Popcorn Radio Electronics and Radio Astronomy
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.Kostas,Greece.
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.Best! Todd