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