RE: [SI-LIST] : stripline PCB board shrinkage?

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From: Dan Swanson (dswanson@bartleyrf.com)
Date: Thu Feb 24 2000 - 04:31:06 PST


Craig,

Once you get down to 1 or 2% error, it can
be really tough to sort out source of center
freq error. You have a measurement of Eps_r,
how about a cross section through one of
the coupled sections. Are the dimensions
what you think they are?

You may also have a small error in one of
the analytical models in Touchstone.

You might do better by substituting in some
field-solver results for some key features, like
open ends adjacent to another line, etc.

Good luck,

Dan

Dan Swanson EMAIL: d.swanson@ieee.org
Bartley R.F. Systems TEL: (978) 241-1091
38 Water Street FAX: (978) 388-7077
Amesbury, MA 01913

-----Original Message-----
From: C Deibele [SMTP:deibele@fnal.gov]
Sent: Wednesday, February 23, 2000 3:48 PM
To: si-list@silab.eng.sun.com
Subject: [SI-LIST] : stripline PCB board shrinkage?

Hi,

I have a question about something that I've heard about recently.

I am making FIR stripline filters - circa 4 GHz, and the physical
dimension of the filter is 5"x30". The filter is manufactured using
Arlon 2.33 LX material, eps_r=2.31 (Arlon measured the dielectric and
loss tangent), and is put together using an etch/acid wash.

I designed the circuit using Agilent's Touchstone package.

My problem: The center frequency of each filter is off by 70 MHz. My
filters are rather narrow band, so this was a problem for me. I simply
made individual sections of my filter a bit longer to accomodate this
frequency offset. Each coupled line in my design was originally 460
mils, I extended each to 465 mils. This implies that the overall length
increase is proportional to 3/460 or about .5%.

Yesterday a colleague told me that it is common with many circuit boards
to shrink when a layer of copper is removed from a single side. This
particular colleague even claimed that a board will shrink around 100
mils or so after etching. Has anyone else observed or heard of an
effect such as this? This definitely would explain why my center
frequencies are off.

Regards,

Craig Deibele

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