iET 1232-A User manual

Operation Manual
Model 1232-A, -AP Tuned
Amplifier&NullDetector
PRECISION INSTRUMENTS FOR TEST AND MEASUREMENT
VIIM-700/cover.p65
TEL: [phone] • [phone] • FAX: [phone]
www.ietlabs.com
534 Main Street, Westbury, NY 11590
IET LABS, INC.
© Copyright- 1992

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Capabilities
•R:20 µΩ-1TΩ
•C: <1 pF - 1 F
•L: 100 µH-100H
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•Accuracy to 1 ppm
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WARNING
OBSERVE ALL SAFETY RULES
WHEN WORKING WITH HIGH VOLTAGES OR LINE VOLTAGES.
Dangerous voltages may be present inside this instrument. Do not open the case
Refer servicing to qulified personnel
HIGH VOLTAGES MAY BE PRESENT AT THE TERMINALS OF THIS INSTRUMENT
WHENEVER HAZARDOUS VOLTAGES (> 45 V) ARE USED, TAKE ALL MEASURES TO
AVOID ACCIDENTAL CONTACT WITH ANY LIVE COMPONENTS.
USE MAXIMUM INSULATION AND MINIMIZE THE USE OF BARE
CONDUCTORS WHEN USING THIS INSTRUMENT.
Use extreme caution when working with bare conductors or bus bars.
WHEN WORKING WITH HIGH VOLTAGES, POST WARNING SIGNS AND
KEEP UNREQUIRED PERSONNEL SAFELY AWAY.
CAUTION
DO NOT APPLY ANY VOLTAGES OR CURRENTS TO THE TERMINALS OF THIS
INSTRUMENT IN EXCESS OF THE MAXIMUM LIMITS INDICATED ON
THE FRONT PANEL OR THE OPERATING GUIDE LABEL.
WARRANTY
We warrant that this product is free from defects in material and workmanship and, when properly used, will
perform in accordance with applicable IET specifications. If within one year after original shipment, it is found
not to meet this standard, it will be repaired or, at the option of IET, replaced at no charge when returned to IET.
Changes in this product not approved by IET or application of voltages or currents greater than those allowed by
the specifications shall void this warranty. IET shall not be liable for any indirect, special, or consequential
damages, even if notice has been given to the possibility of such damages.
THIS WARRANTY IS IN LIEU OF ALL OTHER WARRANTIES, EXPRESSED OR IMPLIED, INCLUD-
ING BUT NOT LIMITED TO, ANY IMPLIED WARRANTY OF MERCHANTIBILITY OR FITNESS FOR
ANY PARTICULAR PURPOSE.

-
1232-A, -AP Tuned
Amplifier & Null Detector
Instruction
Manual
'1992
Bolton, Massachusetts, U.S.A. 01740-1107
February, 1992
Contents
Warranty
Specifications
Introduction -Section 1
Principles
of Operation -Section 2
Operation -Section 3
Service and Maintenance -Section 4
j~IEC417
Symbol L__) on equipment signifies that the manual contains
information to prevent injury
or equipment damage
®lET LABS, INC.
534 Main Street
Westbury, NY 11590
www.ietlabs.com 516-334-5959 F:516-334-5988
Printed in U.S.A. Form1232-0100-00

