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[SOLVED] Ground Vias placed around Signal Vias

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passerby

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Hi

Some layout guidelines said placing a few GND vias around signal vias can reduce the loss of the signal.
Could you please advise the principle behind?

Thanks.
 

I think your questions involves many unsaid premises about the problem. It's apparently assuming a PCB transmission line structure that's continued through vias. If it's so, the answer further depends on the transmission line type, PCB stackup, frequency range...

In other words, I prefer a clear question.
 

There is fringing capacitance where the lead in/lead out line attaches to the via. The via itself is somewhat inductive. So you end up with a lumped element model of a shunt C - series L - shunt C.

For lower frequencies (<10 Ghz) the capacitance and inductance is such that at least 2 ground vias are needed to keep the insertion loss low. Up at 18 Ghz, you find that the inductance of the via is getting significant, so you need to reduce the ground via inductance as much as possible to squeek by--so you see 4 or more ground vias. Too much inductance lowers the cuttoff frequency of the equivalent lowpass filter, and you can not reach the high end.
 
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