,

whereas

in

frequency

range. Be

sure

to use

the

capacitors

the case of

the

Oscillator,

they

need

onlybematched

to

matchedto1%.

NOTE:

One

sel of

capacitors,

having

5%.

Capacitors

purchased

from

P.A.R.C.

for

use in

the

the

value

appropriate

to the

frequency

specified

by

Tuned Amplifier are

matched

to 1%, even

though

they

may

the

customer,issupplied

with

each

Tuned

Amplifier.

be

marked 5%. (This

comes

about

because

they

are

selected

from a large

stockof5%

capacitors.)

Such1%capacitors

(4)

Turnonthe

Model

128A

power.

are marked by

colored

tapetoprevent

them

from

being

•.

confused

with

any

other

capacitors

with

which

they

might

(5)

Set

the

Flat/Selective

slide

SVVIICII

Oil

the

Tuned

bestored.

AmplifiertoSELECTIVE.

TUNED

AMP. FREQ. ADJ.

BLACK(J119)

--a.

YELLOW (J

118)

-~~

RED

(JI20)

--~

WHITE/VIOLET(J130)

WHITE/GREY

(J132)

WHITE/GREEN (J

129)

WHITE /ORANGE (J 131)

NULL

Figure

IV-14.

TUNED

AMPLIFIER

INSTALLED

IV-13

ORANGE

WIRE

FROM

CENTER PIN OF

SIG.

MON. BNC.

CONNECT

TO J128.

BLACK

WIRE FROM
GND. LUG OF SIG. MON.
BNC.

CONNECT

TO J121.

FREQ. RANGE CAPACITORS
MATCH TO 1

0/0.

NOTCH/BANDPASS SWITCH

SELECTIVE/

FLAT

SWITCH

(6)

Set

the

Notch/Bandpass

slide switch to NOTCH.

(7)

Connecta250

mV rrns sinewave (0.7 V pk-pk )

to

both

the

"A"

Input

and Ref.

Inputofthe

Model

128A. This signal

shouldbeat

the

intended

operating

frequency.

NOTE: If

the

unit

is also

equipped

with

the

internal

oscillator

modification,

use

the

internal

oscillator as

the

signal

source.

(8)

Monitor

the

rear-panel SIG. MON.

connector

with

the

oscilloscope and

alternately

adjust

the

two

Tuned

Amplifier

trim-adjustments

for a null in the observed

signal.

The

rear-panel accessible

adjustment

sets

the

center

frequency.

The

one

which is

adjusted

from

the

side sets

theQand

amplitude

response. These adjust-

mentsdointeract

so it will be necessarytogo

back

and

forth

lIll

til no

further

improvement

in

the

observed null can be

obtained.

NOTE:

Any

time

the

operating

frequencyischanged,

the

Null/Amplitude

adjustment

must

also be reset. If

the

Null/Amplitude

adjustment

is

properly

set

for

one

end

of

the

frequency

range,

and

the

tuned

frequencyisthen

shiftedtothe

opposite

endofthe

same range,

the

amplitude

responseofthe

Tuned

Amplifier will be in

errorbyabout

20% unless

the

Null/Amplitude

adjust-

ment

is reset.

(9)

Set

the

Notch/Bandpass

switchtoBANDPASS. Then,

using an

accurate

ac

voltmeter,

establish

the

rms

amplitudeofthe

input

signalat250

mV ±1%.

(10)

Adjust

the

Phase dial (and Phase switch if need be) for

peak panel

meter

indication

(alternatively,

one

could

use a digital

voltmeter

connectedtothe

front-panel

OUT

connector).

(11)

Adjust

the

Model

128A

GAIN CAL. trim-poten-

tiometer,R143,

for

exactly

full-scale panel meter

indication

(+1.000

V on a DVM). R

143ismounted

on

the

Signal Amplifier

board.

The

Model

128Aisnow

tuned

for

bandpass

operation

at

the

intended

operating

frequency,

and is

normalized

for

operation

with

the

tuned

amplifier. Figure IV-15 shows the

phase/amplitude

response of the Selective Amplifier in both

Notch

and

Bandpass

operation.

For

bandpass

operation,

the

Notch/Bandpass

switch

shouldbesettoBANDPASS. For

Notch

operation,itshould

be

settoNOTCH. Neither

position

of

the

Notch/Bandpass

switch

has

any

relevance

unless

the

Selective/Flat

switchissettoSELECTIVE.

Note

that

this

procedure

mustbemodified

somewhat

for

notch

operation,

where

the

frequency

component

to be

50 1005 10 20

01

0205.1

2 5 1 2

NOR~ALIZED

FREQUENCY_

1

,(

Q'

5

2

1

.

2

1

5

NOR~ALIZED

FREQUENCY'

Ille

WHERE I e APPLIED FREQUENt

2

AND 10

'TUNED

FREQUENCY

1

00

00

~

0

5

00

w

It:

r

o

o

5 10 2050100

I

/ \

/ i\.

/

-,

V

1'\

f--

/'

-,

e-/

....

r-,

V NO

....

tIl[O

,.[OU[Oc..,/

..

~

.H[IH,.APPLIED

'''[OU[NCY

:~OOI~"iD

'·f[OC.

z

~

C> 02

w

~

01

~

005

w

>

~

002

001

01

02

05

1 2 5 1

NORMAL! ZED

FREOUENCY-

05

10

BA

NDPASS

AMP.

NOTCH

AMp.

o

NORMALIZED

FREQUENCY_

I

\ Q ' 5

"-

r--f-

--..

-,

NORMALIZED

FREQUENCY'

1/10

WHEREI'APPLIED

FREOUENC

AND

10'

TUNED FREQUENCY

0102.05

.1

.2

5

1

2

10

20

50

10

-90

0

i

-30

0

5 10 20 50 100

---1\

\

1--

0'5

r--I--

\

NQAMAlIZED

nUOIJ(NCl'

I

f/t.,

\-

WH[ltE:

, •

"·"l..I[D

"t(OU[NCY

ANO

t.

I

TUN[O

'"'OUEHe1

-+----+----+------+-

-60

0

_30

0

0

0

-90

0

01 02

05

1 2 5 1

NOR~ALlZED

FREOUENCY--

Figure IV-15.

PHASE/AMPLITUDE

CHARACTERISTICSOFTUNED

AMPLIFIER

BANDPASS

PHASE

IV·14

NOTCH

PHASE

removed is

other

than

the

onetobe measured. As a result,

the signal

frequency

used for setting up

the

Tuned

Amplifier

shouldbethe

one

to be

notched

out,

and

not

the

onetobe measured.

As can be seen

from

Figure IV-15,

the

Notch response is a

sharp

function

of frequency. Hence, any slight

frequency

difference

between

the

"setup"

signal

and

the

"real"

signal

to be

notched

out

may result in the

notch

not

being as

deep

as it could be. Hence,

when

the

setup

for

notch

operation

is

completed, it is generally a

good

idea to

connect

the

signal

produced by the

experimenttothe

input

of the Model

128A and

then,

while

monitoring

the

signal at

the

SIG.

MON.

connector,toadjust

the

Null/Amplitude

adjustment

on the

Tuned

Amplifier as requiredtomake

the signal

being nulled disappear

into

the

noise. It

might

be men-

tioned

that

the

"appearance"ofthe

signal may

not

be as

expected. For example, in measuring

harmonics,

it is

generally desirable

to

notch

out

the

fundamental.

Many

observers are

quite

surprised at

the

appearance

of a square

wave, for

example,

that

has had its

fundamental

frequency

component

removed.

4.8C INSTALLATION
When a Model

128Aisordered

with

the

Tuned

Amplifier

modification, the

instrumentisshipped

with

the

amplifier

already installed and

the

operator

need only

concern

himself with

operating

considerations. If he

should

wish to

operate

without

the

Tuned

Amplifier,

there

are no changes

to make

other

than

to set

the

Selective/Flat

switch to

FLAT, although it may be desirable to

touchupthe

setting

of R143 using a

test

signalofaccurately

known

amplitude.

In the case of a

Tuned

Amplifier which is

ordered

separately,

the

installation is generally made by

the

customer.

Two

items

are supplied, the first being

the

Tuned

Amplifier itself, and

the

second

a BNC

connector

with

two

attached wires

that

terminateinquick-disconnect

contacts.

All of the

interconnections

between

the

Tuned

Amplifier

and the Model

128A

are by means of wires

attachedtothe

Tuned Amplifier. These wires also

terminate

in quick-

disconnect

contacts.

* The following

procedure

can

be used

to make

the

installation.

(1) Remove the

top

cover

from

the

Model

128A.

This

cover is secured by

four

screws,

two

on each side.

(2) Mount

the

BNC

connectorinthe

SIG. MON.

opening

in the rear panel.

(It

will be necessary to first push

out

the plug, which can

then

be discarded.) Be sure to use

the insulated bushings supplied so

that

there

is no

contact

between

the shell of

the

connector

and chassis

ground.

Then

connect

the

orange wire

(attachedtothe

connector)topin

J128

(rear edgeofSignal Amp.

Bd.]

and

the

black wiretoJ121.

·Current

production

units

useadifferent

kindofquick-disconnect

pin

than

was

used

previously.

If

a

current

production

tuned

amplifier

boardistobeinstalled

in an

older

Model

128A.

the

quick-disconnect

terminalsatthe

endofthe

involved

leads

will

have

to be

cut

off

and

replacedbythe

new

type

terminals

(supplied

with

the

Tuned

Amplifier

board).

IV-15

(3) Position

the

Tuned

Amplifier as

shown

in Figure

IV-14

butdonot

snapitinto

place yet. (It is easier to

make

the wire

connections

first.)

(4) On

the

Model 128A, remove

the

jumper

(white/

orange) which

interconnects

J129

and

J130.

Also

remove

the

jumper

(white/violet)

which

interconnects

J131

and

J132.

It may be a good idea to

tape

these

jumpers

somewhere

to a chassis surface inside the

Model

128Atoprevent

their

becoming

lost.

(5) Make the wire

connections

from

the

Selective Ampli-

fier to

the

followinq listed pins.

Wire

Color

Connect

To

black

J119

red J

120

yellow

J118

white/violet

"

J130

whhe/green

J129

white/orange

"

J131

white/gray

J132

(6) Press the

Tuned

Amplifier circuit

board

downsothat

it snaps

into

place.

This

completes

the

installation.

The

Tuned

Amplifier can

now be

operated

as described in

Subsection

4.8B.

4.9 MORE REFERENCE

CHANNEL

OPERATING

HINTS

4.9A

REFERENCE

CHANNEL

SLEWING RATE

When the

input

frequency

to the Reference

channel

changes,

the

internal Reference

circuitry

automatically

trackssothat

detection

is always

with

respect

to the

applied

frequency.

However, the tracking is

not

instan-

taneous,

with

the

result

that

thereissome

phase

difference

between

the

applied signal

and

the

reference drive to

the

detector

while

the

frequency

is changing.

The

maximum

rate at which

the

Reference I

nput

frequency

can change

dependsonhow

much

phase sh itt one is willingtotolerate.

The

relationship linking

these

factors is

df/dt

:: kfO,

where

df/dtisthe

slewing rate in Hz/s, k is a

constant

vS

x 10

-3,

and

0 is

the

phase lag. Example:Ifthe operating

frequency

were nominally 1 kHz, and

the

maximum

0 one

could

accept

were 1

0

,

then

the

maximum

allowable df

/dt

would

be 3 Hz/s.

One

could

as well use this

equation

to solve for

the

angle 0 given some value of df/clt.

4.9B

PHASE

ERRORS

WITH SMALL

REFERENCE

SIGNALS

With reference signals on

the

order

of 1 V rms,

the

firing

point

is very

near

0° (with

respecttothe

zero

crossover of

the

reference

input

sinewave). However, as smaller and

smaller reference signals are applied (speaking

of

sine-

waves),

the

firing

point

becomes

further

and

further

from

the crossover point, introducing phase error. For sinewaves
just barely large enough to provide proper reference
channel operation, the error will

be much nearer to 90°

than to 0°. Any instability in the amplitude of the
reference signal will only compound the problem. Conse-

quently, in any application where phase
important, use of a 1 V rms sinewave reference

signal

is

advised. Other waveforms can of course be used. With a
square wave, this problem is minimized. With a triangular
wave, it is even more severe than with a sinewave.

SECTION V WARNING!

POTENTIALLY LETHAL VOLTAGES ARE PRESENT INSIDE THIS APPARA·
TUS. THESE SERVICE INSTRUCTIONS ARE FOR USE BY QUALIFIED PER·
SONNEL ONLY. TO AVOID ELECTRIC SHOCK, DO NOT PERFORM ANY
SERVICING UNLESS YOU ARE QUALIFIED TO DO SO. ANY ADJUSTMENT,
MAINTENANCE, AND REPAIR OF THE OPENED APPARATUS UNDER
VOLTAGE SHALL BE AVOIDED AS FAR AS POSSIBLE AND, IF UNAVOID·
ABLE, SHALL BE CARRIED OUT ONLY BY A SKILLED PERSON WHO IS
EXPERIENCED IN WORKING ON ELECTRONIC APPARATUS AND WHO IS
AWARE OF THE HAZARD INVOLVED. WHEN THE INSTRUMENT IS CON·
NECTED TO A POWER SOURCE, TERMINALS MAY BE LIVE, AND THE
OPENING OF COVERS OR REMOVAL OF PARTS IS LIKELY TO EXPOSE >.
LIVE PARTS. THE APPARATUS

SHALL

BE DISCONNECTED FROM ALL
VOLTAGE SOURCES BEFORE IT IS OPENED FOR ANY ADJUSTMENT, RE·
PLACEMENT, MAINTENANCE, OR REPAIR. CAPACITORS INSIDE THE
UNIT MAY STILL BE CHARGED EVEN IF THE UNIT HAS BEEN DISCON·
NECTED FROM

ALL

VOLTAGE SOURCES. USERS ARE ADVISED TO WAIT
SEVERAL MINUTES BEFORE ASSUMING THE CAPACITORS ARE DIS·
CHARGED.