Contents
Section 1 INTRODUCTION
1. 1 Purpose . • . .
1.2 Description . • •
1.3 Controls and Connectors 1
2
1.4
Use of External Filters • • 2
Section 2 PRINCIPLES OF OPERATION
21 Preamplifier of the Type 1232-A . 3
2.2 Frequency Selective
Amplifier 4
2.3
Null Network . . 4
2.4 Amplifier-Compressor 5
25 Meter Circuit 5
Section 3 OPERATION
3.1 Use as an Amplifier or
Preamplifier • .
.. 6
3.2 Use as a Null Detector for Bridge
Balancing . . . . . . . 6
3.3
Use in Audio Spectrum Analysis . 7
Section 4 SERVICE AND MAINTENANCE
4.1 GR Field Service 8
4.2 Removal of Cover . 8
4.3 Battery Installation 8
4.4 Internal Noise . . 9
4.5 Transistor Voltage and Resistance
Measurements . • • • . . 10
4.6 Trimmer-Capacitor Adjustment . 10
4.7 Minimum Performance Standards 11
4.8 Knobs . . • . • • 16
------- ----------·------
Instruction Manual Changes
Specifications, Page iii
Power: Nine mercury cells in series have been replaced by eight
11
AA
11
alkaline batteries, manufacturers part number:
Estimated battery life is
250 hours.
Everready E91
Duracell MN1500
Ray-0- Vac B15
Service and Maintenance, Paragraph 4.3, Battery
Installation, Page 8
Remove the instrument cover per paragraph 4.2, Removal of ·
Cover.
Remove the old batteries and replace with eight new ones,
installed
per polarity marking.
Electrical Parts List, Page 20
Nine mercury cells have been replaced by eight II AA II alkaline
batteries, manufacturers part number listed above.

Specifications
TYPE 1232-A TUNED AMPLIFIER AND NULL DETECTOR
Frequency Response: TUNABLE Fl LTERS: 20Hz to 20kHz in 3 ranges; between 2% and
6% bandwidth to 15kHz; 2nd harmonic at least 34 dB down from peak, 3rd at least 40 dB
down; rejection filter on two highest ranges reduces 60-Hz level to at least 60 dB below
peak response (50-Hz level
is down >50 dB). Dial accuracy is ±3%. Fl XED-TUNED FlLTEAS:
50 kHz, 2nd harmonic is 44 dB down; 100kHz ... 53 dB down. FLAT RESPONSE: ±3
dB
from 20Hz to 100kHz.
Sensitivity: See plot. Typically better than 0.1 ,.,.v over most of the frequency range.
Noise Level: REFERRED TO INPUT: See plot. Noise figure at 1 kHz is less than 2 dB
at an optimum source impedance of 27 kn. REFERRED TO OUTPUT: Less than 5 mV
on FLAT filter-frequency position, min gain setting, and -20-dB switch position; less
than 50 mV in MAX SENS position.
Input: IMPEDANCE: Approx 50 kn at max gain; varies inversely with gain to 1 Mn at
min gain. MAX SAFE VOLTAGE: 200 V ac or 400 V de.
Output: VOLTAGE GAIN: Approx 120 dB on the tunable ranges: 100 dB, flat range;
106 dB at 50 kHz; 100 dB at 100-kHz position. LEVEL: 1 V into 10 kn when meter
indication
is full scale. INTERNAL IMPEDANCE: 3 kn. METER LINEARITY: dB dif-
ferences are accurate to :±5% ±0.1 division for inputs of less than 0.3 V. COMPRESSION
(meter switch to LOG): Reduces full scale sensitivity by 40 dB. Does not affect bottom
20010 of scale. ATTENUATION (meter switched to -20 dB): Linear response with 20-dB
less gain than MAX SENS.
Distortion (filter switch in FLAT position): <5% (due to meter rectifiers).
Terminals:
Input, GR874®coaxial connector; output, binding posts.
Available: 1232-P2 Preamplifier
to maintain sensitivity of 1232-A at low frequencies when
operating
from a source impedance above 100 k.n; rack-adaptor sets (see below) convert
1232 alone,
or with companion instruments, to 19-in. rack-mount width.
Power: 12 V de; from 9 mercury (M72) cells in series. Est battery life 1500 hours.
Optionally, a rechargeable battery (non-mercury) can be supplied on special order.
Mechanical: Convertible bench cabinet. DIMENSIONS
(wxhxd): Bench, 8x6x7.5 in.
(203x152x190 mm). WEIGHT: 5.75 lb (2.6 kg) net, 8 lb (3.7 kg) shipping.
Description
1232-A Tuned Amplifier and Null Detector
1232-AP Tuned
Amplifier and Null Detector, with preamplifier
Rack-Adaptor Sets
480-P308,
for 1232-A alone
480-P316, for 1232-A with 1310 or 1311 oscillator or
similar 8-in. wide instrument with convertible-bench
cabinet
480-P317, for 1232-AP (with preamp) and companion
8-in. instrument
Replacement Battery, 9 req'd
20
I I
I I I
-~~-
10
)EST ~IMIT
Catalog
Number
1232-9701
1232-9829
0480-9838
0480-9836
0480-9837
8410-1372
f--
i':
I
0. ~
•• Fluu. jCA" I
I I I
TEST LIMIT
""i FULt \Li
I
v
~. 100
v Minimum
input for full-
scale meter deflection as
a function of frequency.
0.2
~,_..__,__ I I
0. i I 10
10<01 20 ~0 100 200 ~ Ike 2 ~ 10 20 ~ 100
FREQUENCY
iii

