[SI-LIST] : Re: DIFF PAIRS

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From: Kentaylorus@aol.com
Date: Tue Jun 13 2000 - 12:17:39 PDT


Sean, I'm surprised to read that any fudge factor was necessary for the Polar
calculation. In my experience it's very, very reliable. Discrepancies can be
attributed to geometry discrepancies, and to deviation in Er (Usually due to
the glass/epoxy mixture ratio, etc.) In other words, garbage in = garbage out.

This leads me to suspect that some physical factor isn't what it pretends to
be.
a/ Who is measuring the impedance, how, and what with? (In other words, are
you getting reliable measurement results? You can give two people the same
Tek 11800 or HP and get three different answers. These excellent instruments
require expert set up and use.)
b/ Since getting the boards back from your board fab house, have you
microsectioned to see the actual geometry, including etch factor?
c/ Are you using edge coupled, or what geometry? Ground planes where??
d/ Do you have your own copy of the Polar CITS software? If not, you can
download your own free demo copy (www.polarinst.com) and get about fifty free
calculations before it bombs. That could get you some good answers to your
questions.

There are a ton of simple questions and I'll be pleased to assist if you want
to contact me with specifics concerning configuration, dimensions, and
material.

Ken Taylor
ken.taylor@polarinst.com

Date: Mon, 12 Jun 2000 13:34:53 -0700
From: Sean Murray <seandm@mmspec.com>
Subject: [SI-LIST] : DIFF PAIRS

Anyone have any thought on what to use for the following numbers.

Material: CE
Impedance: ZO of the differential trace is 50Ohm and ZOD of each
differential pair traces is 100Ohm.

I built a board using 8 mill traces with a 8 gap between the pair, and
.008 gap between plane to sig and then plane.

Numbers were not what I expected, fabhouse used Polar+fudge factor to
give me the numbers.

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