Engineers in the electronics industry often encounter impedance matching problems. What is impedance matching and why should impedance matching be carried out? This article takes you to explore unexpectedly!
I. What is impedance?
In electricity, the impedance is often called the impedance, which hinders the current in the circuit. The impedance unit is ohm, commonly expressed as Z, which is a complex number Z= R+i (_L-1/(_C)). Specifically, impedance can be divided into two parts, resistance (real part) and resistance (imaginary part). Among them, reactance includes inductance and inductance. The current impedance caused by capacitance is called capacitance impedance, and the current impedance caused by inductance is called inductance.
What is the significance of impedance matching?
Importance of impedance matching
Impedance matching refers to a suitable match between signal source or transmission line and load. Impedance matching has two main functions: adjusting load power and suppressing signal reflection.
1. Adjusting Load Power
Assuming that the excitation source is determined, the load power is determined by the impedance matching between them. For an idealized pure resistance circuit or low frequency circuit, the reactance value caused by inductance and capacitance can be neglected basically. At this time, the impedance source of the circuit is mainly resistance. As shown in Figure 2, current I = U/(r+R) and load power P = I*I*R in the circuit. From the above two equations, P gets the maximum value when R = r, Pmax = U*U/(4*r).
2. Suppression of signal reflection
When a beam of light shoots from air into water, it reflects, because light and water have different photoconductive properties. Similarly, if the characteristic impedance of the transmission line changes abruptly in signal transmission, reflection will also occur. The wavelength is inversely proportional to the frequency, and the wavelength of the low frequency signal is much longer than the length of the transmission line, so the reflection problem is generally not considered. In the high frequency domain, when the wavelength of the signal and the length of the transmission line are of the same magnitude, the reflected signal is easy to overlap with the original signal, which affects the quality of the signal. 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 group resistance, the other is to adjust the transmission line.
To change the impedance force is to adjust the load impedance by series and parallel connection of capacitors, inductances and loads in order to match the source impedance with the load impedance.
To adjust the transmission line is to lengthen the distance between the source and the load, and to adjust the impedance force to zero with capacitance and inductance. At this time, the signal will not emit, and the energy can be absorbed by the load. In high-speed PCB routing, the routing impedance of digital signal is usually designed to be 50 ohms. It is generally stipulated that the baseband of coaxial cables is 50 ohms, the frequency band is 75 ohms, and the pair of strands (differential) is 85-100 ohms.
IV. Application of Impedance Matching
1. Power Amplifier and Speaker
Whether constant impedance power amplifier or constant voltage power amplifier, only when the total power of the loudspeaker and the total power of the amplifier are equal, can the optimal working state be obtained. It is very difficult for the speaker system to achieve complete matching. Its audio components are always changing. Fortunately, the speaker system does not require high impedance matching. The most common indication of horn impedance is 8 ohms, which indicates that when input a sinusoidal wave signal of 1KHz, it presents an impedance value of 8 ohms, or within the range of the horn's frequency response, the average impedance is 8 ohms.