SEVENTH BIMONTHLY REPORT ON THE RT-21 TRANSMITTER PROGRAM

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP78-03424A000800010035-6
• Release Decision: RIPPUB
• Original Classification: C
• Document Page Count: 15
• Document Creation Date: December 22, 2016
• Document Release Date: February 8, 2012
• Sequence Number: 35
• Publication Date: September 8, 1959
• Content Type: REPORT

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Seventh Bimonthly Report on
The RT-21 Transmitter Program
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AUTNh HR 10.2
REASON
Period: 8-Sept.-1959 to 8-Nov.-195
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Prepared by:
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TABLE OF CONTENTS
Par g;No.
I.
Purpose . . . . . . . . . . . . . . . . . .
1
Ii.
Abstract . . . . . . . . . . . . . . . . .
1
III.
Factual Data . . . . . . . . . . . . . . .
1
1. Automatic Transmitter. . . . . . . . .
1
(i) Introduction . . . . . . . . . .
1
(ii) Experimental . . . . . . . . . .
2
2. Automatic Impedance Matching . . . . .
5
IV.
Conclusions. . . . . . . . . . . . . . . .
7
V.
Future Plans . . . . . . . . . . . . . . .
8
VI.
Identification of Key Technical
Personnel. . . . . . . . . . . . . . . . .
8
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I. Purpose
See Bimonthly Report No. 1.
II. Abstract
This report describes work which has been carried out in an attempt
to increase the output power of the transmitter itself while at the same
time decreasing the circuit complexity. Recent work on the automatic imped-
ance matching unit is also described.
Using 2N1337 Pacific Semiconductor transistors an output of 2.5 watts
has been obtained from the transmitter at frequencies up to 30 mc. The
circuit used to achieve this result is described as well as the associated
control circuitry. Construction of the variable elements of the automatic
matching unit has been completed. The motors, gears and variable reactances
have been assembled and the entire unit tested. With the servo loop com-
pleted, including sensors, modulators, amplifiers, motors, torque multipliers
and variable reactances, successful operation of the system has been
achieved at a number of frequencies, using signals supplied by a generator.
However, at the low end of the range, (around 3 me) instability is experi-
enced. Steps are being taken to overcome this difficulty.
III. Factual Data
1. Automatic Transmitter
(i) Introduction
During the past reporting period work has been performed
in an effort to produce substantial power output from high frequency tran-
sistors which are now becoming available from industry sources. Encouraging
results have been obtained with type 2N1337 transistors supplied by Pacific
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Semiconductors. Two of these units connected in parallel have produced
2.5 watts output at 30 mc. Since power gain at this level is low, moderately
high power driving stages are necessary. Some work was performed in an
effort to increase the power level. of the crystal oscillator so as to mini-
mize the number of driver stages required. The experimental work included
the construction and testing of high and low band crystal oscillators, driver
stages, and power output stages.
(ii) Eaerimental
Two test circuits were constructed to develop 2.5 watts
crystal controlled r-f power. One circuit was designed to operate in the
low frequency band of 3 to 15 inc and the other in the high frequency band
of 15 to 30 mc. It was recognized that a 2 position band switch could have
been employed in a single circuit to select the desired band, but to simplify
the experimental construction work individual low and high band test circuits
were employed. Figure 1, which shows the basic circuit configuration of the
units, indicates the component values used for their respective bands.
Transistor Q1 serves as a crystal controlled oscillator which feeds the
driver stage Q2, and Q2 in turn drives the output stage composed of parallel
connected transistors Q3, Q4 and a tapped tank coil L. Series connected
100 and 1100 ohm resistors serve as a 500 ohm load and provide a low impedance
test point for voltage measurements.
Circuit operation is straightforward. Insertion of a crystal may
or may not produce oscillations depending upon the state of switch S and
the magnitude of VS1 which initially is set at a low value. Supply voltage
VS1 is gradually increased until oscillation begins., and switch S, which
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L-CHOSEN FOR RESONANCE
TAPPED 0.3 FROM COLLECTOR
TEST CIRCUIT FOR PRODUCING 2,5 WATTS OF R-F POWER IN A 500 OHM LOAD
9
-Av.- -
i
_~- 05
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would normally be a transistor in the automatic transistor, is set in the
position which provides maximum oscillator output. An additional increase
in VS1 is applied, if necessary, to increase the Q2 amplifier stage signal
power level to a value sufficient to drive the output stage to full power
output. The power stage drive condition may be conveniently monitored by
observing the dc potential developed across the 20 ohm resistors connected
to the Q3, Q emitter terminals. Inductance L is tuned to resonate at the
operating frequency. The design is arranged so that the coupling networks
L1 and L2 are broadband and fixed tuned. The variable inductance L may be
provided by the automatic impedance matching unit. Switch S serves to
couple the fixed capacitor C into or out of the oscillator tank circuit
and so subdivides each frequency band into low and high ranges. The
ambiguity of whether oscillations occur at the fundamental crystal mode or
at higher modes is removed by setting the switch in the position which
yields the highest oscillator output. A transistor flip-flop and an
oscillator output detector may perform the switch function or alternately
the oscillator tuning circuitry indicated in Figure 1 of the Sixth Bi-
monthly Report may be adapted for use with the oscillator.
