Patent ID: 8385458

Claim:
A signal processing circuit for compensating a quadrature modulator for an in-phase/quadrature (I/Q) amplitude mismatch which involves mismatching amplitudes of I/Q-components of an output signal from said quadrature modulator, and for an I/Q phase mismatch which involves a deviation from 90 degrees of a phase difference between the I/Q-components of the output signal from said quadrature modulator, said signal processing circuit comprising: an I/Q mismatch compensating part for correcting an input signal for amplitude or phase, based on a compensation amount for compensating for the I/Q amplitude mismatch or the I/Q phase mismatch, and applying the input signal with the corrected amplitude or phase to said quadrature modulator; a test signal generating part for sequentially generating two sets of combinations of I/Q-components of test signals which are alternating current (AC) signals and applying the test signals to said I/Q mismatch compensating part; a detector for detecting the amplitude of an envelope of an output signal from said quadrature modulator, wherein said detector outputs a signal proportional to first to third power of the amplitude of the envelope of the output signal from said quadrature modulator; a filter for passing therethrough a signal at a cutoff frequency or lower within the output signal from said detector; and a control part for deriving a compensation amount for the amplitude or phase of said I/Q mismatch compensating part such that output values of said filter are equal when the two sets of test signals are respectively generated, and setting the compensation amount in said I/Q mismatch compensating part; wherein: said test signal generating part sequentially generates a first test signal which comprises an in phase component (I-component) that is an AC signal having a first amplitude value, and a quadrature component (Q-component) that is an AC signal in phase with the I-component and having a second amplitude value, and generates a second test signal which comprises an Q-component that is an AC signal having the first amplitude value, and an I-component that is an AC signal in opposite phase with the Q-component and having the second amplitude value; said control part derives the compensation amount such that output value 1 of said filter when the first test signal is generated is equal to output value 2 of said filter when the second test signal is generated; and said detector outputs a signal proportional to 1.5th to 2.5th power of the amplitude of the envelope of the output signal from said quadrature modulator when the output level of the output signal of said quadrature modulator is in a range of −3 dB to +3 dB.