GB797696A - Improvements in or relating to single-sideband radio transmission systems - Google Patents

Abstract

797,696. Radio transmitting and receiving systems. PHILIPS ELECTRICAL INDUS. TRIES, Ltd. Oct. 21, 1955 [Oct. 25, 1954], No. 30157/55. Class 40 (5). In a single-sideband radio system the transmitter comprises a limiter following the sideband generator and a filter to suppress the distortion products of the limiter and the receiver comprises a limiter prior to the demodulator. In one embodiment of transmitter, speech signals from a source 22, Fig. 2a, are fed to the single-sideband modulator 25 through a filter 23 and pre-emphasis network 24. The modulator output is fed to the aerial 32 through a limiter 29, class C amplifier 30 and a singlesideband filter 31. The signal envelope from the single-sideband signal at the output of the modulator 25 is derived in an amplitude detector 33 and fed through a low-pass filter 34, having a cut-off frequency of 300 c/s. or less, to modulate a pilot frequency source 36 in the amplitude modulator 35 the output of which is fed through an amplifier 37 and filter 38 to the aerial 32. The pilot carrier frequency is either about 4 kc/s. different from the main carrier frequency or it may be nearly the same, or the same and derived from the carrier source 27 in the modulator 25. The envelope signal is unidirectional and is applied to the modulator 35 in such a sense that in the absence of speech signals the transmitted pilot signal has its maximum amplitude. The envelope amplitude modulation signal channel 33 ... 38 may be replaced by a frequency modulation channel, Fig. 2b (not shown). At the receiver, the aerial 43, Fig. 3a, feeds the demodulator 55 through amplifiers 44, 46, filter 45, limiter 47, amplitude modulator 52 and amplifier 53. The amplitude modulator 52 is fed with an envelope signal derived from an amplitude detector 50 fed from the amplifier 44 through a filter 48 passing a bandwidth of, for example, 100 to 500 c/s. The output of demodulator 55 is fed to the reproducer 59 through a filter 56, de-emphasis network 57 and amplifier 58. The amplitude modulator 52 produces a decreasing output with increase in envelope modulating voltage. When a frequency modulation envelope channel is used at the transmitter the envelope detector 50 is replaced by a limiter and frequency discriminator, Fig. 3b (not shown). The envelope signal channels at transmitter and receiver may be omitted and an unmodulated pilot frequency, loading the limiters during speech intervals, added to the signal by connecting the main carrier oscillator to the transmitter limiter through an adjustable attenuator, Figs. 1a and 1b (not shown).