[SI-LIST] Re: Trace width @ frequency
- From: "Yuriy Shlepnev" <shlepnev@xxxxxxxxxxxxx>
- To: "'Preethi Ramaswamy'" <preethi.gowtham@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Fri, 25 Jun 2010 08:02:07 -0700
Hi Preethi,
The characteristic impedance of strip line does not change much starting
from about 100 MHz up to about 10-20 GHz for a typical PCB or packaging
trace cross-section (sometime even up to 40 GHz). The smaller the cross
section size, the smaller the change. The smaller the dielectric loss, the
smaller the change. For a strip line, it depends mostly on the dielectric
dispersion - decrease of dielectric constant with the frequency lead to
slightly smaller per unit length capacitance and slightly larger impedance.
Though, it is typically small (within 1%) in the 1-10 GHz frequency range.
The change of characteristic impedance is even smaller in micro-strip
configurations due to the fact that the decrease in dielectric constant is
compensated by the high-frequency dispersion (electric field concentration
in dielectric with the larger permittivity) due to inhomogeneous dielectric.
The high-frequency dispersion effect is also noticeable in some strip-line
configurations with inhomogeneous dielectrics (layers with more glass or
more resin).
To figure out the actual change of impedance over the spectrum of your
signal, I would recommend you to compute it at a few frequencies critical
for your application. Just make sure that the tool you use has appropriate
model for the analysis of t-line with the effect of dielectric dispersion
and high-frequency dispersion for micro-strip configurations. The definition
of the characteristic impedance is also important factor for larger
cross-sections or at high frequencies.
Best regards,
Yuriy Shlepnev
www.simberian.com
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Preethi Ramaswamy
Sent: Thursday, June 24, 2010 3:24 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Trace width @ frequency
Hi All
When I determine the trace width of a 50 ohm single ended signals in
the package, what frequency should I consider?
Say, my signal operates at a fundamental frequency of 1GHz. In the package,
should I determine the trace width at 1GHz or some particular harmonic of
the fundamental frequency or should I look at the rise time?
Appreciate your help
Thanks
Preethi
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