TYPE 1232-A

TUNED AMPLIFIER AND NULL DETECTOR

GENERAL RADIO COMPANY

B

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OPERATING INSTRUCTIONS

TYPE 1232-A

TUNED AMPLIFIER AND NULL DETECTOR

Form 1232-0100-B April, 1962

Copyright 1961 by General Radio Company West Concord, Massachusetts, USA

i. GENER A.L RAD I 0 C 0 MP ANY j

WEST CONCORD, MASSACHUSETTS, USA

Figure 1. Type 1232-A Tuned Amplifier and Null Detector.

SPECIFICATIONS Frequency Response:

Tunable Filters: 20 cps to 20 kc in 3

ranges; 63 bandwidth; 2nd harmonic at least

34 db down from peak , 3rd at least 40 db

down; rejection filter on two highest ranges

reduces 60-cycle level to at least 60 db be­

low peak. Frequency dial accuracy is ±33.

SO-kc and 100-kc Fllten: 2nd harmonic

at least 60 db down.

Flat Response: ±3 db,20 cps to 100 kc.

Sensitivity: One microvolt, full-scale, or

better, over most of the frequency range.

Noise Level at Output Terminals: Less than

50 mv at minimum gain. See Figure 6.

Input Impedance: Approximately 50 kilohms

to one megohm, depending on gain-control

setting.

Max Input Voltage: 200 volts ac or 400 volts

de.

Gain: 120 db on the tunable ranges; 100 db,

flat range; 106 db at 50 kc; 100 db at 100 kc

Output: 1 volt into 10,000 ohms. Internal

impedance is 3000 ohms.

Meter Linearity: Db differences on scale are

accurate to ±53 for inputs of less than 0.3

volt .

External Filter: Source impedance, 700obms .

Compression: Reduces full-scale sensitiv­

ity by 40 db. Does not affect bottom 203 of

SC6le.

Distortion: (In flat position) less than 53,

practically all attributable to the meter rec­

tifiers.

Power Supply: 12 volts de, from 9 mercury

(M72) cells in series. Estimated battery life

is 1500bours. Cost is about0.4 cent per hour.

Transistor Complement: Five 2N 169A, two

2N 1395, one 2N929.

Accessories Supplied: Type 874-C58 Cable

Connector.

Dimensions: Width 8, height 6, depth 7-1/ 2

inches (205 by 150 by 190 mm) over-all.

' .

TYPE 1232-A

'TUNED AMPLIFIER

AND

1. 1 PURPOSE.

NULL DETECTOR

Section 1 INTRODUCTION

The Type 1232-A Tuned Amplifier and Null Detector (Figure 1) is a sensitive, low-noise amplifier continuously tunable from 20 cps to .20 kc, with additional fixed-tuned frequencies of 50kc and lOOkc. Intended primarily as a bridge detector, the Type 1232-A can also be used as a detector of high-frequency modulated signals (with a crystal demodu­lator), a wave analyzer at audio frequencies, and a preamplifier for trans­ducers.

1.2 DESCRIPTION. The Type 1232-A consists of a low-noise preamplifier, a frequen­

cy-selective stage (feedback amplifier and null network), an amplifier­compressor stage, and a meter-rectifier circuit (see block diagram, Fig­ure 2). The total gain of the amplifier is about 120 db. Full-scale meter sensitivity is 1 microvolt or better over most of the frequency range.

OUTPUT

Figure 2. Block schematic of the null detector.

GENERAL.RADIO COMPANY

With the Type 1632-A Inductaace Bridge, this null detector per­

mits inductance balances to a resolution of 1 part in 106. Comparable

precision for capacitance balance can be obtained with the Type 716-C

Capacitance Bridge. The amplifier is powered by 12 TOks de, supplied by nine mercury

batteries m series. The output is 1 volt into 10,000 ohms.

1.3 CONTROLS AND CONNECTORS. The following table lists controls and connectors on the Type

1232-A Tuned Amplifier and Null Detector:

Name Type Function

FILTER TUNING Continuous Tunes filter within selected

FILTER FREQUENCY

GAIN

METER

INPUT

OUTPUT

EXT FILTER

rotary coatrol tuning range.

6-position rotary switch

Rotary control

Toggle switch

Selects desired frequency char­acteristic: tuning-&equency range

of 20·200cps, 200cps-2kc, or 2·20 kc; flat,50-lcc or 100-kc response.

Tums inscrument on or off and controls gain.

Selects linear or logarithmic re­sponse.

Type 874 Coax- Input terminals. ial Connector

Pair of Type Output terminals. 938 Binding Posts

Phone jack Connection for external filter.

