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Characteristic impedance of transmission lines, also known as the characteristic impedance, is a high-speed circuit design concept during our time often mentioned. But many people do not understand this concept, sometimes the wrong understand DC impedance. We understand this concept better high-speed circuit design is necessary. Many high-speed circuit design rules and characteristic impedance are related.
To understand the concept of characteristic impedance, we need to figure out what is the transmission line. Briefly, the transmission line is capable of transmitting a signal cable. Power line, video cable, USB cable, the PCB traces, can be referred to as a transmission line. If the signal transmission line transmission is the low-frequency signal, assumed to be 1KHz , then the wavelength of the signal is 300 km (assuming the signal speed is the speed of light), even if the line length is 1 meter long, the signal is with respect is very short, the signal it can be seen as short-circuited transmission line, transmission line effects on the signal is very small. But for high-speed signal, if the signal frequency up to 300 MHz , the signal wavelength is reduced to 1 m, this time the wavelength of 1 m and a signal transmission line has been completely comparison , it will ripple effect in the presence of the transmission line, the transmission line voltage and current on the different points will be different. In this case, we can not ignore the impact of the transmission line on the signal caused. It is a signal transmission line is relatively long period, we use the long-term transmission in theory to solve the problem.
Characteristic impedance belongs to a long transmission concept. Signal during transmission over the transmission line, in one point of arrival of the signal, will form an electric field between the transmission line and the reference plane, due to the presence of an electric field, produces a small current a moment, each point of the small current exists in the transmission line . At the same time there is a certain voltage signal, so that the signal transmission process, the transmission line at each point will be equivalent to a resistance, the resistance of the transmission line is the characteristic impedance we mentioned . Here we must distinguish a concept that is the characteristic impedance for an AC signal (or a high-frequency signal), it is , for the DC signal , the transmission line has a DC impedance , this value may be far smaller than the characteristic impedance of the transmission line . Upon determining the characteristics of the transmission line (characteristic line width, and the distance between the reference plane and the like), the characteristic impedance of the transmission line is determined. Omitted here equation derivation of a million words, the characteristic impedance is given directly PCB trace formula :
Where L is the self-inductance per unit length of the transmission line, C is the inherent capacitance per unit length of the transmission line. Sure some people will ask, what is a unit of length? Is 1cm, 1mm, or 1mil? In fact, here is the number of unit length is not important. The smaller units higher precision, learned calculus of this concept should be even more clear. Through this simple formula we can see that, to change the characteristic impedance of the transmission line will change inherent inductance and capacitance per unit length of the transmission line . So that we can better understand the several factors affecting the characteristic impedance of the transmission line:
a. linewidth is inversely proportional to the characteristic impedance . Corresponds to the line width is increased to increase the capacitance, the characteristic impedance will decrease, and vice versa
B. Dielectric constant is inversely proportional to the characteristic impedance . Also increase the dielectric constant to increase the capacitance equivalent
C. Transmission line and the distance to the characteristic impedance is proportional to the reference plane . Increasing the distance and the transmission line reference plane equivalent capacitance is reduced, so that the characteristic impedance will decrease, and vice versa
D. Length and characteristic impedance of the transmission line does not matter . Can be seen are the parameters L and C per unit length of the transmission line by the equation, the length of the transmission line does not matter.
E. diameter and inversely proportional to the characteristic impedance . Due to the skin effect of high-frequency signals, affecting small compared to other factors.
Following is a brief talk about transmission line impedance characteristics we often hear is 75 ohms and 50 ohms. Why are these two values, rather than the other values? This is the people to choose two numbers in engineering practice. On the coaxial cable, the inner and outer conductor having the maximum diameter of the power transmission capability than the wire of 1.65, corresponding to this time impedance is about 30 ohms. However, the impedance caused by low signal attenuation is relatively large, considering the attenuation factor of the cable, the impedance of 77 ohms when the minimum attenuation coefficient, for convenience in engineering calculations, calculated to take a value of 75 ohms characteristic impedance, to achieve good attenuation coefficient reduces signal attenuation. If you take the power transfer capability and attenuation coefficients do consider a compromise, then it was 50 ohms, which is easy to calculate the value in engineering. That either 75 ohms or 50 ohms are artificial and consideration of various factors of a compromise choice .
In the actual design of a PCB, a characteristic impedance of a variety of methods. Most EDA design tools will bring their own characteristic impedance calculation tool. Further, a recommended Polar SI9000, small software can easily calculate the characteristic impedance of the transmission line, comprising a single-ended and differential traces traces, etc., high precision calculation, many PCB MILL manufactured by using this tool will feature impedance calculation.
