PROGRESS REPORT NO. 4 CONTRACT NO. RD-53-SA RESEARCH ORDER NO. 1R&D4

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP78-03153A001600010004-2
• Release Decision: RIPPUB
• Original Classification: S
• Document Page Count: 13
• Document Creation Date: December 23, 2016
• Document Release Date: March 5, 2014
• Sequence Number: 4
• Publication Date: April 1, 1954
• Content Type: REPORT

Declassified in Part - Sanitized Copy Approved for Release 2014/03/05: CIA-RDP78-03153A001600010004-2
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PROGRESS REPORT No. 4
Contract No. RD-53-SA
Research Order No.1R&D4
Prepared By:
Approved By:
Project Engineer
Chief Engineer
Period covered by this report - April 1, 1954 to April 30, 1954
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Copy No. 2
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INTRODUCTION
This is the fourth progress report submitted on the
research and development Task IV. The report covers
work done on a pulse time modulation system.. More speci-
fically it summarizes the results of tests performed to
determine security of the equipment developed using this
method of communic tion.
DI SCUSS OF
In this applicatiofl.of pulse coiufli0atiofl systems, the
problem of security is of particular interest. The term
security refers to the inability of conventional AM and
FM receivers to demodulate the pulse signal. Due to
the wave shape of the transmitted signal, as contrasted to
conventional amplitude and frequency modulation, it may
well appear that only unique methods of detection could
convert the pulse to intelligence. Nevertheless there are
components in the modulation envelope that may result in
detection with a conventional receiver.
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Pulse time modulation results in an involved but
important mathematical analysis of the modulation
envelope. (see appendix). The deviation of the
pulse leading edge is one factor in the envelope;
the deviation of the lagging edge, if it is not ident-
ical, is another factor. Any amplitude variation of
the pulse under modulation conditions, introduces a
third factor.
Depending on the rel tive values of the three factors,
there may be present three distinct types of modulation;
amplitude, duration and phase. By careful use of
clipping circuits in the design of the equipment, the
amplitude variation under modulation conditions was
eliminated. This avoided any amplitude modulation in
the transmitted signal. The difference in deviation
between leading and lagging pulse edges was made
negligibly small by successive differentiation of the
pulse. This in turn eliminated pulse duration modula-
tion. Such duration 'modulation is converted to audio
intelligence due to the restricted bandwidth of the
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audio amplifiers of the conventional communications
receivers.
Therefore eliminating this factor is
an important contribution toward security.
Phase nodula tion is the remaining component that
accompanies pulse position or pulse time modulation.
Phase modulation is a form of frequency modulation, as
used in conventional P.M. transmitters. Thus it is to
be expected that an P.M. receiver with sufficient band-
width could convert a P T.M. signal to audio intelligence.
For these tests both an P.M. receiver and an A.M. receiver
were used as detectors. All the tests were performed
with strong signals. It was found that there was security
with ea A.M. receiver, and no security with an F.M.
receiver that was designed with a 50 kilocycle bandwidth.
It would seem that if the first sideband of the trans-
nutted pulse was wide enough to fall outside of the P.M.
receiver bandwidth, security with P.T.M. could be accomplished.
Since the sidebands are multiples of the pulse repetition
rate, this would necessitate a pulse frequency on the
order of 100 kilocycles. This choice, in turn, would
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result in the modification of the duty cycle or ratio
of peak to average H.F. Power from its optimum value.
It is the duty cycle that ultimately determines efficiency,
size and/or range of the transmitter, Nevertheless it is
important to consider the possibilities of atteining
security by so simple a method.
TESTINQ ;+ND EQUIPMTNT,
The following is a description of the tests performed
and equipment designed for an investigation of a 7m1s0
time modulated system: Figure 1 is a block diarram of
a pulse time modulation transmitter. This consists of
two chassis. The first stage is a blocking oscillator
operating at 12 kilocycles. This pulse triggers a one
shot multivibrator. .lso placed on the same multivibrator
grid is the audio signal that provides the modulating
intelligence.
