US 4,380,081 AGranted

Digital receiver for four-phase-modulated carrier

AssigneeCSELT - Centro Studi e Laboratori Telecomunicazioni S.p.A.

InventorPaolo Di Tria

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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.

Claims12

Dependent claims

A receiver of phase-modulated carrier waves arriving over a signal path of a telecommunication system in any of four possible phase positions representing dibits of transmitted data words,

comprising:
a squarer converting an incoming carrier wave into a phase-modulated square wave;
a generator of a reference square wave having a frequency harmonically related to that of the incoming carrier wave;
phase-demodulation means with inputs connected to said squarer and to said generator for emitting a binary sequence of a configuration determined by the relative phasing of said phase-modulated square wave and said reference square wave;
a source of timing pulses controlling the operation of said generator, said timing pulses having a cadence equal to a multiple of the frequency of said reference square wave;
a shift register connected to said phase-demodulation means for receiving therefrom samples of said binary sequence which are taken upon the occurrence of said timing pulses, said shift register being stepped by said timing pulses and having a multiplicity of stages accommodating a number of said samples representing at least one full cycle of said reference square wave;
logic circuitry connected to said source and to stage outputs of said shift register for detecting characteristic bit groupings appearing in said stages, indicative of optimum instants for reading the contents of said shift register, said logic circuitry generating a reading command at said optimum instants;
and
decision means connected to said stage outputs and responsive to said reading command for emitting a dibit decoded from said contents.

6 more elements

Dependencies are inferred from the claim text.