Patent GB-569417-A (GB)
Title: Improvements in or relating to radio receivers
IPC: H04W88/02
Filed: 1943-05-05
Abstract: 569,417. Wireless receiving circuits. UNITED AIR LINES TRANSPORT CORPORATION. May 5, 1943, No. 7123. Convention date, June 22, 1942. [Class 40 (v)] Means for rendering a radio receiver operative only when a signal is being received comprises means for heterodyning selected components of the signal to produce a predetermined audio frequency tone, means for sharply filtering said tone, and means for rendering the receiver operative or inoperative in response to the presence or absence of said audio frequency tone. Fig. 1 shows the invention applied to a superheterodyne receiver in which the signal channel comprises an R.F. amplifier 10, frequency converter 12 and local oscillator 13, I.F. amplifiers 14 and 15, second detector 16 and audio amplifier 17 feeding a loud-speaker or headphones (not shown) through the contacts of a relay 19. The control channel comprises an I.F. amplifier. 22 feeding a frequency converter 23 which also receives the outputs from oscillators 24, 25 having frequencies which differ from the intermediate frequency by about 20 kc. and which differ from one another by about 1 kc. A band-pass filter 26 selects the difference frequencies which differ by 1 kc. and these are mixed in detector 27 to give an output at 1 kc. This is passed through a sharply tuned filter 28 and full-wave rectifier 29 to energize the coil 20 of relay 19. With this arrangement, only when a carrier wave is being received will the 1 kc. tone be present in the output of the control circuit so as to operate relay 19 to render the receiver operative and this tone frequency is independent of the carrier frequency. Thus, filter 27 can be very sharply tuned and so will not pass voltages due to static or thermal noise. In a modification, Fig. 2 (not shown), the control channel is branched from the R.F. stage instead of the LF. stage, but is otherwise similar to Fig. 1. In a further modification, Fig. 3 (not shown), the I.F. beats with the output from a first oscillator and also with the resulting difference product to eliminate the I.F. and the resulting frequency beats with the output from a second control oscillator to give the desired 1 kc. tone for rendering the receiver operative. Fig. 4 shows an arrangement in which the signal channel is combined with the control channel up to the audio frequency stage. The oscillator 51 feeds the converter 50 to give the usual I.F. which passes to stages 53, 54 and 2nd detector 55 and audio amplifier 56 to give the audio signal in the usual manner. In addition, an oscillator 52 whose frequency differs from that of 51 by a high audio frequency, also feeds converter 50. This. gives two difference frequencies which differ by the said high audio frequency and which beat in the second detector 55 to give the high audio. This is filtered out by a filter 57 for use, after rectification, to operate relay 63 to control the muting of the receiver, while a low-pass filter 61 removes the signal for utilization. With this arrangement, stages 53, 54 have to be sufficiently broadly tuned to pass both of the control I.F.'s. Thus, the receiver is not highly selective. A modification is described, Fig. 5 (not shown), in which this objection is met by having separate channels in the I.F. stages. In all the above embodiments, the control tone frequency is independent of the carrier frequency and allows extremely fine tuning of the last tone frequency filter. Fig. 6 shows an arrangement in which the tone frequency is dependent on the carrier frequency. The elements 10 ... 16 of a conventional superheterodyne receiver feed an audio amplifier 80 having output terminals 82. The control channel is taken through an LF. amplifier 83, the LF. being mixed with the output from an oscillator 85 to give a 20 kc. note which, after passing a series of filters 86, is rectified at 87. The rectified voltage appears across resistance 88 which has a compensating resistance 90 arranged so that, in the absence of a 20 kc. tone, no voltage is applied to the grid of a valve 98. The current in anode resistance 94 then puts a prohibitive bias on audio amplifier 80 to mute the receiver. The receiver is opened on the appearance of a rectified tone voltage due to the reception of a carrier.