[SI-LIST] Re: why do I need mixed mode S parameters?
- From: dgun@xxxxxxxxxx
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 21 Sep 2004 13:19:29 -0800
Brad,
I've included only the portions I want to comment on. See below...
From: "Brad Cole" <cole@xxxxxxxx>
> Looking at the mode-conversion (Sdc and Scd) terms can tell you something
> about the susceptibility or likelihood of generating EMI, respectively. In
> the frequency domain you can see bands where you may have problems of this
> nature.
>
> In the time domain, you can look at these parameters to determine the
> location on the device where there is an asymmetry in the structure that may
> be responsible for causing the mode-conversion terms to be non-zero, and
> therefore be responsible for EMI issues.
>
> The trick, of course, is in establishing a limit. In other words, how much
> larger than zero can the mode-conversion terms be before you will have an
> EMI problem. The answer to that question is less straightforward and would
> make a great topic for a technical paper if not a disertation. (Any takers?)
I suspect that the limit would be determined by signal-to-noise ratio.
A 3V digital signal can tolerate a lot of noise. However, a 16-bit A/D could
lose bits if there is too much common-mode to differential-mode conversion.
> On the measurement side, the mode-conversion terms are typically very small
> (< -40dB in the frequency domain) and ideally zero. To calculate them from
> single-ended data you end up subtracting two pairs of large numbers from
> each other. This requires the measurement system to have 1) high dynamic
> range, and 2) very good accuracy. Having a true 4-port VNA and full error
> correction is important. My experience has told me that 1) multiple 2-port
> measurments will not yield very good data, and 2) mixed-mode data from
> TDR/TDT is not accurate. (Shields Up)
Bockelman and Eisenstadt showed that calibration errors are higher for
mode-conversion terms on standard 4-port VNAs. "Pure-mode" VNAs perform
better in this regard. For all other parameters, there isn't much difference.
Unless one is very concerned about mode-conversion accuracy, standard 4-port
VNAs should do the job fine.
The multiple 2-port measurement (round robin) method is inaccurate unless
the terminating loads can be calibrated out. But at low frequencies, it
should work fine with good terminations.
--
Daniel
--
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