referred to as a varactor diode or varactor diode varactor diode 1

Also known as a varactor diode or varicap diode, this tunable circuit utilizes a varactor diode to provide voltage-controlled capacitance. Its advantages in many applications lie in its compact size and low cost. Compared to manually adjusted variable capacitors, varactors have drawbacks such as lower Q-factor, nonlinearity, limited voltage ratings, and a narrower tuning range. You can find background reference materials on varactor diodes. The schematic above shows a basic circuit for testing varactor diodes. A 1M resistor, the circuit isolation, and a DC voltage source are key components. A 0.1uF capacitor blocks the DC bias voltage. I happen to have a 10uH inductor on hand, along with resistors that look like fat resistors, connected via a 220pF capacitor for high-frequency RF signal coupling and unmodulated output. While we cannot measure the varactor diode's capacitance precisely due to stray capacitance in the circuit, we can certainly observe its behavior.diode

Another method closely related to the practical application of variable capacitance is using a capacitance meter, as it does not yield satisfactory results: we can adjust the frequency to find the maximum while observing the voltage of the entire tuning circuit with an oscilloscope. After obtaining the resonant frequency, we then calculate the capacitance and plot a capacitance-voltage curve. You will notice a nonlinear curve, but it indicates a linear relationship between voltage and frequency.diode