Patent ID: 8199864

Claim:
A Quadrature Phase Shift Keying (QPSK) demodulator of a digital broadcast reception system, the QPSK demodulator comprising: an in-phase/quadrature (I/Q) detector generating a baseband I signal and a baseband Q signal by multiplying a modulated QPSK signal by an I/Q carrier signal, wherein the modulated QPSK signal has been generated by performing QPSK modulation on original data; a bandwidth adjustable first filter receiving the baseband I signal and the baseband Q signal generated by the I/Q detector, rejecting imaginary noise included in the received signals and outputting a filtered signal; and a data restoration unit restoring the original data from the filtered signal output from the bandwidth adjustable first filter, wherein the I/Q detector comprises: an analog-to-digital converter (ADC) converting a modulated analog QPSK signal to a digital QPSK signal and outputting the digital QPSK signal; a first Direct Current Removing Module (DCRM) removing a DC component from the digital QPSK signal output from the ADC and outputting the DC-removed digital QPSK signal; a digital automatic gain controller (DAGC) maintaining a signal level of the DC-removed digital QPSK signal at a constant rate by automatically adjusting a gain of the DC-removed digital QPSK signal output from the first DCRM and outputting a gain-adjusted digital QPSK signal; an oscillator adjusting a frequency of the I/Q carrier signal to a frequency offset value detected by the data restoration unit and generating an I/Q carrier signal having the adjusted frequency; and an I/Q separator generating the baseband I signal and the baseband Q signal by multiplying the gain-adjusted digital QPSK signal output from the DAGC by the I/Q carrier signal having the adjusted frequency.