[SI-LIST] : Losses in RF boards

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From: WAUGH,RAY (A-SanJose,ex1) ([email protected])
Date: Fri Apr 28 2000 - 15:20:05 PDT

In open microstrip lines on common plastic substrates, dielectric losses
dominate over conductor losses. In that event, the total loss (mostly
dielectric loss) will measure as a straight line when plotted against
frequency. Where conductor losses are important (as is the case with
thin-film hybrid circuits on alumina substrates), total loss does not plot
as a straight line against frequency. I make such wideband measurements
from time to time, just as a sanity check (and quality check on the material
I buy).

It can be shown from Schneider's work that dielectric losses can be
expressed as a constant loss per wavelength (dB/wavelength). See attached.
If one line is twice as long as another, it will have twice the losses at
all frequencies.

My own measurements of loss tangent on 0.032" thick FR4 microstrip (half
ounce copper lines and ground plane) is 0.030. For thinner material of
uncertain heritage, 0.060 would not be out of line.

      Raymond W. Waugh - WSD Diode Applications
      E-mail: [email protected]

          USPS: Agilent Technologies
              Wireless Semiconductor Division
              39201 Cherry Street, MS NK20
              Newark, California 94560

-----Original Message-----
From: Degerstrom, Michael J. [mailto:[email protected]]
Sent: Friday, April 28, 2000 1:52 PM
To: [email protected]
Subject: RE: [SI-LIST] : RF boards


Isn't your statement that PCB dielectric losses dominate above
1 Ghz only relavent for your particular line length and line width? For
example, the few published comparisons of eye closure for various laminates
tend to base their measurement on .012" wide 12" long lines. My
experience is with .004" wide 2" long lines in FR-4. Simulations
with a .02 loss tangent and sqrt(freq) skin-effect losses predicted
12 Gb/s. I measured 5 Gb/s. I believe the extra loss is attributed
to surface roughness as I don't think a loss tangent of .06 at 5 Gb/s
is realistic.

By the way, with the lower loss laminates, how do
you know that the lower loss is mostly attributed to the lower
loss tangent? Couldn't the surface metal be smoother and/or of better
quality than the metal typically used on FR-4 laminates?


Mike Degerstrom Email: [email protected]
Mayo Clinic
200 1st Street SW
Gugg. Bldg. RM 1042A Phone: (507) 284-3292
Rochester, MN 55905 FAX: (507) 284-9171
WWW: http://www.mayo.edu/sppdg/sppdg_home_page.html

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