an on - chip semiconductor pre - filter device is designed with a prescribed low frequency passband and a prescribed stopband , with a frequency roll - off therebetween . the pre - filter performs a weighted averaging of signal samples taken from an input electrical signal at a suitable sampling frequency and converts the weighted average into an equivalent charge packet for input by injection into a semiconductor charge coupled device driven on the same chip by a clock of frequency f . in this way , the &# 34 ; reflected &# 34 ; parts of the frequency spectrum of the ccd are suppressed from the input to the ccd , thereby suppressing undesired aliasing in the ccd .