iv
SPECIFICATIONS (cont)
TYPE 1232-P2 PREAMPLIFIER
(Accessory supplied with Type 1232-AP and Type 1240-AP only)
Voltage Gain: Approx 0.7.
Noise (referred to input): Open-circuit equivalent 0.1 pA; short-circuit equivalent, 0.3 p.V
(when used with Type 1232-A tuned to 100Hz).
Impedances: INPUT: >100 mn. in parallel with 70 pF. OPTIMUM SOURCE: 3 Mn..
OUTPUT: 10 kn..
Connectors: GR874®on cables, input and output.
Power: 12 V, 200 p.A, supplied by 1232-A.
Mechanical: Special cabinet. DIMENSIONS (wxhxd): 0.75x6x7.5 in. (19x152x190 mm).
WEIGHT: 0.94 lb (0.43 kg) net, 4 lb (1.9 kg) shipping.
Description
1232-P2 Preamplifier
TYPE 1240-A, -AP BRIDGE OSCILLATOR-DETECTORS
Catalog
Number
1232-9602
The Type 1240-A Bridge Oscillator-Detector is a rigid assembly of a Type
1311 Audio Oscillator and a Type 1232 Tuned Amplifier and
Null Detector.
The Type 1240-AP includes the Type 1232-P2 Preamplifier
in addition to the
above instruments. These compact assemblies are convenient for use with audio-
frequency bridges and other null-balance devices.
The units are secured with an Adaptor Plate Set
(PIN 0480-9836 for
1240-A, 0480-9837 for 1240-AP) and bolted together near the rear of the instru-
ments. The assembly fits a standard 19-inch relay rack. For bench use, the two
wings of
the relay-rack adaptor set can be removed and the four rubber feet
can be installed in the corners of the oscil,lator-detector assembly.
Operating instructions for the Type 1232-A or -AP Tuned Amplifier and Null
Detector are given in this book. A separate instruction book is supplied for the Type
1311 Audio Oscillator.
GR 1240-A Bridge Oscillator-Detector.

Introduction- Section 1
Figure 1. Type 1232-A Tuned
AmpI
ifier and Null Detector.
1.1 PURPOSE
The Type 1232-A Tuned Amplifier and Null Detector (Figure 1) is a sensi-
tive, low-noise amplifier continuously tunable from 20 Hz to 20kHz, with addi-
tional fixed-tuned frequencies of 50 kHz and 100kHz. 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 fre-
quencies, and a preampIifier for transducers.
The Type 1232-P2 Preamplifier
can be added to improve the signal-to-noise
ratio and consequently the effective sensitivity of the Type 1232-A when the
latter is to be driven by very high (greater than 100 kQ) impedance sources.
1.2 DESCRIPTION
The Type 1232-A consists of a low-noise preamplifier, afrequency-selec-
tive
stage (feedback amplifier and null network), an amplifier-compressor 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.
Figure 2. Functional diagram of the null detector.
OUTPUT

