Found on a 1964 CQ this circuit is intended for tone frequency change modulation (FSK) telegraphic transmissions. It operates with signals of 2.975 and 2.125 Hz.
The circuit whose characteristics are given in the graph uses two transconductance operational amplifiers...
This circuit was found in the Radio Electronics magazine in an article about emitter-coupled...
This circuit was obtained from an 80s factory manual. The integrated circuits used are uncommon...