Low-Level Low-Frequency Detection System

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Abstract
An amplifier and detector having application in a null detection system or as a servo amplifier in low‐frequency ac instruments is described. The design may be adapted to operate with a carrier frequency in the range from a few cps to a few kc. The unique and apparently unrecognized advantages of operating with a carrier frequency which is an even subharmonic of the principal coherent disturbing signal, such as the power line, are discussed in detail. Design considerations and performance data are presented, permitting an evaluation of the suitability of the device for a given application. For a particular input circuit configuration, it is conservatively estimated that a signal of less than 1.6×10−22 W from 100‐Ω source may be detected with an observation time of less than 1 min. Experiments indicate that, with this device, the sensitivity is primarily limited by thermal noise in the input and by induced signals in the input leads.

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