1、 How to deal with some theoretical conflicts in practical wiring
Q: in actual wiring, many theories conflict with each other; For example: 1. Processing multiple analog / digital ground connections: theoretically, they should be isolated from each other, but in the actual miniaturization and high-density wiring, due to the limitation of space or absolute isolation, the small signal analog ground wiring will be too long, so it is difficult to realize the theoretical connection. My approach is to divide the ground of the analog / digital function module into a complete island, and the analog / digital ground of the function module is connected to this island. Then connect the island with the “big” through the channel. I wonder if this approach is correct?
2。 Theoretically, the connection between the crystal oscillator and the CPU should be as short as possible. Due to the structural layout, the connection between the crystal oscillator and the CPU is relatively long and thin, so it is disturbed and the work is unstable. At this time, how to solve this problem from the wiring? There are many such problems, especially EMC and EMI problems in high-speed PCB wiring. There are many conflicts, which are a headache. How to solve these conflicts?
A: 1. Basically, it is right to divide and isolate analog / digital. It should be noted that the signal routing should not cross the separated place (moat) as far as possible, and do not make the return current path of the power supply and signal too large.
2. The crystal oscillator is an analog positive feedback oscillation circuit. In order to have a stable oscillation signal, it must meet the specifications of loopgain and phase, and the oscillation specifications of the analog signal are easy to be disturbed. Even with ground guard traces, the interference may not be completely isolated. Moreover, if it is too far away, the noise on the ground plane will also affect the positive feedback oscillation circuit. Therefore, the distance between the crystal oscillator and the chip must be closer.
3. Indeed, there are many conflicts between high-speed wiring and EMI requirements. However, the basic principle is that some electrical characteristics of the signal cannot meet the specification due to the resistance capacitance or ferite bear added by EMI. Therefore, it is best to use the skills of arranging wiring and PCB lamination to solve or reduce EMI problems, such as high-speed signal inner layer. Finally, resistance capacitance or ferite bear is used to reduce the damage to the signal.
2、 How to solve the problem of signal integrity in high-speed design? How is the difference distribution line realized? For the clock signal line with only one output, how to realize the difference distribution line?
A: signal integrity is basically a problem of impedance matching. The factors affecting impedance matching include the architecture and output impedance of signal source, the characteristic impedance of routing, the characteristics of load end, the topology architecture of routing, etc. The solution is to rely on termination and adjust the topology of routing. There are two points to pay attention to in the wiring of the differential pair. One is that the length of the two lines should be the same as far as possible, and the other is that the spacing between the two lines (which is determined by the differential impedance) should always remain unchanged, that is, they should be parallel. There are two parallel modes: one is that two lines run on the same side by side, and the other is that two lines run on the upper and lower adjacent layers. Generally, there are many ways to implement the former side by side. To use a differential distribution line, it must be a differential signal at the signal source and receiver. Therefore, the difference distribution line cannot be used for the clock signal with only one output.
3、 About high speed differential signal wiring
Q: when the PCB is close to the parallel high-speed differential signal line pair, in the case of impedance matching, due to the mutual coupling of the two lines, it will bring many benefits. However, some people believe that this will increase the attenuation of the signal and affect the transmission distance. Is that right? Why? I saw on the evaluation boards of some large companies that high-speed wiring is as close and parallel as possible, while some deliberately make the distance between the two lines vary from far to near. I don’t know which kind of effect is better. My signal is above 1GHz and the impedance is 50 ohms. When calculating with software, is the differential line pair also calculated as 50 ohms? Or 100 ohms? Can a matching resistance be added between the differential line pairs at the receiving end?
A: the reason for the attenuation of high-frequency signal energy is the conductor loss, including skin effect, and the dielectric loss of dielectric materials. The influence of these two factors on signal attenuation can be seen when analyzing transmission line effect in electromagnetic theory. The coupling of differential lines will affect their respective characteristic impedance and become smaller. According to the voltage divider, this will reduce the voltage sent by the signal source to the line. As for the theoretical analysis of signal attenuation due to coupling, I have not seen it, so I can’t comment. The wiring of differential pairs should be properly close and parallel. The so-called proper approach is because.