Thursday, 1 January 2015

21.4 MHz VCO frequency modulated with 1 KHz

Testing -- first personal video upload to YouTube. (Content from 2013)


  1. Hi Vasily,
    Nice schematic,as I see it's a series tuned colpitts oscillator.The two MV-209 varicap helps the stability of the circuit or with the 150μH choke are a kind filter?
    A question,
    I'm going to assembly soon,can you tell me how to apply AF signal?
    Thanks for your help and your time.

  2. Greetings and thanks!
    The 150 µH RFC exhibits high reactance (~ 20 k ohms) to the ~ 21.4 MHz carrier frequency to prevent the carrier signal from leaking into the modulator circuits. The 1 µF series C does the same --- this time 7.4 Meg ohms X )
    Further, the 100K decoupling resistor serves the purpose; except it's for both RF + AF (has RF and AF bypass after it].

    The swinging AF AC signal causes the varactor capacitance to vary and in tandem, this varies the deviation of the carrier frequency.

    An op-amp mic amplifier with low output Z with a series output "build out" resistor such as 1K has worked for me. A transistor mic amp also worked fine in the "old days". Adjustable drive.