# Series capacitance added to parallel resonant tank changing the resonant freq?

## Main Question or Discussion Point

HI all,

A parallel tank circuit at resonance has a theoretical infinite impedance. However due to the resistance of L and C some power is dissipated so a small current will flow from the source.

If a parallel tank circuit is oscillated at it's resonant frequency and a capacitor (C2) is added in series the circuit's impedance will change. But will it change the resonant frequency?

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If the circuit has a diode (D1) in it the parallel tank will still resonante since the resonance takes place between the parallel L and C. So if a capacitor (C2) is added to the circuit will the resonant frequency change?

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vk6kro
Putting a capacitor in series with the parallel tuned circuit will not change the parallel resonant frequency, but it will introduce a new series resonance at a lower frequency.

To see an effect, though, you would need to add a suitable resistor in series with the signal source.

I simulated it with 5000 ohms in series with your circuit. Both capacitors were 64 pF and the inductor was 16 µH.

Parallel resonance was at 5 MHz and series resonance was at 3.5 MHz

Adding a diode will cause C2 to charge up and reverse bias the diode for most of the input waveform. So there will be little or no output.