TYPE
1232-A
TUNED AMPLIFIER AND
NULL DETECTOR
GENERAL
B
RADIO
COMPANY
OPERATING INSTRUCTIONS
TYPE
1232-A
TUNED AMPLIFIER AND
NULL DETECTOR
!
j
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 below 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 sensitivity by 40 db. Does not affect bottom 203 of
SC6le.
Distortion: (In flat position) less than 53,
practically all attributable to the meter rectifiers.
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:
2N 1395, one 2N929.
Five 2N 169A, two
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
'TUNED
'
.
AND
NULL
1232 -A
AMPLIFIER
DETECTOR
Section 1
INTRODUCTION
1. 1 PURPOSE.
The Type 1232-A Tuned Amplifier and Null Detector (Figure 1) is
a sensitiv e, low-nois e amplifier continuo usly tunable from 20 cps to .20
kc, with addition al fixed-tuned frequenc ies 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 frequenc ies, and a preamplifier for transducers.
1.2 DESCRIPTION.
The Type 1232-A consists of a low-noise preamplifier, a frequency-selec tive stage (feedbac k amplifier and null network), an amplifiercompres sor stage, and a meter-re ctifier circuit (see block diagram, Figure 2). The total gain of the amplifier is about 120 db. Full-sca le meter
sensitiv ity 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 permits 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 mseries. 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
FILTER TUNING
Type
Function
Continuous
rotary coatrol
Tunes filter within selected
tuning range.
FILTER
FREQUENCY
6-position
rotary switch
Selects desired frequency characteristic: tuning-&equency range
of 20·200cps, 200cps-2kc, or 2·20
kc; flat,50-lcc or 100-kc response.
GAIN
Rotary control
Tums inscrument on or off and
controls gain.
METER
Toggle switch
Selects linear or logarithmic response.
INPUT
Type 874 Coaxial Connector
Input terminals.
OUTPUT
Pair of Type
938 Binding
Posts
Output terminals.
EXT FILTER
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 disconnected. The external filter may be either a series-tuned circuit to
trap out an undesired frequency or an antiresonant parallel-tune d 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 preferably 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.
2 Heuy ~.!J IRE, Tru.sactions on Circuli Theory, Sepiember 1955, Vol CT•2, No. 3,
Rlklio ~rnenler, Vol 35, No. 7, Jwy, 1961, p 8.
P 283;
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
in
(or Y 21 ), is quite.
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
.lin
1 :z)
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 amplifiercompressor 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 distortio n. No de amplific ation was incorporated into the meter
circuit, so that there is no need for a de zero adjustme nt on the front
panel and no possibil ity of de zero instabili ty. High-impedance, crystaltype earphon es can be connecte d to the output terminal s.
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 connecto r. Adaptors for
connecto rs other than Type 874 are availabl e from General Radio (see
table at rear of this manual).
NOTE
For connection lo binding posts, •se a Type B74-R34
(H""' piclt.•p is too great with a Type
Patcb Cord.
.)
Adaptor
B74-Q2
b. Set the METER switch to LINEAR.
c. Set the FILTER FREQUENCY switch for the desired characte ristic: 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
attenuat ion in db is roughly proportional to the rotation angle of the control knob.
e. The OUTPUT terminal s may be connecte d to an oscillos cope or
headpho nes. The red binding post is high, the black binding post is
ground.
The high sensitiv ity of this instrUment permits its use as a preamplifier for transduc er .outputs or oscillos cope input.
6
.3.2 USE AS A NULL DETECTOR FOR BRIDGE BALANCING.
j
1
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 modulation on the high-frequency signal.
.3.4 USE IN AUDIO SPECTRUM ANALYSIS.
The tuned amplifier can be used as an audio-frequency wave analyzer 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 amplifier is first calibrated with a constant-amplitu de, variable-frequen cy
signal. The typical variation of peak response vs frequency Is· shown
in Figure 5.
w+IO
I
Ir
...."
0
/
-·
~
..._
'-
!-10
z
CONSTANT
~NT
..~. .'~ f's
~
'
SOUtlCE
'
r./
'
i.-- --...
~.,,
~
/
CONSTANT VOLTAGE SOURCI ~ L_.....,
D
~
5-zo
CONSTAN{ ~ soJa ~
>
20
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.
2
~~
0.5
...JCl.
bl~
~e
o.z
1-Q
0.1
a111
!!:ffi
Ui._
SHORT CIRCUIT
NOISE11v
,____
II)
-
~
~
~
-
.....
OD5
1-W
~~ 0.02
'
,,
...J
llJ
-
O.OI
IOcpa 20
50
- --
/
,,..,... ""
i---
IOO 2.00
50
/
I
v
~ -NOISE
" ' N CllClllT
JIJIG
I
Ille
2
5
10
20
50
I
IOO•
FREQUENCY
Figure 6.
