[SI-LIST] Re: Dispersion
- From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
- To: <deibele@xxxxxxx>
- Date: Wed, 11 Dec 2002 12:33:58 -0800
This part of the thread (discussion of dispersion) began when I asked =
the question below. It seems that we are back to the original question.
ORIGINAL QUESTION:
When you use the term "dispersive", are you talking about losses =
(resistive, skin effect, dielectric), or about differences in phase =
velocities (page 170 of Pozar's book)? I've heard others refer to loss =
effects as dispersive and have had confusion as a result. Are both uses =
of the term "dispersive" correct?
The explanation of how to measure dispersion (S21 magnitude) implies you =
believe "dispersion" and what I would have termed "effects of conductor =
and dielectric losses" are the same. I have trouble with that, since =
stripline insertion loss magnitude definitely varies with F, and that =
effect is explained without dispersion. I believe "dispersion" is a =
separate effect than conductor and dielectric losses. The only tie =
between them that I've heard of is that Steve Corey (who I am loath to =
contradict) stated "if a material is dispersive, it is also lossy". It =
may be that the converse holds (if a material is lossy, it is also =
dispersive), but I believe the 2 effects are separate (even if one can't =
occur without the other). Maybe Steve would clarify this?
I couldn't follow the explanation of "knobbing" electrical delays until =
S21 phase is flat. Is that legal? ;-)
Jeff Loyer
-----Original Message-----
From: C Deibele [mailto:deibele@xxxxxxx]
Sent: Wednesday, December 11, 2002 10:20 AM
To: dgun@xxxxxxxxxx; si-list
Subject: [SI-LIST] Re: Dispersion
Dispersion can mean a myriad of things.
The discussion about phase is correct - namely that the phase changes =
linearly=20
w.r.t. frequency.
but dispersion also manifests itself in the magnitude of the response. =
lossy=20
materials that have a nice linear phase change are even more dispersive =
than=20
simple aforementioned phase requirements.
a quick way to measure dispersion on a network analyzer is to measure =
s21 --=20
is the magnitude flat over the frequency range of interest? examine the =
phase=20
measurement of s21, and knob the electrical delay until it is "flat" in =
the=20
frequency range of interest. if it is not "flat," then you have =
dispersion. =20
If the phase is "flat" and does not lie at zero or 180 degrees, then the =
device distorts as well.
-craig
>=3D=3D=3D=3D=3D Original Message From dgun@xxxxxxxxxx =3D=3D=3D=3D=3D
>Jeff,
>
>1) ADS can simulate dispersion. However, not all models include =
dispersion. =20
You must check the documentation to found out what models include =
dispersion=20
and what dispersion model is used. In ADS2002 (don't know about earlier =
versions), you can
>choose between three different dispersion models for microstrip lines.
>
>2) You are correct that a non-dispersive transmission line should show =
linear=20
phase response over all frequencies. Therefore, one way to measure =
dispersion=20
is to take the derivative of the phase response with respect to =
frequency=20
("group delay"
>function). In typical VNA measurements, this derivative can be very =
noisy=20
(as derivatives typically are) and mask dispersion (watch out for those =
-180=20
to +180 phase transitions!). You need to carefully smooth the trace. =
VNAs=20
typically do this by
>taking a type of "moving average".
>
>This method won't really give you an accurate dispersion quantity, but =
will=20
tell you how dispersive a trace is, and up to what frequency it is =
relatively=20
non-dispersive.
>
>For typical materials (like FR4 and alumina), microstrip dispersion is =
very=20
well characterized and formulas abound in literature.
>
>- Daniel
>
>From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
>>
>> Thanks for all the information. Not to beat a dead horse, but can =
=3D
>> someone give me the definitive answers on:
>>
>> 1) Whether Hspice or ADS can simulate dispersion?
>>
>> 2) How would dispersion show itself? I would expect to measure the =
=3D
>> effect with a VNA - I would expect my S21 phase rate-of-change to =
vary =3D
>> with frequency. I.E., for a non-dispersive medium, S21 phase appears =
as =3D
>> a regular sawtooth. For a dispersive medium, I would expect the =
period =3D
>> of that sawtooth to vary with frequency (probably very linearly). Of =
=3D
>> course, it would also be seen on a pulse as a rounding of the edges, =
but =3D
>> it couldn't be separated from loss effects in that case.
