Fade-canceling zero beat indicator for reception of standard radio-frequencies

Abstract

A simple circuit is described which enables a local secondary frequency standard to be set quickly to a definite zero beat with a received short-wave standard frequency signal, without the usual uncertainty due to amplitude fading of the short-wave signal. In principle, two identical receivers, both fed by the same antenna, are used. One receiver picks up WWV, for example, and a weak signal from the local standard. The other picks up WWV either alone or together with a second weak signal from the local standard which is 180° out of phase with the first. The outputs of the detectors of the two receivers are subtracted, leaving only the beat note, which is recorded on a strip chart. In practice, one unmodified AM receiver is used on a time-sharing basis by means of a chopper. The band width of the device may be made much less than one cycle per second, and therefore, it is particularly useful when the WWV signal is reasonably stable in phase but far too weak for an audible zero beat to be achieved. Alternatively, the device may be regarded as a simple means of observing fast Doppler shifts of a received standard frequency signal. A schematic diagram and sample recorder charts are included. © 1957 The American Institute of Physics.