GSM-noise "buzz" issue / GPS coupling

Andy Green andy at
Thu Sep 18 10:51:48 CEST 2008

Hash: SHA1

Somebody in the thread at some point said:
| Andy Green wrote:
|> Yeah, waving hands about RF in cans is not understanding: pretty sure
|> about it.
| I think we all would be much more comfortable about our understanding
| of the situation if someone could identify the actual contamination
| path.


| It's one thing to identify violations of good design practice (and to
| avoid them in the future), but to debug an actual problem, we can only
| be sure the true root cause has been found if we have the complete
| story, or at least a plausible theory for every step.
| This, by the way, also applies to the SD vs. GPS interaction. We now
| have what I believe is a thorough understanding of what gets emitted
| and how to silence it, but we never identified where this gets picked
| up and causes trouble.

It's a fair point.  But what I mentioned before about gross difference
in signal level means it is not quite the same.  Once we got the clue
from a customer that SD Card was involved, we could demonstrate good
control over the symptom by stopping SD Card signals, then we had a
decent story about what was going on.  We tailored our workarounds
closely to that understanding and found a good way overall.

For the future, increased GPS unit isolation physically and for tracking
and power, and preparing for slugging routed clocks with caps will
definitely attenuate coupling to GPS unit.  GTA02 workarounds about
simply stopping agressor signal bulk of the time can carry over too.

GPS is a lot more degradable dynamically than audio, the customer is
going to get way more enraged at buzz or click on audio than occasional
dropout of GPS second by second because WLAN did something for example.

For these reasons GPS isn't at the top of my worry list but GSM buzz is
getting closer to it.

| If this is an over-the-air path, we'll know that we'll have to expect
| similar issues in the future, and that we have to provide appropriate
| countermeasures. But if it's just a poorly routed trace, then
| hardening future designs against over-the-air contamination would be
| of limited use and might draw attention from more deserving areas.

There is definitely a story waiting to be uncovered here.  On my GTA02
A5 "skelephone" that is just a PCB, I replaced plastic GSM antenna with
"random piece of wire".  The buzz comes fine.  "Over the air"
contamination does not involve antenna radiation pattern then, it is
suspicious.  It means PCB inherently couples "something" "somewhere"
before antenna.  I keep going back to power rail-borne contamination but
it doesn't fit with beads doing something on headset nets.

- -Andy
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