Patent GB-581782-A (GB)
Title: Improvements in or relating to radio signalling systems
IPC: G01S13/78
Filed: 1943-10-02
Abstract: 581,782. Valve generating circuits; wireless signalling. PREIST, D. H. Oct. 2, 1943, No. 16196. [Class 40 (v)] An oscillation generator for the production of radio frequency pulses of substantially rectangular wave form comprises a multi-electrode valve arranged to generate radio frequency oscillations and is provided with a coupling between the anode and grid circuits such that the pulse width is governed by the natural frequency of the coupling circuit. The radio and pulse recurrence frequencies are variable independently over wide limits without substantial variation of the wave-form which is also unaffected by variation in loading or supply voltages. The generator is applicable for use in a beacon or responder adapted to give a coded pulsed identification signal from an aircraft when interrogated by the pulse exploring wave from radar equipment. Fig. 2 shows an oscillator having tuned circuit L40, C40 and transformer T1 providing the coupling determining the pulse frequency. Alternatively, coupled tuned circuits in the grid and cathode circuits of the valve may be used, Fig. 3 (not shown), or may be connected between the grid and cathode and the grid and plate with the latter earthed, Fig. 4 (not shown). A pushpull oscillator V8, V9, Fig. 6, may be used, with the grids earthed through the secondary of the coupling transformer T1 and the resistance condenser combination C36, R38. In this case the output is taken from the tuned circuit comprising a length of line L10 and a condenser C28 through condenser C26. Such an oscillator may be self-running or may be triggered by positive pulses fed to the grid through a tertiary winding on transformer from an external source, according to grid bias produced in the cathode resistance R28 by resistance R29. The termination of the pulse may be effected by the negative pulses produced by the reversal of the transformer voltage, the pulse period being determined by the leakage inductance of the transformer and the effective shunt capacity. Damping resistance R30 prevents continuous oscillation and together with the smoothing out of small fluctuations by condenser C30 ensures the production of a pulse with a flat top and steeply sloping flanks. Until the charge on the condenser C36 has leaked away through the resistance R38 the circuit is paralysed, determining the maximum pulse recurrence frequency or the interval between the pulses in a self-running generator, The pulse width may be varied by varying the capacity in shunt to the transformer as by switching a condenser C38. Fig. 6 shows the application to a radio beacon or responder in which the feeder F from a common aerial is used both for transmission and reception and is coupled to the output circuit L10, C28 of the oscillator V8, V9 through a condenser C26. A quarter-wave section of line A-A1, couples the feeder to the input circuit L1, C1, of the buffer stage V1 and isolates the receiver during the transmission since this causes the input impedance of the pentode V1 to be low. The transformation due to the tapping on L1 further reduces the impedance while the transformer effect of the quarter-wave line converts this to a high impedance at the remote end of the line and thus prevents absorption of power from the oscillator. The received signal is fed through the buffer stage V1 to the superregenerator stage V2 quenched by a separate oscillator V7. A diode rectifier V3 rectifies signal and noise and the output is fed from a load R6 to an amplifier V4. The positive pulses from the output are applied through a condenser C12 to trigger the oscillators V8, V9 through a tertiary winding on the transformer T1 to give the reply. The noise output is fed to a wide band amplifier V5 which, however, rejects the quench frequency while the turned circuit L11, C16 in the cathode lead ensures that noise alone is amplified by the amplifier V6. In this way the preponderance of quench frequency components of noise in the presence of continuous wave jamming are prevented from causing biassing off of the receiver. Thus the noise level and gain of the receiver are maintained at a steady value-while the receiver is held in its most sensitive working condition just below the point in which it breaks into oscillation, irrespective of aerial loading and supply variations by adjustment of the input voltage to the amplifier V5. Telephones connected in shunt with the cathode resistance R28 of the oscillator V8, V9 indicate that the beacon is operating correctly, the hissing of the superregenerative receiver being audible, and loud when the test key S1 is operated. When the beacon is interrogated a note equal to the pulse recurrence frequency of the transmitter is heard in the telephones. The Provisional Specification describes a modification in which the pulse width is determined by a resistancecondenser combination in the grid circuit of the oscillator and may be varied by connecting an additional condenser in parallel. Specifications 195,987, 273,485, [both in Class 40 (v)], and 409,405 are referred to.