1.4 USE OF EXTERNAL FILTERS. External filters can be connected at the EXT FILTER jack. When

a telephone plug is inserted in chis jack, the built-in shunt filter is dis­

connected. The external filter may be either a series-tuned circuit to

trap out an undesired frequency or an antiresonant parallel-tuned circuit

to imp!'O"Ye the selectivity at the desired frequency. For the purpose of

calculating the Q of the external filter, the source impedance is about

700 ohms. Since the external filter is plugged into the circuit at a point

beyond the 60-cps rejection filter and where there is 80-db gain to the

meter circuit, it is important that the external filter be shielded and pre­

ferably that it use a toroidal iaduccor for miailllWD sensitivity to hum

-!-'···-

Section 2 PRINCIPLES OF OPERATION

2.1 PREAMPLIFIER. The preamplifier stage of the Type 1232-A Tuned Amplifier and

Null Detector is designed to minimize noise from both low-impedance sources, such as inductance bridges at low frequencies, and high-imped­ance sources, such as capacitance bridges at low frequencies. 1 A tran­sistor with a noise figure of 3 to 5 db at an optimum source impedance of 50 lcilohms is used. By use of negative feedback, the input impedance of the preamplifier is also made 50 kilohms, and the noise level indi­cated on the output meter is relatively constant and independent of source impedance.

The input uansistor is protected from possible damage due to large overloads by a limiter consisting of a series capacitor and two shunt silicon rectifier diodes. This circuit effectively prevents signals greater than 1 volt, peak-to-peak, from reaching the input ttansistor and does not conttibute noise or distortion to low-level signals.

Maximum gain of the preamplifier is about 40 db, which is ade­quate to swamp the noise of succeeding stages. After preamplification, the signal passes through a set of series and shunt filters, which are de· signed to reject frequencies above and below the selected tuning range. Typical filter characteristics are shown in Figure 3. On all switch po-

.D .., -•ol---+-~-+-~.j#...~~-l.tC--+--ll~-+~+--++-l-i--W-...j....1.--4-i

-70IO 20

Figure 3. Typical filter characteristics of the Type 1232-A.

1 A. E. Sudersot1 and R. G. Fullca, •A Simplified Noise Theory, and its Applicsrion to the Desip of Lo..,.Nolse Amplifier,• IRE Transacdon on Audio, July-August, 1961.

GENERAL RADIO COMPANY

sitioos ezcept FLAT and 20 - 200 C, another rejectioo filter reduces the response at 60 cps to greater than 60 db below peak response.

2.2 FREQUENCY-SELECTIVE AMPLIFIER. This amplifier consists of three stages with negati•e feedback

through a null necwodc, which has its null at the desired operatiiigftequen· c:y. Since there i• negatiYe feedback at all &equencies but the desired one, the onr-all responae peaks at this frequency and is roughly equiY· alent to that of a tuned circuit with a Q of about 20 (5" bandwidth). The unique feature of this null network is its one• potentiometer taoing .. Many

null networks require three Yariable elements, either ganged capacitors

or ganged potentio~ers. Tills leactll.to maay problems in alignment and tnicldng the three elements to maintain a goOi:f ifull. the Hll1ritu1fnec­work 2. has a perfect null in theory for any position of the ttaaing potenti·

ometer, and it is possible to conr ~ 10:1 wniag raage with a ... u­pooential potentiometer. Tuning capacitors are switched to change raoges, mailtaining the impedanc:e le.el of the null network approziaately coo­... fot the three tuning ranges.

Since the 50-kc and 100-kc null networks need not be tunable, con­ventional twin·T null networks are used.

On the FLAT position of the range switch, all filters are switched out and the frequency response is flat to -within ±Hb from 20 cpa to 100

kc. The onrall gain of the amplifier is reduced by 26 db to keep the noise level on the output meter equal to about 10 percent of fall scale at marimum gain.

2.3 MULL NETWORK. The Type 1232-A Tuned Amplifier and Null Detector uses an RC

null network: with only ooe variable coaponent to adjust the frequency of the null. This aYoicls the use of ganged variable components, which must track closely to maiiltain stability when used in highly selective feedback amplifiers. The network (Figure 4), consisting of three·ter•

figure 4. Null Circuit of the Type 1232-A Tuned Amplifier and Null Detector.

2Heuy ~.!J IRE, Tru.sactions on Circuli Theory, Sepiember 1955, Vol CT•2, No. 3, P 283; Rlklio ~rnenler, Vol 35, No. 7, Jwy, 1961, p 8.

