The Related Knowledge of Impedance Matching

- Oct 26, 2018-

1. What is impedance?

In electricity, the hindrance to the current in the circuit is often called impedance. The impedance unit is ohms, usually expressed as Z, which is a complex number Z = R + i ( ω L - 1 / (ω C)). Specifically, the impedance can be divided into two parts, a resistor (real part) and a reactance (imaginary part). Among them, the reactance includes capacitive reactance and inductive reactance, and the current obstruction caused by the capacitor is called capacitive reactance, and the current obstruction caused by the inductance is called inductive reactance.

2. The importance of impedance matching

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

(1) Adjust the load power

Assuming that the excitation source is fixed, the power of the load is determined by the impedance matching of the two. For an idealized pure resistance circuit or low frequency circuit, the reactance value caused by the inductor and capacitor is basically negligible. At this time, the impedance source of the circuit is mainly resistance.

(2) Suppressing signal reflection

Reflection occurs when a beam of light is directed from the air into the water because of the different light guiding properties of light and water. Similarly, reflections can occur when a characteristic impedance change occurs on the transmission line during signal transmission. The wavelength is inversely proportional to the frequency. The wavelength of the low frequency signal is much larger than the length of the transmission line, so the reflection problem is generally 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.

3. Impedance matching method

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 to adjust the load impedance value through series and parallel connection of capacitor, inductor and load to achieve source and load impedance matching.

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

4. Impedance matching application

Whether it is fixed impedance or a constant voltage output amplifier, the best working condition can be obtained only when the total power of the speaker and the total power of the power amplifier are equal. It is very difficult for the speaker system to fully match, and its audio components are always changing, but the speaker system does not require high impedance matching. The most common horn impedance is 8 ohms, which means that when a 1 kHz sine wave signal is input, it exhibits an impedance value of eight ohms; or within the operating frequency response range of the horn, the average impedance is 8 ohms.

 

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