We found this circuit in an old publication. The components used are rare today. Accuracy depends on the resistors in the input selection network. The circuit operates from 10 to 500 kHz.
This circuit uses an LED (called SSL) to regulate current in a configuration with complementary...
A dual function using common transistors and diodes can be implemented as shown in the figure...
Found on a 1964 CQ this circuit is intended for tone frequency change modulation (FSK) telegraphic...