2
With the Type 1632 Inductance Bridge, this null detector permits induc-
tance balances to a resolution of 1 part in 106• Comparable precision for capa-
citance balance can be obtained with the Type 1615 and 716 Capacitance
Bridges.
The amplifier
is powered by 12 Vdc, supplied by nine mercury batteries
in
series. The output is 1 V into 10,000 ohms.
Front mountings are extendible to tilt instrument face.
The Type 1232-P2 Preamplifier
is a detachable accessory that consists
of a low-noise field-effect transistor used in common-source configuration.
Power
for the preamp is provided by the B +supply of the Type 1232-A.
1.3 CONTROLS AND CONNECTORS OF THE TYPES 1232 AND 1232-P2.
Name
FILTER TUNING
FILTER
FREQUENCY
GAIN
METER
INPUT
OUTPUT
TABLE 1
Type Function
Continuous rotary control Tunes fi Iter within selected tuning range.
6-position rotary switch Selects desired frequency characteristic; tuning·
frequency range of 20-200 Hz, 200 Hz-2 kHz, or
2-20 kHz; flat, 50-kHz or 100-kHz response.
Rotary control
3-position toggle switch
GR874 Locking Coaxial
Connector
Turns instrument on or off and controls gain.
Selects full-gain linear, -20-dB linear,
or logarithmic response.
Input terminals.
Pair of Type 938 Binding Output terminals.
Posts
EXTERNAL FILTER Phone jack
Connection for external fi Iter.
~[HIGH Z, LOW Z
~ INPUT
......
Ill
~ OUTPUT
Toggle switch
GR874 Connector and
Cable Assembly
GR874 Connector and
Cable Assembly
Determines whether Preamplifier is included in
input to Type 1232 (HIGH Z) or shunted {LOW Z).
Input terminals.
Output terminals {to be applied to INPUT of
Type 1232).
1.4 USE OF EXTERNAL FILTERS.
Filters can be connected at the EXTERNAL FILTER jack. When a tele-
phone plug is inserted in this jack, the built-in shunt filter is disconnected. The
external filter may be either aseries-tuned circuit to trap out an undesired fre-
quency or an antiresonant parallel-tuned circuit to improve 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-Hz 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 inductor for minimum sensitivity to hum pickup.

Principles of Operation-Section 2
2.1 PREAMPLIFIER OF THE TYPE 1232-A.
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. A transistor with a noise figure of 3 to
5 dB at an optimum source impedance of 50 kilohms 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 indepen-
dent
of source impedance.
The
input transistor is protected from possible damage due to large over-
loads by a limiter consisting of aseries capacitor and two shunt silicon rectifier
diodes. This circuit effectively prevents signals greater than 1 volt, peak-to-peak,
from reaching the input transistor and does not contribute 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 desig~ed to reject frequencies
above and below the selected tuning range. Typical filter characteristics are
shown in Figure 3. On all switch positions except FLAT and 20 -200 Hz,
another rejection
filter reduces the response at 60Hz to greater than 60 dB
below peak response.
TUNED FILTER SETTINGS
FREQUENCY IN
HERTZ
FIXED SETTINGS
Figure 3. Typical filter characteristics of the Type 1232-A.
3

