Patent GB-865723-A (GB)
Title: Synchronous demodulators
IPC: H03D1/22
Filed: 1957-12-17
Abstract: 865,723. Controlled non-linear inductors and capacitors. RADIO CORPORATION OF AMERICA. Dec. 17, 1957 [Dec. 20, 1956], No. 39184/57. Class 40 (9). [Also in Groups XXXVI, XL (b) and XL (c)] A synchronous demodulator for deriving the modulation information at a predetermined phase of a phase and amplitude-modulated carrier wave, such as the chrominance signal in a colour television system, utilizes a non-linear inductive or capacitive reactance to which the carrier wave is applied together with a demodulating signal of predetermined phase and a frequency equal to the frequency of the carrier or the first sub-harmonic thereof. Fig. 1 shows a demodulator utilizing an inductive reactance provided by a magnetic core 11 carrying a first input winding 13 to which the carrier wave is applied, a second input winding 15 to which the demodulating signal is applied and an output winding 17 from which the modulation information is derived. Since the voltage induced in winding 17 is proportional to the time derivative of the flux in the core, it is stated that the demodulator should be operated in association with an integrating circuit 18, connected either in the output circuit as shown or in the carrier-wave input circuit. The demodulating signal is normally at the same frequency as the carrier wave and a bias source is provided in series with winding 15 to shift the operation to a non-symmetrical part of the magnetization characteristic. As an alternative, the demodulating signal may be at the first subharmonic of the carrier frequency and the bias source dispensed with. The demodulating signal may be of such amplitude that it causes demodulation, in effect by sampling, only during the intervals when the signal passes from one saturation level to the other. Demodulators particularly adapted for colour television purposes are described (see Group XL (b)), Figs. 4 and 6 (not shown), in which more than one output signal is derived with the aid of either a plurality of output windings or a single tapped winding. Two demodulators disposed close together, Fig. 7 (not shown), may have a common carrier-wave input winding embracing both cores. The demodulating winding may be formed in two parts wound on opposite sides of the core and arranged not to develop a signal in the other windings, Figs. 3 and 7 (not shown). Fig. 11 illustrates a synchronous demodulator utilizing a non-linear capacitive reactance provided by a ferro-electric crystal 125, such as barium titanate, bearing electrodes 127, 129, 131, 133 and 135 on its faces. The carrier signal is applied to electrode 127, the demodulating signal is applied between electrodes 129 and 131, and output signals are taken from electrodes 133, 135 and 137 through low-pass filters 140 which select the modulation components and reject the carrier and demodulating signals. The crystal is cut so that the ferro-electric axis passes through electrodes 127 and 135. An output signal of greatest amplitude appears at electrode 135 and signals of lesser amplitude from electrodes 133 and 137 as determined by both their area and angle at which they are positioned off the -ferro-electric axis. Figs. 8 and 10 (not shown) show demodulating circuits in which the non-linear demodulating reactance (illustrated only diagrammatically) is provided by a two-terminal device.