DIGITAL BAND-PASS FILTER

PURPOSE:To filter a signal in flatness up to a DC by adding a phase difference to a beat-down frequency signal and a beat-up frequency signal so as to eliminate the attenuation of a signal near DC. CONSTITUTION:Digital multipliers 201, 202 multiply the rear part cosomega1t and the imaginary part sinthetaw1t of beat-down frequency signals 208, 209 are multiplied in an input signal 200 and the result is fed respectively to digital filters 203, 204. Then the real part cos(omega1t+phi) and the imaginary part sin(omega1t+phi) of beat-up frequencies 210, 211 are multiplied by digital multipliers 205, 206 in the outputs of the filters 203, 204, the outputs of the multiplication are added by a digital adder 207 to form an output signal 212. Since the phase difference phi is added in this way, the signal near the DC is not attenuated. Thus, the signal is filtered in flatness up to the DC.