Patent GB-598644-A (GB)
Title: Improvements in a radio communication system of the carrier frequency shift type
IPC: H04L27/10
Filed: 1945-10-22
Abstract: 598,644. Multiplex radio signalling. DEHN, F. B. (Press Wireless, Inc.). Oct. 22, 1945, No. 27797. [Class 40 (v)] A diplex frequency-modulated radio communication system comprises a common means to render each signal channel alternately and recurrently effective on a transmitter, and means responsive to signals in one channel for raising the frequency of the transmitter a predetermined amount and means responsive to signals in a second channel for lowering the frequency a predetermined amount. In Fig. 1, the frequency of transmitter 1 is varied by frequency shifter 2 which is controlled by the separate signal channels 4 and 5. Control is exercised by oscillator 6 which arranges for channels 4 and 5 to be alternately and recurrently connected to frequency shifter 2, thus enabling the signal in each channel to establish intermittent control of frequency shifter 2. Oscillations in opposite phase are applied through audio-frequency transformer 7 to grid controlled valve 13 and gas valve 12, which have a common anode load 22, so that square topped pulses are produced across the cathode load 23 of gas valve 12. The oscillations are also applied similarly through another secondary coil in transformer 7, to valves 17 and 16, the arrangement then being such that positive half waves produce pulses across resistance 23, and negative half waves produce pulses across resistance 27. The output from cathode load 23 is applied to a second grid of valve 25. When a keyed tone is applied to the input 33<1>, 34 of channel 4, this is rectified by valve 35, thereby applying a negative potential to the grid of valve 37 so that this valve is cut off, thus removing the negative bias on valve 30 so that this valve conducts. The anode of valve 25 is connected in series with the anodecathode circuit of valve 30. When valve 30 conducts, a positive potential is applied to the anode of valve 25, which causes valve 25 to conduct during the periods that positive pulses are also applied to the second grid from resistance 23, thereby producing recurring positive pulses at the oscillation frequency across cathode load 32. Similarly, when a tone signal is applied to channel 5, a recurring positive voltage is developed across the cathode load 33 of valve 29 corresponding to pulses from cathode load 27. When signals are applied simultaneously to channels 4 and 5, the positive voltages are developed alternately across resistances 32 and 33. Resistances 32 and 33 are connected in balanced relation across the input electrodes of valve 49, so that a voltage developed across one resistance increases the normal control grid bias of valve 49, and a voltage developed across the other resistance reduces this bias. The output of valve 49 is applied to frequency shifter 2, such that the frequency of the transmitter is raised in response to signals from one channel, and reduced in response to signals from the other channel. When one channel is sending, the transmitter frequency is thus varied at the frequency of oscillator 6, between the normal and the higher frequencies, and when the other channel is sending, the transmitter frequency is varied between the normal and the lower frequencies. When both channels are sending simultaneously, the frequency varies between the higher and lower frequencies. The receiver portion includes a beat frequency detector 55 for separating the two channel frequencies. In addition, to provide facilities for a third channel, an amplitude modulator 3 may be associated with transmitter 1 to vary the amplitude of the frequency shifted carrier, the receiver then being provided with an amplitude demodulator device 57,