[SI-LIST] Re: (no subject)

Hi Steve and Scott;

Steve - you have to look at the other side of the equation C=3Dlamda =
times frequency, dividing both sides by the sqrt of the dielectric =
constant gives reduction in wavelength , which yields are reduction in =
velocity.

Scott- I will read the article however I have a feeling that dielectric =
dependance on frequency has more to do with manufacturing flaws in the =
substrate - if you are talking about pure dielectrics I still maintain =
that the dielectric constant is not frequency dependent - check out this =
link http://www.rogerscorporation.com/mwu/pdf/rt2121.pdf

Justin

PS we only use FR4 for power and groud layers , never for HF because of =
it's high loss

-----Original Message-----
From: Steve Corey [mailto:steven.corey@xxxxxxxxxxxxxx]
Sent: Wednesday, December 11, 2002 11:37 AM
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: (no subject)



Or perhaps take this approach, since as the modeling guy I always resort =

to equations:

v =3D c/sqrt(er)

v is the velocity of the wave, c is the speed of light in a vacuum, and=20
we assume that the relative permeability is unity for simplicity.

Now, if we have already settled that v can vary with frequency, and=20
hopefully we can agree that c doesn not vary with frequency, that leaves =

only er to vary with frequency.  If er is constant, so is v.

If this doesn't seem authoritative enough, consult any undergraduate=20
physics book or electromagnetics book published over the past 100 years. =

  It will tell you that er varies with frequency.  I find it a good=20
practice to consult such books prior to posting, since half the time the =

person who wrote the book is lurking on this list.

   -- Steve


Scott McMorrow wrote:

> Justin,
>=20
> You are just plain wrong here about FR4.  It is a highly non-linear=20
> material that does have significant changes in Er across frequency =
that=20
> have been characterized in the peer reviewed engineering literature =
for=20
> at least 20 years.  Dielectric losses and changes in Er are linked by=20
> causality as Steve Corey wrote a few days ago, and has been well=20
> discussed in the professional and academic journals.  The latest issue =

> of IEEE Transactions on Advanced Packaging, which includes papers from =

> last years EPEP, has several papers on just this subject.
>=20
> For FR-4, non-uniform dielectric properties throughout the material =
tend=20
> mask the effects of frequency dependent Er (or velocity of =
propagation)=20
> changes.  (i.e. - you get more impedance variation due to material=20
> variation than due to Er changes across frequency.)  For low loss=20
> materials, such as Real Air, the dielectric loss and Er does change,=20
> based upon the relative humidity.  Pure air, however,  does have a =
flat=20
> frequency response and extremely low losses.
>=20
> As for broadband, just launch a gaussian pulse down a microstrip or=20
> stripline trace on an FR4 substrate and see what you receive at the =
far=20
> end.  After you subtract out the effects of variable skin depth=20
> penetration at different frequency (due to the finite conductivity of=20
> the conductor) you will find that there is still pulse distortion due =
to=20
> Er variation.
>=20
> Finally, I would refer you to the material data sheets for many=20
> different vendors versions of FR4, where you will find that the =
measured=20
> Er does truly vary with frequncy, tending to lower at the higher=20
> frequencies.  Since the velocity of propagation of an electromagnetic=20
> wave in a dielectric media is proportional to the square root of the =
Er,=20
> I would expect an accompanying dispersion of any broadband signal =
across=20
> the entire frequency band, with an accompanying change in Impedance.
>=20
> regards,
>=20
> scott
>=20
>=20


--=20
-------------------------------------------
Steven D. Corey, Ph.D.
Time Domain Analysis Systems, Inc.
"The Interconnect Modeling Company."
http://www.tdasystems.com

email: steven.corey@xxxxxxxxxxxxxx
phone: (503) 246-2272
fax:   (503) 246-2282
-------------------------------------------

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