[SI-LIST] Re: Where and how the Ground Noise Come from?
- From: "Ingraham, Andrew" <Andrew.Ingraham@xxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Mon, 3 Feb 2003 09:21:50 -0500
Any time someone talks about noisy ground, I have to ask: how are you
measuring this noise?
Presumably you attached a scope probe or RF meter between one ground
point and another. But I have seen people use remarkably different
approaches when looking for "ground noise," so it's fair to ask. Also,
sometimes people talk about "ground noise" as if it is somehow absolute,
with no reference point.
Is it reasonable to expect no ground noise between the two points you
chose?
At RF, there is no such thing as "ground." Every point is electrically
different from every other, because there is always a non-zero impedance
between all points on a "ground plane;" and if there is any current flow
(which there usually is when things are operating), there will be a
voltage between those "ground" points. Does it matter? i.e., do you
have a circuit that responds to the ground difference between two points
that are widely separated on a board? It's good to keep such things in
mind. Since "ground noise" is inevitable, you should question whether,
and try to understand when, that noise is irrelevant.
Also, do you know the limits of your instruments? Are you seeing noise
that is really in the circuit, or noise picked up by the probe? What if
you attach both probe points to the same "ground" in your circuit? Or
short the probes together, with the same loop size, and place them near
but not touching the circuit?
Antenna mismatch isn't necessarily bad. Mismatch relative to what?
When there is significant coax or transmission line, then mismatch has
relevance because of things like standing waves. But when you have a PA
driving a load without much coax (as is probably the case with a mobile
phone), what does impedance mismatch really mean? The "correct" load
impedance is not much more than the one you wanted it to be, chosen
semi-arbitrarily based on getting good power output while not
overheating the PA. Mismatch also matters to filters, to get the
desired frequency response.
Mismatch might increase the PA output current (which might increase
noise due to IR and/or Ldi/dt), or it might decrease it and increase
output voltage (which might lead to more capacitive crosstalk). But I
think both of them are bounded and can't get very much bigger at the
output of the PA. So it's sort of a gut feeling (but without very much
experience, I fully admit) that mismatch isn't the problem. Also,
having a matched antenna doesn't make either the current or the voltage
zero.
Regards,
Andy
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