PCB News - Classic Q&A sharing on pcb design

1. How to choose PCB board?
The choice of PCB board must strike a balance between meeting design requirements and mass production and cost. The design requirements include both electrical and mechanical parts. This material issue is usually more important when designing very high-speed PCB boards(frequency greater than GHz). For example, the commonly used FR-4 material, the dielectric loss at a frequency of several GHz will have a great influence on the signal attenuation, and may not be suitable. As far as electricity is concerned, pay attention to whether the dielectric constant and dielectric loss are suitable for the designed frequency.

2, how to avoid high frequency interference?
The basic idea of avoiding high frequency interference is to minimize the interference of high frequency signal electromagnetic field, which is called crosstalk (Crosstalk). Can increase the distance between high-speed signal and analog signal, or add ground guard/shunt traces beside the analog signal. Also pay attention to the noise interference from the digital ground to the analog ground.
3. How to solve the signal integrity problem in high-speed design?
Signal integrity is basically a problem of impedance matching. The factors that affect impedance matching include the structure and output impedance of the signal source, the characteristic impedance of the trace, the characteristics of the load end, and the topology of the trace. The solution is to rely on the topology of termination and adjustment of the wiring.
4. How is the differential wiring method realized?
There are two points to pay attention to when wiring a differential pair. One is that the length of the two wires should be as long as possible, and the other is that the distance between the two wires (the distance is determined by the differential impedance) must be kept constant, that is, it must be kept parallel. There are two parallel ways, one is that the two wires run on the same side-by-side, and the other is that the two wires run on two adjacent layers above and below (over-under). Generally, the former has more side-by-side implementations.
5. For the clock signal line with only one output terminal, how to realize differential wiring?
It makes sense to use differential wiring when both the signal source and the receiving end are differential signals. Therefore, it is impossible to use differential wiring for a clock signal with only one output terminal.
6. Can I add a matching resistor between the differential line pairs at the receiving end?
The matching resistance between the differential line pairs at the receiving end is usually added, and its value should be equal to the value of the differential impedance. This way the signal quality will be better.
7. Why should the wiring of the differential pair be close and parallel?
The wiring method of the differential pair should be appropriately close and parallel. The so-called appropriate proximity is because the distance will affect the value of differential impedance, which is an important parameter for designing differential pairs. The need for parallelism is also to maintain the consistency of the differential impedance. If the two lines are suddenly far and near, the differential impedance will be inconsistent, which will affect the signal integrity and timing delay.