PROPOSAL FOR THE DEVELOPMENT OF THE RT-21 TRANSMITTER

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP78-03424A000800010003-1
• Release Decision: RIPPUB
• Original Classification: C
• Document Page Count: 12
• Document Creation Date: December 22, 2016
• Document Release Date: February 8, 2012
• Sequence Number: 3
• Publication Date: June 1, 1958
• Content Type: MISC

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CONFIDEN~~~1.
Proposal for Developonent ,of
DOC _.~ qEV DATE 10 A~ o BYCLLll373
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OHiG CLASS ~ PAGs'5 _ 11cV GLASS
JUST ~~-- NeXT NEV~6ILQ~ AUTH: HR 10.2
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^ QECL ~41-REVW OBI ~-~~~~ _
BY
EXT E3Yi:J 6 YRS
~
REASON
June - 1958
Prepared by
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Table of Contents
I.
Introduction
1
II.
Technical Approach
3
(1)
Automatically Variable Reactances
3
(a) Mechanical
3
(b) Electrical
4
(i) Electrically Variable Capacitors
.4
(ii) Electrically Variable Inductors
6
(2)
Sensing Circuits
.7
(a) Resonance Sensing
.7
(b) Impedance-Match Sensing
.8
(3)
Auxiliary Circuit Investigations
.8
III. Manpower Requirements and Time Schedule
.10
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Proposal for the Development of the
I. Introduction
Specification No. 58-A -1077 enumerates the performance characteristics
of the RT-21 Transmitter. It is understood that at the present time the
requirements cannot be met. The purpose of the proposed activity is to
develop circuitry and device techniques which will, by the conclusion of the
program, lead to the realization of a prototype equipment satisfying the
specifications. It is proposed to divide the program into three phases.
Phase A will be a development phase during which techniques will be devised
in order to achieve the desired circuit functions such as automatic frequency
adjustment and antenna matching. At the conclusion of Phase A, the basic
techniques will be available with which to perform the required functions
although it may be that at that time, device limitations, in particular those
of high frequency, high power transistors, will prevent complete satisfactloa
of the electrical specifications. During the course of Phase B a complete,
integrated electrical design of the transmitter will be carried out, incorporating
the techniques developed during Phase A. The design will be flexible being altered
when and where necessary, to accommodate at all times the most advanced
transistors from the stand point of frequency and power capabilities. At
the conclusion of Phase B an operative transmitter will be delivered in
form meeting the electrical specifications to the extent that
available devices permit. Phase C will be concerned, primarily, with the
packaging of the equipment. At this stage, any electrical design changes or
improvements deemed desirable will also be made. The objective of Phase C
25X1
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will be the realization and delivery of a complete transmitter prototype
satisfying both the electrical and the mechanical requirements set forth is
the previously mentioned Specification.
This proposal, while viewing the three phases as parts of a complete
program, describes in some detail the technical approaches which will ~
made during Phase A. A detailed account of Phases B and C would be isy~seties7.
until such time as the basic circuit techniques have been developed. It can
be stated, however, that only those techniques compatible with the ultirte
objectives of Phases B and C will be studied during Phase A.
The manpower estimates and funding which are included in this_pr~possl
are for Phases A and B only.
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II. Technical Approach
_.r_.... ._~~
As a result of recent work on a similar transmitter, matey of the basic
design problems have 'either been solved or the obstacles to their solutions
clearly defined. Based on presently available circuit techniques the follawLng
additional requirements of the proposed transmitter present the greatest diffi-
culty and will, consequently, call for the ma3or share of the effort during
Phase A.
1. Automatic tuning
2. Automatic antenna impedance matching
The problem of obtaining 10 watts at frequencies up to 30 me with tran-
sistors is of course a ma3or one. However, the limitation in this case is
clearly one of devices. As stated in the Introduction, as improved transistors
become available, they will be incorporated i~ the design.
(1) Automatically Variable Reactances
Both automatic tuning and impedance matching present many common problems.
In particular, each requires the automatic adjustment of one or more reactive
elements in order to produce a desired circuit condition.
