Detection method and application of varactor diode

1. Discrimination between positive and negative poles: Some varactor diodes have a black mark on one end, which is the negative pole, and the other end is the positive pole. Some varactor diodes have yellow and red rings painted on both ends of their casing, with one end of the red ring being the positive electrode and the other end of the yellow ring being the negative electrode.

You can also use the diode mode of a digital multimeter to determine the positive and negative polarity of a varactor diode by measuring its forward and reverse voltage drops. When measuring the forward voltage drop of a normal varactor diode, the reading on the meter is 0.58~0.65V; when measuring the reverse voltage drop, the reading on the meter displays the overflow symbol "1". When measuring the forward voltage drop, the red probe is connected to the positive terminal of the varactor diode, and the black probe is connected to the negative terminal of the varactor diode.

2. To determine the performance, measure the forward and reverse resistance values of the varactor diode using the R × 10k range of a pointer multimeter. A normal varactor diode has both forward and reverse resistance values of ∞ (infinity). If both the forward and reverse resistance values of the tested varactor diode have a certain resistance value or are both 0, it is due to leakage or breakdown damage of the diode.

A varactor diode is an electronic variable capacitor. In other words, the capacitance exhibited by a varactor diode is a function of the reverse bias potential. This phenomenon has led to several common applications of varactor diodes in situations where capacitance factors need to be considered. Figure 1 shows a typical LC oscillator circuit tuned with a varactor diode. The function of the circuit coupling inductor l2 is to input the RF signal into the oscillation circuit when it is used as an RF amplifier. The main LC oscillation circuit includes a main inductor l1 and a series capacitor c1 and cr1. In addition, it is also necessary to consider the stray capacitance CS widely present in electronic circuits. The functions of DC blocking capacitors and series resistors have been introduced earlier. The function of capacitor c2 is to filter the tuning voltage vin.   

The tuning circuit of the variable capacitance diode in the diagram

Because the resonant frequency of the LC tuned oscillating circuit is a function of LC, we found that the ratio of the maximum to minimum resonant frequencies of the oscillating circuit varies with the square root of the capacitance ratio. The capacitance ratio here refers to the ratio of the capacitance at the minimum reverse bias voltage to the capacitance at the maximum reverse bias voltage. Therefore, the tuning characteristic curve of the circuit (bias voltage resonant frequency) is basically a parabola.