TYPE 1232-A TUNED AMPLIFIER AND NULL DETECTOR

minal RC circuits, gives a complete null without being balanced against a voltage divider, and permits frequency adjustment with a single poten· tiometer. The tuning law for this circuit is

(.iJ = o RC "" a(l -a)

In order to span a 10-to-l logarithmic frequency range, the potentiometer must have an e:ii:ponential characteristic of over 100 to 1.

The selectivity of the transfer admittance, E10 (or Y 21), is quite. in

constant as the null frequency is changed. In order to use this charac· teristic, the network ·must be driven by and loaded by low impedances. Therefore, it is used in a feedback circuit with an amplifier having low

input and output impedances and a transfer resistance !2- (or a real Z 1 :z) .lin

that is chosen to give the desired selectivity. This combination pro• vides a second-harmonic rejection of 34 db over each 10-to-l frequency range.

2.4 AMPLI Fl ER-COMPRESSOR. With the METER switch set to the LINEAR position, the amplifier­

compressor functions as a linear amplifier, driving the meter rectifier circuit and supplying the output terminals with about 1.4 volts for full­scale deflectioo of the meter. The de supplied to the last traasiator is sufficient to drive the output meter to full scale, but very little more, so that it is impossible to damage the meter by overdriving the amplifier.

For null detector use, the METER switch is thrown to LOG, effec­tively compressing the upper part of the meter scale. Two pairs of sili­con diodes are switched in shunt with the collector resistors of two tran­sistors to provide a nonlinear collector impedance. Owing to the voltage offset of the silicon diodes, the bottom 20 percent of the meter scale is virtually unaffected. A signal level corresponding to 100 percent de· flection for linear response will drop to SO percent for logarithmic re­sponse. An increase of 20 db increases the reading to 80 percent, and another 20 db raises the reading to 100 percent.

2.5 METER CIRCUIT. The meter circuit uses a full-wave rectifier in order to double the

ripple frequency that passes through the meter and thus to prevent the needle from vibrating visibly at 20 cps. Resistors are used in place of two of the rectifiers in the conventional full·wave bridge in order to linearize the relation between meter indication and signal level, and to

minimize distortion. No de amplification was incorporated into the meter

circuit, so that there is no need for a de zero adjustment on the front

panel and no possibility of de zero instability. High-impedance, crystal­

type earphones can be connected to the output terminals.

Section 3 OPERATING PROCEDURE

3.1 USE AS AH AMPLIFIER OR PREAMPLIFIER. To use the Type 1232-A Tuned Amplifier and Null Detector as an

amplifier:

a. Connect the input signal to the INPUT connector. Adaptors for

connectors other than Type 874 are available from General Radio (see

table at rear of this manual).

NOTE For connection lo binding posts, •se a Type B74-R34

Patcb Cord. (H""' piclt.•p is too great with a Type

B74-Q2 Adaptor.)

b. Set the METER switch to LINEAR.

c. Set the FILTER FREQUENCY switch for the desired character­

istic: FLAT, 20-200 C, 200 C - 2 KC, 2-20 KC, 50 KC, or 100 KC.

d. With the GAIN control, turn the instrument on and adjust the gain

to the desired level. The total range of the GAIN control is 120 db, and

attenuation in db is roughly proportional to the rotation angle of the con­

trol knob. e. The OUTPUT terminals may be connected to an oscilloscope or

headphones. The red binding post is high, the black binding post is

ground. The high sensitivity of this instrUment permits its use as a pre­

amplifier for transducer .outputs or oscilloscope input.

6

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1

.3.2 USE AS A NULL DETECTOR FOR BRIDGE BALANCING. To use this insuument as a detector for bridge measurements:

a. Conne~t the JNPUT terminals of the Type 1232-A to the DE·

TECTOR terminals of the bridge. b. Set the METER switch to LOG.

c. Set the FILTER FREQUENCY and FILTER TUNJNG controls

to the desired frequency. d. With generator and unknown connected to the bridge, set the

GAIN control of the Type 1232-A for approximately half-scale deflection

of the output meter, and tune the FILTER TUNJNG control for maximum

output. The bridge balance can now be made in the conventional manner,

readjustment of the GAIN conuol as balance is approached.

3 • .3 USE AS A DETECTOR-DEMODULATOR. For the detection of modulated high-frequency signals, the Type

1232-A Amplifier and Null Detector can he used with the Type 874-VQ

Voltmeter Detector. Sensitivity is approximately 200. µ.., full-scale up

to about 2000 Mc. The crystal output of the Type 874-VQ Voltmeter De·

tector should be connected through a Type 874-R.22 Patch Cord to the

JNPUT connector of the Type 1232-A. The FILTER FllEQBENcY and

FILTER TUNJNG controls should he set to the frequency of the modu­

lation on the high-frequency signal.

