10 58 to 10 74 MHz VFO Oscillator Circuit

Figure 1, below, shows the 10.58 to 10.74 MHz VFO oscillator circuit. Yes, this is the same circuit that was presented last time, but I’ve re-drawn it to show the Colpitts oscillator more clearly. “L” is approximately 1.5 uH; 19 turns in a T50-6 core (yellow). The value of C6 is found experimentally. I used 69 pF (a 47 pF and a 22 pF in parallel). More about this during check-out. 

“L” and the V V C along with C6 make up the parallel-tuned circuit that determines the frequency of the VFO. The V V C is an MV2104. Capacitors marked with an asterisk must be silvered mica, COG, or NPO.

You might wonder how the component values for the resonant circuit are determined. Well, when Jupiter is aligned with Mars, and Mercury is on the cusp of a new moon, if the inductive reactance equals the capacitive reactance, then we have a resonant circuit. For any given frequency there are endless combinations of L and C that will produce a resonant circuit. Experience shows that variable capacitors and V V C’s in the 5 pF to 300 pF range are altogether practical. Combine this with the fact that a 1.5 uH coil and a 150 pF capacitor resonate near the frequency of interest, and you have the component values for this VFO’s resonant circuit. You can find formulas for figuring all this out in your Handbook, if you like to do arithmetic. The component values actually used in the circuit take into account “stray” inductance and capacitance that occurs in the real world. More about this during check-out.

RESONANT CIRCUITS are fascinating critters! While I’m NOT going to get into a theoretical discussion, there are some fundamental characteristics of resonant circuits that you (and I) should keep in mind when building VFO’s, filters, RF amplifiers, etc.

Two types of resonant circuits are commonly used: series resonant circuits and parallel resonant circuits, as shown below. Here are two important things to keep in mind: PARALLEL resonant circuits attenuate the flow of AC current at the resonant frequency (and pass all other frequencies); SERIES resonant circuits pass AC current at the resonant frequency (and attenuate all other frequencies).