PROGRESS REPORT NO. 2 CONTRACT NO. RD-53-SA RESEARCH ORDER #1R&D4

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP78-03153A001600010002-4
• Release Decision: RIPPUB
• Original Classification: S
• Document Page Count: 19
• Document Creation Date: December 23, 2016
• Document Release Date: March 5, 2014
• Sequence Number: 2
• Publication Date: February 1, 1954
• Content Type: REPORT

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PROGRESS REPORT NO. 2
CONTRACT NO. RD-53-SA
Research Order #1R&Dii.
Prepared by:
Approved by:
Project Engineer
Chief Engineer
50X1
50X1
50X1
Period Covered by This Report - February 1, 1954 to February 28, 1954
Copy #2
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WRODUCTION,
This is the second progree report to be submitted on th a project.
It describes the work done on a pulse modulation system. A unit
using pulse amplitude modulation was investigated 40 a means of
communication with a view toward seeurity and efficiency, The
deeign and testing of the units on a system beet* is discussed.
The following are some of the design considerations n into
account during the development of the circuite.
Pulse emplitude modulation concists of a process wherein the
amplitude of a pulse carrier, is varied in accordance with the
value of an audio modulating wave.
The Nyquiet criterion for sampling a signal states that in order
to determine uniquely the value of the sampleiwave at all times,
the minimum sampling rate required is two pulse samples per audio
cycle. This equipment was designed on a more conservative basis,
in which the ratio of pulse repetition rate to highest audio
frequency was 2.5. This bad the advantage of simplifying the
necessary filters at the demodulator, and accommodated an audio
frequency band of 3200 cycles per second.
One ad, nt in pulse modulation systems is the us.
of t hi oak to average power for more efficient
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operation Of the transmitter. In this application the pulse
cycle was designed for an interval of 125 microseconds, and
the pulse width was 10 microseconds. This resulted in a trans-
mitter duty cycle of 8 percent, which would permit a peak
power on the order of 12.5 times the average power used in con-
ventional amplitude modulation systems. Theoretically the peak
power could be increased still further, as the pulse duration
Is reduced, while maintaining a fixed average power out of the
transmitter.
The major disadvantage of pulse modulation systems is the
bandwidth requirement. The conventional amplitude and fre-
quency modulation systems have a bandwidth determined by the
audio side bands. Pulse system bandwidths are determined on a
more critical basis. Once the pulse Is modulated with the audio
signal, the video stages following the modulator must here a
frequency response determined by the rise time of the pulse.
For this equipment, the pulse WAS designed with a rise time of
0.5 microseconds measured from the 10 percent to 90 percent
amplitude points' In order to pass this pulse without dia.-
torting the wave shape, a video amplifier with a frequency
response of 1 megacycle would be required. In a similar m nn r
the transmitter output bandwidth would be 2 megacycles. This
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could be confined to 1 megacycle It single side band trans-
mission was utilized, but this in turn would require critical
filtering at the transmitter.
Particularly in pulse amplitude modulation, the use of wider
frequency bands do not improve signal-to-noise ratio. Theo-
retically, with a fixed average transmitter power and noise that
has a uniform power density spectrum over the acceptance band,
a greater frequency range affords no improvement In signal-to-
noise ratio.
The theoretical minimum bandwidth is defined as that required
for the audio side bands in a conventional modulation system.
Por the condition Where peak instead of average power is uti-
lized, whenever the bandwidth occupied by a pulse amplitude
modulation system exceeds the theoretical minimum, the resulting
signal-to-noise ratio is less. The wider the band, the smaller
is the ratio. Thus, for a specified signal-to-noise ratio at
the output of the system, more peak sower is required as the
band is widened. Howdver, although a wider band implies a larger
signal power, other requirements are eased. This follows since
the distortion tends to become unreasonably severe unless the
occupied band is wide enough to accommodate the pulse rise time,
which is considerably wider than the theoretical minimum.
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There are several methods of pulse amplitude modulation avail-
able. These include single polarity pulses, double polarity
pulses, sinFle polarity flat-top pulses, and double polarity
flat-top pulses. The first method, single polarity pulses was
chosen for this application because of the relative sim2licity
of the modulator. The wave shapes of this form of modulation
are demonstrated in the oscillograms included in this report.
The nreatest diaadvanta3e of pulse amplitude modulation as com-
pared to the other pulse modulution systems, concerns signal-
to-noise ratio. The other types of pulse modulation involve a
pulse of constant amplitude. Thus, as they pass through the
receiver and pick up noise, the pulse can be treated by clipping
and shaping at various stages to improve signal-to-noire ratio.
The 7'. A. T. system in contrast has the best signal-to-noise
ratio at the first stage of the receiver; the following stages
can only cause deterioration, and there is no means of noise
limiting applicable to the system
EQVIPMENT A7D TPSTS
The transmitter and receiving equipment were completed in this
period, and tests were made on a system basis.
Figure I represents a block diagram of the entire transmitter,
which actually conorises two units, the modulator and the R.7.
chassis.
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Reference to he oa illographa as labeled, indicates the
functioning of the stages in the stem:
) Point A- The positive pillse at one plate of the
twin triode functioning as a free running
muItivibr'et
Point B - The other plate of the multivibrator. This
is a negative pulse and is the one used
a source.
