RE: [SI-LIST] : Merits of low dielectric constant

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From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Fri Jun 23 2000 - 12:36:18 PDT


>>I think this "correction factor" you speak of, is for a capacitive "stub"
>>hanging off a main transmission line.
>>
>>If you are using an electrically short, unterminated coax, attached to the
>>middle of a longer transmission line, then this is a consideration.
>>
>>If you are using terminated coax transmission lines and no unterminated
>>stubs, then I think it's irrelevant.
>>
>>Andy
>
>
>I believe (though I won't argue the point too strongly) that the correction

>factor also applies to capacitive loads (i.e. devices) placed along the
>line and at the end of it.
 
True. That is where the correction factor is most commonly used. I am just
treating the short stub like a small lumped capacitive load.

> Then it would become relevant in high-speed designs.
 
Well, yes, but it may or may not be relevant to THIS problem.

If you used the low dielectric constant cable as an unterminated stub
attached to some other transmission line, then low cable Er means smaller C
(of the small load attached to the line), which means a smaller correction
factor to get the effective Zo.

If you used the low dielectric constant cable as your main transmission
line, and attached other devices to it at specific points along the line,
then low Er means smaller C/meter, which means that each small added C
(which didn't change) is more significant compared to the now-smaller
C/meter (which did change), which means you have a LARGER correction factor.

If you use the same low-Er cable for both, then I think there is no net
change.

So it depends on where you use the cable: as an electrically long line with
devices/stubs on it, or as an electrically short stub hanging off some other
line.

Regards,
Andy

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