Patent GB-506085-A (GB)
Title: Improvements in or relating to modulated carrier wave receivers
IPC: H03J7/04
Filed: 1938-02-10
Abstract: 506,085. Phase and frequency modulation; automatic tuning correction; piezoelectric crystal mountings; impedance networks. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Feb. 10, 1938, No. 4244. Convention date, Feb. 10, 1937. [Classes 40 (iii) and 40 (v)] In a phase-modulated wave receiver the side-bands on both sides of the carrier are attenuated so that the depth of phase modulation is inversely proportional to the modulation frequency. The wave has then the characteristics of frequency modulation and may be received on a frequency modulation receiver without audiofrequency correction being applied. The phase-modulated carrier (intermediate frequency) is passed through a correction filter comprising a sharply-resonant circuit with a response (as shown) at A, Fig. 4, and the resultant wave is passed through a circuit with response (as shown) at B. The output, which is amplitude modulated, is detected in the normal manner. The two filters may be combined in a single circuit which has a response (as shown) at C. The correction filter takes the form of a sharply resonant circuit or tuned amplifier with or without negative resistance ; but is preferably a crystal filter, Fig. 5, the input transformer 50 being coupled to the valve 62 and output transformer 66 by the piezo-electric crystal element 59 on which are mounted the plates 56-58. The sloping characteristic filter and detector units for frequency modulation may be constituted by a back to back arrangement, Fig. 6. The output from the correction filter is applied in phase to the valves 50, 51 having oppositely-sloping characteristic filter circuits L1C1CO, L2C2LO fed from their anodes. The high potential ends of the filter circuits are connected to the grids of detectors 52, 56. High resistance loads R1, R2 are placed in the cathode circuits, high resistances R3, R4 shunting the grid circuits, and the detectors act as infinite impedance diodes, the coupling condensers K passing the audio-frequency to the push-pull amplifiers 80, 81. For the reception of amplitude modulation the correction filter would be switched out and the output of valves 80, 81 taken in parallel by reversal of switch S. The combined effect of the correction filter and sloping filter may be obtained by arrangements of crystal filters, Figs. 8 and 9 (not shown), in which the holder capacity is partly neutralized by a condenser, or an adjustable electrostatic shield is used to obtain the same effect. Complete neutralization of holder capacity results in a symmetrical response as in curve A, Fig. 4 ; partial neutralization by adjustment of the condenser or shield results in an asymmetric response as in curve C. Fig: 2 shows the invention applied to an automatic frequency-control system for a superheterodyne receiver. The H.F. amplifier 1, first detector 2, local oscillator 3, I.F. amplifier and amplitude limiter 31, and correction and sloping filters 9 are followed by the second detector 26. The output from 31 is applied in phase opposition, and the output from 9 in phase, to two diodes 11, 14. Voltages of opposite polarity developed across resistances 12, 13, are fed via resistance-condenser combinations 15, 16, to the grids of valves 18, 19. The anode circuits of the valves are in parallel and together in series with condenser 21 across the tuned circuit of the local oscillator. When the receiver is out of tune. the balance of voltage will be upset due to the filters 9, and the resultant impedance of the valves 18, 19, changes and so varies the oscillator tuning. In a modification, Fig. 3 (not shown), the rectified voltages from the A.F.C. diodes are combined and charge a condenser, the voltage being employed as bias on a single valve in the frequency-determining circuits.