[SI-LIST] Re: High speed signal on top layer

Hi Larry,

Thanks for your comment,
1, Currently we limit the delta of the pos/minus signal lentgh to less
than 50 mil, is that enough? (Per my caculation, it create around 10ps
different delay)
2, I don't understand the EMI issue very clearly. Do you mean the 3G
signals will couple to the adjacent power plane, then make EMI? Any
other pitfalls besides EMI problem?

Best Regards,
George
---original message---
From:Larry Miller ; Mon, 30 May 2005 10:15:19 -0700
Subject:[SI-LIST] Re: High speed signal on top layer 
> 
> On May 30, 2005, at 4:48 AM, George Dai wrote:
> 
> > Hi all,
> >
> > We are layout one 3.0Gbps high speed backplane. The original design  
> > style
> > is routing the edge coupling differential signals in the internal  
> > layer,
> > with adjacent GND plane. Now we met the layer limitation, and here  
> > is the
> > questions,
> > 1. Can we routing the differential pair on the top layer with GND  
> > plane
> > on the second layer? In my knowledge, it seems functional but odd.
> > Please give your advice.
> 
> That will work, but you have to be very careful to keep your routes  
> exactly the same length and parallel. If there is any unbalance it  
> radiates. Be sure to take into account the difference in propagation  
> velocity compared to striplines if you have critical timing.
> 
> > 2. Can we use one continuous power plane (12v or 5v) as the reference
> > plane to the high speed signals? For example, if 3.0Gbps signals on  
> > top
> > layer works, can the second layer be +12v power plane?
> 
> I would not do this. It is practically impossible to make the power  
> plane at AC ground (by bypassing or capacitance between planes) at  
> 3G. You would probably have EMI problems.
> 
> Larry Miller
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