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Is hint 2 in Agilent application note 1291-1B valid?

Question asked by drkirkby on Jan 31, 2013
Latest reply on Feb 4, 2013 by drkirkby
The Agilent note 1291-1B, has the title +10 Hints for Making Better Network Analyzer Measurements+. 

Hint #2 is +Compensate for time delay for better cable measurements+ 

The basic point made in hint #2 is that if using long cables (the example uses 12 feet), it is necessary to slow the sweep speed down below the default value. 

I mentioned this to someone on a mailing list devoted to the cheap ($700 including cal kit) WVWA, which is one of the many cheap portable VNAs aimed at the amateur radio market, which has no display, but connects to a PC via USB.  See http://sdr-kits.net/VNWA3_Description.html  

The developer of that cheap VNA (Tom, DG8SAQ) has said I was wrong, and it is not necessary to slow the speed speed on long cables. He worked out the time of the two way path of 15 metres of coaxial cable and arrived at 100 ns. He actually forget to include the velocity factor of the cable, but adding an extra 30% or so does not change the argument. His comment was 

+"In a decent VNA you don't measure while sweeping. You set the LOs to the new frequency, let the DUT and the VNA settle to the new stimulus (about 1/IF bandwidth or DUT settling time, whatever is larger) and measure. My VNWA can sweep pretty fast at 160us/data point of which 80us are reserved for DUT and VNA to settle at this sweep speed. So, you see that 1us DUT settling time is indeed irrelevant."+

I can see his argument and it makes some sence to me in a modern VNA, which would use a synthesizer. I'm wondering if the hint in that application note is only valid if the frequency of the VNA source is swept continuously, rather than in descrete steps, which may have been the case in older VNAs, but which I doubt is true in a PNA. The application note is based on the 8714ET network analyzer, and shows an even more bizarre measurement on an 8753ES, which is not quite an antique, as it was supported until 01/12/2011 -  (which I assume is the 12th of Jan 2011, though in England we would consider it the 1st of December 2011).

Some basically I'm asking what is the validity of hint #2 in application note 1291-1B with regard to 

* The latest VNAs like the PNA series and FieldFox. 
* Older VNAs like the 8753A and 8720D that I own. (I did at one point own a FieldFox N9923A, but no longer do).

I've attached a copy of the Agilent note 1291-1B, since it seems to be unavailable at the Agilent web site today, although it was yesterday. If it is still available, it is not at the link Google gives, and I can't be bothered to hunt around any more, given I have a copy. 

Dave  

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