IV·16

Sensitivity:

100

mV

Input

Selector

switch:

"A"

Lo Pass

switch:

MAX.

Hi Pass

switch:

MIN.

Phase

switch:

270

0

Phase dial:

90

0

Reference Mode

switch:

f

Time

Constant:

MIN.

Zero

Offset

switch:

OFF

(center

position)

dial:

0.00

(fully

counterclockwise)

Fast/Slow

switches

(locatedonRef.

board):

FAST

DC Prefilter

switch:

OUT

(2) Remove

the

DVM

from

C309

and

transferitto

the

negative

endofC310.

The voltage

shouldbe-15.5

V

±0.2

V.

(3)

Remove

the

DVM

from

C310

and

transfer

it to

the

positive

endofC312.

The

voltage

should

be +5 V

±0.2V.Remove

the

DVM.

5.4

PROCEDURE

5.4A

+15 V ADJUST (R310),

-15

V CHECK,

and

+5 V CHECK

(1)

Connect

the

DVMtothe

positive

endofcapacitor

C309.

Then

adjust

R310

(+15 V ADJ)

for

a DVM

reading

of

+15.50

V.

5.4B REFERENCE

BOARD

ADJUSTMENTS

(1)

Set

the

controls

as follows:

SECTION V

ALIGNMENT

READ SAFETY NOTICE ON FACING PAGE BEFORE PROCEEDING

5.1

INTRODUCTION

(4)

Connect

BNC

shorting

plugstoboth

inputs.

The Model

128A

Lock-In Amplifier is a reliable conserva-

tively designed

instrument.

High

quality

stable

components

have been used

throughout

in its

construction

and

one

can

reasonably

expect

a long

periodoftroublefree

operation

without

any

need

for

realignment. However,tobe assured

of

continued

high

confidenceinthe

experimental

data

obtained

with

the

Model

128A,itmay

be advisable to

run

through

the

following

alignmentatone

year

intervals,

and

after doing a

repaironthe

instrument.

Duetopossible

interactions

between

someofthe

adjustments,

it is neces-

sary

that

they

be carried

outinthe

indicated

sequence.

Any

decisiontomake

a partial

alignment

should

be reserved

to

someone having

sufficient

knowledge of

the

Model

128A

to

fully

understand

all possible

interactions.

Figure V-l

identifies

the

adjustments

and

testpoints.

(2) Sinewave oscillator, providing

both

an

adjustable

output

andafixed

amplitude

output,

with

the

two

to

be in phase.

The

fixed

output

is used as

the

reference

drive

to

the

lock-in amplifier and need

not

be a

sinewave.

One

suitable

oscillator

wouldbethe

Krohn-

hite Model

4200.

One

could

also use a single

output

oscillator followed by a 10: 1

attenuator.

(3) Digital

Voltmeter

suchasthe

Fairchild Model

7000.

Note

that

this

alignmentisnot

intendedtobe used in

troubleshooting. If

the

instrumentissuspectedofmalfunc-

tioning, go

directly

to

Section

VI,

which

deals

with

troubleshooting.

The

instrument

mustbeworking

properly

before it can be aligned.

5.2

REQUIRED

EQUIPMENT

(1) General

purpose

oscilloscope having a sensitivityofat

least 1 mV

fcm

with

a 10: 1

attenuator

probe.

(4)

Two

shorting

plugs, CW-159fU

(Amphenol

or equiva-

lent).

(5) Cables

for

interconnecting

the

above

items.

(6) Three small

jumper

cables.

5.3

PRELIMINARY

STEPS

(1) Remove

the

top

cover, which is

secured

by

four

screws,

twooneach

side.

(2) If

the

unit

containsatuned

amplifier

or an

internal

oscillator,

take

the

necessary stepstorender

these

accessories inactive,

that

is,

the

Model

128A

should

be

operatedinthe

external

reference

mode

and

the

signal

channel

response

should

be flat.

(3) Check

and,

if necessary,

adjust

the

mechanical

zero

of

the panel

meter.

Then

plug in

the

Model

128A,

turn

on the power,

and

allow a fifteen

minute

warmup.

V-1

(2)

Schmitt

Trigger

Symmetry

Adjust

(R405)

(a)

Connect

the

oscillatortothe

Ref. In

connector.

Set

the

amplitudeto3 V

pk-pkat400

Hz.

(b)

Monitor

the

signal at

TP401

with

the

oscillo-

scope.

The

observed signal

should

be a

400

Hz

square

wave

with

a pk-pk

amplitude

of

about

3 V.

(c) Gradually

reduce

the

amplitudeofthe

applied

signal until

"rounding"

of

the

square

wave is

observed. As

the

amplitudeisfurther

reduced,

the

symmetry

of

the

observed

waveform

may

become

degraded.

Adjust

R405

(Schmitt

Trigger

Symmetry

Adj) as

requiredsothat

ideal sym-

metryismaintained.

Keep

reducing

the

ampli-

tudeofthe

input

signal to

the

point

where

R405

is

adjusted

for

best

symmetry

with

the

lowest

possible

reference

input

which gives

proper

oper-

ation.

J 128 A2 OUT

SIG.

BO.----+-

TRACKING RATE

R49lE0ADJ.

Al OUT

RI43

GAIN

PREAMP

OUT

ClOG HF CMR

SYM. ADJ.

,i*

\1

i·

REF.

BO.

FINAL AC AMP

OUT 2

(J232)

FINAL

AC AMP

OUT 1

(J

231 )

MIXER
POWER

SUPPLY

BO.

TP202
R20l

AC BAL.

C210 HF ZERO
TP201

R234

AMP

1

ZERO

~

o

.0.,

R242

A2 ZERO

: 00.

R247

METER CAL.

~

••

243

ZERO

SUPP.

CAL.

1"P204
TP203

(d) Increase

the

amplitudeofthe

reference

input

to

about

3 V

pk-pk

,

(3) A3 Level

Adjust

(R498)

(a) Being sure

to

use

the

10: 1.

attenuator

probe,

connect

the

oscilloscopetoTP407.

The

oscillo-

scope

should

be dc

coupled.

(b)

Adjust

R498

(A3

Level Adj)

foranindicated

voltageof-4V.Then

waitaminute.Ifthe

voltaqe

changes,

readjust

R498asrequired

to

obtain

the

desired-4V reading.

(4)

E0

Adjust

(R491)

(a)

Transfer

the

oscilloscope

to ac

coupling

and

transferitfrom

TP407toTP405.

