[SI-LIST] Re: VNA and characterization of long cables
- From: Peter.Pupalaikis@xxxxxxxxxx
- To: markfilipov81@xxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Thu, 19 Apr 2012 13:56:29 -0400
Hi Mark:
I'm not a VNA expert, but there are two things I know of that you'll need to
be careful with in such a measurement (and some VNAs might not be able to
handle these things properly - but I'll let the VNA experts respond to
that).
1) Dwell time - since a VNA measures incident and reflected standing waves,
the waves must truly be standing - meaning they have stabilized to steady
state behavior. With a very long cable, you will need to ensure that the VNA
dwells at frequency in the sweep for a sufficient period of time to
stabilize. With small circuits, you probably don't even think about this,
butwith long cables, you will need to. I believe in many VNAs there is a way
to program that, but I don't know how long you can program it to wait and
themenus might be hidden away somewhere.
2) frequency resolution - on long devices, you will need to ensure that the
data has been taken with sufficient frequency resolution to avoid
"time-aliasing". If you are just looking at a plot in the frequency domain,
this time-aliasing might not be so much of an issue (although it still can
be), but when using the resulting Touchstone file for simulations -
especially time-domain simulations - you will need to take care of this.
Thiswas partially the subject of my DesignCon paper this yea - see:
http://cdn.lecroy.com/files/whitepapers/designcon2012_lecroy_s
-parameters_discrete_frequency_continuous_response.pdf[1]
Hope this helps,
Pete
Mark Filipov ---04/19/2012 01:44:57 PM---Hello! I have heard that in
practice it's is not possible to to VNA
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Mark Filipov <markfilipov81@xxxxxxxxx> [IMG]
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Date:[IMG]
04/19/2012 01:44 PM [IMG]
Subject:[IMG]
[SI-LIST] VNA and characterization of long cables [IMG]
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Hello!
I have heard that in practice it's is not possible to to VNA
measurements on cables that are around 7 meters long or longer.
Can someone explain why is it so?
How do we otherwise obtain simulation models of long cables if, they
cannot be measured on a VNA and saved in Touchstone format?
Thanks! /Mark
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