FreeLists / si-list / [SI-LIST] Re: Differential microstrip with coplanar ground traces

[MikonCons@xxxxxxx]

NOTE:  This is a re-send of an earlier transmittal as I did not see it get 
posted. Perhaps I deleted the original posting inadvertently. If so, just 
dump this one.

Mike Conn
*************
Bob:

You appear to have a few misconceptions as evidenced by your trace 
descriptions. Without knowing trace lengths or dielectric materials, here are 
a few initial observations for you to consider.

"Our digital signals are 2.5Gb/s with the fastest rise time at 100ps."

The fundamental is 1.25 GHz and, at your noted rise time, greater than 90% of 
the signal energy is contained within (1/PI*Tr) = 3.18 GHz. (100 ps sounds a 
bit slow for this signal; I would expect ~70 ps.) The minimum harmonic 
frequency you should design for is the third harmonic, and 3*1.25 GHz = 3.75 
GHz. 

For effective performance in field capture, lowered crosstalk, and shielding 
protection (for lower jitter susceptibility), the guard traces should have 
vias to the ground plane at about (1/20) lambda (wavelength) at 3.75 GHz. 
Assuming an Er of ~4, the 0.5" spacing you noted is >(1/10) lambda at the 
fundamental and is ~(1/3) lambda for the third harmonic.... Bad News!

"We expected the coplanar grounds to better contain the fields, and allow 
closer pair to pair spacing without the risk of large crosstalk."

For surface traces/microstrip, you have multi-mode propagation at different 
speeds; i.e., most of the energy travels in the dielectric medium, but some 
travels in the air above the surface. The wavelengths are different and the 
composite signal at the end/destination point suffers rise time degradation 
which increases jitter. But perhaps worse, the surface fields are not 
effectively captured/terminated by the coplanar guard traces (per the above 
comments) and will easily couple to other surface traces to yield substantial 
crosstalk.

Relative to microstrip, use of stripline construction will slow the 
propagation speed of the wavefront; however, you will experience 
substantially less rise time degradation as only one speed of propagation is 
present, thereby preserving the signal fidelity. This effect keeps any 
contribution to signal jitter to a minimum. The use of stripline also results 
in closer coupling of all signal fields to the image/reference planes and 
thereby produces lower crosstalk than microstrip construction. Using guard 
traces with the stripline pairs further increases crosstalk reduction (per my 
experience, ~4-8 dB added reduction), and the ground via spacing becomes much 
less critical.

Good luck on your next board spin.

Mike

Michael L. Conn
Owner/Principal Consultant
Mikon Consulting
Cell: (408)821-9843

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


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