This 80's Exar circuit has a carrier detector for frequencies up to 500 kHz. The circuit uses a company 567 which is still a common component.
This circuit was suggested by General Electric in a 1969 documentation. The transistors are of the...
This QST circuit generates 4 frequency signals according to the crystals used. Transistors admit...
This 4-20 mA current interface was found on a 90's EDN. The components used are not common today and...