.3.4 USE IN AUDIO SPECTRUM ANALYSIS. The tuned amplifier can be used as an audio-frequency wave an­

alyzer with a sensitivity of 1 µv and a bandwidth of about 5 percent. For approximate measurements, the gain can be assumed to be constaat

with frequency. More accurate measurements can be obtained if the am­

plifier is first calibrated with a constant-amplitude, variable-frequency

signal. The typical variation of peak response vs frequency Is· shown

in Figure 5. w+IO

I Ir 0

" .. .. .. !-10 z D

~ 5-zo >

20

-· ~ ... _ CONSTANT ~NT SOUtlCE

~ ... f's .. - ' i.-- --... '- '~ ~

/ ' r./ ' ~.,, ~ / CONSTANT VOLTAGE SOURCI ~ L_.....,

CONSTAN{ ~ soJa ~

llO IOD ... 2DO SOO Ille Z I IOllc 20 llO IOOllc

Figure 5. Typical variation in peak response with frequency for

constant gain-control setting.

7

GENERAL RADIO COMPANY

Section 4 SERVICE AND MAINTENANCE

4.1 GENERAL. The two-year warranty given with every General Radio insttWDent

attests the quality of materials and workmanship in our products. Wheo difficulties do occur, our service engineers will assist in any way pos­sible.

In case of difficulties that cannot be eliminated by the use of the.se service instructions, please write or phooe our Service Department, giving full information of the trouble and of aeps taken to reaedy it. Be sure to mention the serial and type numbers of the instrument.

Before returning an instrument to General Radio for service, please write to our Service Department or nearest district office(see backco•er), requesting a Returned Material Tag. Use of this tag will ensure proper handling and identification. For instruments not co•ered by the war­ranty, a purchase order should be forwarded to a•oid unnecessary delay.

4.2 REMOVAL OF COVER. To open the instrument for access to transistors and components,

loosen the two fluted caprive screws at the rear of the instrument and slide the U-shaped dust cover away from the panel. All transistors and components are now accessible.

4 •. 3 BATTERY REPLACEMENT. The Type 1232-A Tuned Amplifier and Null Detector is powered

by nine M72 mercury batteries (Mallory RM·4R or equivalent), which will lastfor over 1500 hours with normal use. For a simple t:heck of the bat· teries, measure the de voltage between anchor terminal 6 and ground. This should be 12 volts.

To replace the batteries, remove the cap (twist councerclockwise and pull out) on the right-hand side of the instrument under the M72 CELLS engraving. Batteries must be inserted with the positive ( '*' ends facing into the instrument.

TYPE 1232-A TUNED AMPLIFIER AND HULL DETECTOR

4.4 INTERNAL NOISE. At certain frequencies the sensitivity of the Type 1232-A Tuned

Amplifier and Null Detector greatly exceeds the catalog specifications. With maximum gain at these frequencies, the noise generated by ttan· sistor QlOO normally causes large deflection of the output meter. For maximum useful gain, the GAIN control should be set low enough so that the noise level at the output is not greater than 10 percent of full scale.

For minimum sensitivity to hum pickup and stray fields, input con· nections must be shielded. Do not use an adaptor from the Type 874 IN· PUT connector to binding posts; use a shielded Type 874 cable.

If, with compl~ely shielded input connections, the noise level of the Type 1232-A greatly exceeds the values given in Figure 6, the noise is probably caused by QlOO. If it is necessary to replace this transistor, a replacement with a high /3 (beta) will usually give the lowest noise levels.

It is possible, but not likely, that QlOl will contribute signifi· candy to the internal noise. If it is necessary to replace QlOl, select a Type 2Nl69A transistor with a /3greater than 60.

Due to the large amowit of amplification that follows the GAIN control, there is always a small amoum: of noise present at the output terminals. At minimum gain, this should be less thaa 50 mv and should cause no more than 1 percent deflection of the output meter. This noise is generally caused by Q201, which may be caused by Q200, which can be interchanged with Q202, or replaced. Occasionally, the noise may be caused by Q200, which can be interchanged with Q203, Q204, or Q205 or replaced.