(a) Mechanical
There are two basic approaches which can be taken. The first is the
mechanical method suggested in the Specification, utilizing small servo motors
to rotate relatively conventional variable capacitors and inductors. This
method has been used for many years. Before it would be suitable for the
present application, however, considerable effort will be necessary in order
to reduce the physical size so that it would be compatible with the overall
transmitter requirements. This work will include further development of minia-
ture variable capacitors of the barium titanate and polyethylene types, similar
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to those produced during the Radio Circuit Study. Work will also be done on
novel nmotor~~ mechanisms which would lend themselves to this application. The
motion which is required to alter capacitance need not be rotational. Studies
may show that a "motor's resulting in lateral displacement would be more de-
sirable than a rotating device from a miniaturization and packaging standpoint.
(b) Electrical
The second approach to the problem of an automatically ad3ustable reactance
is the electrical method in which there are no mechanically moving parts. This
approach is novel and will call for the development of special solid state de-
vices. This work will require the application of some of the improved materials
which are becoming available in the form of ceramic dielectrics and ferrites.
(i) Electrically Variable Capacitors
An example of material which is available at the present time for the fabri-
cation of electrically variable capacitors is a variety of barium titanate, the
dielectric constant of which is strongly a function of the voltage impressed
across the material. For a change of impressed voltage of 32 voltsfmil. a
change in dielectric constant from 6000 to 1200 is typical. One of the factors
which has limited the application of material of this type is its temperature
dependence. In a system of the type required for the RT-21 transmitter this
problem is of considerably reduced significance. In both the tuning and im-
pedance matching applications the effect of temperature would be that, for a
given frequency or antenna configuration, the desired conditions would be ob-
twined with a different voltage across the capacitor at different temperatures.
This would be of no consequence as long as the range of variation was sufficient
to accommodate all desired frequency and antenna configurations at all desired
temperatures.
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The amount of capacitance change obtainable is a function of temperature,
exhibiting a peak around the Curie temperature. In order to broaden the
temperature range over which a strong variation in dielectric constant can be
obtained with applied voltage, two approaches are possible. The first approach
is one of materials development. The second approach would be to use capacitors
of several different materials, each peaking at a different temperature, con-
nected in parallel.
A second problem associated with the use of voltage variable capacitors is
that of isolation between the control voltage and the signal voltage. Using a
capacitor of this type to tune the output tank of a vacuum tube transmitter is
difficult due to the large voltage excursions of the signal frequency. If the
control voltage is to be very large compared to the signal voltage, with vacuum
tube stages, impractically high potentials are required. In the RT-21 trans-
mitter operation is limited to transistors so that the signal voltage excursions
wall be very much reduced. This fact in combination with some of the isolation
techniques proposed for vacuum tube circuits should result in a practical de-
vice. A method of isolation which has been considered relies on the geometry
of the barium. titanate. The signal and control voltages are applied to
electrodes on different faces of the rectangular capacitor so that the bias
field is at right angles to the signal field. The bias field is across the
thin dimension so that its effect on the dielectric constant, being a function
of volts/ mil., is greater than that of the signal which is applied across the
thick dimension. Other techniques require the addition of a small component
of the signal frequency shifted 90? in phase, to the control voltage. The
control voltage can be generated by a do-dc converter with little difficulty
since no power is required, each capacitor representing essentially an open
circuit.
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(ii) Electrically Variable Inductors
Electrically variable inductors have been available in the form of satu-
rable reactors for a long time. Their use in applications of the type required
in the RT-21 transmitter has been impossible due to materials limitations.
With the advent of ferr.ites and in particular the CQ series of high frequency
ferrates the picture has been changed radically. The use of square.loop
materials provides the possibility of anew technique for automatic tuning.
During the first year of the Radio Circuit Study, work was done on a
remote control system in which an attempt was made to control the frequency
of an oscillator by varying the state of saturation of a square loop ferrite
core in a number of discrete intervals. In this case accuracy of resettability
precluded use of the system for its original purpose. In the RT-21 transmitter
the steps of inductance change could be regarded as roughly analogous to band
switching. The error signal could be used to activate a blocking oscillator
which would pulse the core until the setting was approximately correct. The
error signal would then be insufficient to cause the blocking oacillator to
fire, the fine adjustment being by means of a voltage variable capacitor.
The advantage of using square loop materials for the variable inductors is
that power does not have to be dissipated in order to maintain the inductance
at its desired value since memory is .inherent in the device. If high overall
operating efficiency is important, as is .invariably the case with battery-
operated equipment, this feature is significant.
It is proposed to study both the electrical and mechanical methods of vary-
ing reactive components. The two approaches will be investigated simultaneously.
