GENERAL RADIO COMPANY 874-GAL Operating Instructions Manual

OPERATING

INSTRUCTIONS

TYPE

874-GAL

ADJUSTABLE

ATTENUATOR

DESCRIPTION

The

Type

874-GAL

Adjustable

Attenuator

is

of

the

wave-guide-

below-cutoff

type operating

in

the

TE

1~mode

(inductive

coupling).

The

waveguide

is

excited

byacoaxial

trankmission

line,

as

showninFig

-

ure1,and

the

output

voltage

is

induced

inaloop mounted

onasliding

plunger.

One

endofthe

loopisterminated

ina50-ohm

resistor

and

the

otheris

connected

to

the

output

cable.

This

arrangementmakes

the

output

impedance

of

the

attenuator

approximately

50

ohms.

The

attenuation

is

varied

by

rotating

the

barrel

whichmoves

the

plunger

and

loop

along

the

tube.

The

barrel

is

calibrated

in

relative

attenuation

in

dB.

The

attenu

-

ators

can

be

adjusted

from

the stopatthe

minimum

attenuation

position,

which

occurs

at

about

-9dBonthe

calibrated

scale,tothe

stopatthe

max

-

imum

attenuation

end

of

the

range,

which

occurs

at

about

140

dB on

the

scale.

Each

revolution

of

the

barrel

changes

the

attenuation

20

dB.

In

most

applications

the

maximum

usable

attenuation

is limited

by

leakage

from

various

partsofthe

driving

circuit

and

notbythe

rangeofthe

atten

-

uator.

In

this

typeofvariable

attenuator,

as

the

attenuation

is

varied

the

phase

at

the

output

connection

is

essentially

constant

with

respect

to

the

input

signal

phase.

The

method

of

coupling

to

the

coaxialline

permits

the

attenuator

to

beused

asastandard

calibratedattenuator

orasa

device

to

couple

a

small

amount

of

energy

fromahigh-level

circuit

for

monitoring

or

meas

-

uring

purposes

without

introducingalarge

discontinuity

in

the

main

line.

The

Type

874-GAL

Coaxial

Attenuator

employs

the

new

Type

874—BL

Locking

Coaxial

Connectors.

These

connectors

canbeused

interchange

-

ably

with

the standard

GR874~

Connectors.

When

mated

with

another

locking

connector,

a

firm

mechanical

coupling

of

the

two

is

achieved

when

the

coupling

shell

is

engaged.

The

shielding

is

also

improved

sig

-

nificantly,

and,ingeneral,

the

leakage

is

reduced

byatleast

50

dB.

The

“quick-disconnect”

feature

of

thestandard

GR874

Coaxial

Con

-

nectors

is

retained

in

the

locking

type

if the

coupling

shell

is

noten-

gaged.

However,inthis

case

the

shielding

is

notaseffective.

ERRORS

In

all

waveguide

attenuators,

the

excitation

of modes

other

than

the

desired

one

distorts

the

theoretical

attenuation

characteristic,

particularly

at

low

attenuation

levels.

In

the

Type

874-GAL

Attenuator,the

TM

0

1

mode

(capacitive

coupling)

is

the

most

serious

offender.

This

mode

is

attenuated

more

rapidly

with

distance

from

the

source

endofthe

wave-

guidethanisthe

desired

mode

and

its

intensity

is

proportional

to

the

volt

-

age

across

the

exciting

lineatthe

coupling

point,while

the

intensity

of

the

desired

modeisproportional

to

the

current

flowinginthe

exciting

line

at

the

pointofcoupling.

The

effect

of

the

undesired

modeonthe

attenu

-

ation

characteristic

therefore

decreases

as

the

attenuation

increases.

Since

the

output

from

the

attenuator

is

the

sumofthe

voltage

generated

in

the

loopbyboth

modes,

serious

errors

can

result

if the

voltage

in

the

exciting

lineatthe

coupling

point is

high

compared

to

the

current;

that

is,

if the

impedanceis high.

An

error

correction

procedure

is

required

to

achieve

the

stated

accuracy

for

the

874-GAL

Attenuator.

CORRECTION

PROCEDURES

There are

two

configurations

that

may

be

employed

in

using

the

874-GAL.

First,it

maybeterminated

ina50-ohm

termination

(874-W5OBL)

as shownin

Figure

2,in

which

case

the

special

correction

given

in

Fig

-

ure2is

required,

and,inaddition,

thefrequency

correction

of

Figure3is

required.

