[SI-LIST] Re: Diff.Pairs
- From: MikonCons@xxxxxxx
- To: bmgman@xxxxxxxxxx, scott@xxxxxxxxxxxxx
- Date: Mon, 13 Oct 2003 17:48:04 EDT
Scott & Mike:
You guys definitely have the right idea(s). One thing you forgot to mention,
or that others on this thread might not have thought of, is that a reference
plane between the traces of a "differential" pair provides excellent
decoupling/isolation of signals on the two traces. In addition to the
variations in
effective dielectric constant (and its negative impacts on skew and even-mode
impedance), the plane provides electrical isolation between the top and bottom
surface plane currents via the skin effect. Even a 1/2-ounce copper plane would
provide 26 dB reduction in magnitude of the image plane return current from one
side of the plane to the other at only 120 MHz. The isolation is virtually
complete for the GHz data rates that this thread addresses. I echo Scott's
disdain for any high-speed design for differential pairs to use this approach.
RE: Scott's comment below,
"For vias spaced larger than 150 mils away from a ground via (most signal
vias), there is about -0.8 dB insertion loss at 3 GHz. This is
comparable to a an 8.8% reduction in the signal amplitude of a 100 ps
edge. At 7 GHz (50 ps rise time) there is a -1.5 dB loss (16% amplitude
reduction.) These may or may not be significant losses in a design, but
are extremely significant to the noise and EMI profile of a board, since
all of this energy is being injected into the planes where it is not
well contained. The implication for 2.5, 3.125 and 10 Gbps PCB designs
is profound. Decouple the differential vias and get ready for increased
losses, increased noise and increased EMI."
I couldn't agree more. As supporting evidence over the last four years, I was
the third (and only successful) consultant to analyze and define routing
rules for multi-GHz boards and backplanes for Nokia Networking. I found it
necessary to use closely-coupled, coplanar pairs in open field routing, and
widened,
symmetrical traces within pin fields that bracketed other differential pairs
(to obtain canceling common-mode coupling fromn each +/- pair bracketed). My
first guidance was to NOT use vias if at all possible. For unavoidable
conditions, I devised a via structure that widened the narrowly spaced pair
traces and
bracketed two ground vias before coming back to close spacing on the new
coplanar routing layer. The structure created a vertical two (round) wire pair
of
100 Ohms differential impedance for minimal signal disturbance at the layer
transition. Other consultants had tried multiple loosely-coupled techniques for
1.5 years without acceptable results (names withheld as they are too
recognizeable). In other words, tight coupling was the only viable solution,
along with
carefully designed via-pair transitions.
Keep up the good comments for the less experienced designers out there! Good
thoughts also for Wyland and Knighten contributions.
It's 70 degrees, sunny, and the trout are rising. Bye.
Mike
Michael L. Conn
Owner/Principal Consultant
Mikon Consulting
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