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PCB design skills classic problem

2019-11-25 16:29:51

1. How to choose PCB board?

Choosing PCB boards must strike a balance between meeting design requirements and mass production and cost. Design requirements include both electrical and institutional components. This material problem is often more important when designing very high speed PCB boards (greater than GHz). For example, the current FR-4 material, which has a large dielectric loss at several GHz frequencies, can have a large effect on signal attenuation and may not be useful. As far as electrical is concerned, it is important to note whether the dielectric constant and the dielectric loss are combined at the designed frequency.

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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 fields, which is called crosstalk. You can use the distance between the high-speed signal and the analog signal, or add ground guard/shunt traces next to the analog signal. Also pay attention to the digital ground noise interference to the analog ground.

3. How to solve signal integrity problems in high-speed design?

Signal integrity is basically a matter of impedance matching. The factors affecting the impedance matching are the architecture of the signal source and the output impedance, the characteristic impedance of the trace, the characteristics of the load end, and the topology of the trace. The solution is to terminate and adjust the topology of the traces.

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4. How is the differential wiring method implemented?

There are two points to note about the wiring of the differential pair. One is that the length of the two lines should be as long as possible, and the other is that the spacing between the two lines (which is determined by the differential impedance) should remain the same, that is, to be parallel. There are two ways to go parallel. One is that the two lines are on the same side-by-side, and the other is two lines that are on the top-by-side. In general, the side-by-side implementation of the former is more.

5. How to implement differential wiring for a clock signal line with only one output?

To use differential routing, it must be meaningful that both the source and the receiver are differential signals. Therefore, differential wiring cannot be used for a clock signal with only one output.

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6. Can I add a matching resistor between the differential pairs at the receiving end?

The matching resistance between the differential pair of the receiving end is usually added, and its value should be equal to the value of the differential impedance. This signal quality will be better.