[SI-LIST] Re: VNA and characterization of long cables
- From: Mark Filipov <markfilipov81@xxxxxxxxx>
- To: Jim Nadolny <jim.nadolny@xxxxxxxxxx>
- Date: Thu, 19 Apr 2012 21:05:45 +0200
Thanks Jim!
The VNA I'm using is an Agilent N5230A, it goes from 10MHz to 20GHz.
Will it help if I use low IF-BW, say 1kHz, dwell time of 1ms?
I'm also using 6400 points in the sweep, so I end up with pretty big
Touchstone files.
The tool I'm using HyperLynx, it does vector fitting on S-parameter
models, I don't know however how well they can extrapolate to from
10MHz to DC, since that portion of cable is not modeled.
But as you say it will have big impact simulation.
Even though I'm running HyperLynx FastEye in statistical mode, it
still has to to SPICE simulation in order to obtain impulse response,
I don't know if I can then trust my simulation, like you say, it's
problem with phase when characterizing long cables.. ?
Thanks you for your good advice!
BR /Mark
2012/4/19, Jim Nadolny <jim.nadolny@xxxxxxxxxx>:
> Mark -
>
> Oh man, have I been burned by this one.
>
> I used a VNA with a start frequency of 50 MHz and my Touchstone models were
> seriously messed up. The insertion loss curves looked fine...until you look
> at the phase response. The phase changes rapidly on a long cable and a
> start frequency of 50 MHz does not capture the low frequency behavior. It
> is so poorly represented that you can't easily extrapolate/interpolate. In
> short- you have a problem.
>
> One solution - use a VNA with a lower start frequency (like 300 KHz).
> Another solution - use a TDR and model extraction software (as per Tom D's
> suggestion)
>
> If the cable is well behaved, I will match a model to fit the high frequency
> response. I use some of the built in models in ADS and tune the loss
> parameters and Dk to match up the attenuation and delay parameters. Once
> that is dialed in on the higher frequency, you trust it for <50 MHz. Cables
> are pretty easy to model so long as you are not trying to match 2nd order
> effects.
>
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