[SI-LIST] Re: (no subject)
- From: "Rich Peyton" <p2rich@xxxxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Thu, 12 Dec 2002 09:06:05 -0500
FYI:
Here's some info regarding Er vs. Freq.:
http://www.rogers-corp.com/mwu/techtip9.htm
http://www.dupont.com/kapton/general/spfreq.html
I had a very good report on the subject, but I cannot find it after tearing
apart my cubical (Oh well). The above are from doing a quick search.
Rich
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]On
Behalf Of Scott McMorrow
Sent: Wednesday, December 11, 2002 1:45 PM
To: justin.tabatchnick@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: (no subject)
Justin,
You are just plain wrong here about FR4. It is a highly non-linear
material that does have significant changes in Er across frequency that
have been characterized in the peer reviewed engineering literature for
at least 20 years. Dielectric losses and changes in Er are linked by
causality as Steve Corey wrote a few days ago, and has been well
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.
For FR-4, non-uniform dielectric properties throughout the material tend
mask the effects of frequency dependent Er (or velocity of propagation)
changes. (i.e. - you get more impedance variation due to material
variation than due to Er changes across frequency.) For low loss
materials, such as Real Air, the dielectric loss and Er does change,
based upon the relative humidity. Pure air, however, does have a flat
frequency response and extremely low losses.
As for broadband, just launch a gaussian pulse down a microstrip or
stripline trace on an FR4 substrate and see what you receive at the far
end. After you subtract out the effects of variable skin depth
penetration at different frequency (due to the finite conductivity of
the conductor) you will find that there is still pulse distortion due to
Er variation.
Finally, I would refer you to the material data sheets for many
different vendors versions of FR4, where you will find that the measured
Er does truly vary with frequncy, tending to lower at the higher
frequencies. Since the velocity of propagation of an electromagnetic
wave in a dielectric media is proportional to the square root of the Er,
I would expect an accompanying dispersion of any broadband signal across
the entire frequency band, with an accompanying change in Impedance.
regards,
scott
--
Scott McMorrow
Teraspeed Consulting Group LLC
2926 SE Yamhill St.
Portland, OR 97214
(503) 239-5536
http://www.teraspeed.com
Tabatchnick, Justin wrote:
>Sandor;
>
>One more thing to consider
>
>Air is a dielectric same as alumina , rogers , etc. - it has a =
>dielectric constant of 1 - according to you the dielectric constant will =
>vary with frequency- then how can you explain white light which is a =
>composite of a number of frequencies all traveling at the same speed. =
>Also if the dielectric constant is frequency dependent then stripline =
>and microstrip characteristic impedance will also vary with frequency =
>since it is a function of the dielectric constant - all who have =
>designed plain TL know that it is extremely broad band and that the =
>characteristic impedance is constant as long as the geometry remains the =
>same as well as the substrate.=20
>
>Justin
>-----Original Message-----
>From: Sandor [mailto:sandor@xxxxxxxxxx]
>Sent: Tuesday, December 10, 2002 9:09 PM
>To: justin.tabatchnick@xxxxxxxxx
>Cc: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: (no subject)
>
>
>Hi Justin,
>
>
>
>>Hi Sandor;
>>=20
>>I beg to differ - I don't know who wrote the article but they=20
>>are wrong =3D
>>- They mention that the dielectric constant varies with=20
>>frequency - is =3D
>>it a linear or nonlinear variation
>>
>>
>
>In real life, the relationship is probably nonlinear, but does it really =
>
>matter? The important thing for the arguments sake is that it varies=20
>(with frequency, given a certain dielectric material). That's probably=20
>where our opinions diverge...
>
>
>
>>- I don't think they even know =3D
>>because they are wrong -are you telling me that the wavelenth=20
>>does not =3D
>>change in a medium that it is the dielectric constant or are=20
>>you saying =3D
>>that the frequency changes with the dielectric constant -=20
>>
>>
>
>Is it a trick question? :-)
>
>Everything changes. What we are comparing is the ideal case of Er being=20
>constant versus being variable with frequency. For a given frequency,=20
>well frequency is constant but the wavelength will change with any=20
>difference in Er compared to the ideal case due to a resulting different =
>
>propagation speed.
>
>
>
>>think of this =3D
>>- if the frequency changes at the medium interface ,
>>
>>
>
>It's the tail wagging the dog again. I did not say, at least I never=20
>meant to say that the frequency changes at the medium interface. What=20
>did I say that implied this?
>
>
>
>>it goes=20
>>from higher =3D
>>to lower or reverse then at the interface there is a=20
>>discontinuity that =3D
>>can not be imagined. I have better analogy let's discount any=20
>>losses, =3D
>>energy equals plank's constant times the frequency . Energy=20
>>conservation =3D
>>dictates that the energy crossing a boundry will be the same on both=20
>>
>>
>=3D
>
>
>>sides in a lossless medium which means the frequency has to remain =3D
>>constant.
>>=20
>>Justin
>>
>>
>
>I can not possibly argue against the conservation of energy but I'm not=20
>quite sure how that ties back to the original argument about dispersion. =
>
>If the system is lossless, you can pump in a certain amount of energy=20
>in and the same will come out in the end, but nothing says it all has to =
>
>come out with the same timing profile.
>
>Perhaps what we had been saying were simply on two different planes=20
>which intersect somewhere...
>
>I see Er as a complex number, a bit like <insert feverish internet=20
>search here> what is described in=20
>http://www.sbu.ac.uk/water/microwave.html , and well, that article=20
>explains everything better then I could. I couldn't quite work out at=20
>the time, but perhaps it's something related to this that you were=20
>referring to at the end of your original contribution to this thread,=20
>when you mentioned the loss tangent.
>
>It's just when you said...
>
>
>
>>- you can =3D3D
>>explain light dispersion not by a variation of dielectic with=3D20
>>frequency =3D3D
>>but a variation of speed in the medium with the relative=20
>>dielectric =3D =3D3D
>>constant . Velocity =3D3D3D frequency times wavelength ,=3D20
>>wavelength is equal =3D3D
>>to the wavelength divided by the square root of the relative=3D20
>>dielectric =3D3D
>>constant- white light is made up of a spectrum of varying=3D20
>>wavelengths =3D3D
>>all traveling at the same speed in air however in a medium=3D20
>>their speeds =3D3D
>>differ and that is why you see a seperation.
>>
>>
>
>..I could not work out how different frequency components of light can=20
>travel at different speeds unless the Er they are "seeing" is different. =
>
>The speed is different because the Er is different. This may be a=20
>question of viewpoint, and I'm sure first the variability of speed was=20
>measured and then Er was "invented" to model the phenomenon with a nice=20
>equation. I guess now we are arguing chicken and egg.
>
>The point is, I don't see how you can have different speeds in a medium=20
>(and I regard air as a medium) unless the Er is different.
>
>
>
>>the only way you will see a =3D3D
>>variation in the dielectric constant is in a non-homogenous=3D20
>>medium.
>>
>>
>
>Let's take a perfectly _homogeneous_ piece of glass in a shape of a=20
>prism. It would still bend different frequency light differently. We=20
>seem to agree that different spectral parts of the white light will=20
>travel at different speed. The actual speed of the light is=20
>1/SQRT(Eo*Er*Mu). How can we have different speeds for the different=20
>spectral components without a difference in relative permettivity?
>
>Regards,
>
>Sandor
>_____________________________________________________________
>
>
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