The

suggested

horizontal

sensitivity is 1

ms/cm.

(b)

Rotate

R491 (E0 Adj) fully

counterclockwise.

(c) Increase

the

vertical sensitivityofthe

oscillo-

scope

until

small

spikes

are

observed.

Then

adjust

R491

(E0

Adj) until

the

spikes

disappear.Ifthe

adjustment

is

turned

too

far,

the

spikes will

reappear

but

with

reversed

polarity.

Remove

the

oscilloscope.

5.4C

SIGNAL

BOARD

ADJUSTMENT

(1)

Preamp

DC Bias

Adjust

(R111)

(a)

Connect

the

DVMtoTP 101.

(b)

Adjust

R111

(Preamp.

DC Bias Adj)

for

+3 V.

Remove

the

DVM.

Initially,

the

meter

will

probably

be against

one

"stop"orthe

other.Asthe

adjustment

is

turned,apoint

will be

reached

where

the

meter

indication

"snaps"tothe

other

ex-

treme.Ifthe

adjustmentisthen

turned

in

the

opposite

direction,

about

a half

turn

will

typicallyberequiredtomake

the

indication

"snap

back

'',

The

correct

settingismidway

through

the

"dead

zone".Inother

words,

one

must

adjust

the

pot

until

the

indication

"snaps"

to

the

other

extreme,

then

stop,

andgoback

half waytothe

setting

required

to

make

the

meter

snap

back.

4.

Remove

the

two

jumpers

connectedinsteps

1

and

2.

5.

Select

for

R273

that

resistor

which

yields a

DVM

indication

of

"0"

±0.2

mV.

The

resistor

value

should

be

one

megohm

or

smaller.

(2) DC

Amp.1Zero

Adj.

(R234)

(a)

Set

the

Sensitivity

switchto2.5

mV.

(b)

Remove

the

jumper

from

J221.

(c)

Adjust

R234

(DC

Amp.1Zero

Adj)

for

"0"

on

the

DVM.

(d)

Set

the

Sensitivity

switchto100

mV.

(3) Meter Cal.

Adjust

(R24

7)

(a)

Set

the

Zero

Offset

Polarity

switch

to

"<",

5.40

MIXER

ADJUSTMENTS

(1) DC Amp. 2

Zero

Adj

(R242)

(b)

Adjust

the

Zero

Offset

dial

clockwise

(about

one

full

turn)

for a DVM

indicationof+1.00

V.

(a)

Connectajumper

from

J221toJ223

(ground).

(c)

Adjust

R247

(Meter

Cal. Adj) so

that

the

panel

meter

reads

exactly

full scale to

the

right.

(b)

Connect

the

DVM to

the

front-panel

OUT

connector.

(c)

Adjust

R242

(DC

Amp2Zero

Adj) for

"0"

on

the

DVM.

NOTE:

If

the

instrument

has

been

repaired

and

Q206

A-B

replaced,

the

following

procedure

should

be

usedtoselect

R273

(select-by-test

resistor).

(b)

Connect

the

oscilloscopetothe

front-panel

OUT

connector.

(e)

Remove

the

DVM.

(d)

Set

the

Zero

Offset

Polarity

switchtothe

center

(OFF)

position.

(a)

Set

the

Sensitivity

switchto2.5

mV.

(c) Decrease

the

reference

frequencyto40

Hz.

(4) AC Bal. Adj.

(R201)

After

performing

stepsaandb,connect

a

jumper

from

TP204toTP205.

I.

2.

Connect

another

jumper

from

TP203

to

J223

(or

any

ground).

3.

Adjust

R242

(DC

Amp2Zero

Adj)

for

"0"

panel

meter

indication.

NOTE:

Th is is a

sensitive high-gain

adjustment

and

setting

a

true

zero

will

probably

prove

impossible.

V-3

(d)

Adjust

R201 (AC Bal. Adj) for

minimum

square

wave signal

observedatthe

oscilloscope.

NOTE:

This

square

wave will

drift

around

because

of

short

term

temperature

fluctuations

caused

by air

currentsonthe

components.

Remove

the

oscillo-

scope.

(5) HF

Zero

Adj. (C210)

(a) Increase

the

reference

signalto100

kHz.

(b)

Adjust

trim-capacitor

C210

(HF

Zero

Adj)

for

"0"

panel

meter

indication.

(c)

Reset

the

reference

frequencyto400

Hz.

5.4E

OTH ER

ADJUSTMENTS

(1)

Reference

0° Phase Adj.

(R438)

(a)

Set

the

Sensitivity switch to

100

mV.

(b)

Remove

the

shorting

plug

from

the

"A"

Input.

Then

connect

the

signal

generator

output

(400

Hz;

280

mV pk-pk ]tothe

"A"

Input.

The

Reference

Input

should

still be driven

from

the

same

signal

generator.

Set

the

Input

switch

to

(c)

Note

the

panel

meter

indication.Itshould

be

near

full-scale to

the

right.

(d)

Set

the

Phase

switchto90°

and

the

Phase dial

to

0° (fully

counterclockwise).

The

meter

indication

should

go to very

near

"0".

(e)

Adjust

R438

(00 Adj) for

exactly

"0"

on

the

panel

meter.

(2)

Reference

90°

Phase Adj.

(R437)

(a)

Set

the

Phase

switchto0°

and

the

Phase dial

to

90.0°

(nine

turns

clockwise

from

the

fully

counterclockwise

position).

(b) Again

the

panel

meter

indication

should

be

approx

imately

"0".

Adjust

R437

(90°

Adj)

for

exactly

"0"

panel

meter

indication.

(3) Gain Adj.

(R143)

(a)

Connect

the

DVMtothe

OUT

connector.

(b)

Adjust

the

level of

the

input

signaltoexactly

100

mV rms. If necessary, use a

calibrated

ac volt-

metertobe assured

that

the

input

signal level is

accuratetoat

least 1%.

(c)

Set

the

Time

Constant

switch

to .3 SEC.

(d)

Set

the

Phase

switchto2700and

the

Phase dial

to

90°.

Then

adjust

the

dial for

peak

output

as

indicatedbythe

DVM.

(e)

Adjust

R143

(Gain Adj) for

1.000

V at

the

DVM.

(4)

Zero

Offset

Cal.

(R243)

(a)

Set

the

Sensitivity

switchto10 mv.

(The

100

mV rms signal

should

still be

applied.)

V-4

(b)

Set

the

Zero

Offset

Polarity

switch

to

"+".

Then

rotate

the

Zero

Offset

dial to

the

fully clockwise

position.

(c)

Adjust

R243

(Zero

Offset

Cal.)

for

0.00

V on the

DVM.

The

panel

meter

will also

indicate

"0".

(d)

Set

the

Zero

Offset

Polarity

switchtoOFF

and

the

Sensitivity

switch

to

100

mV.