In

the second

configuration

recommendedto

achieve

the

best

accuracy

above2GHz,

the

termination

is

an

adjustable

stub

employed

to

placeavoltage

minimumatthe

coupling

points.

The

frequency

correction

graph

of

Figure3is

required

onlyinthis

case.

The

detailed

procedure

is

giveninthe

following

paragraphs.

If

the

exciting

lineisterminated

in

its

characteristic

impedance,

for

instance

withaType

874-W5OBL

Termination,

the

error

resulting

from

the

coupling

of

the

TM01modecanbecorrected

by

using

the

correction

chart

of

Figure

2.

The

~ign

of

the

correction

depends

on

which

end

of

the

exciting

lineisconnected

to

the

sourceofrf

power.

If

the

source

is

con

-

nected

to

the

end

marked

with

the

letterS,use

the

correction

curve

labeled

NORMAL

and

subtract

the

correction

found

for

initial

setting

of

the

atten

-

uator

from

the

initial

scale

reading.

Then

subtract

the

correction

found

for

the

finalsetting

of

the

attenuator

from

the

final

scale

reading.

The

trueattenuation

is

the

difference

in

thecorrected

values.

If

the

source

is

connected

to

the

opposite

end,

addtothe

indicated

values

the

corrections

read

from

the

curve

labeled

REVERSED.

When

these

correction

charts

are

used,

the

indicated

relative

attenuation

is

accurate

within

±(O.015

times

the

difference

+0.2)dB

at

frequencies

upto2000MHz.

For

accurate

results

2 GENERAL

RADIO

at

higher

frequencies

for

small

values

of

attenuation,

the

method

outlined

in

the

following

paragraph

shouldbe

used.

To

eliminatethe

correctionandtomake

the

indicated

relative

atten

-

uation

agree

very

closely

with

the

actual

attenuation

between

—9

and

140

dBonthe

scale,

the

voltage

across

the line,

at

the

actual

coupling

point,

should

approach

zero.

To

accomplishthis,

connect

an

adjustable

short-

circuiting

stub (Type

874-D2OLor-D5OL)tothe

endofthe

main

lineop-

posite

the

source.

Adjust

the

stub

untilavoltage

minimum

appears

at

the

coupling

point.

To

find

this

stub

adjustment,

substitute

a

Type

874-VRL

Voltmeter

Rectifier

andaType

874-VI

Voltmeter

Indicator,

oraType

874-

TL

Tee

andadetector,

for

the

attenuator.

Adjust

the stub

until

the

indi

-

cated

voltage

is

minimized;

then

replace

the attenuator.

(If

the

Type

874-

VRLisused,connect

the

end

containing

the

series

resistor,

engraved

R,

to

the

generator.)If

the

frequency

is

changedthe

stub

mustbe

reset,

but

a

frequency

calibration

can

be

marked

on

the

stub,

for

convenience

when

a

numberofmeasurements

aretobe

made.

At

frequencies

below

300

MHz,

a

Type

874-WNL

Short

Circuit can

be

substituted

for

the

stub,

without

fur

-

ther

adjustments.

With

the

Type

874-WNL

Short

Circuit,

accurate

results

can

be

obtained

at

scale

readings

greater

than

zero.

At

frequencies

below

1500

MHz,

the

indicated

attenuation

is

accurate

at

scale

readings

as

low

as—9dB,

and,atfrequencies

between

1500

and

4000 MHz,

readings

above

0

dBonthe

calibrated

scale

are

accurate

if

the

stubisset

as

previously

outlined.

An

alternate

method

of

setting

a

voltage

minimum

at

the

coupling

point,forfrequencies

above

1500MHz,

utilizes

the

factthat

the

attenuation

constant

for

the

desired

modeissmaller

than

thatofthe spurious

modes.

If

the

attenuator

is settoa

relatively

large

valueofattenuation,

thestub

can

be

properly

set

by

adjusting

it

until

maximum

outputisobtained

from

theattenuator.

However,avoltage

minimum

does

not

occuratthe

coupling

point

unless

the

effective

sourceimpedance,

seen

looking

back

toward

the

generator

from

the

coupling

point,

isapure

resistance.

When

the

genera

-

tor

and

the

detector

are

not

matched,

the

above

condition

can

be

obtained

by

the

use

of

suitable

pads

(Type

874-G1OL

or

-G2OL)atthe generator

and

the

detector.

At

the

higher

frequencies

the

attenuation

is

reduced

as

the

wave-

guide

approaches

its

cutofffrequency.