The blocking oscillator determines the Instant of
initiation or triggering of the multivibrator, and as
a result the instant of pulse leading edge remains
virtually in dependent of modulating signal. However,
once the tube is triggered, and the fast blocking
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oscillator pulse is over, the audio signal controls the
current flow in the first section of the multivibrator.
Since there ts a common cathode resistor for the two
halves the audio signal thus determines the bias for
the Second half. The pulse duration which would normally
be controlled by the RC constant, which would determine
the voltage on the grid of the second section, is thus
modulated by the audio signal. This results In .a pulse
duration Modulated signal.
The pulse is then passed through a.differentiating network
which converts it to two pulses of opposite polarity
corresponding to the leading and lagging edges of the
P.D.M. signal. This Is pulse time modulation, where
in the pulse co responding to the lagging edge has its
position varied relative to its unmodulat d time of?
occurrence.
The transmitter was designed to accommodate single polarity
pulses This required that the P.T.M. pulses be converted
to uni-direction pulses. The clippers and amplifier
accomplished this. The second differentiator reshaped
the reference pulse to make it more similar to the
P.T.M. pulse both as to shape and size.
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The signal leaves the modulator as a negative pulse,
passes through two video amplifiers, Figure 3, on the
transmitter chassis into a power amplifier that performs
the modulating function. The R.P. section consists of
a tuned plate tuned grid oscillator operating at 74
megacycles. This is a completely shielded stage. The
oscillator is 11 ,htly coupled to a buffer amplifier
which is fed to the R.F. power output stage. The plate
of this stage is coupled to the modulator through a
pulse transformer, utilizing plate modUlition. However,
there is no voltage on the R.F. stage plate except for
the interval of pulse duration. This prevents R.F.
output between pulses and insures a high ratio of peak
to average power. The R.F. is loop coupled to a whip
antenna.
The receiver, Figure 4, was a television receiver, modified
for this specific application. This choice was made for
a number of reasons, It provides complete coverage for
the frequencies under consideration, SO N.C. to P16 N.C.,
it Is sufficiently sensitive, and contains a minimum
bandwidth of 4 11. C. throughout the video or A.H. channel.
In order to pass the pulse without mis-shaping or other.
wise distorting, a minimum bandwidth of 2 N.C. is re-
quired. In addition the receiver can be readily
adapted to work into a P.T.N. detector.
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The sound section of the receiver functions as a
narrow band frequency modulation detector, with an
I.F.bandwidth of 50 1.00. The audio amplifiers were
re-arranged to be switched to either the P.M. sound
stages or the A.M. video stages. This enabled a simple
means of testing the P.T.M. signal for security on both
A.M. and F.M. channels
o c usron AND
A pulse time modulated transmitter was designed and
tested. Checks were made for security in communication
using a -A.N. receiver. ?Theory was confirmed
n that security was obtained with an A,M, receiver,
and found lacking in an F.M. detector,
it is planned to investigate further the possibiiLty of
omplishing security with a F.T.M. system, against an
receiver. The F.T.M. demodulator will be completed
and the unit will be tested on a system basis It will
be compared to the previously tested pulse amplitude
modulated system. Further testing will be done to
compare the signal reception of this system to an F.M.
receiver, under weak signal conditions.
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BLOCKING 1 5NOT P.T.M.
OSCILLATOR
AUDIO
,--???73111.1.
MULTI VIBRATOR DIFFERENTIA-TOR
INPUT
CLIPPER
DIFFERENTIATOR AMP.
a-1r-
CLIPPER
PULSE TIME MODULATOR
74 MC TRANSMITTER
MODULAToR VIDE_O VIDEO
-
AMP.
AM P. AMP.
R. F. OUTPUT
P.T. MODULATED.
FIG.1 - PULSE. TIME 1\AC)DuLATED TRANISMCITER
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BLOCKING OCSILLATOR
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