>>
>> Jeff Loyer
>>
>> -----Original Message-----
>> From: Loyer, Jeff=3D20
>> Sent: Tuesday, December 10, 2002 9:55 AM
>> To: 'lifehappiest@xxxxxxxxxxx'; steven.corey@xxxxxxxxxxxxxx;
>> si-list@xxxxxxxxxxxxx
>> Subject: RE: [SI-LIST] Re: (no subject)
>>
>>
>> I've been trying to better understand dispersion and its effects on =
=3D
>> signal integrity. Allow me to state what I believe, and feel free to =
=3D
>> correct any falacies I've developed (or clarify what I've stated).
>>
>> 1) The key factor "creating" dispersion is the fact that, for a given =
=3D
>> medium, different frequencies will travel at different velocities. =
=3D
>> I.E., the Er for a material varies, depending on frequency. This =
same =3D
>> effect is responsible for prisms' breaking light into its constituent =
=3D
>> components and, interestingly enough, causes distortion of earthquake =
=3D
>> waves as they travel through the earth.
>>
>> 2) I've heard the term associated with microstrip, but don't know why =
it =3D
>> couldn't be present in stripline.
>>
>> 3) The net effect on our (digital S.I. types) signals of interest =
would =3D
>> be that a pulse would appear rounded after passing through a =
dispersive =3D
>> medium.
>>
>> 3) I don't think Hspice can simulate this phenomena, and I don't know =
if =3D
>> ADS can, either.
>>
>> 4) The way I would expect to measure the effect would be with a VNA - =
I =3D
>> would expect my S21 phase rate-of-change to vary with frequency. =
I.E., =3D
>> for a non-dispersive medium, S21 phase appears as a regular sawtooth. =
=3D
>> For a dispersive medium, I would expect the period of that sawtooth =
to =3D
>> vary with frequency (probably very linearly). I haven't measured the =
=3D
>> effect this way (that I know of, it might have been too subtle for me =
to =3D
>> notice), nor have I seen data demonstrating the effect.
>>
>> I would love clarification on the terms "medium dispersion" vs. "wave =
=3D
>> dispersion" vs "phase dispersion". I thought there was only a single =
=3D
>> dispersion effect of phase velocities being different for different =
=3D
>> frequencies.
>>
>> Thanks for your insights...
>>
>> Jeff Loyer
>>
>>
>> -----Original Message-----
>> From: Xin Wu [mailto:lifehappiest@xxxxxxxxxxx]
>> Sent: Tuesday, December 10, 2002 6:54 AM
>> To: steven.corey@xxxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
>> Subject: [SI-LIST] Re: (no subject)
>>
>>
>>
>> Steve, it is not a good idea to mix things together. Wave =
dispersion=3D20
>> definitely has nothing to do with loss, conceptually. Dispersion is =
=3D
>> related=3D20
>> to phase velocity, but the loss is related to energy. Of cause, =
because=3D20
>> signal pulse consists of a bunch of frequency components, =
different=3D20
>> components propagate at different speed, this makes the pulse shape =
=3D
>> change=3D20
>> at the reception. However, the total energy of the pulse won't =
be=3D20
>> dissipated. Dispersion of medium is another concept. If we talking =
about =3D
>> a=3D20
>> dispersive medium, this means that the dielectric constant of the =
medium =3D
>> is=3D20
>> frequency dependent, the medium is dispersive. Kramer Kronig =3D
>> relationship is=3D20
>> to explain the medium dispersion rather than wave dispersion.
>> Of cause, when the medium is frequency dependent, both wave =
dispersion =3D
>> and=3D20
>> signal loss will be happening.
>>
>>
>>
>>
>>
>> Xin Wu
>> Ph.D student,
>> University of Maryland, College Park
>> Http://www.wam.umd.edu/~xwu
>>
>>
>>
>>
>>
>>
>> >From: Steve Corey <steven.corey@xxxxxxxxxxxxxx>
>> >Reply-To: steven.corey@xxxxxxxxxxxxxx
>> >To: si-list@xxxxxxxxxxxxx
>> >Subject: [SI-LIST] Re: (no subject)
>> >Date: Mon, 09 Dec 2002 16:44:28 -0800
>> >
>> >
>> >This illustrates an interesting point, which is that if you have
>> >dispersion, then you have to have frequency-dependent losses as =
well.
>> >The Kramers-Kronig relationship (a.k.a. causality) states that the =
real
>> >part and the imaginary parts of the dielectric constant are tightly
>> >related to each other. As a result, frequency-dependent phase =
velocity
>> >is always accompanied by frequency-dependent attenuation, and vice
>> >versa. So you can easily make the argument that describing =
something =3D
>> as
>> >"dispersive" is the same as describing it as "lossy".
