DIGITAL FILTER

PURPOSE:To control the pass band of a digital filter over a wide range only by changing a sample frequency, by controlling a cut-off frequency of a low-pass filter for AD and DA conversion proportional to the sampling frequency. CONSTITUTION:An analog signal is inputted from a terminal 1, the band is limited through a low-pass filter section 2 located at the pre-stage so that the frequency reflexing due to sampling is not produced, and converted into a discrete data with equal interval at an AD converting section 3, and the filter transfer function is operated at a digital filter operating section 4. The digital converted signal is inverted into an analog signal at a DA convering section 5, a harmonic component is eliminated at a low-pass filter 6 located at the post- stage, and the result is outputted to a terminal 8. Since a control section 7 generates a sampling pulse S1, cut-off frequency control signals CS1, CS2 proportional to the sampling frequency and a sequence control signal S2, the pass band is controlled only by changing the sampling frequency.