# Resonant frequency of a tuning circuit with varactors

This is the circuit that I'm simulating in LTSpice:

I measured the experimental resonant frequency of the circuit by finding the frequency at which $$\V_\text{out}\$$ peaks for different values of $$\V_1\$$, as shown below.

I then found the theoretical resonant frequencies of the circuit by finding the capacitance of the diodes using the capacitance vs reverse voltage curves provided by the datasheet for the MBRS360 diode (shown below) for each value of $$\V_1\$$ and evaluating the following equation using $$\C=\$$ 1/2 of that value (since the two back-to-back Schottky diodes that are being used as varactors here would be the equivalent of two capacitors in series) and $$\L=\$$ 1 mH:

$$f=\frac{1}{2\pi\sqrt{LC}}$$

However, when $$\V_1\neq\$$ 0 V, there were fairly significant discrepancies between the theoretical and experimental resonant frequencies found, as evident in the table below. Would someone know why this is happening?

• You're asking much from a LTspice model. My default diode library includes only two varactors - of these, I'd expect model parameters to be optimized for capacitance versus DC voltage. Yes, I've used 1N4000-series diodes as varactors, even avalanche diodes and LEDs, but I'd not expect their SPICE models to be accurate for this application. Dec 2, 2022 at 15:02

At higher applied AC voltages, one diode conducts for the positive half cycle and, the other diode conducts for the negative half cycle. This means that the effective varactor capacitance is altered by a factor of two because the conducting diode would be equivalent to a short-circuit or infinite capacitance. And, of course, as the control voltage rises from 0 volts this gets muddled up.

I'm unsure if you factored that complication into your calculations. Apart from anything else, the typical values in the data sheet are not going to be very accurate.