Understand the Q and D values ​​of the capacitor

When doing RF, choose the inductor and capacitor to pay special attention to their Q value, then what is the Q value? What does the Q value mean and why is it important?

Quality factor Q: A quality indicator that characterizes an energy storage device (such as an inductor, capacitor, etc.), the energy stored in the resonant circuit, and the energy lost per week. The larger the Q value of a component, the better the selectivity of the circuit or network formed by the component.

Or Q = reactive power / active power, or the ratio of characteristic impedance to loop resistance.

The higher the Q value, the smaller the loss and the higher the efficiency;

The higher the Q value, the higher the frequency stability of the resonator and, therefore, the more accurate. The factor Q is an important parameter representing the quality of the coil. The size of the Q value indicates the loss of the inductor coil. The larger the Q value, the smaller the loss of the coil; otherwise, the greater the loss.

The quality factor Q is defined as the ratio of the inductive reactance exhibited by the coil to the DC resistance of the coil when the coil is operated at an AC voltage of a certain frequency. It can be expressed as follows:

Where: W - working angle frequency L - coil inductance R - total coil loss resistance

The requirements for the quality factor Q vary depending on the application. For the inductive coil in the tuning loop, the Q value is higher, because the higher the Q value, the smaller the loss of the loop, the higher the efficiency of the loop; the Q value can be lower for the goose coil; Or high frequency chokes, you can not do so.

In fact, the increase in Q value is often limited by factors such as the DC resistance of the wire, the dielectric loss of the bobbin, the loss caused by the core and the shield, and the skin effect at high frequencies. Therefore, the Q value of the coil cannot be made very high, usually the Q value is tens to 100, and the highest is only four or five hundred.

The Q value is equivalent to the reciprocal of the D value. The loss angle is the D value: Generally, the electrolytic capacitor has a high D value because of its large internal resistance. The specification depends on the capacitance value, which is 0.1-0.24 or less. The plastic film capacitor has a lower D value, which is determined by the material to be 0.001- Below 0.01. Ceramic capacitors are determined by their materials. Hi-K type and S/C type are below 0.025. The specifications of T/C type are higher than 400-1000 in terms of Q value.