Hi KC,

Effective dielectric constant comes into play when you have a build that
is not homogenous. For example, microstrip board (double sided), where
you have dielectric and air as well as stripline (3 layer board) if the
bonding film/prepreg is not of the same dielectric constant as the rest
of the material. It is true that dispersive effects can alter the
effective dielectric constant vs frequency, but this usually does not
come into play until you exceed 2 GHz. As frequency increases, so would
the effective dielectric constant. Now in the case of FR4 you have the
the dielectric constant of the material itself is changing vs frequency
(decreases) at a greater rate than effective dielectric constant. Let me
know if there are any other questions.

Art Aguayo
[log in to unmask]

KC Chan wrote:
>
> Hi all
>
> Correct me if I am wrong.
>
> Effective dielectric constant of a material changes vs frequency, no
> matter what the dielectric is.  When frequency increases, the effective
> dielectric constant would get close to the material dielectric constant.
> The reason why we have the effective dielectric constant is due to the
> E-field trapped into the substrate.
>
> By the way, does someone know about any news group or web sites about
> Microwave or High frequency stuff.
>
> KC Chan
> IMS
>
> > ----------
> > From:         Arturo J. Aguayo[SMTP:[log in to unmask]]
> > Reply To:     [log in to unmask]
> > Sent:         Sunday, May 24, 1998 11:02 PM
> > To:   [log in to unmask]
> > Subject:      Re: [TN] Frequency Performance of FR-4
> >
> > Hi to All,
> >
> > Been following this long string of emails and just wanted to add a
> > comment or two from an RF/microwave point of view (which from some of
> > the comments I think probably already know). I also work for a high
> > frequency material supplier and have come across designs that use FR4
> > at
> > 2 GHz and PTFE materials at 880 MHz. The case is that each application
> > is different and there is no set limit at which one type of material
> > ends and the other starts. One trend we have seen is that if the
> > application is digital, the switch to high performance materials is
> > sooner. The reason is that a square signal is made of of a series of
> > sine waves with a fundamental and a series of harmonics. When the
> > dielectric constant of a material changes vs frequency (which FR4
> > does),
> > the speed of each components is different. Faster rise times create
> > the
> > need for higher level harmonics and these can start getting quite high
> > into the RF/microwave range (where dielectric losses are quite
> > critical). Hope this helps.
> >
> > Art Aguayo, Technical Support Manager
> > Rogers Corporation
> > [log in to unmask]
> >
> >
> > Fred Watt wrote:
> > >
> > >         Thank you all for your inputs and thoughts on the subject of
> > Frequency performance of FR-4 material.  This is an interesting
> > Discussion.  Hope it continues.
> > >
> > >         The Rise time of 175 psec was degraded to 375 psec.
> > >
> > > Moral:
> > >         Use proper equipment and understand the theory before
> > jumping to conclusions:
> > >
> > > Tenative conclusion:
> > >         FR-4 is acceptable for rise times of 500 psec and
> > frequencies up to 1 GHz.
> > >
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