NOISE TEST FOR CRYSTAL VIDEO PREAMPLIFIERS USED WITH CRYSTAL VIDEO RECEIVER MODEL 3502

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP78-03535A000500060001-8
• Release Decision: RIFPUB
• Original Classification: K
• Document Page Count: 6
• Document Creation Date: December 27, 2016
• Document Release Date: September 13, 2012
• Sequence Number: 1
• Content Type: REPORT

Declassified and Approved For Release 2012/09/13: CIA-RDP78-03535A000500060001-8
NOISE TEST FOR CRYSTAL VIDEO PREAMPLIFIERS
USED WITH CRYSTAL VIDEO RECEIVER MODEL 3502
This document is part of an integrated
file. If separated from the file it must be
Objected to _Jodi. _oystematiolov_lon.
The following test is recommended as a means for obtaining a rough
measurement of the noise level of each crystal v
preamplifier used
with crystal video receiver model 3502. The preamplifier noise level,
after the system has been installed in the aircraft, should approach the
laboratory noise level as closely as possible. In order to provide a
simple, reproducible method of measuring this noise level, laboratory
tests were performed on six of the preamplifiers using the equipment
shown in Fig. 1. The end product of these measurements consists of two
vacuum tube voltmeter readings:
1. V = V2 - 40 db = noise voltage at the preamplifier output
connector with the input connector shorted.
2. V4 = (V - 80 db) = input signal voltage, at 1 Kc, necessary
to increase preamplifier output voltage
indication (signal plus noise) to
2 V1 = (2V2 -40 db).
Instructions for performing the noise test are as follows:
To meaSure V1? -
1. Connect the preamplifier being tested to any preamplifier power
connector at the rear of the junction box for the model 3502 receiver.
Disconnect all other preamplifiers from the system.
2. Short the input connector for channel A with a modified PL-259
connector in which the center conductor has been grounded to the outside
of the connector by means of a soldered buss wire.
3. Allow the receiver to warm up for approximately one minute, then
connect a sensitive vacuum tube voltmeter having a bandwidth of at least
150 Kc to the output connector for a channel A and read the voltage V1.
If a voltmeter with a .001 volt scale is not available it will be necessary
to use an additional amplifier as shown in Fig. 1. V1 may then be
obtained by dividing the voltage V2 measured at the output of the second
amplifier by the gain of the second amplifier.
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STEP 1:
Input .
Shorted;
Hewlett-
Packard
Model 410 B
VTVM
VI = noise voltage at the out-
put of the preamplifier.
V2 - 40 db.
4 =
Crystal
Video
Preamplifier
+40 db
v1
?0?
STEP 2:
Tektronix
Preamplifier
Model
121
Ballantine
Model
300
VTVM
V
2
r. m, s. voltage (1000 cps)
necessary to produce 2V1 at
the output of the preamplifier
where 2V1 = 2V2.- 40 db.
Ballantine
Model
300
VTVM
-80 db
2V
+40 db
2V2
Hewlett-
Packard
Audio Oscillator
Model 200D
= 1000 cps
Attenuation
Network
Crystal
Video
Preamplifier.
Tektronix
Preamplifier
Model
121
?0
10 Meg.
lk
?0?
Fig. 1 - Noise Test for Crystal Video Preamplifier Used With
rrli-gtn1 ViCiFtre R eceiver Model 3502
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4. Repeat 2 and 3 for channels B and C.
To measure V4??
1. Build an 80 db resistive attenuator as shown in Fig. 1 by en-
closing the two one-half watt resistors in a small metal box and providing
coaxial connectors for the input and output of this attenuator. (Amphenol
SO-239 connectors).
2. Check the attenuation of this network at 1000 cps by introducing
a 10 volt r, m. so signal at the input of the attenuator from an audio
oscillator which has an effective output impedance of approximately 600
ohms. This 10 volt r.411, s. input signal may be measured with the
Hewlett Packard model 410B vacuum tube voltmeter and the output signal,
.001 volt r. m. s., may be measured with the Ballantine model 300 vacuum
tube voltmeter, the two meters shown in Fig. 1.
