Frequency stabilizing circuit for a .pi./4 shift QPSK signal rec

Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Synchronizing

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Details

327156, 329306, H03D 304, H03L 718

Patent

active

054402590

ABSTRACT:
A frequency stabilizing circuit for a .pi./4 QPSK (Quadrature Phase Shift Keying) signal receiver has a voltage controlled oscillator (VCO) for outputting an oscillation signal whose frequency changes in response to a control voltage applied to the VCO. A mixer changes the frequency of an input .pi./4 shift QPSK signal. A counter measures the frequency of the .pi./4 shift QPSK signal from the mixer. A timing generator generates, on the basis of the oscillation signal from the VCO, a timing indicative of an interval during which the counter is to measure the frequency. A set data generator generates a frequency signal having a predetermined set frequency. A comparator compares the frequency measured by the counter and the set frequency of the set data generator to thereby output the resulting frequency deviation. A voltage data generator changes voltage data in association with the frequency deviation. A digital-to-analog converter transforms the voltage data to the control voltage. A demodulator demodulates the .pi./4 shift QPSK signal from the mixer. An integrator adds up phase shifts output from the demodulator over the interval generated by the timing generator. An adder/subtracter selectively increases or decreases the set frequency of the frequency signal output from the data generator in association with a cumulative phase shift produced by the integrator. The circuit is capable of stabilizing the frequency of a .pi./4 shift QPSK signal receiver even when the mean frequency differs from the carrier frequency due to the offset of data.

REFERENCES:
patent: 4773083 (1988-09-01), Baumbach et al.
patent: 5097220 (1992-03-01), Shimakata et al.
patent: 5121070 (1992-06-01), Tomita

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