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Abstract

A direct conversion receiver for a radio system has an antenna and blocking filter connected to an amplifier. The input signal is split and mixed with an in-phase signal and a quadrature phase signal generated by an oscillator in a mixer circuit. An output from each mixer circuit is applied to a low pass filter and to an input of a limiting circuit. The output from each low pass filter is applied to respective circuits, a first of which is arranged to sum the in-phase and quadrature phase signals, and the second of which is arranged to subtract the in-phase and quadrature phase signals to generate a respective output signal having an axis intermediate the in-phase and quadrature phase signal axes. These signals, together with the in-phase and quadrature phase signals are passed through a limiting circuit respectively, to a decoder circuit for recovering the data. The output of the limiting circuits represent signals quantized to eight possible phase states separated by 45.degree.. In an alternative embodiment a ratiometric combiner may be used instead of the summation circuits, particularly for .pi./4-DQPSK modulation, where the phase excursion per symbol is .+-.45.degree. or .+-.135.degree. depending upon the bit pair combination. In the latter embodiment a minimum of eight axes are generated giving sixteen sectors and 22.5.degree. phase resolution.

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First Claim