The automatic adjustment of VS1 during transmitter tuneup may be
performed by a voltage control transistor, shown dotted in Figure 1, connected
between terminals VS1 and VS2. Figure 2 illustrates a method for performing
the desired control. The voltage VS1 is determined in part by the current
flow through resistor R and transistor Q5. A minimum value of VS1 will
result if Q5 is nonconducting, and a maximum value, nearly equal to VS2,
will result if Q5 is fully conducting. Any intermediate value may be obtained
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L2 - WINDING ON 02 OUTPUT COIL L2
AUTOMATIC VOLTAGE CONTROL CIRCUITRY
FIGURE 2
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by appropriate control of the conduction of Q5. The degree of conduction
of Q5 is determined by the base current available from resistor R1. If Q6
and Q7 are nonconducting, then the full current from R1 is applied to the
Q5 base and full conduction of Q5 results. On the other hand if Q6 or Q7
conduct, the base current of Q5 is reduced, and consequently, the voltage
VSl is reduced. The circuit is arranged so that initially, without crystal
oscillator output, Q6 is conducting and VS1 is at its minimum value due to
nonconduction of Q5. When a crystal is inserted, oscillations begin and the
i
L2 D4 detector circuit produces a dc potential which reverse biases the
base emitter of Q6 causing Q6 to become nonconducting. The voltage VS1
can rise provided Q7 is not conducting. The conducting condition of Q7 is
determined by dc signals from the automatic tuning unit, and the emitters
of Q3 and %. The automatic tuning unit provides a positive dc signal
whenever the unit is in an untuned condition. This is fed to the base of
Q7 via the diode Dl. Thus, Q7 conducts and maintains the VS1 voltage at
a low value whenever the automatic impedance matching unit is not tuned.
This prevents excessive output stage r-f drive during the tuneup period
which might otherwise result in damage to the output transistors Q3 and Q Q.
Once the tuning unit has attained a tuned condition the positive potential
at D1 disappears, and Q7 conduction decreases. The resulting rise in VS1
increases the driving signal fed to the output transistors Q3 and Q%, and
this produces a rise in dc potential at their respective emitters. When the
emitter potentials have risen to a value sufficient to cause conduction in
D2 and D3, Q7 becomes conducting again, thus preventing a further increase
in V31. The drive signal to Q3 and Q4 accordingly stops rising, and the r-f
output is stabilized at its designed maximum value.
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2. Automatic Impedance Matching
During the past reporting period the mechanical assembly of
the variable inductor., variable capacitor and the associated gear trains
and motors has been virtually completed. A modification of the variable
inductor has been found to give improved electrical performance. Instead
of using a rotating wheel to tap a portion of the total inductance, the
modified inductor incorporates a brass rod in conjunction with the ferrite
rod. The inductance is varied by winding or unwinding turns from the
ferrite with the brass rod acting as a "take-up" reel. This method of
varying the inductance eliminates the difficulties due to self-resonance
of the coil within the frequency band of interest.
The ranges achieved by the variable capacitor and the variable
inductor are shown in Figure 3. The variable capacitor has two taps,
thereby allowing a portion of the total capacitance to be removed by the
band switch in the 15-30 me band. The band switch also changes the fixed
output capacitor of the pi network.
A block diagram representation of the impedance matching system is
shown in Figure 14. The manner in which this system produces a purely con-
ductive input admittance is as followso The operation of the system requires
that L2 be at its maximum position when the tuning cycle begins. This is
accomplished automatically by means of a relay which, when unactuated,
applies a signal which causes L2 to increase. When L2 has reached its
maximum position., a limit switch then actuates the relay and the tuning
cycle begins. The output of the phase detector tends to drive C1 to the
position which cancels any phase angle associated with the input admittance.
