**From:** Eric Bogatin (*eric@bogent.com*)

**Date:** Mon Oct 30 2000 - 06:36:39 PST

**Next message:**Ingraham, Andrew: "RE: [SI-LIST] : spread spectrum clock drivers"**Previous message:**Aric Hadav: "[SI-LIST] : spread spectrum clock drivers"**Next in thread:**John Howard: "Re: [SI-LIST] : effective dielectric constant as bulk dielectric constant error"**Reply:**John Howard: "Re: [SI-LIST] : effective dielectric constant as bulk dielectric constant error"

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.

--eric

(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

Training for Signal Integrity and Interconnect Design

v: 913-393-1305

f: 913-393-1306

e: eric@bogent.com

web: <http://www.bogatinenterprises.com/>

ftp: ftp://ftp.BogatinEnterprises.com

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**Next message:**Ingraham, Andrew: "RE: [SI-LIST] : spread spectrum clock drivers"**Previous message:**Aric Hadav: "[SI-LIST] : spread spectrum clock drivers"**Next in thread:**John Howard: "Re: [SI-LIST] : effective dielectric constant as bulk dielectric constant error"**Reply:**John Howard: "Re: [SI-LIST] : effective dielectric constant as bulk dielectric constant error"

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