Digital receiver for four-phase-modulated carrier

Abstract

A signal receiver at the subscriber end or the exchange end of a telephone line loop, over which messages are transmitted in digitized form by a carrier with four possible phase positions representing as many two-bit combinations or dibits, includes a coherent phase demodulator in which samples of the incoming carrier (converted to a square wave) are logically multiplied with coincident samples of a locally generated reference wave of like or subharmonically related frequency to produce a binary sequence of a configuration depending upon the relative phase. A multiplicity of samples of that sequence, encompassing two cycles of the reference wave, pass through a corresponding number of stages of a shift register; two interleaved groups of nonadjacent stages of this shift register are connected on the one hand to a decision circuit, designed to recognize the dibit represented by the current carrier phase, and on the other hand to respective coincidence circuits of a time base determining the exact instance when the contents of the shift register are to be read by the decision circuit. A phase-locking circuit is connected to a further group of register stages in order to detect spurious phase shifts and to make compensatory adjustments in the phasing of the locally generated reference wave. The receiver forms part of a modem whose transmitting section operates with an outgoing carrier of a frequency harmonically related to that of the incoming carrier.