Typical noise levels as a function of frequency.
TYPE 1232-A TUNED AMPLI FIER 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.
C242
S201
PREAMPLIFIER
BOARD (SEE
PAGE 14 )
CIOO
RI03
250
Ml
BATTERY TUBE
MAIN ETCHED BOARD (SEE PAGE 14 l
Figure 7. Bottom interior view of the Type 1232-A.
General Radio Type 1232-A Tuned amplifier and Null Detector Full Page Schematic Diagram
Schematic Page 1
Schematic Page 2
Schematic Page 3
\
I
\
1232-2711
,----
R20:J
IK
1232-270/
-
I
I
C/02 +
3311F,X
.,..
+
C207
.:I: 60pF
v
I
R204
220K
Q201
czOI
220
I
I
I
I
I
C/03
6.8pF
!
R2/I.
/,$/(
R/03A
500K
C2/9
0.46f
.2K
//8W
C220
0.464
R235
3.32K
//BW
·-·-.-
2.2K
i'
j
C221
j0.464
R2.J6
, 3.32K
l/BW
A.i:
11
--1
R228
2.7K
5.1/K
C2
41
//BW
C222
46~
/.IK.
I/SW
R238
IK
AJ:
AJ:
T
R23!J
R221
R237
I
L_
I
R206\
/K
R205
220K
•1-
I/
WH-BK
0.0/5
0.(}()22
Figure 8. Schematic diagrallj of the Type 1232-A Tuned Amplifier an
,
er1 r - - - ,
~ :~.--..-~...,.....~~~~~---<•-~~~~~~~::::.::.::~::::::.::~::::::.::~===~-=--==~~~ ~-
C/02
:~:;
~:''
- _J ;~o:
+
.J.:JµF~
+
1232-2nl
C207
~60µF
---..,
R2J2
/OK
C218
5µ~
R2.JO
9. IK
0. 22~
R2.J2
9 .IKI
J201
I
C2/0
+
I
R2.JI
IOpF
22K
C20/
220
R217
22K
R205
220K
J200
A.rt I
C209
C21.J
680
R206'
IK
I
R224
12K
+
I
,.,,
Ml
·: 1-
+
I
SH:£1~.
·'~
R227
I . BK
C2H
5pF
0201- 0202
A .T.
2
C219
0464
2 .2K
i C221
j 0.464
+
R2:J6
:J.32K
• I/SW
A}'.
g
__\
MITER
16
R242
R243
I ";:.
5./IK
5.llK
::;-k
'lt'
64
46
I
R2.J8
IK
~'f ,
T
R228
2.7K
r1
R2:J5
3.!J2K
//BW
R240
R221
R2.J7
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//SW
C220
0.464
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//BW
0/01
0200
0204
""
AJ:
I/
A . T.
12
0100
0203
0205
20·200C
'"~·:::,
·-J~
WH-Y£
WH-BK
FILTER FREQUENCY
J
GAIN
S20I
WH-BL
f:5iZ1c:cz~:~rfc
SIOO-RIO.JA-R/038
POSlr/ON
5201 SWITr:HING DIAGRAM
'SWITCH POSfflON
.,,246
.J.:JK
Figure 8. Schematic diagra"i of the Type 1232-A Tuned Amplifier and Null Detector.
Zif
;JF
.JR
.,.
1-2
7·8
7-8
l·Z
7·8
1-z
8-IO
/·.J
7·9
1-4
6-10
lf·IO
1·4
T·IO 1-4
7-// I-$
SWffCH SECr/ONS
IF
,_.JR
FLAT
7-IZ
20-200C
7-12 1-Z.8
ZF
7-8
7- 8
200C-2KC
7-12 l-.J-8 8-10
2-20KC
!JOKC
7. g
1-10
1-S
8·/D
8 · /D
IOOKC
7-//
1·6
8 ·/0
I·•
l·.J
1-•
a - 10
7-12
""
1· 6
/
INPVT
L/111/TE:R
CRIOO
CRIOI
VAR. GAIN
2-srAGEAMP
QIOO
Q/01
:JOIJF
S20I PANEL LEGEND
•.
0-:..-0
~Fll!,TERS
t
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.
Figure 10. Etched Board Layout of the
Type 1231A Tuned Amplifier and Null Detector
Notes on the
General Radio Type 1232-A
Tuned Amplifier and Null Detector
Replacement transistors:
Quantity in use
Original Specification
Replacement type
5
2N169A
NTE 101
1
2N929
NTE 123A
2
2N1935
NTE 100
Alternate
NTE 123AP
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