DEMODULATOR FOR DIGITALLY MODULATED WAVE

摘要

A QPSK modulated wave is inputted to an in-phase detector and an orthogonal detector. The detected components are converted to substantially a base band, and each component is digital-converted by A/D converters. Each digital component is spectrum-shaped by digital LPFs. The outputs of digital LPFs are inputted to a complex multiplier and calculated by use of first and second reproduction carriers and expressed as first and second calculation outputs, and inputted to a phase detector. The phase detector obtains phase difference data between the phase expressed by the first and second calculation outputs and a predetermined phase and quadrant data of the phase. The phase difference data is used for a PLL. The phase difference data is inputted to a frequency error detection circuit detecting a frequency error. The frequency error output is smoothed by a filter of an AFC loop, and used as a control signal controlling the oscillation frequency of the local oscillation unit. If the frequency error is large, the frequency error detection circuit controls the AFC loop to be in an operation state and the PPL loop to be fixed state. If the frequency error is small, the frequency error detection circuit contains the control state of the AFC loop and switches the PLL loop to be the operation state.