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.
Many still like to hear medium waves but live in places with difficult reception with many...
This common operational PLL circuit was found in a Popular Electronics 90’s. The power supply need...
In the figure we have a low-pass filter, with a cutoff frequency of 10 kHz, also suggested by...