The

DVM

should

indicate

1 V ±5

mV.Ifit

does,

go on to

the

following

step.

If it

does

not,

it is because of

interaction

between

R143

(Gain Adj) and R243

(Zero

Offset

Cal). If necessary,

repeat

steps

3 and

4 as

requiredtoachieve

the

desired

adjustment

objectives.

(e)

Reset

the

Sensitivityto100

mV. Also, set the

Offset

Polarity

switch

to

the

center

(OFF)

position

and

rotate

the

Offset

dial fully

counter-

clockwise.

Remove

the

DVM.

(5)

Low

Frequency

CMR Adj

(Rl?4)

(a)

Remove

the

shorting

plug

from

the

"-B"

Input.

Then

connect

the

100

mV rms signal

from

the

generator

simultaneouslytoboth

inputs.

(b)

Set

the

Input

Selector

switchto"A-B".

(c) Increase

the

level of

the

input

signalto1 V rms

(2.8

V pk-pk).

The

Sensitivity

should

remain set

to

100

mV.

(d)

Set

the

Sensitivity

switchto100

/lV.

Then

adjust

R

124

for

"0"

panel

meter

indication.

(e) Increase

the

Sensitivity

to 10 /lV. Again adjust

R

124

for

"0"

panel

meter

indication.

(6) High

Frequency

CMR Adj

(Cl06)

(a)

Set

the

Sensitivity

switchto250

mV.

(b) I

ncrease

the

signal

frequencyto100

kHz and set

the

amplitudeofthe

input

signal to

250

mV rms

(0.7 V pk-pkl.

(c)

Set

the

Sensitivity

switchto25/lV.

(d)

Connect

the

oscilloscope

(with

probe)toJ128

(quick-disconnectatrear

of Signal

board).

(e)

Adjust

trim-capacitor

Cl06

for

minimum

ob-

served signal.

It

should

be possibletoadjust

the

observed

signaltoless

than60mV

pk-pk,

(f)

Set

the

Sensitivity

switchto100

mV.

(7) High

Frequency

Phase Adj

(R521)

(a) Make

provision

for

supplying

in-phase signals to

the

Model

128A.

The

signaltobe

applied

to the

Reference

Input

should

have an

amplitude

of

nominally

1 V rrns, while

that

appliedtothe

Signal

Input

should

be

100

mV rms. It is

absolutely

essential

that

the

two

signals be in

phase. This is assured by using a single

source

and

a

simple

series 10: 1 divider. A

910

ohm

com-

position

resistor in series

witha100

ohm

composition

resistor

makesasuitable

divider. No

great

divider

accuracy

or precision is

required.

Ordinary

5% resistors are

quite

adequate.

A signal

having a

nominal

one

volt rms

amplitude

is

appliedtothe

divider

andtothe

Reference

Input

of

the

lock-in

amplifier,

while

the

signal

applied

to

the

Signal I

nput

connectoristaken

from

the

junctionofthe

two

divider

resistors.

(b)

Set

the

signal

frequencyto100

kHz

and

adjust

the

signal

source

output

amplitudesothat

the

signal

appliedtothe

Model

128A

"A"

Input

is

100

mV rms.

V-5

(c)

Set

the

Input

Coupling

switchto"A".

Then

remove

the

cable

connectedtothe

"B"

Input.

(d)

Set

the

Phase

switchto0°

and

the

Phase dial

to

90.0°.

(e)

Adjust

R521 (High Freq. Phase Adj)

for

"0"

on

the

panel

meter.

(f)

Set

the

Sensiti vity to

10mVand

adjust R521

again, this

time

for a 1

Q<){)

of full scale

meter

deflection.

This is a

meter

indicationtothe

left

of

"0"

(-.1onthe

upper

mete

r scale).

This

completes

the

alignment.

The

test

equipment

can

now

be

removed

and

the

cover

secured

in place.

SECTION VI WARNING!

POTENTIALLY LETHAL VOLTAGES ARE PRESENT INSIDE THIS APPARA·
TUS. THESE SERVICE INSTRUCTIONS ARE FOR USE BY QUALIFIED PER·
SONNEL ONLY. TO AVOID ELECTRIC SHOCK, DO NOT PERFORM ANY
SERVICING UNLESS YOU ARE QUALIFIED TO DO SO. ANY ADJUSTMENT,

MAINTENANCE, AND REPAIR OF THE OPENED APPARATUS UNDER
VOLTAGE SHALL BE AVOIDED AS FAR AS POSSIBLE AND, IF UNAVOID·
ABLE, SHALL BE CARRIED OUT ONLY BY A SKILLED PERSON WHO IS

EXPERIENCED IN WORKING ON ELECTRONIC APPARATUS AND WHO IS
AWARE OF THE HAZARD 'NVOLVED. WHEN THE INSTRUMENT IS CON·

NECTED TO A POWER SOURCE, TERMINALS MAY BE LIVE, AND THE
OPENING OF COVERS OR REMOVAL OF PARTS IS LIKELY TO EXPOSE

LIVE PARTS. THE APPARATUS

SHALL

BE DISCONNECTED FROM ALL

VOLTAGE SOURCES BEFORE IT IS OPENED FOR ANY ADJUSTMENT, RE·

PLACEMENT, MAINTENANCE, OR REPAIR. CAPACITORS INSIDE THE
UNIT MAY STILL BE CHARGED EVEN IF THE UNIT HAS BEEN DISCON·
NECTED FROM

ALL

VOLTAGE SOURCES. USERS ARE ADVISED TO WAIT
SEVERAL MINUTES BEFORE ASSUMING THE CAPACITORS ARE DIS·
CHARGED.

Input

Selector: A

Sensitivity:

100

mV

Phase

switch: 0°
dial: 0°

Reference

mode:

f

Zero

Offset

switch:

OFF

(center

position)

dial: fully

counterclockwise

Time

Constant:

.3 SEC.

DC Prefilter:

OUT

Power: ON

Reference Tracking-Rate switches (internal): FAST

(2) If

the

unregulated

supply

levels are

incorrect

(nominal-

ly ±24 V),

check

the

unregulated

supply

components

(line fuse,

transformer,

rectifiers,

and

filter capaci-

tors).

Note

that

the pass

transistor

for each of

the

three

regulator

circuits is

bolted

directlytothe

rear

chassis,

which

acts as a

heat

sink.

(2)

Set

the

oscillator

controls

to provide a

280

mV pk-pk

(100

mV rms) sinewave at

400

Hz.

(3)

Connect

the

output

of the

oscillatortothe

Reference

Inputofthe

Model

128A.

(4)

Connect

the

oscilloscope (use 10: 1

probe)toTP401.

The

observed sitJnal

should

be a

400Hzsquare

wave

having its

lower

level at

about

---O.G

V and its

upper

level at +2.5 V. If this signal is as

indicated,

one

can

assume

that

the

Input

Schmitt

Trigger

circuit

is

working

correctly.