The mechanical method is more likely to lead to successful results in a shorter
time. However the electrical method is, from the overall standpoint, the more
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desirable with probably superior miniaturization possibilities.
(2) Sensing Circuits
.f~~ -~
In the RT-21 transmitter two servo systems are required. The first will
regard at~y condition other than resonance of the tuned circuits in the trans-
mitter .itself as constituting an error. The second will produce an error
signal whenever the antenna is not matched to the transmitter output resistance.
Circuitry must be devised to perform these functions.
(a) Resonance Sensing
A method which has been utilized with vacuum tube circuitry will be studied
for possible application to the RT-21. The method relies on the fact that at
resonance a tuned circuit appears purely resistive. Under these conditions the
voltage at the grid of a tuned amplifier will be exactly out of phase with the
voltage at the plate. then the plate tank circuit is other than resonant it
~,ti.ll be reactive causing a phase shift other than 180?. This condition can be
detected by a phase detector resulting i.n an error signal. An attempt will be
made to obtain similar results using transistors. In the case of transistors,
however, the problem is complicated by the fact that phase shift through the
transistor will be a function of frequency to some extent. It may be necessary
to have some compensation built into the phase detector so that the condition
of balance changes with frequency.
At resonance the voltage swing developed across the tank circuit will be
a maximum. A second approach will utilize this feature by comparing a small
portion of the tank circuit voltage swing with a reference voltage from the un-
tuned oscillator. By making the portion of the tank circuit voltage always
smaller than that from the oscill ator, even at resonance, and adding the two in
such a manner that they are out of phase, resonance will be indicated by a mini-
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mum resultant voltage. Essentially the same results could be obtained by
rectifying a portion of the tank circuit voltage and comparing it with a
reference do voltage of opposite polarity.
(b) Impedance Match Sensing
One approach to the problem of sensing the condition of correct matching
between antenna and transmitter utilizes a standing-wave bridge. The operation
is as follows. It will be assumed that the output tank circuit has been ad-
justed for resonance either manually or by one of the techniques described
above. The output of the transistor consequently appears to be purely resist-
ive. This apparent resistance is arranged to form one arm of abridge. Look-
ing towards the antenna from the input of the impedance matching network, an
impedance is seen which depends upon the antenna and the adjustment of the
matching network. If the network was adjusted correctly for the particular
antenna and frequency, the impedance seen would be resistive and equal to the
apparent transmitter resistance. Consequently, by making this impedance the
second arm of the bridge an output will be obtained for all conditions other
than the desired one. This output or error signal may be used to adjust the
reactances of the impedance bridge.
(3) Auxiliary Circuit Investigations
As a result of work recently completed the basic tools are available for
the design of the remaining portions of the RT-21 transmitter. Experience has
been obt~.ned in the techniques for combining transistors to increase their
output capabilities. The limiting factor for output power is the availability
of a suitable device. As improved devices become available modified design
techniques will be developed to utilize them to best advantage.
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Depending upon the relative degrees of success of the different approaches
to automatic transmitter adjustment described above a need will develop for the
investigation of a large number of auxiliary circuits such as phase detectors,
standing wave bridges, discriminators, do amplifiers, blocking oscillators etc.
The functions to be performed by these circuits axe not new. The circuit de-
signs wi11, however, have to be tailored to the particular requirements of the
RT-21 so that they can be used to form an integrated piece of equipment.
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III. Manpower Requirements and Time Schedule
1. Phase A. Circuit Development
This program will run for 12 months, during which time the following
areas will be investigated.
Variable Reactances (a)
Mechanical
(b)
Electrical
Sensing Circuits
Auxiliary Circuits
(a)
(b)
Resonance Sensing
Impedance Match Sensing
Phase A will require the following engineering manpower
Engineers 125 man weeks
Technicians 75 man weeks
2. Phase B. Construction of the Electrical Model
At the conclusion of Phase A a period of 6 months will be spent in
construction of an electrical model. This model will incorporate the most
satisfactory methods developed during Phase A. Phase B will terminate with
the delivery of a complete transmitter meeting the electrical requirements
listed in the Specification to the extent that devices then available permit.
Phase B will require the following engineering manpower
? Engineers
Technicians 20 man weeks
Reports will be submitted in accordance with the Schedule given in
Specification No. 58-,A-1077-~,.
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