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Two major problems in high speed PCB design
Posted:12:29 PM October 30, 2018 writer: G

Q: The software we use in high-speed PCB design is only to check the set EMC and EMI rules, and the designers should consider the EMC and EMI rules from those aspects. How to set the rules?

A: Generally, EMI/EMC design needs to consider both radiated and conducted. The former belongs to the higher frequency part (>30MHz) and the latter is the lower frequency part (<30MHz). So you can't just pay attention to the high frequency and ignore the low frequency part. A good EMI/EMC design must consider the position of the device at the beginning of the layout, the arrangement of the PCB laminate, the important online routing, the choice of the device, etc. These have no better arrangements beforehand. Afterwards, the solution will be half the effort and increase the cost. For example, the position of the clock generator should not be close to the external connector. The high-speed signal should go as far as possible to the inner layer and pay attention to the characteristic impedance matching and the continuity of the reference layer to reduce Reflection, the slope of the signal pushed by the device is as small as possible to reduce the high-frequency components. When decoupling/bypass capacitors are selected, pay attention to whether the frequency response meets the requirements to reduce the power layer noise. In addition, pay attention to the high-frequency signal. The return path of the current makes the loop area as small as possible (that is, the loop impedance is as small as possible) to reduce the radiation. It is also possible to control the high frequency noise by dividing the ground plane. Range. Finally, suitable for PCB ground point of the housing (chassis ground).


Q: When the high-speed differential signal pair is paralleled on the pcb, in the case of impedance matching, there are many benefits due to the mutual coupling of the two wires. However, there are opinions that this will increase the attenuation of the signal and affect the transmission distance. Why? I have seen some high-speed wiring on some large companies' evaluation boards as close and parallel as possible, while others intentionally make the distance between the two lines too close. Which effect would be better? My signal is above 1 GHz and the impedance is 50 ohms. Is the differential pair calculated in 50 ohms when calculated in software? Still counting 100 ohms? Can I add a matching resistor between the differential pairs at the receiving end?

A: The reason for the attenuation of the high-frequency signal energy is the conductor loss of the conductor itself, including the skin effect, and the dielectric loss of the dielectric material. These two factors can be used to analyze the effect of their influence on signal attenuation when electromagnetic theory analyzes the transmission line effect. The coupling of the differential lines affects the respective characteristic impedances and becomes smaller. According to the voltage divider, this causes the signal source to be sent to the line with a small voltage. As for the theoretical analysis of signal attenuation due to coupling, I have not seen it, so I can't comment. The wiring of the differential pairs should be properly close and parallel. The proper proximity is because this spacing affects the value of the differential impedance, which is an important parameter in designing the differential pair. Parallelism is also required because the consistency of the differential impedance is maintained. If the two lines are too close, the differential impedance will be inconsistent, which will affect signal integrity and timing delay. The differential impedance is calculated as 2(Z11 - Z12), where Z11 is the characteristic impedance of the trace itself, and Z12 is the impedance between the two differential lines due to the coupling, which is related to the line spacing. Therefore, to design a differential impedance of 100 ohms, the characteristic impedance of the trace itself must be slightly greater than 50 ohms. As for how much bigger, it can be calculated by simulation software. 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.

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