3) Point C - The output of the shaper. This was a well
formed pulse. It has the positive pdla-
rity necessary to gate the following etage.
The O. microsecond rise time and the 10
microsecond pulne width were clearly
demonstrated. This pulse was applied to
the limiter grid of the modulator.
4) Point D The audio signal applied to the quadrature
grid of the modulator. This was provided
by an audio oscillator, and functioned as
the modulating signal.
5) Point E The modulator output. The modulation
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envelope was well represented. The pulse
repetition rate was clearly indicated as
8000 cycles per second. The percentage of
modulation was approximately 0 percent, but
this could be readily adjusted by the
separate bias controls on the two grids of
the modulator. It was also possible to
adjust theee bias controls so that there
would be no output unless a modulating
signal was present. This in turn would re-
sult In no transmitter output unless a
signal was to be communicated.
modulated pulse was essentiall
f the modulator chassis.
This.
the output
The output of the video ampli r as seen
at the secondary of a pulse transformer.
This was a pulse without audio modulation.
It demonstrated pulse shape after passing
through a transformer, and before being
applied to the final R.P. amplifier.
7) Point 0 The R. P. Output of the buffer amplIfier
as seen at the grid of the power amplifIer
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without pulse applied. This demonstrated
the R. P. frequency of the crystal oscillator.
8) Point 0 - Composite picture of the R.F. superimposed
on the pulse. The pulse was not audio
modulated in order to demonstrate more clearly
the type signal applied to the grid of the
power amplifier as grid modulation.
9) Point H . Output of transmitter as a radiated signal.
In this oscillograph was summed up many of
the features of pulse amplitude modulation,
namely: Transmitter duty cycle, pulse
repetition rate, modulating signal and
percentage of modulation.
10) Point H - Enlargement of pulse shape at output, with
R.F. modulation but without audio modulation
for picture simplicity.
Figure 2 represents a block diagram of the entire receiver. This
actually comprises three units the first four stages of a com-
mercial Hammarlund communications receiver, a small victoreen
chassis that plugged into the last I.F. amplifier of the receiver,
and a demodulator chassis.
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The transmitted signal was .received within the confines of a
room; no tet of range was undertaken. The signal was in.
telligible and was transmitted with a frequency response in
accordance with the design, and uith acceptable fidelity. Of
major significance was the fact that the signalwas received by
an ordinary communications receiver as well as the specifically
designed receiver unit. This clearly indicated that there was
no security with a conventional type of pulse amplitude modulated
system. Nevertheless, it is possible to combine systems for
additional security, such as amplitude modulated pulsing of an
P.4. transmitter.
Point I . Output of th
pre.selecto At this
point the effect of narrow bandwidths be-
comes apparent. The pulses have been
greatl mis-shaped; but the general out-
line of pulse amplitude modulated R,F is
readily discernible. The second stag
of the receiver, the converter, had so
narrow a bandwidth that the I.P, output
was barely recognizable as P.A N.
nevertheless, the intelligence of the
signal was preserved.
12) Point Output of the cathode follower. These
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remaining tests were made on a closed cir-
cuit basis, in order to better demonstrate
the operation of the demodulator section.
By this means, the mia-shaping of the re-
ceiver's narrow bandwidth R.F. stages, was
avoided.
13) Point K - Output of a commercial low pass filter. The
pulse was pretty well eliminated, leaving
the audio signal as the intelligence' A
low impedance input to the filter was re-
quired for proper operation. Thus a cathode
follower stage preceded it.
110
Point L - Output of an M-derived constant-K low pass
filter. This was designed as an infinite
impedance device for the pulse repetition
rate frequency. The pulse essentially was
eliminated at this point.
15) Point M . The audio signal input to an external loud.
speaker. The sinusoidal wave shape is of
particular interest.
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Figures 3 and t are schematic diagrams of the R.F. chassis and
the demodulator chassis. They are included because significant
modifications have been made in their original design as re-
presented in progress report ro. 1.
CONCLUSIONS tND FUTURE ,'LANS
A pulse amplitude modulated transmitter and receiver was de-
signed and tested on a system basis.
entirely intelligible.
The received sIgnal was
It was noted that a commercial co=unications receiver could
receive the transmitted signal adequately. This negates the
!Possibility of security in communications with a conventional
pulse amplitude modulated system.
The requirement of large bandwidth for pulse systems was demon-
strated. The efficiency of utilizing peak pulsed power as
against large average power of conventional modulation systems
was indicated.
The signal-to-noise ratio of the sinal, as it appears at the
input to the receiver, could not be imnroved as it passed
through the receiver. This disadvantage is not true of other
pulse modulation systems.
Oscillographs of the signal were taken at various stages of
Pae ,!'10
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the transmitter and receiver and the wave forms demon rated,
It Is planned to design a system of pulse position modulation,
and investigate its characteristics. It will be compared to
the pulse amplitude modulated system already tested.
A transmitter will be designed to operate at 50 megac cies .
with wide bandwidth capabilities. This will be used as the
standard for a comparison of all pulse modulation systems
wide band receiver to opetate in co junction with the 50 mega-
cycle transmitter will be designed,
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