PCB Technical - How to draw performance of PCB copy board?

How to draw a good differential pair on PCB copy board:

Before describing the importance of differential alignment, we must first understand the factors that affect the impedance of the transmission line: line width, line length, line thickness, sidewall shape, resistive layer coverage and transmission line medium are obvious influencing factors. The dielectric constant and dielectric thickness will also affect the impedance accuracy of the transmission line. For the specific calculation formula, please refer to related books on signal integrity analysis.

Understanding these influencing factors can help us design good wiring, especially when we need to clear the line impedance, by adjusting the line width and thickness through related software, we can complete the fixed impedance of the line. A differential pair is called a differential pair because the signal sent above is equal to the difference between two complementary and mutually referenced signals, thus greatly reducing their external interference. The differential correspondence is routed in an appropriately close and parallel manner. The so-called appropriate proximity is because the interval affects the value of the differential impedance (differential impedance), which is an important parameter in the design of a differential pair.

Need to be wired in parallel. In the circuit design, if all signal lines use single-ended lines, please do the impedance design. Generally, once the signal lines at both ends of the ground potential are different, the system can work normally, even if the gap is relatively large, it will cause the system to fail to work properly., And using differential alignment or routing is an effective solution.

The PCB copy board has the same length and the same impedance of the differential line, and passes through the same environment, which has inherent advantages for stable signals.

In the schematic design of PCB copy board, the differential signal is usually marked with "_n" and "_p" as backward loss. The differential line can effectively solve the lack of a good reference connection between the signal source and the load, thereby suppressing the interference of electronic products And reduce the electromagnetic interference (EMI) generated by the signal line to the outside world. So why can differential lines effectively eliminate noise? First, let's take a look at the general requirements of the fabric differential line: the differential line is designed to ensure that the length of the two lines is equal, usually within 5%. There is a distance of 3w between the two differential lines, and the surrounding cladding around the differential line has good design experience.

In this way, on the one hand, the magnetic fields generated by the two signal lines of the differential line offset each other to reduce EMI. On the other hand, if the differential pair signal line introduces external noise interference signals at the same time, it can be very good because of different results. Ground noise is eliminated, which is similar to the traditional three-op amp. When drawing the wiring of the PCB differential pair, try to be in the same layer as much as possible. The differential alignment of the wiring layer will introduce impedance discontinuity due to the increase of holes. Secondly, if the layer is changed, the loop current will not have a good low-impedance circuit, and there will be an RF circuit. If the differential pair is longer, the common-mode RF energy will have an impact. Another reason is that the difference pairs have different signal transmission speeds between different board layers, and in the signal integrity analysis related data, it can be seen that the signal transmission is faster on the microstrip line than the stripline, which will also lead to A certain time delay. In terms of connectivity, you also need to pay attention to the connection problem of the differential pair. If the load is not a direct load but a capacitive load, you can introduce the connection problem of the differential pair.

In the PCB copy board circuit design, it is also necessary to pay attention to the impedance matching of the terminals to prevent reflection transmission and introduce EMI problems. Regarding terminal impedance matching, we usually use differential mode for differential signal transmission, and one transmission mode is common mode.

The reference design of its terminal resistance pairing is shown below

A method of drawing differential pairs with good performance

The impedance analyzer is usually used to measure and calibrate the size of the matching resistance. For our commonly used differential chip terminal matching resistance, usually 50 ohms or 100 ohms are selected, and further matching is required in practice. Because the signals transmitted in the differential mode are referenced to each other and not related to ground, there is no common mode RF energy. It can also be set as a common-mode differential mode model at the beginning of the PCB design, and it is a good way to compare it with different modes in the subsequent debugging process.