[SI-LIST] Re: Via impedance
- From: "Yuriy Shlepnev" <shlepnev@xxxxxxxxxxxxx>
- To: <sukumarm@xxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Wed, 27 Nov 2013 13:32:15 -0800
Hi Sukumar,
When you create a model of via-hole, the primary design parameter should be the
reflection from the via. The reflection is frequency-dependent and better vias
have lower reflections over wider frequency band. Lower reflection means that
more power is transmitted through the via (assuming that the via is localized).
Ideal non-reflective via is completely transparent for the signal. In reality
all vias reflect signals and there must be a goal for maximal possible
reflection over the target frequency range. Reflection below -20 dB over the
frequency bandwidth of your signal is an example of such goal.
Though, an effective impedance of a via may be helpful and can be estimated
with simulated or measured S-parameters as follows:
1) As the input impedance of the via terminated with the target impedance on
both sides: Zvia=Zt*(1+|S11|)/(1-|S11|), where Zt is the target impedance of
the channel, |S11| is magnitude of the reflection parameter either in terminal
space for single-ended vias or differential in the mixed-mode space for
differential vias (|Sdd11|). This impedance will be frequency-dependent and may
be useful to estimate quality of the via. It looks like you have used this
approach below.
2) Build approximate transmission line model of via as outlined in L.
Simonovich paper "Method of Modeling Differential Vias" available at Lambert's
web site http://lamsimenterprises.com. The impedance of the t-line may be
considered as the effective impedance of the via. The impedance in this case is
not frequency-dependent and may be good only if via geometry is very close to a
quasi-periodic structure.
3) Create a linear network with two short segments of transmission line and via
in the middle, compute TDR response and plot it as Z(t) (in terminal space for
single-ended vias and differential in the mixed-mode space for differential
vias). Short segment of real or ideal t-line with the target characteristic
impedance are needed to make the structure electrically larger. Without
additional segments, a very short rise time may be needed to resolve the via on
TDR plot, that will require prohibitively large frequency bandwidth for the
analysis. In general, line segments with flight time larger than the rise time
may be acceptable to have some impedance value at the via location.
The last approach may be preferable, because of it can be compared with the
measured TDR. Though, there are a few caveats here. First of all, the TDR rise
time must be correlated with the bandwidth of the electromagnetic model. On the
other hand, the resolution of the via details depends on the rise time and
frequency content of the step function and frequency bandwidth of the model.
Reducing the rise time resolves the smaller details, but only within the
frequency band of the electromagnetic model (or measurement). With very small
rise time you may see very large variation of the impedance that either
characterize via behavior at very large frequencies (or even completely
artificial if the spectrum of the excitation is outside of the frequency band
of the model). Such impedance estimation may be too pessimistic. On the other
hand, if slow rise time is used or if long real lossy lines are used and act as
the low frequency filters - you may see the via impedance at lower frequencies
(will be always close to the target impedance) and this estimation may be too
optimistic. The approach with TRD require some considerations before being
applied to a particular via.
Best regards,
Yuriy
Yuriy Shlepnev, Ph.D.
President, Simberian Inc.
3030 S Torrey Pines Dr. Las Vegas, NV 89146, USA
Office +1-702-876-2882; Fax +1-702-482-7903
Cell +1-206-409-2368; Virtual +1-408-627-7706
Skype: shlepnev
www.simberian.com
Simbeor – Accurate, Fast, Easy and Affordable Electromagnetic Signal Integrity
Software
2010 and 2011 DesignVision Award Winner
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Sukumar MR
Sent: Wednesday, November 27, 2013 10:51 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Via impedance
Hi Experts,
I want to control via impedance. 50 Ohm for Single ended signal
via &100 Ohm for Differential signal via. I have S parameter (.snp) file for
via. How can I calculate the via impedance at particular freqency?
May I use below formula for Single ended,
Via impedance= 50 *((1+Return Loss in Linear form)/ (1-Return Loss in Linear
form))
Please share your ideas.
Regards,
Sukumar
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List forum is accessible at:
http://tech.groups.yahoo.com/group/si-list
List archives are viewable at:
//www.freelists.org/archives/si-list
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List forum is accessible at:
http://tech.groups.yahoo.com/group/si-list
List archives are viewable at:
//www.freelists.org/archives/si-list
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
