GB1110750A - Balanced constant impedance modulators and demodulators - Google Patents

Abstract

1,110,750. Amplitude modulation; frequency changing circuits. STANDARD TELEPHONES & CABLES Ltd. 26 May, 1967, No. 24579/67. Headings H3F and H3R. A balanced constant impedance modulator comprises sampling means SW1 having N output channels (two in Fig. 1), each connected to one of the output terminals 3, 4 via one winding L of an inductor having N windings and each connected to earth through an impedance R, each of the N sampling periods being delayed in time on the preceding one by an amount T/N when T is the period of the sampling frequency. Fig. 1 shows the modulator; a demodulator (Fig. 2, not shown) is the same circuit in reverse with the input applied to terminals 3 and 4 and the output taken from switch SW1 through a high impedance amplifier. When there are more than two channels the input switch may be an N-position rotary switch (Fig. 3, not shown) or may be replaced by N simple switches (Fig. 4, not shown). The impedances R may be resistances or capacitors. Frequency changing circuits.-The modulator/ demodulator may be used in the frequency translation system described in Specification 1,081,240 (Fig. 6, not shown). When N is an even number, pairs of channels may share a common filter (Fig. 8, not shown). For a threephase system (Fig. 9, not shown), the three channels may share a common three-phase filter. The system may operate as an N-path filter by making the modulating and demodulating frequencies the same.