Phase-Sensitive Detection for Direct Measurement

To understand the advantages inherent in this method, it is appropriate to discuss briefly the detection technique. 

Phase-sensitive detection may be accomplished by the sequential operation of multiplexing and time-averaging circnits. The multiplexer serves effectively to multiply the input sine wave e, with a reference square wave e f. We can represent Cref in terms of its Fourier components 

In normal practice, cOr and y, are derived from a common source (i.e. oOr = to,), 

Only the first term in Eq. (22) is time-independent and, when applied to the time-average circuit, will result in a nonzero output 

This is obviously a phase-sensitive dc ontpnt voltage, which is a maximum at ( = 0. 

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