Barry,
I would be careful with taking your discussion below to
heart. Your equation for frequency is actually a Bandwidth
equation. Granted, if your calculating the HIGHEST
frequency of the bandwidth, that's fine. But, frequency
does play a significant role in the "impedance" that one
wants. Ask anyone dealing with board constructions with FR4
above 800 MHz.
Also, your assumption is that all the frequencies required
for the transition time are in sync with one another (from
Fourier). Bad assumption. Group delays and shifts in phase
based on frequency are only part of what will determine the
actual outcome. At best, you'll get a smeared edge at fast
edge rates.
When board construction for controlled impedance is most
important, there's a whole other side to controlled
impedance that can get nasty.
doug
______________________________ Reply Separator _________________________________
Subject: Re: Controlled Impedance - Dielectric Constant- Comment
Author: [log in to unmask] at internet-mail
Date: 5/21/96 10:39 PM
Hi Luis
The dielectric constant of soldermask is slightly
different to that of FR4 however in practice the
variation is neglibible.
(-1.55*H'/H)
>From IPC-D-317 e=er[1-e ]
I also noticed from the discussion that it has been
mentioned that the frequency of operation will
determine the impedance of the substrate. That is
not exactly correct as it is in fact the rise time
of the leading edge of the pulse that is the
influencing factor.
f=0.35/Tr
where f is in GHz
Tr is in nS
At the end of the day what I do is just specify in
the PCB Spec that the trace widths and prepreq
thickness may be varied to obtain the desired
impedance. And really who cares what material is
used in the board manufacture as long as the
impedance is what we want!
Cheers,
Barry Olney, In-circuit Design, Australia
Luis Rivera wrote:
>
> Bob,
>
> You are correct, the dielectric constant of a material does not change
> between microstrip, embedded microstrip or stripline configurations.
>
> Avner,
>
> Some general words of caution:
>
> Dielectric constant does change depending on resin and reinforcement
> type, resin content, frequency, temperature and humidity. Typically,
> dielectric constant decreases as frequency increases.
>
> On microstrips, the fact that there is air on one side of the
> transmission line lowers the effective dielectric constant. Solder
> mask coatings counteract this effect to some degree.
>
> Please call off-line if you would like to discuss further.
>
> Luis Rivera
> [log in to unmask]
> CompuRoute, Inc.
> Dallas, TX
> (214) 340-0543
>
> >
> >
> > Avner--
> > If I am wrong, I hope someone out there will make me smarter, but
> > the dielectric "CONSTANT" is just that--constant for that type of
material.
> > IMOP, when calculating impedance of microstrip or stripline, the
dielectric
> > constant of a material doesn't change, less the material's unique
tolerance
> > range REF: IPC-D-275 Table 3-2. Therefore, unless proven wrong, the
constant
> > DOES NOT change, but the impedance value WILL vary depending on which
> > formula you are using depending on you pwb construction. Hope that helps.
> > If wrong, please correct me via E-mail,
> >
> > Bob Vanech
> > E-Mail : [log in to unmask]
> >
> >
> >
--
___________________________________________________
Barry Olney
Managing Director
In-Circuit Design Pty Ltd Ph: +61 3 9205 9595
VeriBest Solutions Centre Fax:+61 3 9205 9410
Suite 211, Princess Tower Mbl:+61 4 1117 0827
1 Princess St,
Kew, VIC 3101, Australia Email:[log in to unmask]
___________________________________________________
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