Stabilizing a variable frequency oscillator (VFO) can be done by using an additional circuit when the VFO has an input through which its frequency can be controlled.
Luckily, most VFOs are varicap controlled.
The signal coming from the VFO is sampled and processed by the stabilizer circuit featured here. The amplified signal is then fed to flip-flop 1 (FF1). This flip-flop is
controlled by a clock signal coming from 4060. The clock frequency is determined by a jumper joining points A and C. The FF1 is also controlled by another signal with
a frequency which is 1/4 of the clock frequency. To achieve this, a jumper is needed between the point B and C but always one stage lower than the points C and A.
When the input signal is stabilized, the LEDs D6/D/ are off. Otherwise, a LED lights up and shows the direction of the signal shift. When the input signal is not stabilized,
a difference voltage appears at the ouput which can be used as correction control for the main VFO. This VFO stabilizer circuit can operate up to 100 MHz.
Electronic Circuits volume 1.0 - Circuit Nr. 43
The complete data of the electronic circuit described above can be found in the following book and is available from Amazon.com.
Click on the image to view the book.