Classification of Crystal Diodes (2) 3

8. Varactor diodes are small power diodes used for automatic frequency control (AFC) and tuning. There are also many other names for Japanese manufacturers. By applying a reverse voltage, the electrostatic capacitance of its PN junction changes. Therefore, it is used for automatic frequency control, sweep oscillation, frequency modulation, and tuning purposes. Usually, although silicon diffusion diodes are used, special fabricated diodes such as alloy diffusion type, epitaxial bonding type, and double diffusion type can also be used because these diodes have a particularly large rate of change in electrostatic capacitance for voltage. The junction capacitor changes with the reverse voltage VR, replacing the variable capacitor, and is used as a tuning circuit, oscillation circuit, phase-locked loop. It is commonly used in the channel conversion and tuning circuit of TV high-frequency heads, and is mostly made of silicon materials.

9. For the frequency doubling effect of diodes, there are frequency doubling relying on varactor diodes and frequency doubling relying on step (i.e. rapid change) diodes. The varactor diode used for frequency doubling is called a variable reactor. Although the working principle of the variable reactor is the same as that of the varactor diode used for automatic frequency control, the structure of the reactor can withstand high power. A step diode, also known as a step recovery diode, has a short reverse recovery time trr when switching from conduction to shutdown. Therefore, its advantage is that the transition time to quickly turn off is significantly short. If a sine wave is applied to a step diode, the output waveform will be abruptly pinched off due to the short transition time (tt), resulting in the generation of many high-frequency harmonics.