Patent GB-1206094-A (GB)
Title: Improvements in or relating to radio communication systems
IPC: H03J7/06
Filed: 1969-03-12
Abstract: 1,206,094. Radio signalling; automatic frequency control. C.I.T.-COMPAGNIE INDUSTRIELLE DES TELECOMMUNICATIONS. 12 March, 1969 [19 March, 1968], No. 12958/69. Headings H3A and H4L. Means are provided for reducing the effect of carrier frequency deviations, such as is caused by Doppler shift, in single sideband or separate sideband radio communication systems. Transmitter, Fig. 1.-Signals from a microphone 10 are supplied via a filter 11 passing a range 300 to 2500 Hz to filters 12, 13 passing bands of 300 to 600 Hz and 600 to 2500 Hz respectively. The outputs of filters 12, 13 are supplied via frequency dividers 14, 15 dividing by four and ten respectively, to an adding circuit 16 providing an output within the range 60 to 250 Hz. This output and an auxiliary carrier of 2750 Hz from an oscillator 17 are supplied to a ring modulator 18 suppressing the carrier, and the resulting sidebands in the range 2500 to 3000 Hz are supplied via a filter 19 to be combined at 20 with the speech signals from filter 11 to modulate the carrier wave of the S.S.B. transmitter 21. In a modification, the output of the filter 11 is supplied to a threshold device, which replaces the filters 12, 13 and dividers 14, 15, and this controls a gate to allow the output of a noise generator to pass to the modulator 18. Receiver, Fig. 3.-The I.F. amplifier 51 provides an output at 250 KHz modulated by the 300 to 3000 Hz band and also by a deviation e arising from Doppler or other effect which is supplied to filters 52, 53 passing bands 252À5 to 252À7 and 252À8 to 253 KHz respectively representing the lower and upper side bands produced by the modulator 19 but shifted in the same direction by the deviation frequency e. Filters 52, 53 supply a modulator 54 whose output is fed via a filter 55 passing a narrow band centred about 505À5 KHz (twice the I.F. of 250 KHz plus twice the frequency of 2750 Hz produced by oscillator 17) to a modulator 56 receiving also a signal of 5À5 KHz. The output of filter 55 is two narrow bands of equal width equally disposed above and below the centre frequency if no Doppler shift is present but any Doppler shift will move both bands in the same direction so that one is nearer the centre frequency than the other. The output of modulator 56 is at a frequency of 500 KHz plus twice the deviation frequency e and this is fed via a filter 58, clipper 59 and divide-by-two circuit 60 to a phase discriminator 61 receiving its other input at 250 KHz from a controlled oscillator 69. The output of the phase discriminator is supplied via amplifier 62 to an AND gate 66 and to a store 65 so that as long as there is a frequency present at point Q, i.e. there is a speech modulation signal, a rectifier 63 provides the second input to AND gate 66 and the control signal is passed to the oscillator 69 via an OR gate 68. The output of the rectifier is inverted at 64 so that under these conditions an AND gate 67 is blocked. Should the speech signal cease the gate 67 is opened and the signal stored at 65 is used to control the oscillator 69. A filter 71 passes the I.F. modulated by the speech signals and the deviation frequency e to a modulator 70 supplied also with the output signal of the oscillator 69, the resulting speech signals being passed by a filter 72 and amplifier 73 to the loudspeaker 74.