[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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