3. Connect the 80 db attenuator, the model 410B voltmeter, and the
audio oscillator to the input connector for channel A of the crystal video
preamplifier as shown in step 2, Fig. 1. If the 1 Kc audio output voltage
of the oscillator is reduced to a minimum, the new V1 or V2 is only
slightly higher than the voltage measured in step 1 with the input con-
nector shorted.
4. Increase the 1 Kc audio input voltage until voltage indication at
the output of the preamplifier is twice the value obtained with the input
shorted. The output is therefore 2V or 2V2 where V1 is the direct
output voltage for a shorted input when a sensitive vacuum tube voltmeter
is available, and V2 is the output voltage for a shorted input when the
auxiliary amplifier and a less sensitive voltmeter are used.
5. V3 is read directly from the Hewlett Packard model 410B vacuum
tube voltmeter and V4 is calculated by dividing by 10,000. That is,
-20 log10 VV3 -80 db.
=
6. Repeat 3, 4 and 5 for channels B and C.
In the laboratory test V1 and V4 were measured for six preampli-
fiers using the equipment as shown in Fig. 1. The attenuation of the 80 db
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attenuator was measured as previously described and the gain of the
Tektronix preamplifier was checked by introducing a 1 Kc voltage
through a 40 db attenuator to the input of the amplifier and measuring
equal voltages at the input to this attenuator and the output of the Tek-
tronix amplifier with the model 410B voltmeter.
Measured values of V2 and V3 and the calculated values of V1 and
V4 for the six preamplifiers are shown in Table 1. When this noise test
is performed in the field V2 and V3, and therefore V1 and V4 for any
given preamplifier, should approach the values shown in Table 1 as
closely as possible. V1 which is a measure of the preamplifier noise
should be approximately 500 microvolts. V4 which is determined by
both noise and gain in the preamplifier should be approximately 20 micro-
volts.
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Declassified and Approved For Release 2012/09/13: CIA-RDP78-03535A000500060001-8
Pre-
amplifier
Number
\ Channel A
Channel B
Channel C
V1
V2
V3
V4
VI
V2
V3
V4
V1
V2
V3
500r
. 050 v
0200v 1
201u.v
6001.2v
. 060 v
. 200v
20r
7001w
. 070 v
?200v
201w
2
460r
. 046 v
0180v
18 rv
460 v
. 046 v
. 160 v
161w
40v
_ . 046 v
. 160 v
16 r
\
410r
,041v
0160v
16r
5401iv
. 054 v
. 160 v
16r
5001iv
. 050 v
. 200 v
20 rv
4
520r
. 052v
. 160 v
,
16r
5201iv
. 052v
, 150 v
15r
540r
. 054 v
. 160 v
161w
450 v-
. 945v
. 180 v
18 v
540 rv
, 054 v
. 200 v
20 v
480 v
. 048 v
. 180 v
181w
6
450 v
. 045 v
. 180 v
18r
410r
.041v
.180v
18r
500 ry
. 050 v
. 180 v
0-
18r
All voltages = r. m. s.
Table 1
Results of Laboratory Noise Tests on Preamplifiers for Crystal Video Receiver Model 3502
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Declassified and Approved For Release 2012/09/13: CIA-RDP78-03535A000500060001-8
SUGGESTIONS FOR IMPROVING THE PREAMPLIFIER NOISE LEVEL
If the voltages V1 and V4 obtained in the noise test are considerably
higher than the laboratory measurements for V1 and V4 the following
procedure is recommended for reducing the noise level:
1. Check the noisy preamplifier for broken ground wires. Make
certain that preamplifiers are well grounded.
2. Check for excessive microphonism in the 12AT7 input stage and
replace this tube if necessary.
3. Check for faulty filter condensers by shunting the filter con-
densers in the preamplifier with a new condenser to determine whether
V1 is considerably reduced. Note that condensers C422 through C426
near the preamplifier power connectors at the rear of the junction box
are also used for filtering the B+ voltage to the preamplifiers. These
should be checked in the same manner as previously described.
4. If objectionable noise such as ignition noise and 400 cycle hash
is detected in the earphones of the crystal video receiver, even though
the preamplifiers have passed the noise test, the antennas should be
insulated from the fuselage electrically so that the only input ground is
at the preamplifier itself. This procedure has been found to be necessary
in numerous crystal video installations.
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Declassified and Approved For Release 2012/09/13: CIA-RDP78-03535A000500060001-8