Werner Almesberger wrote:
> ?lvaro Lopes wrote:
>> One idea is to have a separate ground for the RF part, and connect
>> it to digital ground at one point, forming some sort of ground peninsula,
>
> Whether to split ground planes or not seems to vary mainly with the
> designer, less with the design.
>
> To take random examples from a quick google search, the pro-split side
> has a quite convincing example here:
>
http://www.signalintegrity.com/Pubs/news/9_04.htm>
> And so does the contra-split side (also check out the link to the 2001
> article):
>
http://www.hottconsultants.com/techtips/split-gnd-plane.htmlSame guy. :)
"You must also arrange the common connection so that it does not encourage ground currents circulating between the analog and digital regions (or else you
defeat the whole point of isolating the grounds). That's the tricky part about having separate DGND and AGND plane regions: It doesn't help to separate them
unless you also prevent high-speed currents from passing from one to the other. "
That's why junction of the two ground planes should be as close as possible to the PMU. Remember, ground actually "feeds" electrons onto the system.
These analysis are however only for common mode. in RF, most of the signal does not actually travel on the copper, but in the dielectric. Currents flowing
nearby cause electromagnetic fields, which disturb the transmission.
Personally I'd like to not split grounds, but to achieve the same "effect" we might have to change layout a bit. This was primarily intended to avoid SD card
interference.
>
> My interpretation is that splitting ground planes gives you more
> control, but at the cost of increasing complexity and the risk of
> unintended consequences.
>
>> We should also have these
>> ground planes (RF) filled with through-hole vias, to avoid EMI emission
>> (and maybe mitigate interferences).
>
> Putting that RF into a "cage" sounds good to me. This should also help
> to reduce ground impedance.
>
> - Werner
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