Square wave reference digital lock-in detection using non-orthogonal demodulation.

Digital lock-in detection technique is commonly used to measure the amplitude and phase of a selected frequency signal. The technique which uses a square wave as the reference signal has an advantage of easier implementation and higher computational efficiency compared to that uses a sine wave. However, one constraint for a square wave reference digital lock-in is that the sampling rate must be the integral multiple of 4 of all the signal frequencies. As the sampling clock may not always be able to set to the integral multiple of 4 of the signal frequencies, the constraint brings inconvenient for the implementation. Presented in this paper is a novel algorithm for square wave digital lock-in detection in which the sampling frequency doesn't have to meet the constraint. The algorithm allows frequency sweep measurements with a fixed sampling rate. For different relationships between the sampling rate and the signal frequency, different calculation equations are provided in this paper. Simulations and actual experiments show the feasibility of the proposed algorithm.