Circuit selecting highest amplitude of two AC signals - uses open loop operational amplifier, to which are applied summed AC signals, with zero crossing counter and weak signal filter

摘要

Tram car communication is provided through the lines in information zones, each zone having a different carrier frequency. The circuit selects the carrier frequency which has the greater amplitude using simple level detector. The signals corresponding to two frequencies are summed by an open loop amplifier and the zero crossings at the output are counted. The counter output is used to switch into circuit an appropriate filter to reject the lower amplitude signal. The period of the square wave delivered to the counter is directly proportional to that of the signal having the greatest amplitude, assuming that the counter has a predetermined count capacity. Thus, the device remains insensitive to the weakest signal and the signal having the greatest amplitude is always considered to be the useful one.