[SI-LIST] Re: impedance relation with frequency...
- From: Martyn Gaudion <martyn.gaudion@xxxxxxxxxxxxxxxxxxxx>
- To: jonpowell@xxxxxxxxxxxx
- Date: Thu, 25 Sep 2003 17:07:55 +0100
Hi Jon,
The answer is "it depends" - the non homogenous nature of
FR4 ( and all woven glass reinforced materials ) mean that
Er will vary with both scale and structure...
For an example have a look at this note:
www.polarinstruments.com/support/cits/AP139.html
Kind regards
Martyn Gaudion
www.polarinstruments.com
At 16:05 25/09/2003, you wrote:
>Actually I wasn't referring to Er variation in a given batch so much as
>variation across a large manufacturing run. How about a counter question:
>What is the observed Er of FR4? That is, given any random sample, what is
>the range?
>
>
>-----Original Message-----
>From: Loyer, Jeff [mailto:jeff.loyer@xxxxxxxxx]
>Sent: Thursday, September 25, 2003 7:55 AM
>To: Jon Powell
>Cc: si-list@xxxxxxxxxxxxx
>Subject: RE: [SI-LIST] Re: impedance relation with frequency...
>
>
>Hi Jon,
>Your posting made me a bit curious - it seems to imply that you've seen
>large Er variations in PCB's.
>
>I agree that I expect much larger variation of Z0 than is shown in the 2nd
>graph of Martyn's posting, so the effect of Er variation on Z0 is
>insignificant compared to other effects - etching and thickness variations,
>etc.
>
>On the other hand, I personally have seen very little variation in Er, as
>derived from Vp measurements using TDR (or S21 phase w/ VNA). Have you seen
>substantial Vp variations (or some other variation that you would directly
>attribute to Er variation)?
>
>Jeff Loyer
>
>-----Original Message-----
>From: Jon Powell [mailto:jonpowell@xxxxxxxxxxxx]
>Sent: Thursday, September 25, 2003 7:37 AM
>To: martyn.gaudion@xxxxxxxxxxxxxxxxxxxx; Loyer, Jeff
>Cc: si-list@xxxxxxxxxxxxx
>Subject: RE: [SI-LIST] Re: impedance relation with frequency...
>
>
>I would expect to see greater ranges in Er and Zo across a board (or set of
>boards) just from manufacturing tolerances. Is this sort of effect really
>big enough to enter into the equation? Isn't this magnitude of change
>completely swamped by other (more or less random) considerations? For
>instance, what is the tolerance of Er for FR4 for any given manufacturing
>line or run? (or even the tolerance for thickness across a board).
>
>
>If you have little or no control over a large variable, then why bother with
>worry over a small contributor?
>
>jon
>
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx
>[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Martyn Gaudion
>Sent: Tuesday, September 23, 2003 3:29 PM
>To: jeff.loyer@xxxxxxxxx
>Cc: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: impedance relation with frequency...
>
>
>Hi Jeff,
>
>A number of posts have discussed loss on silicon and
>on longer PCB traces, however loss also starts to become significant
>on fine line (3mil and less) traces especially where copper weights
>under 1/2 ounce are employed.
>
>
>On broader traces where dc and ac series loss is insignificant
>you can get an idea of the expected variation over frequency
>by graphing the change in Zo due to Er change, - many laminate
>manufacturers will have the graphs. Zo varies as 1/sq root Er so
>in a lossless case the variation is small. Most of the change
>occurs as Andy points out at the lower end of the frequency
>spectrum.
>
>Here is a link to a fairly generic graph of Zo Vs Frequency for
>FR4
>
>http://www.polarinstruments.com/support/cits/images/ap155_1.jpg
>
>And the resultant modelled variation in impedance (lossless case)
>
>www.polarinstruments.com/support/cits/images/ap155_2.jpg
>
>Kind regards
>Martyn Gaudion
>www.polarinstruments.com
>
>
>
>
>At 12:40 PM 24/09/03 -0700, you wrote:
> >Hi Andrew,
> >I would like some help understanding the difference between board traces =
> >and "chip lines". My experience has been that I can TDR a trace using a =
> >35ps risetime, or through 100 and 400ps filters, and measure the same Z0 =
> >for that trace. This would seem to be backed up by the fact that there =
> >is no compensation made when measuring traces with different TDRs, =
> >regardless of their risetime.
> >
> >I just confirmed that again, measuring the same 3" microstrip trace with =
> >no filter, and 100ps and 400ps filters, and finding the TDR and TDT =
> >waveforms stabilize at the same level, regardless of the risetime. Of =
> >course, there's significant impact to the risetime of the TDT, but the =
> >Z0 of the trace (as indicated by the DC level of the TDR trace) remains =
> >constant.
