It is called a varactor diode or a varactor diode

Also known as a varactor diode or a varactor diode. The variable capacitance electronic control capacitor used in this tunable circuit is provided. What makes it advantageous in many applications is its small size and low price. If compared to manually controlled variable capacitors, this type of variable capacitor has disadvantages such as lower Q values, non-linearity, low voltage levels, and a more limited range. You can find background reference materials for varactor diodes. The above schematic diagram is used to test the basic circuit of a varactor diode. The key is to isolate the DC voltage source of the 1M resistor and variable capacitance diode circuit. A 0.1 uF capacitor prevents DC bias voltage. I happen to have a 10UH inductor in my hand, which looks like a fat resistor and is coupled and unmodulated output of an RF signal generator through a 220pF capacitor. We cannot accurately measure the capacitance of the varactor diode due to circuit stray capacitance, but of course, we can see its action.

Another method closely related to the application of variable capacitance is to use a capacitance meter, as it does not produce satisfactory results: we can change the frequency to find the maximum while observing the voltage of the entire tuning circuit and oscilloscope. After obtaining the resonance frequency, calculate the capacitance record and draw a capacitance voltage curve. You will find that there is a non-linear curve, but it implies a linear relationship between voltage and frequency diode