type 1232-a tuned amplifier and null detector · type 1232-a 'tuned amplifier and 1. 1...
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TYPE 1232-A
TUNED AMPLIFIER AND NULL DETECTOR
GENERAL RADIO COMPANY
B
! j
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
j
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
+ C207 .:I: 60pF v
R204 220K
czOI 220
Q201
I
\ \
R205 220K R206\
/K !
R237
.2K ' //8W i
C2/9 C220 j C221 0.46f 0.464 j0.464
R235 R2.J6 3.32K , 3.32K //BW l/BW
A.i: 11 T ·-·-.- --1
R2/I. /,$/(
R221
2.2K
R238 IK
AJ: •1-
0.0/5
0.(}()22
1232-2711
R228 2.7K
R23!J
5.1/K //BW C2
C222 41
46~
AJ: I/
WH-BK
/.IK. I/SW
Figure 8. Schematic diagrallj of the Type 1232-A Tuned Amplifier an
er1 r-- -, , ~ :~.--..-~...,.....~~~~~---<•-~~~~~~~::::.::.::~::::::.::~::::::.::~===~-=--==~~~ ~-
- _J ;~o: ~:'' :~:;
C/02 + .J.:JµF~
+ C207 ~60µF
C20/
+
220
R205 220K
C219 0464
A}'. g
.,,246 .J.:JK
R206' IK I
R2.J7
.f0.2K //SW
C220 i C221 0.464 j 0.464
R2:J5 R2:J6 3.!J2K :J.32K //BW • I/SW
T
J
R2.J8 IK
~'f \ __ ,
WH-BL
R2J2 /OK C209
0.22~ C2/0 + IOpF
16
R217 22K
r-1 I
R221
2 .2K
MITER
1232-2nl
R228 2.7K
R224 C21.J 12K 680
+ C2H 5pF
R240
5.llK 5./IK
::;-k 'lt' I ";:. 46 64
/.IK "" //BW
AJ: I/
WH-BK
Figure 8. Schematic diagra"i of the Type 1232-A Tuned Amplifier and Null Detector.
R227 I .BK
R242
R2.JO 9.IK
R243
A. T. 12
C218 5µ~
R2.JI
22K
WH-Y£
---.., J200
A.rt I R2.J2 9 .IKI
I J201
I I Ml
·: 1- + ,.,, I
·'~ £1~ SH: . A.T. 2
20·200C
0201- 0202
0/01 0100 0200 0203 0204 0205
'"~·:::, ·-J~
FILTER FREQUENCY S20I GAIN
f:5iZ1c:cz~:~rfc SIOO-RIO.JA-R/038 POSlr/ON
5201 SWITr:HING DIAGRAM
'SWITCH POSfflON SWffCH SECr/ONS
IF JR ZF Zif ;JF .JR .,. "" FLAT 7-IZ ,_. 7-8 7·8
20-200C 7-12 1-Z.8 7-8 1-2 7-8 l·Z 7·8 1-z 200C-2KC 7-12 l-.J-8 8-10 l·.J 8-IO /·.J 7·9 1-4 2-20KC 7. g I·• 8·/D 1-• 6-10 1·4 T·IO 1-4 !JOKC 1-10 1-S 8 · /D lf·IO 7-// I-$
IOOKC 7-// 1·6 8 ·/0 a -10 7-12 1· 6
INPVT L/111/TE:R CRIOO CRIOI
VAR. GAIN 2-srAGEAMP
QIOO Q/01
:JOIJF
•.
/
~-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