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Aviation History
1953
1953 - 1403.PDF
This drowing, which is based on the display panel exhibited by the M.o simplified form. The components are ennumerated in A, helical aerial; B, main receiver; C, demodulator (delay approx. 3 micro-sec); D, signal amplifier and sync-separator time base; E, strobe unit; F, drum of 24in- wide film recording traces of 15 c.r. tubes (from the top: tele, bay; D.C. voltages; pi tot static, starboard; pi tot static, port; primary fuel, port; air blast; combustion chamber; wall static, port; main fuel, port; main fuel at block; primary fuel, starboard; D.C. voltage and 1g switch; acceleration; main fuel, starboard; and wall static, starboard); G, high-speed camera; H, c.r. tube; J, section of high-speed In the case of the inductance-responsive modulator, a normal triode oscillator is used, the pick-up providing the variable induct ance of the tuning circuit. The output of the oscillator is then amplified to give the power needed to modulate the r.f. oscillator. In the second modulator, responding to voltage inputs only, the frequency of oscillation of a tuned-grid triode is controlled by a conventional reactance valve which forms part of its tuned circuit, the output frequency being again amplified to modulate the r.f. oscillator. To provide a combined circuit responding to both voltage and inductance inputs, these two circuits are combined in a four- valve version. The first valve acts as an oscillator for inductance inputs, and the second as an amplifier which locks the third valve frequency to that of the first. The fourth valve is a straight ampli fier. In the case of voltage inputs, the first valve is rendered passive by the damping of its tuned circuit and the voltage applied to the input to the second valve which, acting as a reactance valve, determines the frequency of oscillation of the third valve. The fourth valve is, again, a straight amplifier. In all cases the range of frequencies generated is 130-160 kc which, in the case of voltage inputs, is covered by a change of 6 volts. To provide adequate power for all ranges, while keeping the D.C. demand as small as possible, three oscillators were designed for use in the system, with outputs (measured on a C.W. basis) of 0.2, 1 and 8 watts respectively. The earliest version was the 0.2 watt oscillator, built in the form of a push-pull tuned plate, tuned grid oscillator, using a CV858 (6J6). This is now obsolescent and is being replaced by the 1 watt version, which has a similar circuit arrangement but is built around a specially-developed sub- miniature valve. The larger 8 watt unit, used for longer ranges, consists of a tuned - anode, tuned- grid oscillator built around the CV 397a disc-sealed triode. As the modulator cannot control this oscillator unaided, a driver-unit, supplying the neces sary power, is included in this model. Both the 1 watt and 8 watt oscillators can be arranged to work either C.W. or pulsed, the choice depending largely on the avoid ance of interference with other r.f. circuits associated with the oscillators in missiles. The fact that these oscillators m ust be easily controlled in fre quency and permit coupling to whatever A typical P.P.M. trans mitter for installation in a test rocket. .S. at the S.B.A.C. Display, represents a typical time-multiplex system in the key below. Missile K houses units L to U inclusive. record; K, missile; L, accelerometer (variable inductance for longitudinal and transverse acceleration); M, voltage-type pressure transducer (for fuel pressure, etc., range 0-4,000 Ib/sq in); N, standard calibration inductance; O, variable- inductance pressure transducer (range 0-200 Ib/sq in); P, voltage-type pressure transducer (for fuel and hydraulic pressure, range 0-600 Ib/sq in); Q, R, angle-of- attack head and socket (variable inductance measuring pitch and yaw); S, high speed rotary sampling switch; T, transmitter; and U, modulator. aerial system is available and, at the same time, be stable over a wide range of temperatures and pressures, involved major prob lems of design. This was particularly the case with the high- power oscillator, where high voltages combined with low pressures necessitated considerable development work to prevent flash-over. As a by-product of this work, the design of r.f. power-measuring and impedance-measuring equipment at these frequencies had to be developed from scratch, as no suitable units existed for testing purposes. The ground equipment of the system provides two records, a histogram (i.e. high-speed record) and the main record. For the latter, the signal received from the airborne sender is changed, after demodulation, to a series of direct-current levels, and the step function thus produced is applied simultaneously to the Y-plates of 15 cathode ray tubes. From the output of the r.f. receiver a filter separates out the synchronising signal, which is then used to generate the time base for a large monitor tube and also to produce strobes coinciding with each channel. These pulses are used to brighten the traces on the 15 cathode ray tubes. Any number of them can be connected to each of the display tubes so that any channel or any combination of channels can be displayed on any of the tubes. All 15 tube-faces are photographed side-by-side on a moving film 24in wide, as shown in the diagram above. Timing pips at each tenth-of-a-second are recorded on each tube, as also is the firing pulse. On the completion of each firing, short calibration lines from a crystal-controlled frequency genera tor are added to the record. The histogram recorder is designed as a self-contained unit for producing records from single-channel senders or histogram traces from multi-channel senders. It contains a receiver and discriminator similar to those used in the main equipment, and is fed from a separate helical aerial. The display consists of three cathode ray tubes, one large and two small, which are photo graphed simultaneously by a 35 mm cine-camera. The large tube displays the single-channel record, or the series of 24 direct-current levels corresponding to the channels in the sender. One of the small tubes is used to display a time scale, the lateral position of which is calibrated in terms of field strength, and the other small tube indicates any radio-frequency deviation throughout flight. The P.P.M. System.—In this method, channel information is given by the time interval between the trailing edge of a refer ence pulse (about 50 micro-seconds long) and the incidence of a one-mifto-second channel pulse. The carrier frequency is pulse modulated by the reference pulse and the channel pulses, the recurrence frequency of the complete pulse train being between 2,000 and 5,000 per second, according to the number of channels in the sender. Up to 20 channel-pulses, each of which can vary over a range of 400 micro-seconds, can be accommodated between successive reference pulses. Normally, however, each group of pulses is con fined to consecutive intervals of 80 micro-seconds, the range of time displayed on each of the five cathode ray tubes in the ground equipment. Each channel is arranged to produce, either directly or via a transducer, a variation of voltage or capacity which is used to control the time interval between the trailing edge of a refer ence square-wave and the generation of the respective channel pulse. The airborne transmitting equipment of the set, the main parts of which are a modulator and an oscillator, is made in the form of a cylinder, with a diameter of 4|in and an overall length of 6iin for a 12-channel set. For ease of manufacture and
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