...J llJ

2

~~ 0.5 ...JCl.

bl~ ~e o.z 1-Q 0.1 a111 !!:ffi OD5 Ui._

1-W

~~ 0.02 II)

O.OI

SHORT CIRCUIT ,____ NOISE11v ~

----

' ,, /

/ ~

~ ..... --I

v ,,..,... ""

~ -"' N CllClllT i--- NOISE JIJIG

I I IOcpa 20 50 IOO 2.00 50 Ille 2 5 10 20 50 IOO•

FREQUENCY

Figure 6. Typical noise levels as a function of frequency.

TYPE 1232-A TUNED AMPLIFIER AND HULL DETECTOR

b. With the GAIN and FILTER FREQUENCY controls unchanged, apply a lOO·kc signal (of the same amplitude as the 1-kc signal) to the INPUT connector and adjust the trimmer capacitor, C202, for a meter in­dication of ~ db. Check the frequency response below 100 kc to make sure it is within the soecified limits.

PREAMPLIFIER BOARD (SEE

PAGE 14 )

CIOO

RI03

Ml

C242 S201

BATTERY TUBE MAIN ETCHED BOARD (SEE PAGE 14 l

Figure 7. Bottom interior view of the Type 1232-A.

250

General Radio Type 1232-A Tuned amplifier and Null Detector Full Page Schematic Diagram General Radio Type 1232-A Tuned amplifier and Null Detector Full Page Schematic Diagram

Schematic Page 1

Schematic Page 2

Schematic Page 3

I

1232-270/ ,---- -I I I

I I I I I I L_

C/03 6.8pF

R/03A 500K

C/02 + 3311F,X .,..

R20:J IK

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R205 220K R206\

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R237

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2.2K

R238 IK

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1232-2711

R228 2.7K

R23!J

5.1/K //BW C2

C222 41

46~

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WH-BK

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Figure 8. Schematic diagrallj of the Type 1232-A Tuned Amplifier an

er1 r-- -, , ~ :~.--..-~...,.....~~~~~---<•-~~~~~~~::::.::.::~::::::.::~::::::.::~===~-=--==~~~ ~-

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R228 2.7K

R224 C21.J 12K 680

+ C2H 5pF

R240

5.llK 5./IK

::;-k 'lt' I ";:. 46 64

/.IK "" //BW

AJ: I/

WH-BK

Figure 8. Schematic diagra"i of the Type 1232-A Tuned Amplifier and Null Detector.

R227 I .BK

R242

R2.JO 9.IK

R243

A. T. 12

C218 5µ~

R2.JI

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0/01 0100 0200 0203 0204 0205

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FILTER FREQUENCY S20I GAIN

f:5iZ1c:cz~:~rfc SIOO-RIO.JA-R/038 POSlr/ON

5201 SWITr:HING DIAGRAM

'SWITCH POSfflON SWffCH SECr/ONS

IF JR ZF Zif ;JF .JR .,. "" FLAT 7-IZ ,_. 7-8 7·8

20-200C 7-12 1-Z.8 7-8 1-2 7-8 l·Z 7·8 1-z 200C-2KC 7-12 l-.J-8 8-10 l·.J 8-IO /·.J 7·9 1-4 2-20KC 7. g I·• 8·/D 1-• 6-10 1·4 T·IO 1-4 !JOKC 1-10 1-S 8 · /D lf·IO 7-// I-$

IOOKC 7-// 1·6 8 ·/0 a -10 7-12 1· 6

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VAR. GAIN 2-srAGEAMP

QIOO Q/01

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~-Fll!,TERS

t

S20I PANEL LEGEND

0-:..-0 FLAT

0 20~2GOC

0 200-2000C

0 2-20KC

0 50KC

0 IOOKC

Figure 9. Block diagram of the Type 1232-A Tuned Amplifier and Null Detector.

F i g u r e 1 0 . E t c h e d B o a rd L a y o u t o f t h e T y p e 1 2 3 1 A T u n e d A m p l i f i e r a n d N u l l D e t e c t o r

Notes on the General Radio Type 1232-A

Tuned Amplifier and Null Detector

Replacement transistors:

Quantity in use Original Specification Replacement type Alternate

5 2N169A NTE 101

1 2N929 NTE 123A NTE 123AP

2 2N1935 NTE 100

Note well: This unit requires a 12 volt power supply. If Vcc is less than 12 volts, positive peaks flat

top at ≥ 60% on the meter scale. 9-volt batteries are not sufficient. It is easiest to remove

the battery holder tube and replace it with three holders containing 2 AA cells each (or one

quad holder and one 2-cell holder).

The following link describes an alternative rechargeable power supply:

http://members.rpa.net/~choffman/GR1232A.htm