NOTE: Here

and

throughout

the

remainderofthe

troubleshooting

procedure

the opera-

tor

should

concern

himself primarily

with

gross

discrepancies. Generally,

whenacircuit

malfunctions,

the

"error"inthe

output

signal of

that

circuit

is so

great as

to

leave no

doubt

of a

malfunction.

Much

time

may

be saved by going

through

the checks fairly

rapidly,

without

wasting

undue

time

and

effort

trying

to

verify

that

each

signal of voltage

conformstothe

value

indicated

downtothe

"last

decimal

point".

(5)

Connect

the

DVMtoTP403.

The

voltage should be

-4.5Vto.l

V. If this voltage is

correct,

one can

reasonably assume

that

the

AMP 1 circuit,

the

AMP 2

circuit,

and

the U401

switching.circuits

at

the

output

of

the

Input

Schmitt

Trigger are all

working

normally.

Remove the DVM.

6.5

Rf:FERENCE

CHECKS (schematics on

pagesV

11-6

and V

11-7)

(1)

Set

the

controls

as follows.

VI-1

(2) Oscillator (sinewave) to provide a

100

mV rms signal

at

400

Hz.

SECTION

VI

TROUBLESHOOTING

READ SAFETY NOTICE ON FACING PAGE BEFORE PROCEEDING

6.1

INTRODUCTION

supplies

the

reference voltage

for

both

the +5 V and

This section consists of a series of

procedures

to be

-15

V regulators.

Thus,

any

trouble

with

the

+15 V

followed in

troubleshooting

the

Model

128A.

The

purpose

regulator

would

cause loss of regulation in

the

-15

V

of the

procedure

is to

narrow

the

trouble

downtoone

of

and

+5 V circuits as well.

the three plug-in

circuit

boards

by making voltage and

waveform check'S at critical points. Once

the

faulty

board

has been identified,

the

operator

can

contact

the

factory

or

the authorized representative in his area for advice on

how

to get the

instrument

back

into

operationinthe

shortest

possible time. In

the

caseofunits

still in Warranty, it is

particularly

important:

that

the

factory

or

one

of its

representatives be

contacted

before

doing

any

workonthe

board itself, because any damage

that

occurs

as a

result

of

unauthorized

work

could

invalidate

the

Warranty.

Although

past

experience

indicates

that

some

instrument

failures

turn

outtobe

the

fault

of a specific

component

failure on

oneofthe

boards,

it is of course

perfectly

possible

that

some

component

other

than

one

located on a

circuit board could go bad. Where this is

the

case,

the

person

troubleshooting

will have to

appropriately

adapt

the

procedure to isolate

the

faulty

component.

6.2

EQUIPMENT

REQUIRED

(1) Digital

Voltmeter

suchasthe

Fairchild Model

7000.

(2) If the

unit

containsatuned

amplifier or an internal

oscillator,

take

the necessary stepstorender

these

accessories inactive,

that

is,

the

Model

128A

should be

operated in

the

External Reference

mode

and the

signal channel response

should

be flat.

(3) General

purpose

oscilloscope with 10: 1

probe.

(3) Plug in

the

Model

128A,

turnonthe

power,

and

allow

a fifteen

minute

warmup.

6.3

INITIAL

STEPS

(1) Remove

the

top

cover, which is secured by

four

screws,

two

on each side.

6.4 POWER SUPPLY CHECKS
(schematic on page

VII-11)

(1) On the Power

Supply

board,

check

for: (a)

+15.5

V

to.lVat

the

positive

endofcapacitor

C309,

(b)

-15.5Vto.2Vat

the

negative

endofcapacitor

C310,

and (c) +5 V

to.2Vat

the

positive

endofcapacitor

C312. If

these

voltages are

correct,goto

Subsection

6.5.Ifany of these voltages are

incorrectormissing,

proper

power

supply

operation

must

be established

before any

further

checks can be made. Note from

the

schematic on page

VII-ll

that

the

+15Vrequlator

(6)

Connect

the

oscilloscope

to

TP402.

The

observed

signal

should

be a

negative

sawtoothat400

Hz.

The

upper

"plateau"

should

be at

+10Vand

the

lower

"points"

shouldbeat

+4.5V.The

"plateau

intervals"

should

have

the

same

durationasthe

sawteeth.Ifthis

signal is as

indicated,

one

can

reasonably

assume

that

all of

the

circuits

depictedonthe

page

VII-6

schematic

are

functioning

normally.

(7)

Connect

the

oscilloscopetoTP404.

The

signal

there

should

be a

1600Hzsquare

wave

having

its

upper

level

at

about

+4 V

and

its

lower

at 0 V. If

this

signal is as

indicated,

one

can

reasonably

assume

that

the

"4f

Oscillator"

and

the

oscillator

control:circuitry

(0427

·0428

and

all

componentstothe

"left"onthe

schematic)

are

working

properly.

It

mightbeinstructivetocheck

the

voltageatTP407.

The

allowable

voltage

rangeatthis

point

extends

from

oV

to~.8 V.

However.

under

the

measurement

conditions

established,

this

voltageisnominally

--4 V.

The

voltageatTP406

shouldbeabout

-0.6

V relative

to

thatatTP407.

The

correction

signal

can

be

observed

at

TP405.

However,

when

the

loopisoperating

correctly.

there

is

little

to

observe.

aile

might

see small

amplitude

"fuzzy

blips"at400

Hz.

When

the

loopisperturbed

by

changing

the

frequency

of

the

applied

reference

signal.

these

blips

transform

into

definite

spikes

which

can be

either

positive

or negative

accordingtothe

sense of

the

correctiontobe

made.

(8)

Monitor

J413

with

the

oscilloscope.

One

should

observea400Hzsquare

wave

having

its

upper

level

at

+4 V

and

its

lower

level at 0 V. If

this

waveform

is as

indicated,

one

can

conclude

that

the

logic

circuits

which

provide

the

Ref. Mon.

output

are

working

normally.

If all of

these

vol taqes

and

signals are as

indicated,

one

can

reasonably

assume

that

all of

the

tracking

circuits

are

functioning

normally.

The

only

remaining

reference

circuit

is

that

which

controls

the

Reference

Unlock

lamp.

6.6

SIGNAL

CHANNEL

AMPLIFIERS

6.6A

PREAMPLIFIER

(schematic

on page

VII-3)

(1)

Connect

the

oscillator

output

(still set to

280

mV

pk-pkat400

Hz)tothe

"A"

inputofthe

Model

128A.

This

signal

should

still be

applied

to

the

ReferenceInput

as well.

(2)

Connect

the

oscilloscope

to

the

negative

end

of

capacitor

Clll.

NOTE:

This

capacitor

is

shown

schematically

on page VII-4.

The

observed

signal

should

be a srnewave

withapk-pk

amplitudeof2.8

V,

indicatinq

that

the

preamplifier

has

the

expected

gain

of

ten.

