filtered signal with a time synchronized reference voltage waveform and averages the resulting voltage the time average of randomly varying noise thus processed goes to zero after a long enough time whereas the true signal component which is synchronized with the reference waveform produces a dc output proportional to the desired signal component the reference waveform which is multiplied with the signal must have good time and polarity correlation with that of the signal the lockin amplifier incorporates circuitry to generate this reference wave form from a user supplied input reference voltage which has periodic rising or falling edges which have precisely the same period as the signal source excitation i e the same period as the square wave controlling the solenoid current to obtain optimum time overlap correlation between the signal and reference waveform a manual adjustment is provided to adjust the time delay between the reference voltage edges supplied and the symmetric reference waveform generated by the instrument a good procedure for making this adjustment is to process a strong representative signal while adjusting the time shift to maximize the averaged output this time shift adjustment and reference input are then left fixed and the signals of interest processed more particularly and as illustrated.