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0.3-40 ?h
L2
BAND SWITCH
10-450 25-1450
??f CO ~? f
360 1-
P)l f T Ca
IMPEDANCE MATCHING Pi NETWORK
FIGURE 3
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IMPEDANCE
PHASE
SENSOR
IMPEDANCE
MAGNITUDE
SENSOR
D.C. SIGNAL
TO -
DECREASE L2
OFF
DC VOLTAGE
o
SUPPLY ON
DC-AC
MODULATOR
f-
SERVO
AMPLIFIER
D.C. SIGNAL
i-/-~
Ci MINIMUM
LIMIT SW.
L2 MAXIMUM
LIMIT SW
SERVO MOTOR
a
SPEED REDUCER
(3000=1)
DC - AC
MODULATOR
VARIABLE
INDUCTOR
L2
SERVO
AMPLIFIER
SERVO MOTOR
a
SPEED REDUCER
(300-1)
BLOCK DIAGRAM OF AUTOMATIC IMPEDANCE MATCHING SYSTEM
VARIABLE
CAPACITOR
CI
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The output of the magnitude detector tends to drive L2 to the position
which produces the desired input conductance, provided that C1 was not
driven to its minimum position in an attempt to maintain zero phase while
L2 was +"prepositionedtt to its maximum. When C1 is driven to its minimum
position, a limit switch is actuated. This limit switch applies a signal
which causes L2 to decrease as long as C1 is at its minimum position. The
limit switch is released when C1 leaves its minimum, and the control of L2
is returned to the magnitude detector. The tuning cycle is completed as
the phase and magnitude error signals drive C1 and L2 to their proper
positions.
The complete impedance matching system is currently being evaluated
in the Laboratory. When driven by an RF generator, satisfactory tuning has
been achieved at 15 and 30 mc, but the servo system tends to become unstable
as the frequency approaches 3 mc, The following points contribute to the
fact that the stability of the system depends upon the transmitter frequency.
(i) The magnitude of the signal. derived from the phase sensor
is frequency sensitive. This means the speed with which
the capacitor responds to a phase angle error is greater
at high frequencies than at low frequencies.
(ii) The change in input admittance produced by a specified
angular rotation of either the capacitor or inductor
shaft is greater at high frequencies than at low frequencies.
The complete significance of these points is not readily apparent since the
manner in which the capacitor and the inductor interact is an extremely
complicated function of capacitance, inductance, frequency, terminating
impedance, and the mechanical parameters of the system.
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The Laboratory evaluation of the impedance matching unit indicates
that one method of stabilizing the system is to reduce the speed of the coil.
The disadvantage of this approach is that the tuning time is lengthened.
However, if a suitable method can be devised which causes the coil to be
slowed only in the neighborhood of its tuned position, the increase in
tuning time will be negligible. Another approach to stabilizing the system
would be to synthesize a compensating network to insert into the servo loop.
The disadvantage of this method is that, unless the transfer functions of
the various components is derived (a task which, if possible at all, would
be extremely difficult), the compensation must be done in a "cut and try"
manner. No satisfactory compensating network has been devised, but work
along this line is being continued. Further work will be directed towards
determining the best system by which to achieve system stability.
IV. Conclusions
As far as the transmitter itself is concerned, the circuit shown in
Figure 1 has been tested for both low and high band operation. An output of
2.5 watts was obtained at 8 me in the low band and 30 me in the high band.
An overall efficiency for the transmitter in excess of 6007o was achieved at
30 mc. The oscillator and driver stages of both low and high band units
provided full driving power over their respective frequency ranges.
Successful operation of the automatic impedance matching unit has
been achieved over the majority of the 3-30 me range. Instability has been
observed around 3 mc. The redesigned variable inductance performs well,
providing a relatively high Q1 over the full range without the resonance
effects observed in the original design. Similarly, the specially designed
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variable capacitor has been completed. Assembly was carried out with con-
siderable care in order to keep the necessary torque to a minimum. In the
completed servo system, the motors have more than sufficient power to turn
the capacitor and variable inductance--so much so that it was felt desirable
to construct a slipping clutch in order to avoid damage to the capacitor
when it reaches the limit of its travel.
Automatic voltage control circuitry such as that indicated in
Figure 2 and oscillator switch circuitry will be tested and installed in
the experimental units. The automatic antenna tuning unit may then be
coupled to the low and high band circuits, and preliminary testing of the
interconnected principal components of the transmitter can begin. An
effort to obtain improved high power, high frequency transistors suitable
for use in the transmitter output stage will continue.
Before connecting the transmitter to the matching network, the
instability experienced at the low frequency end of the 3-30 me range
will have to be eliminated. It is to be hoped that this can be done with
a minimum of modification. Several remedies suggest themselves and these
will be explored in turn, starting with the simplest.
VI. Identification of Key Technical Personnel
The following name should be added to those given in previous reports-
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