The 353 Phase-Lock Amplifier uses the principle of synchronous detection to obtain very narrow effective bandwidths and hence high noise rejection without tuned circuits. It detects the average value of the signal at the frequency and phase of an external reference signal to which the instrument is synchronized. The basic systems consists of a Type B1 Amplifier to provide signal amplification and a Type C1 Demodulator which contains both the synchronous switches and the reference signal phase-shifter.