>> >
>> >Yu Liu wrote:
>> >
>> > > Jeff,
>> > >
>> > > By definition, I think dispersive effect is the frequency-related =
=3D
>> effect
>> > > (phase velocities at different spectrums). However, since the =
=3D
>> losses
>> > > are also frequency-dependent, sometimes people just use the term
>> > > interchangbly, which causes confusions. Distinguish the two terms
>> > > (dispersive and loss) would make things clearer.
>> > >
>> > > Yu
>> > > =3D3D=3D3D=3D3D
>> > >
>> > > Loyer, Jeff wrote:
>> > >
>> > >
>> > >>Yu,
>> > >>When you use the term "dispersive", are you talking about =
losses=3D20
>> >(resistive, skin effect, dielectric), or about differences in =
phase=3D20
>> >velocities (page 170 of Pozar's book)? I've heard others refer to =
loss =3D
>>
>> >effects as dispersive and have had confusion as a result. Are both =
=3D
>> uses of=3D20
>> >the term "dispersive" correct?
>> > >>
>> > >>Jeff Loyer
>> > >>
>> > >>
>> > >>-----Original Message-----
>> > >>From: Yu Liu [mailto:yu_liu@xxxxxxxxxxxxx]
>> > >>Sent: Friday, December 06, 2002 9:32 PM
>> > >>To: xlzhou@xxxxxxxxx
>> > >>Cc: 'Xin Wu'; si-list@xxxxxxxxxxxxx
>> > >>Subject: [SI-LIST] Re: (no subject)
>> > >>
>> > >>
>> > >>
>> > >>Mick,
>> > >>
>> > >>To answer your question: In theory, Yes. In practice, No.
>> > >>
>> > >>The fundamental principle for SPICE model approach is using =
lumped
>> > >>elements (RLC) to model distributed effect (S-parameter). For =
lower
>> > >>frequency and small size, it is possible. But for broad-band =3D
>> frequency
>> > >>and large size (say, 20' FR-4), it is very hard to model the =3D
>> dispersive
>> > >>effects accurately. That's why people start to use =3D
>> frequency-dependent
>> > >>elements, or better yet, import S-parameter directly into Spice =
=3D
>> engine.
>> > >>
>> > >>
>> > >>Regards,
>> > >>
>> > >>Yu
>> > >>=3D3D=3D3D=3D3D
>> > >>
>> > >>Apache Design Solutions
>> > >>web: www.apache-da.com
>> > >>
>> > >>
>> > >>
>> > >>Zhou, Xingling (Mick) wrote:
>> > >>
>> > >>
>> > >>
>> > >>>Xin Wu
>> > >>>
>> > >>>Thanks for your information. It will be valuable.
>> > >>>However, it still contains something like (from the example on =
the =3D
>> web=3D20
>> >you
>> > >>>referred)
>> > >>>
>> > >>>E11 11 12 FREQ {V(10,5)}=3D3D
>> > >>>
>> > >>>which will not be accepted by some circuit simulators such as =
ADS. =3D
>> It=3D20
>> >is
>> > >>>similar to Ansoft fullwave SPICE. I know HSPICE and PSPICE can =
=3D
>> handle=3D20
>> >it.
>> > >>>
>> > >>>Although it is still a research topic in terms of efficiency, =
=3D
>> stability=3D20
>> >and
>> > >>>accuracy etc., I know the problem [S] ---> SPICE model (reduced =
=3D
>> models)=3D20
>> >is
>> > >>>solvable without the following limitations in several ways. But, =
=3D
>> can we
>> > >>>generate broadband SPICE models without frequency dependent =
lookup=3D20
>> >tables
>> > >>>such as " FREQ " ? Or is it still solvable without components =
=3D
>> "FREQ"=3D20
>> >etc. ?