To

correct

for

this effect,

multiply

the

indicated

attenuation

by

the

factor

found

in

the

frequency

correction

graph,

Figure3,corresponding

to

theoperating

frequency.

The

impedance

of

the

adjustable

armofthe

attenuator

is

nominally

50

ohms.

However,

the

impedance

varies

appreciably

with

frequency,

as

shown

by

the

plotinFigure4of

the

VSWR

seen

looking

back

into

the

out

-

put

connector

foratypical

unit.

The

tolerance

on

the

dc

resistance

of

the

50-ohm

resistor

is±10%.

TYPE 874-GAL ADJUSTABLE

ATTENUATOR

3

APPLICATIONS

(1)

The

attenuator

can

be

used

withaType 874-YRor-VRL

Volt

-

meter

Rectifier

andaType

874-VI

Voltmeter

Indicator

to

provide

the

level-

monitor

and

variable

attenuator

ofasignal

generator,

when

combined

with

a

General

Radio

Oscillator.

TheTypes

1362,

1363

and

1218

are

recom

-

mended.

A

block

diagramofatypical

setup

is

showninFigure

5.

In

this

case

the

stubisadjusted

asindicated

inaprevious

paragraph

and

the

adjustable

line

1

(Type

874-LAL)

is

adjusted

to

obtainamaximum

indication

from

the

Type

874-VI

Voltmeter

Indicator.

The

voltage

indi

-

cated

with

the

load

connected

is

the

effective

open-circuit

output

voltage,

and

the

source

impedance

is

accurately

50

ohms.

The

output

canbereduced

by

adjusting

the

attenuator;the

actual

voltage

at

any

attenuator

setting

is

the

indicated

numberofdb

below

the

original

calibrating

value.

Sincethe

source

impedance

is

the

sameasthe

characteristic

imped

-

ance

of

the

line,

the

same

effective

open-circuit

voltage

and

source

im

-

pedanceare

obtained

at

the

endofany

lengthof50-ohm

line

connected

to

the

outputof

the

voltmeter

rectifier.

If

the

lineisnot

lossless,

theef-

fective

open-circuit

voltage

willbereduced

by

the

lossinthe

line.

(2)

Theattenuator

can

also

be

used

to

measurethe

attenuation

of

a

network

by

the

substitution

method,

as

indicated

by

the block

diagramin

Figure

6.

Readings

of

the

detector

output

and

attenuator

setting

are

first

made

without

the

circuit

under

test

connected.

The

circuit

is

then

inserted

and

the

Type

874-GAL

Adjustable

Attenuator

is

readjusted

to

give

the

same

detector

indication.

Theattenuation

of

the

unknown

circuit

is

thenthe

difference

in

the

attenuator

readings.

The

pads

indicated

are

used

to

make

the

source

and

load

impedances

very

close

to50ohms. Oneor

both

can

be

omittedifthe

source

and

detector

impedances

are

matched.

(3)

For

monitoring

purposes

the

input

line

of

the

attenuator

can

be

connected

in

series

with

the

line

under

test

without

introducing

an

appre

-

ciablereflection

or

loss.

The

changes

in

level

in

the

main

line

can

be

measured

accurately

using

the

attenuator

andanuncalibrated

detector

which

may

be1)a

Type

874-VRL

Voltmeter

Rectifier

withadc

microammeter

or

an

audio

amplifier

such

as

the

General

Radio

Type

1232-A

Tuned

Ampli

-

fier and

Null

Detector

if the

signal

is modulated

or 2)

if the

signal

is

not

modulated,

the

General

Radio

Type

1241

Heterodyne

Detectors

may

be

‘At

the

lowerfrequencies,

it

maybenecessary

to

add

lengths

of

Type

874-L30

AirLineinseries

with

the

adjustable

lineto obtain

the

maximum

output.

4

GENERAL

RADIO

employed.

The

VSWR

introduced

inaflat

line

by

the

insertion

of

the

at-

tenuator

is

showninFigure

7.

(4)Itcan

be

used

asaphase

inverter

if the

flexible

leg

is driven,

producing

equal

outputs

180°

outofphase

at

the

remaining

legs.

RW

~

5.0

~

~30

~

~

H20

~

1.0

0.9

0.8

0.7

0.6
05

~-~U-~ ~

~ ~, ~GE~ ~ ~ TYPE

874-W5OBL

~

~

H

TYPE

874-GAL

~ ~

~

~~L#~H

~t- ~ ~j~i1Jl~J111~

-~

L~

~ ~

—~

When

sourceIsconnectedtoendofat-

-~

- -

—

tenuator

markedS,subtract

correction

-

labeled

NORMAL

from

reading.

