Patent GB-948810-A (GB)
Title: Improvements relating to demodulators
Inventor: DAVIDSON JOHN
IPC: H03D1/10
Filed: 1961-07-31
Abstract: 948,810. Demodulating circuits. WALLACE, DAVIDSON & PARTNERS Ltd. Oct. 31, 1962 [July 31, 1961], No. 27735/61. Headings H3L, H3Q and H3T. Means for demodulating an amplitude modulated carrier wave comprises an input terminal to which it is applied and a phase-sensitive demodulator having two input circuits each connected to the input terminal and being adapted to derive two further amplitude modulated waves from the input, at least one of which differs in phase from the input wave, the demodulator being adapted to derive an output signal from the two further waves having an amplitude determined by both the relative phase and amplitude variation of the further waves, at least one of the input circuits including frequency responsive means whereby the two input circuits have different frequency responses such that the relative phase is determined by the frequency of the input wave, the arrangement being such that the ratio of the output amplitude to that of the input exhibits alternate maxima and minima occurring at frequencies which are dependent upon the characteristics of the input circuits. In one embodiment the demodulator comprises a pair of diodes 18, 19, Fig. 1, feeding into loads comprising resistors 20, 21 respectively, and corresponding capacitors 22, 23 ; the output is taken across terminals 24, 25 and the diodes so poled that their outputs are opposed to one another. The diodes are fed from either end of a centretapped tuned circuit 10, 11 which is inductively coupled to an inductor 12 in the anode circuit of pentode 13. The centre tap is connected via a D.C. blocking capacitor 15 to resistor 14 which is in series with inductor 12. In a second embodiment the demodulator comprises a ring modulator 33, Fig. 2, one diagonal of which is fed from either end of a centre-tapped tuned circuit 31 which forms with tuned circuit 30 a tuned coupled pair in the collector circuit of transistor 34. The other diagonal is fed from either end of the centre-tapped secondary of a wide band transformer 32, the primary of which is in the emitter circuit of the transistor. The output is obtained at terminals 36, 37, across a resistor and smoothing capacitor connected between the two centre taps. The arrangement may be adapted so as to give variable selectivity, by tuning the secondary of the transformer 32 with a capacitor to resonate at the same frequency as circuits 30, 31 and varying the decrement of this circuit by means of a variable resistor connected in shunt. In another embodiment the demodulator 44, Fig. 3, is fed in push-pull from either end of a centre-tapped inductor 41 which is series resonant with capacitor 40 connected between one end and the input terminal, and in phase by the voltage across a capacitor 43 which is series resonant with inductor 42 which is connected between one end of this capacitor and the input terminal. In a further embodiment the demodulator comprises a pair of diodes connected to either end of a centre-tapped inductor (Fig. 4, not shown), being driven in phase from an aerial which is directly connected to the centre tap. A larger push-pull drive is also applied to the diodes, the aerial being additionally coupled to a tuned circuit which forms the input to an amplifier, the output of which comprises a second tuned circuit inductively coupled to the centre-tapped circuit. The amplifier may be of the super-regenerative type. The two inputs to the demodulator of different phase and amplitude characteristics may alternatively be derived from two separate aerials with different characteristics. In another embodiment an aerial feeds two tuned circuits 50, 51, Fig. 5, of different characteristics, which form the inputs to two mixers 53, 52 respectively. Each mixer is also fed from a local oscillator 54, the intermediate frequencies so produced being fed (i) from mixer 53 to one input of demodulator 58 over filter 55 which removes the difference frequency and (ii) from mixer 52 to the other input of the demodulator via an optional high-pass filter 56 and amplifier 57.