>> > >>>
>> > >>>Regards,
>> > >>>
>> > >>>Mick
>> > >>>
>> > >>>-----Original Message-----
>> > >>>From: Xin Wu [mailto:lifehappiest@xxxxxxxxxxx]
>> > >>>Sent: Friday, December 06, 2002 8:38 PM
>> > >>>To: scott@xxxxxxxxxxxxx; xlzhou@xxxxxxxxx
>> > >>>Cc: si-list@xxxxxxxxxxxxx
>> > >>>Subject: Re: [SI-LIST] Re: (no subject)
>> > >>>
>> > >>>
>> > >>>Hi, XL,
>> > >>> If you want to convert S-matrix into a spice simulator, a lot =
of
>> > >>>commercial software can do that--that's the simplest way. =
However, =3D
>> it=3D20
>> >is
>> > >>>possible for yourself to make a equivalent subcircuit which can =
=3D
>> include=3D20
>> >the
>> > >>>timing and frequency info and then import to a spice like =3D
>> simulator.=3D20
>> >Here
>> > >>>are some application notes:
>> > >>>http://wwwinfo.cern.ch/ce/ae/Maxwell/apps/2stripem/2stripem.html
>> > >>>If you want to get involved in more details, I remembered that =
=3D
>> somebody=3D20
>> >did
>> > >>>his Ph.D work on this in UIUC around early 1990s'.
>> > >>>
>> > >>>
>> > >>>
>> > >>>Xin Wu
>> > >>>Ph.D student,
>> > >>>University of Maryland, College Park
>> > >>>Http://www.wam.umd.edu/~xwu
>> > >>>
>> > >>>
>> > >>>
>> > >>>
>> > >>>
>> > >>>
>> > >>>
>> > >>>
>> > >>>>From: "Scott McMorrow" <scott@xxxxxxxxxxxxx>
>> > >>>>Reply-To: scott@xxxxxxxxxxxxx
>> > >>>>To: xlzhou@xxxxxxxxx
>> > >>>>CC: "'si-list@xxxxxxxxxxxxx'" <si-list@xxxxxxxxxxxxx>
>> > >>>>Subject: [SI-LIST] Re: (no subject)
>> > >>>>Date: Fri, 06 Dec 2002 13:45:41 -0800
>> > >>>>
>> > >>>>
>> > >>>>Mick,
>> > >>>>
>> > >>>>There are two solutions which I use:
>> > >>>>
>> > >>>>1) Apache Nspice is an Hspice compatabible simulator that can
>> > >>>>co-simulate with Touchstone S-parameter files.
>> > >>>>http://www.apache-da.com/
>> > >>>>
>> > >>>>2) Sigrity BroadBand Spice can convert Touchstone s-parameter =
=3D
>> files=3D20
>> >into
>> > >>>>spice black boxes.
>> > >>>>http://www.sigrity.com/
>> > >>>>
>> > >>>>Both are scary accurate in both the frequency and time domain.
>> > >>>>
>> > >>>>regards,
>> > >>>>
>> > >>>>scott
>> > >>>>
>> > >>>>--
>> > >>>>Scott McMorrow
>> > >>>>Teraspeed Consulting Group LLC
>> > >>>>2926 SE Yamhill St.
>> > >>>>Portland, OR 97214
>> > >>>>(503) 239-5536
>> > >>>>http://www.teraspeed.com
>> > >>>>
>> > >>>>
>> > >>>>Zhou, Xingling (Mick) wrote:
>> > >>>>
>> > >>>>
>> > >>>>
>> > >>>>
>> > >>>>>Hello,
>> > >>>>>
>> > >>>>>Did anybody successfully generate broadband (DC-20GHz) SPICE =
=3D
>> model=3D20
>> >from S
>> > >>>>>matrix for complex structures using the SPICE generator in ADS =
? =3D
>> Or=3D20
>> >any
>> > >>>>>other alternatives. I know Ansoft fullwave SPICE does the =
work.
>> > >>>>>Unfortunately, some circuit simulators do not support the =3D
>> frequency
>> > >>>>>dependent lookup table sources. We can definitely use [S] =3D
>> directly,=3D20
>> >but
>> > >>>>>
>> > >>>>>
>> > >>>>>
>> > >>>>some
>> > >>>>
>> > >>>>
>> > >>>>
>> > >>>>>users still prefer SPICE models. Any experience to share ?
>> > >>>>>
>> > >>>>>Thanks,
>> > >>>>>
>> > >>>>>Xingling(Mick) Zhou, PhD
>> > >>>>>Signal Integrity Technologist
>> > >>>>>Agere Systems
>> > >>>>>
>> > >>>>>Tel: 610-712-7462
>> > >>>>>Fax: 610-712-4081
>> > >>>>>
>> > =3D
>> =
>>>>>------------------------------------------------------------------
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>> >
>> >--
>> >-------------------------------------------
>> >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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>
>
>--
>Daniel
>ZZZ-dgun-ZZZ-@xxxxxxxxxxxxxxx
>(remove the Z-'s to reply--they're what I do when I read spam)
>
>--
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