[SI-LIST] Re: Differential microstrip with coplanar ground traces
- From: "Rohit Mittal" <rmittal@xxxxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Wed, 2 Oct 2002 10:32:27 -0700
A friend of mine had mentioned that for speeds less than ,say 7G, the
microstrip and CPWG have equally good characteristics but since CPWG is more
difficult to design (similar to what people have mentioned on this thread),
people prefer microstrip
However at 10G and above, he mentioned that a lot of the fields might escape
into the air. Hence a CPWG is better because it captures those fields as
well. I am not sure about this. Does it mean that CPWG has less loss than
microstrip at 10G and above
thanks
----- Original Message -----
From: <MikonCons@xxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Wednesday, October 02, 2002 10:03 AM
Subject: [SI-LIST] Re: Differential microstrip with coplanar ground traces
> 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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