Radio teletype detector circuit

Claim 1

A circuit for detecting the differential amplitude of a pair of frequency tones in a frequency-shift-keying receiver, comprising: a pair of filter circuits, each filter circuit responsive to a different one of said frequency tones, each of said pair of filter circuits including a filtering element of identical construction, one of said pair of filter circuits additionally comprising: means for heterodyning one of said pair of frequency tones to provide a heterodyned signal to the filtering element in said one of said pair of filter circuits. a pair of squaring and detecting circuits, each connected to the output of a different one of said filter circuits for squaring and detecting the presence of one of said frequency tones, said squaring and detecting circuits increasing the sensitivity of said differential amplitude detecting circuit to minor differences in amplitude of said pair of frequency tones, each of said squaring and detecting circuits comprising: a multiplier having two inputs, each of said inputs being coupled to the same output signal from its respective filter circuit; and a subtracting circuit responsive to the relative amplitude of the outputs of said pair of squaring and detecting circuits to produce a signal the polarity of which is determined by which of said pair of frequency tones has the higher amplitude.