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I am trying to design a variable frequency oscillator for use in a superheterodyne FM receiver and have been reading First Steps in Radio (http://www.arrl.org/first-steps-in-radio). In this example a varactor diode (D1) is being used as a variable-voltage capacitor to vary the resonant frequency of the circuit. I am having trouble understanding which capacitors in the overall circuit would be used in the resonance equation.

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Of course C1 and D1 would be included but what about C2? or C3. The same goes for calculating the bandwidth using the following equation.

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What resistances do I use.

Any sources to help me in understanding circuit design and to answer questions like these would be useful.

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  • \$\begingroup\$ by the way, the inductor shorts out the varactor bias \$\endgroup\$ – analogsystemsrf Aug 24 at 4:20
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Start with drawing the AC equivalent circuit, and see what capacitance(s) is in parallel with the tank. Don't forget to include the BJT internal capacitances (base to emitter, and base to collector) which play a role at higher frequencies.

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  • \$\begingroup\$ and the load resistance/capacitance \$\endgroup\$ – analogsystemsrf Aug 24 at 4:21

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