4
2.2 FREQUENCY-SELECTIVE AMPLIFIER.
This amplifier consists of three stages with negative feedback through a
null network, which has its null at the desired operating frequency. Since there
is negative feedback at all frequencies but the desired one, the over-all response
peaks at this frequency and is roughly equivalent 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 tuning. Many null networks require three variable elements,
either ganged capacitors or ganged potentiometers. This leads to many problems
in alignment and tracking the three elements to maintain a good null. The Hall
null network has a perfect null in theory for any position of the tuning potenti-
ometer, and
it is possible to cover a 10:1 tuning range with a40-dB exponential
potentiometer. Tuning capacitors are switched to change ranges, maintaining
the impedance level
of the null network approximately constant for the three
tuning
ranges.
Since the 50-kHz and 100-kHz null networks need not be tunable, con-
ventional 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 ±3 dB from 20 Hz to 100kHz. The over-
all gain of the amplifier is reduced by 26 dB to keep the noise level on the out-
put meter equal
to about 10 percent of full scale at maximum gain.
2.3
NULL NETWORK.
The Type 1232-A Tuned
Amplifier and Null Detector uses an RC null net-
work with only one variable component to adjust the frequency of the null.
This avoids the use of ganged variable components, which must track closely to
maintain stability when used in highly selective feedback amplifiers. The net-
work (Figure 4), consisting of three-terminal RC circuits, gives a complete null
without being balanced against avoltage divider, and permits frequency adjust-
ment with a single potentiometer. The tuning law for this circuit is
·1
wo=
HC v' a( 1 - a) (1 + 2k)
In order to span a 10-to-1 logarithmic frequency range, the potentiometer must
have an exponential characteristic of over 100 to 1.
I
The selectivity of the transfer admittance, E~ {or y
21
) is quite constant
In
U+K)(I+2K) R
K
~
lt!)R
0
0
Figure 4. Null circuit of the Type 1232-A
Tuned Amplifier and Null Detector.

as the null frequency is changed. In order to use this characteristic, the network
must be driven by and loaded by low impedances. Therefore, it is used in a feed-
back circuit with an amplifier having low input and output impedances and a
E
transfer resistance -1•0 (or a real z
21
) that is chosen to give the desired selec-
m
tivity. This combination provides a second-harmonic rejection of 34 dB over each
10-to-1 frequency range.
2.4 AMPLIFIER-COMPRESSOR.
With the METER switch set to either linear position, the amplifier-com-
pressor functions
as a linear amplifier, driving the meter rectifier circuit and
supplying the
output terminals with about 1.4 V for full-scale deflection of the
meter. The
MAX SENS position'provides full gain for very low-level applications.
When
maximum sensitivity is not required, use the LINEAR -20 dB position
for a less noisy output. With this switch setting, the noise generated in the input
and selective amplifier stages is attenuated. The de supplied to the last transistor
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 deflection for linear response will drop to 50
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 Hz. 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 amplifica-
tion 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 termi-
nals.
5

6
Operation-Section 3
3.1 USE AS AN AMPLIFIER OR PREAMPLIFIER.
To use the Type 1232-A or -AP Tuned Amplifier and Null Detector as an
amplifier:
a. Connect the input signal to the INPUT connector of the Type 1232 or,
if used, the Type 1232-P2. Adaptors for connectors other than Type 874 are
available from General Radio. If the output impedance of the source of this signal
is greater than 100 k!2, set the switch on the Type 1232-P2, if used, to HIGH Z;
otherwise set the switch to LOW Z.
NOTE
For connection to binding posts, use a Type 874-R34
Patch Cord. (Hum pickup is too great with aType
874-Q2 Adaptor.)
b. Set the METER switch
to LINEAR -20 dB (unless maximum sensi-
tivity is required.)
c. Set the Fl LTER FREQUENCY switch for the desired characteristic:
FLAT, 20-200 Hz, 200Hz-2kHz, 2-20kHz, 50 kHz, or 100kHz.
d. With the GAIN control, turn the instrument on and adjust the gain
to the desired level. The total range of the GAl N control is 120 dB, and attenu-
ation in dB is roughly proportional to the rotation angle of the control knob.
e. The OUTPUT terminals may be connected to an oscilloscope or head-
phones. The red binding post is high, the black binding post is ground.
The high sensitivity
of this instrument permits its use as a preamplifier
for transducer outputs or oscilloscope input.
3.2
USE AS A NULL DETECTOR FOR BRIDGE BALANCING.
To use this instrument as a detector for bridge measurements;
a. Connect the INPUT terminals of the Type 1232-A or -AP to the
DETECTOR terminals of the bridge as in paragraph 3.1, step a.
b. Set the METER switch to LOG.
c. Set the Fl LTER FREQUENCY and Fl LTER TUNING controls to the
desired frequency.
d. With generator and unknown connected
to the bridge, set the GAl N
control
of the Type 1232-A for approximately half-scale deflection of the out-
put meter, and tune the FlLTER TUNING control for maximum output.
The bridge balance can now be made in the conventional manner, by re-
adjustment of the GAIN control as balance is approached.

