[SI-LIST] Re: Discontinuities on PCB
- From: "Itai Frenkel" <itaifrenkel@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Fri, 13 May 2005 10:48:12 -0000
Hello Charles,
In order to simulate and understand such an effect you could use a
3D electromagnetic solver. Ansoft has already an HFSS parametric
model of a differential via on their Online Technical Support site.
There is also a paper descirbing the design assumptions behind the
3D model.
SI engineers ussually continue the stripline/microstrip away from
the via so the higher modes will fade, and then use deembeding to
mathematically get closer to the via.
In your case,however, I recommend to get closer to the via, and
change the port settings to calculate higher modes. For each mode
you get a schematic field diagram or draw an accurate field
distribution for each mode, the mode impedance (Z0), the attenuation
constant real(Gamma), and the propagation constant im(Gamma). For
the first two modes (diff/common) the attenuation constant will be
small and the propagation constant high (as in sin(x) function). For
the other modes you will have a bigger attenuation constant and a
small propagation constant (as in a decaying exponent function).
The resulting S-matrix contains the transmission coefficient. For
example, if the impedance of the via matches the impedance of the
line, you will see a clean transmission (with attenuation ofcourse).
If however, the via stub is one quarter of a lambda you will see
that the via will return all of the energy in this frequency. In
order to understand this effect you could animate the surface
currents as a function of the phase and "see" the standing wave on
the via stub. The S-matrix has also information on mode conversion.
For example, how much energy started as differetial mode and ended
up in common mode.
A few technical notes:
1) Change the default port solver accuracy from 2% to 0.1%.
2) Notice that re(Gamma) units are in Nepper
3) If you "fear" a 3D EM tool, you should consult with your RF
colleague. I am sure they are fammiliar with these type of tools.
I hope this helps,
Itai
--- In si-list@xxxxxxxxxxxxxxx, Charles Harrington
<ch_harrington@xxxx> wrote:
> Hello,
>
> When a TEM (or quasi-TEM) signal propagating on a
> microstrip trace meets a discontinuity (e.g., a via)
> on a substrate at microwave frequencies, higher-order
> modes are excited at the discontinuity, leading to
> reflections.
>
> Can anyone tell me exactly what kind of modes are
> excited and why they are excited. How far away from
> the discontinuity can they propagate and how does this
> depend on the operating frequency.
>
> I'm a postgraduate student currently working on
> discontinuities on PCB. My search for relevant
> literature led me to this list. So, I'll appreciate
> any
> help.
>
> Charles.
>
>
>
> Yahoo! Mail
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