a self - tuning filter that is well suited for use as a output digital filter in a direct conversion delta - sigma transmitter is constructed as a high pass finite impulse response filter having a cutoff frequency of twice the desired carrier frequency . the filter is clocked using the same clock as used for the commutatation within the transmitter . the aliasing effect of the digital filter produces a passband centered around the carrier frequency which allows the information contained in the spectrum around the passband to be transmitted , while effectively filtering out the quantization noise produced by the commutation . when the commutator clock frequency is changed in order to change the carrier frequency , the passband automatically moves to track the new carrier frequency . the output filter may be constructed using series connected flip - flops with analog taps and an analog summer connected to respective q and q outputs , thereby producing an analog output .