GB1013761A - Improved compatible single-sideband system - Google Patents

Abstract

1,013,761. Single sideband systems. L. R. KAHN. June 8, 1962 [June 14, 1961], No. 22371/62. Heading H3R. In a compatible S.S.B. transmitter an S.S.B. wave (without carrier) is produced and applied to a pair of networks 26, 28 non-linearly processing its amplitude so that the ratio of its amplitude to the original modulation amplitude increases with the latter; and then each of the network outputs added in summation circuits 30, 46 to pure carrier of amplitudes respectively equal to and twice the maximum amplitude level of the two processed S.S.B. waves (without carrier). This produces 1 PM and 0À5 PM single sideband plus carrier waves respectively, at 100% modulation (PM denoting phase modulation), which are then added together in a mixer 44 and transposed up in frequency from a 500 kc/s. oscillator 56. This output is then limited at 68 and transposed up to transmitter carrier frequency by an oscillator 74 before being modulated by amplitude modulating voltage, preferably derived from the S.S.B. filter 18 by a product demodulator 86 with carrier input. The non-linear networks 26, 28 ensure that at 100% modulation the phase modulation corresponds to 1PM + 0À5PM but at lower modulation levels the phase modulation is less and approximates to that of an unmodified single sideband plus carrier wave. The non- linear networks may each comprise three parallel connected paths, two of them each including a reverse biased diode oppositely poled in each case to pass only positive and negative extremes of the modulation amplitude to a phase shifting tuned network. The bias of each network is then adjusted to give the required total phase deviation as a function of the original amplitude modulation index and the square of that index, i.e. of the amplitude of the S.S.B. (without carrier) wave.