3.3 USE IN AUDIO SPECTRUM ANALYSIS.
The tuned amplifier can be used as an audio-frequency wave analyzer
with asensitivity of 1 p.V and a bandwidth of about 5 percent. For approximate
measurements, the gain
can be assumed to be constant with frequency. More
accurate measurements
can be obtained if the amplifier is first calibrated with a
constant-amplitude, variable-frequency signal. The typical variation of peak
response vs frequency is shown in Figure 5.
+10
CONSTANT CURRENT SOUR.::E
~---
---
-
...... r--s~~
.........
,..,.---
- ........
-
... , ~·
-
/ ' / '
~rjf
'
0
r
7-
CONSTANT VOLTAGE SOURCE-<!__..,.
-10
CONSTANf CURR~NT SOU~CE .-!!!_
--"'
-20
-30
20
50 100 200
500
10 20 50 100
FREQUENCY- HERTZ
Figure 5. Typical variation in peak response with frequency for
constant gain-control setting·.
7

8
Service and Maintenance-Section 4
4.1 GR FIELD SERVICE.
Our warranty attests the
quality of materials and workmanship in our
products. When difficulties do occur, our service engineers will assist in any way
possible.
If the difficulty cannot be eliminated by use of the following service
instructions,
please write or phone the nearest GR service facility (see back page),
giving full information
of the trouble and of steps taken to remedy it. Describe
the instrument
by type, serial, and ID numbers. (Refer to front and rear panels.)
Before returning
an instrument to General Radio for service, please ask
our nearest office for a "Returned Material" number. Use of this number in
correspondence and on a tag tied
to the instrument will ensure proper handling
and identification. After the initial warranty period, please avoid unnecessary
delay by indicating
how payment will be made, i.e., send a purchase-order num-
ber or (for transportation charges) request "C.O.D."
For return shipment, please use packaging that is adequate to protect the
instrument
from damage, i.e., equivalent to the original packaging. Advice may be
obtained from any GR office.
4.2
REMOVAL OF COVER.
To open the instrument for access to transistors and components, loosen
the
two fluted captive 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 INSTALLATION.
The Type 1232-A Tuned Amplifier and Null Detector is powered by nine
M72 mercury batteries (Mallory RM-4R
or equivalent), which will last for over
1500 hours with normal use. For a simple check of the batteries, measure the
de voltage between anchor terminal 6 and ground. This should be 12 V.
To replace the batteries, remove the cap (twist counterclockwise and pull
out) on the right-hand side of the instrument under the M72 CELLS engraving.
Insert the
1% in. spacer (P/N 1232-6010) in the opening on the right-hand side
of the instrument, under the M72 CELLS engraving. Push it in to the end of the
compartment. Then, insert the nine batteries
with the positive(+) terminal as
marked on the battery facing into the instrument. Finally place the cap (battery-
keeper assembly, P/N 1650-2120) in position, twisiting
it approximately% turn
in either direction to lock it in position.

