Patent GB-817059-A (GB)
Title: Improvements in or relating to apparatus for demodulating amplitude modulated electric signals of the kind including a vestigial sideband
IPC: H03D1/24
Filed: 1956-09-25
Abstract: 817,059. Automatic phase control systems. GENERAL ELECTRIC CO. Ltd., HARLING, D. W., and HINE, R. G. B. Dec. 23, 1957 [Sept. 25, 1956], No. 29294/56. Class 38 (4). [Also in Group XL (c)] Relates to an arrangement for initially setting up the demodulator in a receiver in an amplitude modulation vestigial side-band system in order to suppress the unwanted quadrature carrier component which is caused by the asymmetry of the side-bands. As shown in Fig. 1, during normal operation, the input signal from 1 is applied to a demodulator 2 together with local oscillations at the carrier frequency from an oscillator 3, the output from the demodulator being passed via a low-pass filter 4 and amplifier 5 to the output circuit. A portion of the signal from the input 1 is fed via a variable phase-shifting network 6 and amplifier 7 to a phase discriminator 8 where it is compared with a portion of the output from the oscillator 3 to control the frequency and phase of the oscillator 3, the phase relationship with the carrier being dependent upon the setting of the phase-shifter 6. When .initially setting up the apparatus the normal vestigial side-band carrier signal is replaced by a test signal consisting of a carrier having the same frequency as the normal carrier and two equal side-bands which are relatively close to the carrier frequency, e.g. the test carrier may be amplitude modulated by a 1000 c.p.s. signal. A 90 degrees phase-shifting network 10 is connected between the input and the demodulator 2 by switches 11, 12, and the output of the demodulator 2 is passed via a selective amplifier 13 which is tuned to a frequency of 1000 c.p.s. to an indicating device 14, the phase-shifter 6 being adjusted for zero reading of the device 14. Under these conditions the oscillations supplied by the oscillator 3 to the demodulator 2 are in phase quadrature with the carrier frequency component of the test signal supplied to the demodulator so that the carrier frequecy of the test signal received at 1 and the oscillations supplied by the oscillator 3 are in phase or antiphase. The phase-shifting network 10 is then switched out of circuit and the apparatus is ready to receive normal vestigial side-band signals. The frequency of the oscillator 3 is locked by the carrier signal as before, the oscillations supplied to the demodulator 2 by the oscillator 3 and the carrier component of the carrier signal being in phase or antiphase and the effect being that the quadrature component of the signal produces no output from the demodulator 2. The network 6 alternatively may be connected in the position shown as 6<SP>1</SP>. If the modulation ratio is greater than unity the phase of the carrier may be reversed during certain periods of the signal and it is necessary to provide a frequency doubler in the path feeding oscillations from the oscillator 3 to the discriminator 8 and in the path supplying a portion of the input from 1 to the discriminator 8 and the amplifiers 7 and 9 may be arranged to act as frequency doublers. In an alternative arrangement, Fig. 2 (not shown), the test signal is locally generated. When setting up the apparatus a portion of the output of the oscillator 3 is applied via a phase-shifting network to a modulator where it is modulated by a signal having a frequency of 1000 c.p.s. and the output of the modulator is applied instead of an incoming signal to the phase-shifter 10.