[SI-LIST] Re: How can I simulate the influence of power plane noise on the signal trace ?
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: jeff.loyer@xxxxxxxxx
- Date: Sat, 14 May 2005 18:18:26 -0400
Jeff,
You need to rethink this. Power/ground planes form a parallel plate
waveguide cavity, associated with the capacitance and inductance of the
structure. (You make the error of thinking of power/ground planes as
having only capacitance.) Any power system di/dt that shows up across
the planes, becomes voltage noise in the cavity. Any signals in the
cavity will be coupled into this field. In addition, any single ended
and differential traces that traverse this cavity will also inject noise
into the cavity which will be picked up by any traces in the cavity.
If you still don't believe, imagine that instead of planes, power and
ground are carried by two traces above and below the differential
trace. Obviously there is broadside coupling between the power and
ground traces and the differential pair, which results in common mode
noise. Now increase the width of the power/ground traces. What
happens? The impedance goes down and the currents spread across the
entire surface. This results in a smaller voltage across the two
power/ground traces. Now enlarge the traces to the size of a plane.
What happens? Impedance goes down. Noise waves can now travel in two
dimensions. If these happen to be the planes for 3.3 Volt power
distribution, and you've designed your power distribution system to keep
the noise below 5% of VCC, then you might just measure 165 mV of noise
potential difference at any point in space or time across the planes.
But, if you've done a lousy job at keeping the noise down on these
planes, then you might see something much larger.
Will this cause a problem with differential signalling. Yes, if it
forces the receiver out of the common mode range. If not, it will most
certainly reduce your receiver noise margins and increase BER. And if
the signal goes out of box, you can bet that there is an EMI nightmare
just lurking. And what about that really high frequency noise caused by
high speed signal vias traversing the planes and injecting power into
them, which can never be adequately bypassed with capacitors? Well, all
that noise just rattles back and forth in the resonant cavity, and shows
up as noise peaking on the differential pairs at the oddest places.
Imagine what might just happen to received jitter when the power/ground
planes resonate at a frequency within the encoding bandwidth of the
signalling standard?
Scott
Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax
http://www.teraspeed.com
Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC
Loyer, Jeff wrote:
>Hi Steve,
>I'm also a bit curious about your comment, and wonder if others agree.
>Do you have any papers I could refer to that clearly document this
>occurring? Maybe I'm missing something, but assuming the traces are
>closer to the ground plane than the power plane (or at least no further
>than), it seems like if:
>1) the planes are very close together, the inter-plane capacitance would
>keep the AC potential between the planes at 0, or=20
>2) the planes are far apart, there wouldn't be substantial noise coupled
>from the power plane to the traces.
>
>Hmmm - I wonder what would happen if I went in the lab and, for the
>stackup described, hooked up a signal generator between the power and
>ground planes. What would I see on a TDR (which is referenced to
>ground)? I don't think I'd see effects from the induced noise. Perhaps
>if I injected the signal using ground as a reference and then used the
>power plane as the reference plane at the receiver - bad. The amplitude
>of the signal would vary with the noise.
>
>But I'd sure like to see where this effect is quantified (which is what
>I think the original intent was). Besides a rule-of-thumb that says
>"don't ever do it!", how can we characterize it and decide when it's ok
>and not?
>
>For Zhangkun - I'd be leery of referencing high-speed signals to power.
>As has been pointed out on this list before, at some point you probably
>have to get back to ground reference, and that can be problematic.
>
>
>Jeff Loyer
>
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
>On Behalf Of zhangkun 29902
>Sent: Friday, May 13, 2005 5:22 PM
>To: weirsi@xxxxxxxxxx
>Cc: jgxian@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: How can I simulate the influence of power plane
>noise on the signal trace ?
>
>Steve
>
>In the realistic, there have to be some signal refered to power plane.
>Moreover, I do not think it is very cirtical if the decoupling for the
>power delivery system.
>
>I have use 3D field solver to simulate this kind of situation. There is
>displacement current between planes, which gives rise to problem. If the
>displacement current could be lessen, I think it is no critical for the
>signal to refer to any plane.
>
>Best Regards
>
>Zhangkun
>2005.5.14
>
>----- Original Message -----
>From: steve weir <weirsi@xxxxxxxxxx>
>Date: Saturday, May 14, 2005 1:43 am
>Subject: [SI-LIST] Re: How can I simulate the influence of power plane
>noise on the signal trace ?
>
>
>
>>Jia, if you assume that:
>>=20
>>1. The geometry of each line is identical
>>2. The noise does not violate the CM range of the receiver,
>>3. The CM rejection of the receiver is perfect
>>=20
>>Then you will not see any effects whatsoever. Put in more=20
>>realistic models=20
>>for each point above and you will find:
>>=20
>>1. Differential noise injection
>>2. Very bad if this happens, don't let it!
>>3. Increased jitter
>>=20
>>For these reasons, it is a poor idea to reference high performance=20
>>links=20
>>against multiple voltage rails. It doesn't mean that you cannot=20
>>do it=20
>>successfully, but it does mean that you have a lot of additional=20
>>homework=20
>>to do, and may not be able to meet a given performance target.
>>=20
>>Steve.
>>11:47 AM 5/13/2005 +0800, Jia Gongxian wrote:
>>
>>
>>>Dear all,
>>>
>>>I'd like to simulate the influence of power plane noise on the=20
>>>
>>>
>>signal=20
>>
>>
>>>traces, which are differential stripline, using the power plane=20
>>>
>>>
>>as one=20
>>
>>
>>>reference plane.That is,
>>>
>>> ----------------------------------- ground plane
>>>
>>> --- --- differential traces
>>>
>>> ----------------------------------- power plane + noise
>>>
>>> Any suggestions will be welcome.
>>>
>>>Thanks.
>>>
>>>Jia Gongxian
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>>=20
>>=20
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>>=20
>>=20
>>
>>
>
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