GB539793A - Improvements in or relating to fading compensation in radio receivers - Google Patents

Abstract

539,793. Valve circuits ; frequency modulation. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. March 18, 1940, No. 5040. Convention date, March 18, 1939. [Classes 40 (iii) and 40 (v)] In a system using a radio-frequency carrier which is modulated by a frequency modulated sub-carrier, interference is reduced by providing two channels for receiving respectively the sub-carrier and its second harmonic which are passed for frequency detection, means being provided for blocking that one of the channels which carries the weaker signal. The invention is described with reference to a picture transmission system. The transmitter is shown in Fig. 1. The variation in the light received from the picture 5 by the photo cell 6 changes the impedance of valve 8 and hence varies the frequency of oscillation of the neon tube-condenser arrangement 16, 13 in accordance with the brightness of the picture. This frequency modulated audio-frequency sub-carrier is applied by a transformer 19 to amplitude modulate a radio-frequency carrier in a transmitter 20, 201, 21 the signal being radiated by an aerial 22. Fig. 2 shows a receiver in which selective fading is overcome by employing separate channels B, A for the fundamental sub-carrier and the second harmonic respectively. The signal is received by an aerial 23 and applied to receiver 24 the output of which comprises the frequency modulated sub-carrier and harmonics. Band pass filter 27 passes only the second harmonic of the sub-carrier while band pass filter 28 passes only the fundamental. The subsequent stages of the two channels comprise limiters 29, 30 and frequency detectors 31, 32 the output from which comprise amplitude modulated waves which are combined and applied to an amplifier and rectifier 34 the output from which passes to a picture recorder 35. A suitable form for the limiters 29, 30 is shown in Fig. 5. The input consists of two coils 50, 51. connected to the anodes of a double rectifier 54 The live ends of the load resistances 55, 56 are connected to the grids of two valves 60, 61 the anodes of which are connected in push-pull to the primary of a transformer, the secondary 75 of which comprises the input of a frequency detector such as is shown in Fig. 6. This is in effect a low pass filter consisting of inductances 76, 77 and condensers 78, 79, 80 and possessing a characteristic having a straight part covering the band of modulation frequencies. In a modification, Fig. 3 not shown, a frequency doubler is inserted in channel B after the band-pass filter 28 so that, there being now only one frequency, the subsequent stages are common to both channels. To overcome phasing difficulties valve means are provided for blocking the channel having the weaker signal. A special circuit arrangement for selecting the preferred signal channel is described with reference to Fig. 9 not shown. Specifications 252,798, [Class 40 (iii)], and 352,621 are referred to.