[SI-LIST] Re: Signal flight time through via
- From: Istvan Novak <istvan.novak@xxxxxxx>
- To: Kelvin Harding <kelvin.harding@xxxxxxxxxxxxxxx>
- Date: Mon, 04 Jul 2011 18:09:41 -0400
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
> ------------------------------------------------------------------
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