Design criteria for low-level second-harmonic magnetic modulators
Earl J. Kletsky · Transactions of the American Institute of Electrical Engineers Part I Communication and Electronics · 1959
Major difficulties are encountered when d-c amplifiers are used to amplify low-level low-frequency signals. These difficulties are mainly problems of zero-drift and noise. One of the most successful techniques in circumventing the drift and noise problems is to convert the low-frequency intelligence to a frequency permitting conventional a-c amplification. The means for accomplishing the frequency conversion have been mainly mechanical: choppers, vibrating capacitors, and similar devices. These devices, by their very nature, must be operated at relatively low sampling rates and are subject to the usual failures of moving equipment.