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 *** ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu