[SI-LIST] Re: Differential microstrip with coplanar ground traces

  • From: MikonCons@xxxxxxx
  • To: si-list@xxxxxxxxxxxxx
  • Date: Tue, 8 Oct 2002 14:34:17 EDT

In a message dated 10/8/2002 10:11:47 AM Pacific Standard Time, 
rmittal@xxxxxxxxxxxxx writes:


> Continuing on the same topic - I was wondering if you would say the same
> thing for 10G. At 10G, I think stripline will not be as effecient because
> you will have to use vias and unless you make 50ohm vias, the distortion
> from the vias will be too significant. I have always used microstrip at 
> 10G.
> Do you agree?

As usual, "it depends." 

If you mean RF at 10 GHZ versus 10 Gbps the digital folks talk about that 
really means 5 GHz fundamental, most RF circuits at this frequency are 
relatively isolated from other circuits (sometimes even compartmentalized 
stage-to-stage) and contained within a small, RF-tight enclosure. Microstrip 
is great for these conditions. 

If other circuits are immediately adjacent, stripline in PCB pseudo-cavity 
construction (for isolation and shielding) is appropriate. And, yes, 50-Ohm 
via construction IS needed for maximum efficiency. At these frequencies, 
teflon-impregnated glass dielectric is generally needed, especially if 
efficiency is critical to successful circuit performance.

For the digital case, and especially for a backplane, the approach I would 
recommend is differential stripline with buried vias in teflon-impregnated 
glass dielectric. Multiple pairs are generally involved; therefore, crosstalk 
considerations are critical and lead one to stripline rather than microstrip.

Lots more can be said, but I have to go.

Mike

Michael L. Conn
Owner/Principal Consultant
Mikon Consulting

                   *** Serving Your Needs with Technical Excellence ***


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