[SI-LIST] Re: impedance relation with frequency...
- From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
- To: <andrew.c.byers@xxxxxxxxxxxxxx>
- Date: Wed, 24 Sep 2003 12:40:25 -0700
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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