Found in American documentation from 1968, this circuit has an acute cut-off of the frequency to which it is tuned and which depends on RC. Transistors may be modern equivalents of general use.
The figure shows how to implement an alarm buzzer in an LM3914 to warn about crossing the scale...
This circuit can act as a shield being controlled by a microcontroller by pin 1 or 2. The...
This circuit was obtained from a 1980's QST magazine. The circuit can generate 1 to 15 MHz signals...