To measure phase accurately in the presence of harmonics for both noisy input signals, a technique based adaptive bandpass filter (ABF) and the method of interchanging signal channel is presented.

A frequency translation circuit (172-188) abuts the multiple channel bandpass filtered analogue signals in the frequency domain without any guard bands between adjacent channel signals.

These narrowband multi-channel filters and narrow bandpass filter arrays show good filtering features and can be utilized in many optical applications.