4.4 INTERNAL NOISE.
4.4.1 Noise Indicated by Meter.
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 transistor Q100 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 connec-
tions must be shielded. Do not use an adaptor from the Type 874 INPUT con-
nector to binding posts; use a shielded Type 874 cable.
If, with completely 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 Q100. 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 Q1 01 will contribute significantly to the
internal noise.
4.4.2 Noise From the Type 1232-P2.
The equivalent noise sources
of the Type 1232-AP with the FILTER
FREQUENCY set to 100 Hz are a current source of less than 0.1 p.p.A and a
voltage source
of less than 0.3 p.V. If the noise levels of the Type 1232 alone do
not exceed the values of Figure 6, yet the Type 1232-AP will not meet the
above specifications, replace the transistor in the Type 1232-P2.
..J
w I
~;; 0.5
..Jo..
w~
6~ 0.2
~8 0.1
5~
.....___
I I
SHORT-CIRCUIT /
NOISE ~V
i\.
..,~
"
.......
~
) /
'l
"'
./ ~ ~
~
~-CIRCUIT j
~~ 0.05
._!.a::
~~ 0.02
~---
(/) :>.01
lOHz 20 50 100 200 500 IkHz 2
FREQUENCY
NOISE pA
I I
5 10 20
100
Figure 6. Typical noise )e..vels as a function of frequency.
4.4.3 Output Noise.
Due
to the large amount of amplification that follows the GAIN control
there
is always a small amount of noise present at the OUTPUT terminals. At
minimum gain, and with the FILTER FREQUENCY control set to FLAT or with a
FlLTER FREQUENCY of 5kHz or lower, this noise should be less than 50 mV
with the METER switch set to MAX SENS and should cause no more than 1 per-
9

10
cent deflection of the output meter. With the METER switch set to LINEAR
-20 dB, this noise should be less than 5 mV.
If the noise is greater than 50 mV with minimum gain and MAX SENS
switch setting, the trouble is probably caused by 0200, 0201, or 0202. If this
noise level
is less than 50 mV and the noise with LINEAR -20 dB switch setting
is greater than 5 mV, the faulty component is probably 0203, 0204, or 0205.
4.5 TRANSISTOR VOLTAGE AND RESISTANCE MEASUREMENTS.
Table 2 gives the normal voltage and resistance measurements from transis-
tor terminals to ground. A deviation of 10 percent from any of these values is not
necessarily abnormal.
1. Before making transistor voltage measurements, make sure that the bat-
tery voltage is approximately 12 V (refer to paragraph 4.3). Turn the instrument
on
but leave the GAIN control counterclockwise, set the Fl LTER FREQUENCY,
switch
to FLAT and the METER switch to LINEAR -20 dB. Measure voltage
with a high impedance voltmeter.
2. Before making resistance measurements, remove all the batteries and
short-circuit anchor terminal 6
to ground. Remove ail transistors. Set the GAIN
control to OFF, the Fl LTER FREQUENCY switch to FLAT, and the METER
switch
to LINEAR -20 dB.
Transistor Term.
(Type)
Q100 c
E
B
· Q101 c
E
B
Q200 c
E
B
Q201 c
E
B
QlOl of
D
Type 1232-P2 s
G
TABLE 2
VOLTAGES AND RESISTANCES
De Volts
Ohms To
Transistor
To Ground Ground (Type)
7.8
222 k
0202
3.6 220 k
3.7 3.3 M
11.6 2.2 k
Q21)3
7.7 40 k
7.8
222 k
7.4
34 k Q204
3.6
33 k
3.7 120 k
3.3 22 k Q205
7.8
23 k
7.4
34 k
12.0
Term. De
Volts
To Ground
c 0
E 3.5
B 3.3
c 1.7
E 0.4
B 0.5
c
3.6
E 1.6
B
1.7
c
7.2
E 3.5
B 3.6
3.2 k
0.7 to 2.5
lOk
* Impedance too high to permit
* 1 G
measurement.
4.6 TRIMMER-CAPACITOR ADJUSTMENT.
Ohms To
Ground
0
llk
22 k
100 k
4.7 k
42 k
47 k
10 k
100 k
4.4 k
3.3 k
47 k
Normally, the factory-set trimmer capacitor adjustment (C202) will not
require attention. However, adjustment may be necessary if transistor 0201,
Q203, or Q204 is replaced. If the frequency response for the FLAT characteris-
tic is outside the specified limits, adjust the trimmer capacitor as follows:

a. With the FILTER FREOUENCY switch set at FLAT and a 1-kHz sig-
nal at the INPUT connector, adjust the GAIN control for a 5-dB meter indica-
tion.
b. With the GAIN and Fl LTER FREQUENCY controls unchanged, apply
a 100-kHz signal
(of the same amplitude as the 1-kHz signal) to the INPUT con-
nector and adjust the trimmer capacitor, C202, for a meter indication of 5 dB.
Check the frequency
response below 100kHz to make sure it is within the
specified limits.
c. With same settings, use continuously variable source to check that
response is flat within ±3 dB from 20 Hz to 100 kHz. Adjust with C202, if
necessary.
4.7 MINIMUM PERFORMANCE STANDARDS.
4. 7.1 General
The following tests are provided to demonstrate compliance of the 1232-A
with published specifications at incoming inspection or following repair or
calibration.
4.7.2 Test Equipment
The items listed in Table 3
are recommended for performance of the
ensuing tests.
Table 3
Instrument Recommended Type*
Decade Voltage Divider
GR 1455-B
Counter
GR 1192-B
Coaxial Adaptor
GR 874-010
Patch Cord
GR 776-A
Patch Cord
GR 776-B
Patch Cord
GR 274-NP
Shielded Patch Cord
GR 874-R34
Oscillator
GR 1310-B
Voltmeter hp 400E
Short-Circuit Termination
GR 874-WNL
Open-Circuit Termination
GR 874-WO
Insertion
Unit
GR 874-X
5.1 Mn resistors, 5% (2 required)
•or equivalent.
4.7.3 Noise Check
Connect the Voltmeter to the 1232-A OUTPUT terminals (via the
776-A Patch Cord and
set controls as follows:
a. Voltmeter RANGE switch to 0.1 volts.
b. 1232-A METER switch
to MAX SENS.
c. 1232-A FILTER FREQUENCY switch to FLAT.
d. 1232-A GAIN control on, but fully ccw.
The voltmeter indication (noise) should be .03-V maximum.
11

12
4.7.4 FilterTuning Dial.
Connect equipment as shown in Figure 7.
Set controls as follows:
a. Set oscillator to 1 kHz at 1 V output.
b. Set 1455-B to .00100 (1 mV output.)
c. Set 1192 in FREQUENCY mode.
d.
Set 1192 range switch to 1 sGATE TIME.
e. Set 1192 DISPLAY control at 1 s.
f. Set 11921NPUT ATIEN at 1:1.
g. Set 1232-A FILTER FREQUENCY switch to 200Hz-2kHz.
h. Set 1232-A METER switch to MAX SENS.
Set the FlLTER TUNING dial on the 1232-A to each of the numbered points
(2,
3, 4, 5, 7, 10, 15, 20) and adjust the 1232-A GAl N and the 1310 frequency for
a peak reading on the 1232-A meter. In each case, the frequency read on the 1192
should
be within ±3% of the nominal value on the 1232-A FILTER TUNING dial.
(See Table 4).
Table 4
OSCILLATOR FREQUENCY SETTINGS
Hz
Hz
194-206 679-721
291-309 970-1030
388-412 1455-1545
485-515 1940-2060
Repeat the test at 100Hz, 20Hz, and 200Hz on the 20-200 Hz range,
and at 2, 10, and 20 kHz on the 2-20 kHz range. Also check 50- and 100-kHz
(via the 1232-A FILTER FREQUENCY switch). In
each case set the switch to
its proper position before setting the frequency. The Fl LTER TUNlNG dial is
not in the circuit at 50- and 100kHz. At each step the FILTER TUNING dial
should be accurate to within ±3% of the frequency read on the 1192.
4.7.5 Frequency
Response
Set the following controls (same set-up as above):
a. Set oscillator to 1 kHz at 1 V output.
b. Set 1455-B to .01000 (1 0 mV.)
c. Set 1232-A FILTER FREQUENCY switch to FLAT.
GR 1310.8 GR 1455-B
GR 1232·A
GR 874·A34 INPUT OUTPUT
r -o-+---+-o
GR 274-NP
GR 874-010
GR 1192-B
GR 776-B
L------+-o A
1232-28
Figure 7. Filter Tuning Dial Test Set-up.