Hi Brian,
A 2D field solver may be sufficient to start design of coplanar link. Just make
sure that it works properly. Either correlate the results with the published
data (better) or do a numerical experiment. Set the guard conductors as
reference and try to move them closer to the signal conductor - the impedance
should go substantially down as the distance to the reference conductor becomes
smaller than the distance to the traditional reference plane.
The configuration you are trying to analyze is called conductor-backed coplanar
waveguide (CB-CPW). Comparing to microstrip, it has some advantages (lower
dispersion, reference in the same plane,...) and disadvantages such as the mode
conversion. To suppress unwanted common mode in CB-CPW configuration and to
make the reference strips actually the reference, you have to put sufficient
number of stitching vias connecting two coplanar reference conductors with the
common reference conductor below (more vias:-). Distance between vias along
the wave propagation direction and in the cross-section should be close to a
quarter of wavelength at the highest frequency of interest. That is about 70-80
mil for 20 GHz frequency range and 35-40 mil for 40 GHz. The mode conversion in
CPW takes place at the discontinuities. Electromagnetic analysis of CPW as
quasi-periodic structure and CPW discontinuities may be a good idea. The vias
may reduce the impedance a few Ohms further.
See more at
W.R. Deal, Coplanar waveguide basics for MMIC and PCB design, Microwave
Magazine, v. 9, No4, 2008, p. 120-133.
G. Gronau, A. Felder, Coplanar-waveguide test fixture for characterization of
high-speed digital circuits up to 40 Gbit/s, Electronics Letters, v. 29, No 22,
1993 , p. 1939 - 1941.
R. W. Jackson, Mode conversion at discontinuities in conductor-backed coplanar
waveguide, IEEE Trans. On MTT, 1989, v 37, No 10, 1582-1589.
Best regards,
Yuriy
Yuriy Shlepnev, Ph.D.
President, Simberian Inc.
Office +1-702-876-2882; Fax +1-702-482-7903
Cell +1-206-409-2368; Virtual +1-408-627-7706
Skype: shlepnev
www.simberian.com
Simbeor – Accurate, Productive and Cost-Effective Electromagnetic Signal
Integrity Software
2010 and 2011 DesignVision Award Winner, 2015 Best In Design&Test Finalist
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Moran, Brian P
Sent: Thursday, November 5, 2015 2:52 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Characterizing PCB Impedance in Coplanar Structures
Hi Folks,
Hopefully this is an easy one for someone out there, but I need some insight.
I'm using a 2D field solver to calculate the Zo of a micro-strip which has both
a traditional broadside reference plane and ground guard traces on one or both
sides.
The solver does allow me to define the guard traces as references. However,
even when I do, the field solver reports the same Zo as without the guard
traces. The only metric that does change is Zself, but Zself does care if they
are guard traces or normal signal traces. So I'm wondering, should the solver
include the coplanar reference traces in its Zo calculation, or does Zo, by
definition, only consider the broadside reference. Should I be looking at
Zself to characterize the trace?
Or should I be looking for a different solver. How do people generally
characterize coplanar t-line structures?
Brian Moran
Intel Corporation
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