VI-2

6.6B

INTERMEDIATE

AC

AMPLIFIERS

(schematiconpage

VII-4)

(1)

Connect

the

oscilloscopetodisconnect-pinJ128.

The

observed

signal

should

be a

400

Hz sinewave

with

a

pk-pk

amplitude

of

130

mV.Ifthis

signal is as

observed,

one

can goonto

6.6C.Ifthe

signal is

not

as

indicated,

the

following

checks

can be

made

to

determineinwhichofthe

two

Intermediate

Ampli-

fiers

the

troubleislocated.

(2)

First

connect

a 1

krl

resistor

in series

with

the

oscilloscope

probe.

Then

monitor

the

signal at the

negative

endofcapacitor

C135.

The

signal observed

should

be a

sinewave

withapk-pk

amplitude

of 1.4 V.

If th is signal is as

indicated.

one

can

conclude

that

the

first

Intermediate

Amplifierisworking

correctly.

(3)

Connect

the

oscilloscope

to

quick-disconnect

pin

J13l.

The

signal

there

should

be a

sinewave

with

a

pk-pk

amplitudeof1.4 V. If

this

signal is as

indicated,

the

second

Intermed

iate

Amplifier

is also

functioning

normally.

6.6C

FINAL

AC AMPLI FIE R

(schematiconpage

VII-10)

Connect

the

osci

tloscopetoeachofthe

two

outputsofthe

Final AC

Amplifier.

The

signal

observedatboth

points

should

be a .7 V

pk-pk

sinewaveat400

Hz.Ifthis

signal is

as

indicated,

one

may

conclude

that

allofthe

Signal

Channelacamplifiers,

including

the

two

final amplifiers,

are

functioning

normally.

6.7

MIXER

(schematic

on page VII-10)

(1)

Set

the

Phase

switchto2700and

the

Phase diat· to

90

0

(2) Verify

that

the

280

mV

pk-pk

signal

from

the

oscillator

is still

applied

to

both

the

Signal and

Reference

inputsofthe

Model

128A.

(3)

Connect

the

oscilloscope

to

TP201.

The

observed

signal

should

be a

positive

full-wave

rectified

sinewave

withapeak

amplitudeof+0.35

V. A

slight

adjustment

of

the

Phase dial

may

be

required

to

obtain

this

waveform.

If

this

signal is as

indicated,

one

may

conclude

that

the

Mixer

circuit

(0201

through

0204)

is

functioning

normally.

If

this

signal is

incorrectormissing,

check

the

Mixer

Reference

drive

signal,

which

canbemonitored

at

TP202.

One

should

observea400

Hz

square

wave

having

its

upper

level at +6 V

and

its

lower

level at

-12

V. If th is signal is as

indicated,

one

rnav

conclude

that

the

Reference

Schmitt

Trigger,

which

directly

supplies

the

reference

signal to

the

Mixer

circuit,

is

functioning

normally.

6.8 DC

AMPLI

FIERS

(schematic

on page

VII-l0)

(1)

Connect

the

DVMtoquick-disconnect

pin

J222.

With

the

Phase

dial

adjusted

for

maximum

DVM

indication,

the

voltageatJ222

should

be

-5

V

±.2

V. If

this

voltage is as

indicated,

the

First

DC

Amplifier

is

functioning

normally.

(2)

The

outputofthe

final arnpl ifier

canbemeasured

at

either

the

Recorder

Output

or

at

the

front-panel

Output

connector.

At

either

place

the

voltage

should

be +1 V

±50

mV

(assuming

the

input

signal is

the

specified

amplitude).

Also,

the

panel

meter

should

indicate

full scale. If

readinqs

other

than

those

indicated

are

obtained,

the

troubleisassociated

with

the

final de

amplifier.

Th is

completes

the

troubleshooting

procedure.Ifno

indica-

tionoftrouble

has

been

foundtothis

point,

the

instrument

is

either

functioning

normally,orthe

problemisbeyond

the

scopeofthis

procedure.

For

additional

aid or

advice,

the

operator

is

advised

to

contact

the

factory

or

the

authorized

representative

in his

area.

VI-3

APPENDIX A

MODEL 128A/90A MODIFICATION

The Model 128A/90A is a Model 128A

modified

to

operate at

two

different

combinations

of frequen-

cies.

Both

a Tuned

Amplifier

and an

Oscillator

are

incorporated

into

the Model 128A/90A. However,

instead of

setting

the tuned

frequency

of these ac-

cessories

by means of

capacitors

mounted

on the

accessory

boards themselves as explained in the

instruction

manual, the

capacitors

are

mounted

on rear-panel

switches.

One

switch

allows

the

Tuned

Amplifier

to be tuned to

either

2 kHz or 17

kHz.

Another

allows

the

oscillator

frequency to be

set to

either

1 kHz or 17 kHz. The

two

are operated

together

so that, when the

oscillator

frequency is

1 kHz, the tuned

amplifier

is tuned to 2 kHz, and

when the

oscillator

frequency is 17 kHz, the tuned

amplifier

frequency is

also

17 kHz.

In

addition

to the changes required to achieve

two-frequency operation, changes have

also

been

made in the

time

constant

circuitry.

The range of

available

time

constants

is changed and the Exter-

nal

Time

Constant

capabilityiseliminated.

A plate

TUNED FREQUENCY ADJUST

SYMBOLIZATION
MODEL

129A/84

TUNED AMPLIFIER MODIFI CATlON BOARD

FAB

IF

6973-

MD-

A

OSCILLATOR
FREOUENCY
ADJUST

AMPLITUDE

ADJUST

SYMBOLIZATION

MODEL

129A/95

MODIFICATiON

INTERNAL

OSCILLATOR

BOARD

FAB

6662

-MD-B

A-1

with

the new Time-Constant

switch

symbolization

is added to the panel so

that

the Time-Constant

switch

pointer/symbolization

indicates

correctly.

The new range

extends

from 300

P.s

to 30 s in

1-3-10

sequence.

Several

component-value

changes have been made in

implementing

the

new

time

constants.

Resistors R239 and R235 are

changed in value to 3

MOand 301 kO respectively.

Capacitors

C1

through

C9 are all decreased in

value by a

factor

of ten, and a new capacitor, one

microfarad, has been added to achieve the 30 SEC

time

constant

(EXT.

position

in standard in-

struments).
The diagrams

below

illustrate

the

wiring

of the

rear-panel

switches

to the spring-loaded

contacts

of the individual accessory boards.

Schematics

of

the

affected

circuits

are

also

provided. Because

the

time-constant

changes

only

involve compo-

nent-value changes as

described

above, no

separate

schematics

are

furnished

for the Time-

Constant

circuits.

NOTCH/BANDPASS SWITCH

SELECTIVE/FLAT

SWITCH

BRNiWHT
BLU/WHT

~:

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ALLI~RESISTORS

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COPYRIGHT

JJ.ll.

