GSM-noise "buzz" issue
joerg at openmoko.org
Wed Sep 17 17:27:39 CEST 2008
Am Mi 17. September 2008 schrieb Andy Green:
> Somebody in the thread at some point said:
> | Actually Andy explained the pin3/pin5 issue.
> | From what I understand, with removed receptacle and 3-5 short, your buzz
> | actually vanished, no? That's good news.
> | For the suggested rework, that's exactly why I bought a Dremel couple
> of days
> | ago ;-)
> | You described my plans to the smallest detail.
> | Contrary to Andy I don't hope for eventual enlightenment somewhere in
> | Especially since no one than me is examining the whole issue, and I'm
> | there *is* *no* other way for existing devices.
> | If you got the fox on the farm, you don't investigate how it's actually
> | killing the chicken, rather you simply fix the hole in the fence.
> Our incomplete understanding does not lead to a solution for headset mic
> buzz nor any security about future designs.
You're sure about that?
> One way or another we need
> to penetrate those mysteries a bit deeper up to WM8753.
If anybody thinks he needs to learn why it's bad to route RF into a can that's
designed to keep RF outside, go ahead! Those cans are used *because* not even
the chip designers can predict what will happen when their chip is flooded
with 1800MHz. If they could, they could change chip design in a way we
wouldn't need cans anymore ;-)
Basically it's always the same problem: RF hits a non-linear component (like
crystal-detector back in ~1900, or any P-N-zone [aka diode] in a 2- or 3-pole
(chip-)component nowadays), and there it gets rectified and
introduces "DC"-current (actually audio-frequency LF-current) in parts of the
circuit where we absolutely don't want to see it. You can't do anything else
than just stop RF *before* it reaches this critical point. Typical points to
stop RF are entrance of external "antennae" (=short or long unshielded wires)
to PCB, and **entrance of traces into can**.
We don't want to learn how to make RF "behave" inside a can, simply because
we'll never win. RF is a beast, mostly you just can't tell what it will do
until you actually tried.
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