From: John Howard (firstname.lastname@example.org)
Date: Mon Oct 30 2000 - 01:00:04 PST
Don't neglect the error contribution from FR4 anisotropy and
(for cheap FR4) inhomogeneity.
Eric Bogatin wrote:
> Thank you all for your comments on my posting related to possible
> measurement error in TDR. The thread was moving toward predictability of Zo
> for microstrips, which, though an interesting topic, wasn't where I had
> intended to go.
> I was actual interested in the specific case of a board shop using a
> measurement of the time delay (TD) of a microstrip test trace to get the
> dielectric constant they would use for later Zo simulation.
> What they would measure from a microstrip is the effective dielectric
> constant, which could be as low as 3.8 for FR4, compared to the bulk value
> of 4.5. If they used 3.8 for their field solver predictions of Zo, even
> including trace thickness and solder mask effects, they would still be off
> by about 8-10% because they have the wrong value of the bulk dielectric
> constant. Not knowing this, it might cause them to question their tool or
> add some in-house fudge factors to get better agreement between what they
> simulate with their tool or favorite approximation and what they measure.
> In fact, their problem was interpreting the dielectric constant measured
> from the microstrip as the bulk value rather than as the effective value.
> Does anyone know of any board shops that might be making this effective
> dielectric constant as bulk dielectric constant error, or is it an obvious
> problem that is always avoided?
> thanks for your comments.
> (ps- for those wondering about how well you can predict the Zo of test lines
> with a field solver, I have some papers on my web site that discuss this)
> Eric Bogatin
> BOGATIN ENTERPRISES
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