> >
> >This implies to me that the Z0 of the trace is constant for 10GHz , =
> >3.5GHz, or 875MHz (35ps, 100ps, 400ps risetimes, respectively).
> >
> >My experience with VNA seems to substantiate this - S11 typically =
> >remains fairly constant (other than resonances at lambda/4, etc.) while =
> >S21 varies with frequency due to loss effects.
> >
> >Is there something else I'm missing?
> >
> >Jeff Loyer
> >
> >-----Original Message-----
> >From: andrew.c.byers@xxxxxxxxxxxxxx
> >[mailto:andrew.c.byers@xxxxxxxxxxxxxx]
> >Sent: Wednesday, September 17, 2003 12:05 PM
> >To: jonpowell@xxxxxxxxxxxx; kbagga31@xxxxxxxxx; si-list@xxxxxxxxxxxxx
> >Subject: [SI-LIST] Re: impedance relation with frequency...
> >
> >
> >Concerning Zo relation with frequency:
> >
> >Once again, depends where you live. On boards, typically the
> >frequency-dependent impedance change starts leveling out at much lower
> >frequencies. Essentially you are approaching the sqrt(L/C) impedance,
> >because your omega*L overwhelms your R. *Usually* by 1MHz your Zo curve =
> >is
> >flat. But if you are modeling chip lines, your R value for the line =
> >might be
> >comparable (or greater) than omega*L up to a couple GHz or so. Then you
> >cannot ignore this frequency dependent behavior. I have seen a typical =
> >line
> >on chip go from about Zo=3D100ohms @100MHz, to Zo=3D63ohms @1GHz, to =
> >Zo=3D55ohms
> >@10GHz. Measurement, simulation, theory, literature, and gut feel all =
> >back
> >this up.=20
> >
> >So the bottom line (as it always is in the world of interconnect =
> >modeling)
> >is it depends on how high you go in frequency, the dimensions of line =
> >you
> >are using, and if you are designing in a narrow band or a wide band.
> >HOWEVER, as Jon pointed out, you can often see greater variations due to
> >coupling from nearby traces. Plus you have to remember that impedance
> >control is an issue too - usually +/- 10% is as good as it gets for
> >run-of-the-mill PCBs out there (but money talks).=20
> >
> >To get a feel for the numbers I got above, you can use a 2D field solver
> >that handles the frequency dependent behavior of R and L (ansoft =
> >spicelink
> >or some other flavor). Or you can dig up equations and plug them into a
> >matlab or mathcad. Calculate your R and L and C (usually G is =
> >non-existant
> >or insignificant...) and crunch away.
> >
> >salud,
> >Andy Byers
> >
> > =20
> >
> >-----Original Message-----
> >From: Jon Powell [mailto:jonpowell@xxxxxxxxxxxx]=20
> >Sent: Wednesday, September 17, 2003 9:09 AM
> >To: kbagga31@xxxxxxxxx; si-list@xxxxxxxxxxxxx
> >Subject: [SI-LIST] Re: impedance relation with frequency...
> >
> >
> >Karen,
> >It is my feeling that the frequency related impedance changes on a =
> >signal
> >will be second order considerations compared to the impedance changes =
> >caused
> >by crosstalk from neighboring wires. These effects can be shown with =
> >most
> >good SI engines. Intel has often recommended (for instance) calculating =
> >the
> >effective impedance when the coupled wires on either side of the target =
> >wire
> >switch simultaneously with the target wire in both the same direction =
> >(all
> >going high and low) and opposite (target going high and low and coupled
> >going low and high).
> >
> >hope this helps (and if I am wrong, I am sure someone will scream at me =
> >so
> >wait a couple of minutes).
> >
> >regards,
> >jon
> >
> >
> >-----Original Message-----
> >From: si-list-bounce@xxxxxxxxxxxxx
> >[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of karan bagga
> >Sent: Wednesday, September 17, 2003 2:24 AM
> >To: si-list@xxxxxxxxxxxxx
> >Subject: [SI-LIST] impedance relation with frequency...
> >
> >
> >Hi
> >
> > >From the telegraphic equations on Transmision lines it seems the =
> >impedance
> >of the Trace varies with frequency.
> >
> >In my design specifications it is specified that my trace should be on =
> >(25
> >+/- 10%) Ohms.
> >How will I do it ? How will I do these kind of analysis?
> >
> >The frequency of the signal is high and also the rise time is =
> >significantly
> >low.
> >Will FFT be of some help here ?
> >
> >Regards
> >Karan.
> >
> >
> >
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