BY

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PftN:[TQItI91~

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APPENDIX B

MODIFICATION

1281/70

INSTRUCTIONS FOR OPERATING THE MODEL 128A OR MODEL 129A

WITH

THE

MODEL 189 INCORPORATED AS A TUNED AMPLIFIER

.NOTE:

Before

proceeding,

first

verify

that

the

lock-in

amplifier

is in

good

working

order. One

way of

doing

this

is to run

through

the

initial

checks

procedure

provided

in the

lock-in

amplifier

:

instruction

manual.

Since

the

underlying

assump-

tionisthat

the

lock-in

amplifier

has been

specifi-

cally

modified

for

operationinconjunction

with

a

Model

189, be

sure

the

rear-panel

switch

is in the

FLAT

positioninperforming

the

Initial

Checks.

TUNING PROCEDURE

The

procedure

that

followsiswritteninterms

of

tuning

the

system

to 450 Hz.

This

same

procedure

couldbeusedtotune

the

systemtoother

oper-

ating

frequencies.

The

only

difference

would

be

that

the

oscillator

and the

Model

189

wouldbeset

to

the

new

frequency

instead

of to 450 Hz.

(1) Set the

lock-in

amplifier

controlsasfollows.

(a)

CONTROL

SETTINGS FOR M128A

ONLY

Input

switch:

A

Sensitivity:

100 mV

Filters:

MIN

and

MAX

Phase

switch:

270

0

Phase dial: 900(9

turns

from

fully

ccw

position)

Reference Mode: FUND. f

Zero

Offset

dial: 0.00

(fully

ccw)

Zero

Offset

switch:

OFF

Time

Constant:

0.3 SEC.

de Prefilter: OUT

(b) CONTROL SETTINGS FOR M129A

ONLY

Input

switch:

A

Sensitivity:

100 mV

Filters:

MIN and

MAX

Phase

switch:

270

0

Phase dial: 900(9

turns

from

fully

ccw

position)
Reference Mode: FUND. f
Vector

switch:

2 PHASE

Zero

Offset

dial: 0.00

Zero

Offset

switch:

OFF

Time

Constant:

0.3 SEC.

both

channels

Output

Expand: x 1

de Prefilter: OUT

(2) Set

the

SELECTIVE/FLAT

switch

(on

the

rear

panel of the

modified

Model

128A or

Model

129A) to the SELECTIVE

position.

B-1

(3)

Connectacable

from

the

Model

189 INPUT

conneotortothe

"TO

M189

IN"

connector

on

the

rear of

the

lock-in

amplifier.

(4) Set

the

controls

of the

external

oscillator

as

requiredtoprovide

a 100 mV

rms

sine

wave

output

at 450 Hz. Then

connect

this

signal

to

both

the

"A"

and Reference

Inputs

of the

lock-in

amplifier.

(5)

Set

the

Model

189

controlsasfollows.

Pushbuttons:

all to OUT

position

Frequency

control:

4.50

Frequency

range: x 100

(6) Turn the

power

on at

both

the

Model

189 and

lock-in

amplifier.

(7)

Monitor

the

Bandpass

output

of

the

Model

189

withanoscilloscope.

The observed

signal

should

be a 450 Hz

sine

wave.

Carefully

adjust

the

inner

dialofthe M189

dual-concentric

Fre-

quency

control

for

maximum

observed

signal.

The

amplitudeofthe

observed

signal

should

be

about

125 mV pk-pk.

(8) Remove

the

oscilloscope

and

connectacable

from

the

BANDPASS

output

of the

Model

189

to

the

"TO

M189

OUT"

jack

on the rear panel

of

the

modified

Model

128A or

Model

129A.

(9) The panel

meter

on the

Model

128A (IN

PHASE

meterinthe

case of a M129A)

should

indicate

near

full

scaletothe

right.

(10)

Adjust

the

lock-in

amplifier

Phase

dial

for

peak

meter

indication.

The

lock-in

amplifier

M189

systemisnow

tuned

to

450 Hz. As

explained

previously,

this

same

proce-

dure

couldbeusedtotunetoother

frequencies

as

well.

If

the

lock-in

amplifier

is to be

operated

as a

flat

frequency-response

instrument,

simply

set the

switchatthe

rear panel of the

lock-in

amplifier

to

FLAT.

It is

not

necessarytodisconnect

the

Model

189,

although

it can be

disconnected,

if desired.

With

the

switch

in the

FLAT

posttlon,

the

lock-in

amplifier

functionsasdescribed

in the

instruction

manual.

AC LINE FREQUENCY OPERATION

It is never a good idea to operate a lock-in ampli-

fier

at the power line frequency or one of

its

har-

monics. This is

particularly

true if a tuned

amplifier

such

as the Model 189 is

incorporated

into

the system.

Significant

pickup

and measure-

ment error

will

occur

if operation at the

power

fre-

quency or

its

harmonicsisattempted.

B·2

EXTRA

GAIN·OF·TEN

OPERATION

The

Model

189 has a gain-of-one when

both

of the

Model

189 GAIN

pushbuttons

are in the

out

posi-

tion. The

maximum

sensitivity

that can be select-:

ed at the

front

panel of the lock-in

amplifier

is one

microvolt.

By operating

with

the Model 189

PREAMP GAIN

pushbutton

depressed,

this

sensi-

tivity

can be increased to 100 nV. Do

not

operate

the

system

with

the

Model 189 BANDPASS GAIN

pushbutton

depressed. The result

willbesignifi-

cantly

increased noise and, at

high

frequencies,

increased phase

shift.

JI

INPUTA

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J9

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REFERENCE

IIOARD

1260-1900

CHASSIS WIRING DIAGRAM

SECTION

VII

SCHEMATICS

Signal

Amplifier

Board Parts Location

Signal

Amplifier

Board (7457 -O-SO, sheet 1 of 2)

Signal

Amplifier

Board

(7457-0-S0,

sheet 2 of 2)

Reference

Board Parts Location

Reference

Board

(6622·0-S0,

sheet 1 of 2)

Reference

Board

(6622-0-S0,

sheet 2 of 2)

Mixer

- Power Supply Board Parts Location

ixer

- PowerSupply Board (7444-0-S0, sheet 2 of 3)

ixer- PowerSupply Board

(7444-0-S0,

sheet 3 of 3)

assis

Wiring Diagram (7589-E-SO) . .

ternal

Oscillator Board Parts Location

ternal

Oscillator

Modification

(6645-C-SO)

uned

Amplifier Board Parts Location . .

ned

Amplifier Modification (6646-C-SO)

VII·'

Page

VII-l

VII-2
VII-3
VII·4
VII-5
VII-6

VII·7
VII-8

VII-9
VII·10
VII-l1

VII-12
VII-13
VII·14
VII·15
VII·16

":

S101-A

HFCMR

Rx 3

(shouldbehigh

ohms

resistortomlnlmlze)o->

_

shunting

effect

on '// input

impedance)

..

R x 1

(two

possible

position

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