According to why you need to test the impedance mentioned here, if you need to get a qualified impedance, you need to perform impedance matching. If your USB interface is to be used for data transmission and the speed is in the high-speed range, you need to connect the USB interface data cable on the PCB For impedance matching, you can specifically design a differential impedance of about 90 ohm, which is only for a pair of lines that transmit data; if the speed requirement is not high, of course it is not a big problem without impedance, but in high-speed applications, stability and speed will have an impact Many high-speed signal lines, such as CVBS signal lines with a characteristic impedance of 75 ohms, LVDS data signal lines with a characteristic impedance of 100 ohms, and USB high-speed data lines with a characteristic impedance of 90 ohms. In the process of signal transmission, the path Each step has a corresponding transient impedance. If the transient impedance felt by the electrical signal transmitted along the interconnection line changes, then one part will continue to go on, and the other part will be reflected back to the source. Because the required characteristic impedance of each signal line is inconsistent, and the impedance of the signal source does not match. When the internal resistance of the source is less than the internal resistance of the transmission line, there will be ringing, that is, overshoot and overload the transmission line. Excessive overshoot tends to damage the device. And if the internal resistance of the source is greater than the impedance of the transmission line, undershoot will occur, which will cause the circuit logic to be in an indeterminate state, which may lead to misjudgment or signal loss.
