Why perform impedance matching

First, what is the impedance?

In terms of power, the obstruction of current in the circuit is often referred to as impedance. The unit of impedance is ohms, usually denoted by Z, and is the complex number Z = R + i(ωL-1 /(ωC)). Specifically, the impedance can be divided into two parts, a resistor (real part) and a reactor (imaginary part). Among them, the reactance includes capacitive reactance and inductance, and the current blocking caused by the capacitor is called capacitive reactance, and the current blocking caused by the inductance is called inductive reactance.


Second, the importance of impedance matching

Impedance matching refers to the proper match between the signal source or transmission line and the load. Impedance matching has two main functions, namely adjusting the load power and suppressing signal reflection.

1, adjust the load power

Assuming the excitation source is fixed, the power of the load is determined by the impedance matching of the two. For an idealized purely resistive or low frequency circuit, the reactance values caused by the inductor and capacitor are essentially negligible. At this point, the impedance source of the circuit is primarily a resistor. As shown in Figure 2, the current I = U / (r + R) in the circuit, the load power P = I * I * R. From the above two equations, P can get the maximum value of R = r, Pmax = U * U / (4 * R).

2, suppress signal reflection

Due to the different light guiding properties of light and water, reflection occurs when the light beam is introduced into the water from the air. Similarly, reflection occurs when a characteristic impedance change occurs on the transmission line during signal transmission. The wavelength is inversely proportional to the frequency, and the wavelength of the low frequency signal is much larger than the length of the transmission line, so the reflection problem is usually not considered. In the high frequency field, when the wavelength of the signal is the same as the length of the transmission line, the reflected signal is easily aliased with the original signal, affecting the signal quality. High frequency signal reflection can be effectively reduced and eliminated by impedance matching.


Third, the method of impedance matching

There are two main methods of impedance matching, one is to change the resistance of the group, and the other is to adjust the transmission line.

Changing the impedance is by adjusting the load impedance value by series and parallel capacitors, inductors and loads to achieve source and load impedance matching.

The transmission line is adjusted to extend the distance between the source and the load and to adjust the impedance to zero through capacitance and inductance. At this point, the signal is not emitted and energy can be absorbed by the load. In high-speed PCB layouts, the trace impedance of digital signals is typically designed to be 50 ohms. The coaxial cable baseband is typically specified as 50 ohms with a frequency band of 75 ohms and a twisted pair (differential) of 85-100 ohms.