For

t

-

opposite

connection,

add

REVERSED

a

-

-

-

~

correction.

ft

it~t.

1I~

I

II

II

-10

0 0

20 30

40

ATTENUATOR

READINGA,dB

50 60

874

0.12

Figure2.Correction

chart

for

use

with

Type

874-GAL

Adjustable

Attenuator

when

exciting

lineisterminated

in

its

characteristic

impedance.

Figure

1.

Cutaway

viewofadjustable

attenuator.

TYPE 874-GAL

ADJUSTABLE

ATTENUATOR

5

874

G-lO

(I)

>

g

0

0.06

0.05

— —

— ——-

-

0.04

—

—

—

— —

-

0.03

—

—

—

- -

-

-

/

-

o.or

ii

2

I

III

0.0!

—

E

E

—

—

—

——-

—

O.OOT

0.005

0.00~

—

—

——-

—

~‘

Actual

AttenuatIon

=

(1-K)aIndicated

AttenuatIon

,

1000

2000 3000 4000

FREQUENCY,

MHz

Figure

3.

Frequency

Correction

Factor.

Figure

4.

VSWR

seen

looking

back

into

out

put

con

-

nectorofa

typical

Type

874-GAL

Ad.

justable

Attenuator.

FREQUENCY.

GHz

874 G.8

6 GENERAL

RADIO

Figure

6.

Block

diagramofsetup

for

measuring

the

attenuation

ofanetwork.

TYPE

874—R22LA

PATCH

CORD

1209-5

Figure5.Block

diagramofa

simple

signal

generator

usingthe

adjustable

attenuator

and

other

GR874

Coaxial

Elements.

TYPE

874—D2OL

OR

—D5OL

874

G-14A

TEST

TYPE 874-GAL ADJUSTABLE

ATTENUATOR

7

(I)

>

Figure7.VSWR

introduced

in a

flat

line

by

the

insertion

ofaType

874-GAL

Adjustable

Attenuator.

SPECIFICATIONS

A

waveguide-below-cutoff

type, useful as a calibrated
attenuatororas a sampling device. Calibratedindecibels,
onamicrometer-type

scale.

Absolute

attenuation

is the

sum

of insertion

loss

and

scale

reading.

Phase

shift

is

essentially constant as

attenuation

isvaried. The

main

line

is a

short

coaxial

section

with

locking

GR874

connectors,

one

end

for

source,

the other for

load.

It introduces

mini

-

mum discontinuity when

insertedina 50-ohm

line.

The

loop

outputisbrought

out

through

three feet of 50-ohm

cable with locking GR874 connector.
Calibrated

Range:

120dB(relative

attenuation)

with

input

line

terminatedin50 0;

129dBwith input line

terminated

in

adjustable stub to

minimize

the

electric

field at the

coupling

point

(scale

reads—9to

120

dB).

Insertion

Loss

(from

input

connector to end of output cable

at 1 GHz, when

signal

source impedanceis50

0):

With

input

line

terminatedin500,and

scale

set

at 0

dB,

30.4dB±2

dB;

setat—9

dB, 17±2dB

(settings below 0

are not accurate).

With input line

terminatedinadjustable

stub

(Whichex-

tends the

range

over which the calibration is accurate to

the—9dB

scale

setting), 19±2dB

minimum.

(Insertion

lossisapproximately

inversely

proportional

to

frequency up to 1

GHz.)

Insertion

Loss

Directly

Through

Tee:

Negligible.

Accuracy of Attenuation:

Stub-terminated

input,

±(0.0l

times differenceinattenu

-

ation

reading

+0.2)

dB,

direct-reading.

50-U

terminated

input,

±(0.015

times differenceinatten

-

uation

reading

+0.2)

dB,

when corrected.

Correction

chart

supplied.
VSWR

Introduced

into Line:

<1.03at1

GHz;

<1.12

from 1

to 4 GHz.
VSWRofOutput:<4at 1

GHz;<5from 1 to

4GHZ.

Max

Power:

Input power limit

inversely

proportional

to

square root of frequency.

Power

should not exceed 300

W

at 1 GHz. Output power should not exceed

1/2

W.

Patent

No.

2,548,457.

GENERAL

RADIO

COMPANY

*WEST

CONCORD,

MASSACHUSETTS

01781

FREQUENCY.

GHZ

8

74.0.13

Form

No.

0874.0210-I

6/68

Printed

in U.S.A.