[SI-LIST] Re: Signal flight time through via
- From: "Lambert Simonovich" <bertsimonovich@xxxxxxxxxx>
- To: "'Istvan Novak'" <istvan.novak@xxxxxxx>, "'Kelvin Harding'" <kelvin.harding@xxxxxxxxxxxxxxx>
- Date: Mon, 4 Jul 2011 23:12:07 -0400
Kelvin,
To further Istvan's explanation, if you know the loop inductance (L) per
unit length and capacitance (C) per unit length, then the propagation delay
through the via can be calculated as tpd=sqrt(L*C). If your unit length is
inches, then tpd will be in s/in.
Alternatively, if you know the effective dielectric constant (Dkeff), then
tpd=sqrt(Dkeff)/C; where C is the speed of light. If you want tpd to be in
s/in, then C=1.18E10 in/sec.
The trick is determining Dkeff. The distributed capacitive loading of the
plane anti-pads to via barrel has an effect of lowering the overall via
impedance. Since via impedance is inversely proportional to capacitance and
therefore dielectric constant, Dkeff is the equivalent dielectric constant
value needed to give the same via impedance but without the anti-pads. Dkeff
is generally higher than the bulk dielectric constant published in data
sheets.
If you are working with differential vias, I invite you to review a White
Paper I wrote along with Eric Bogatin and Yazi Cao titled, "Method of
Modeling Differential Vias" available from my web site:
http://lamsimenterprises.com/
In it you will find the equations to calculate the odd-mode via impedance
and Dkeff of a differential via structure based on it's geometry.
BR/
Bert Simonovich
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Istvan Novak
Sent: July-04-11 6:10 PM
To: Kelvin Harding
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Signal flight time through via
Kelvin,
Flight time will be frequency dependent. You can make reasonable
approximations and estimates at low frequencies, but for very high speed
signals, you need an appropriate field solver to get the right answer. At
low frequencies you can start the approximation by looking at the length of
the via portion that is in series to the signal path (NOT the stub
portion) and get its approximate delay from the speed of wave through the
particular dielectric you use. The speed could be in the 160-180ps/inch
range for typical dielectrics.
As a next layer of approximation you need to compensate for the capacitive
loading of the plane antipads and via stubs. The low-frequency capacitance
of the stub portion could be estimated from delay (calculate as above) and
its estimated characteristic impedance, which you could assume to be in the
range of 30-60 ohms, dependent on barrel/pad/antipad geometry and dielectric
constant. For frequencies where any portion of the via gets closer to the
quarter-wave resonance, you need a field solver.
On top of all the above you have the frequency dependence of the dielectric
constant, which typically drops slightly as frequency goes up. According to
popular models, the drop is linear on a logarithmic frequency scale; higher
dielectric loss means faster drop.
Regards,
Istvan Novak
Oracle
On 7/4/2011 3:57 PM, Kelvin Harding wrote:
> How can I estimate the flight time though a via? I am trying to
> compare flight times of two signals, one on later 3 of 16 the other on
> layer 14 of 16. Both have vias to bgas on each end. The trace on layer
> 14 has approx more 1.1mm extra via stub at each end. Regards Kelvin
> ------------------------------------------------------------------
------------------------------------------------------------------
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 technical documents are available at:
http://www.si-list.net
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 technical documents are available at:
http://www.si-list.net
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:
