VNA for cable characterization

[pdenisowski]

I had trouble figuring out how to zoom in on the NanoVNA, ...

Check whether your NanoVNA has a "TDR mode", i.e. an IFFT on the the frequency domain data.

Thank you for putting "TDR mode" in quotes   I still don't understand why the NanoVNA (software) developers didn't just call this "FDR" (or better yet, "DTF") mode.  I just did a couple of videos on DTF mode on our instruments and I have a "technology" video on DTF coming out next week and TDR vs FDR is one of the things I talk about.

That said, I'm not sure what relevance "TDR" mode has to measuring cable impedance (rather than length) or zooming in on the Smith chart display.  Sorry if that's a dumb question - I don't use my NanoVNA very often these days.

[pdenisowski]

There is an interesting analysis on mixing 50Ω and 75Ω plugs and sockets on cables and devices (link). Although it is only available in German it should be translatable with one of the free online pdf translators.

Danke   As always, a great article from HF-Praxis.  I like the fact they mention ("for the sake of completeness" - lol), the old ARCNET cables.  I actually picked up one of these when I was experimenting with measuring cable impedance for the video I mentioned above

[Pinörkel]

Danke   As always, a great article from HF-Praxis.  I like the fact they mention ("for the sake of completeness" - lol), the old ARCNET cables.  I actually picked up one of these when I was experimenting with measuring cable impedance for the video I mentioned above

Ah, good old 300Ω "UKW-Antennenkabel" on an SMA connector and THT termination, a classic.

[joeqsmith]

Thank you for putting "TDR mode" in quotes   I still don't understand why the NanoVNA (software) developers didn't just call this "FDR" (or better yet, "DTF") mode.  I just did a couple of videos on DTF mode on our instruments and I have a "technology" video on DTF coming out next week and TDR vs FDR is one of the things I talk about.

That said, I'm not sure what relevance "TDR" mode has to measuring cable impedance (rather than length) or zooming in on the Smith chart display.  Sorry if that's a dumb question - I don't use my NanoVNA very often these days.

I used TDR to measure the various cable impedance in my post.  Z=ZO(1+S11)/1-S11). 

I am guessing they referred to it at TDR as it seems the industry refers to it as TDR and TDT.   Still though, I didn't want to see this but rather the method you show in your video that you feel was the better choice.   If you want to include other methods, great but don't leave out the one you recommended.

Sorry, I don't know how to use zoom, or most features of the NanoVNAs firmware.   Seems I was able to get their standard return loss or something to zoom in and it was a nightmare.  I wrote about how poor it was.  It really was this poor UI that drove me to use the PC and wash my hands of it.  Because I already had a decent code base to work from, it wasn't a big deal to support the NanoVNA.  The LiteVNA's protocol was much better IMO.  It's too bad they didn't just settle on that for all the low cost VNAs. 

[joeqsmith]

RS paper on TDR.  Describes measuring impedance.
https://scdn.rohde-schwarz.com/ur/pws/dl_downloads/dl_application/application_notes/1ez44/1ez44_0e.pdf

If you plan to publish a new paper/video, you may want to mention that  Copper Mountain Tech uses the chirp Z rather than DFT/IDFT.  You may want to cover the pro's and con's of each.   
https://coppermountaintech.com/inverse-chirp-z-transform-for-vna-time-domain-processing/

[jjoonathan]

Whoops, looks like FieldFox defaults the TDR y-axis (for rho measurements) to start:0 stop:2 rather than center:0 span:2!

Firmware build is A.12.64, 2023-11-30.19:26GMT

[pdenisowski]

RS paper on TDR.  Describes measuring impedance.
https://scdn.rohde-schwarz.com/ur/pws/dl_downloads/dl_application/application_notes/1ez44/1ez44_0e.pdf

That's a great paper, but from 1998 -- all of the products listed on the first page are at least two generations old.

I'll look and see if we have something more modern

[tggzzz]

RS paper on TDR.  Describes measuring impedance.
https://scdn.rohde-schwarz.com/ur/pws/dl_downloads/dl_application/application_notes/1ez44/1ez44_0e.pdf

That's a great paper, but from 1998 -- all of the products listed on the first page are at least two generations old.

I'll look and see if we have something more modern

Have the fundamentals changed since then?

If only the details have changed, I would hope people could make the appropriate adjustments.

[pdenisowski]

Have the fundamentals changed since then?

No   But measurement methodologies sometimes become "obsolete" over time in the sense that they are no longer implemented in T&M instruments.  And of course there are types of instruments that are no longer made.

For example, you can still measure antenna impedance (well, resonance) using a grid dip meter or a noise bridge - I've done these myself many times - but no one manufactures them anymore.  The general idea of generating a signal and observing how much is "coupled into" something at a given frequency is not that different from a "modern" S11 measurement on a VNA, but you can have a very successful career measuring antenna impedance without knowing anything about how a "dipper" works.

[tggzzz]

Have the fundamentals changed since then?

No   But measurement methodologies sometimes become "obsolete" over time in the sense that they are no longer implemented in T&M instruments.  And of course there are types of instruments that are no longer made.

Ah yes, the "you must buy this months/years new shiny toy" argument 

Software sales (mis)practices invading the electronic engineering world

For example, you can still measure antenna impedance (well, resonance) using a grid dip meter or a noise bridge - I've done these myself many times - but no one manufactures them anymore.  The general idea of generating a signal and observing how much is "coupled into" something at a given frequency is not that different from a "modern" S11 measurement on a VNA, but you can have a very successful career measuring antenna impedance without knowing anything about how a "dipper" works.

Oh, drat. A solid well-argued counter-case You might add wavemeters as a similar example. (I still regret never using a slide rule in a business meeting, just to unsettle the opposition)

Mind you, using a dipper without understanding what's happening really would be a case of just following established procedures, "monkey see monkey do". While there's a place for that (e.g. military field operations, while under fire), I think someone at the start of their career should be considering aiming for more than that.

Another point is to consider what oscilloscope you should use to test and verify the world's fastest oscilloscope. Or how to test the linearity of an 8.5-digit DVM. Or how to build a 100GHz transceiver. When operating in such domains, you have to be inventive, and will often end up using variants of very old techniques.

[joeqsmith]

... you can still measure antenna impedance (well, resonance) using a grid dip meter or a noise bridge - I've done these myself many times - but no one manufactures them anymore.  ...

Certainly possible to use that meter below 1MHz.

Goal though is for you to use it to measure the coax's impedance.

[joeqsmith]

Have the fundamentals changed since then?

Has the math changed ...?  Nope. 

[Pinörkel]

My testing of a liteVNA64 for basic cable validation will take a little bit longer than expected. I got one delivered yesterday, but unfortunately, that one had a broken 3-way switch and corroded contacts. So it immediately got sent back. Not a great first impression of quality.

[joeqsmith]

We have bought 5 Lite's now with no quality issues.  Did you buy it direct from the OEM?

[Pinörkel]

We have bought 5 Lite's now with no quality issues.  Did you buy it direct from the OEM?

Kind of. Since the Aliexpress offers of Zeenko are seemingly blocked for customers in the EU, I took the Zeenko recommended EU distributor, with which I have made only good experiences in the past. If there is an issue, they are very helpful and customer-friendly, so I have no doubts that this will be resolved without problems.

[joeqsmith]

Comparing the same three cables measured with Agilent E8357A, different cables, adapters, standards, but using the ideal model like I had done with the LiteVNA.  I would call it a wash.   Also shown is a home made 25 ohm 70mm Beatty standard.   It is an airline but made from hobby brass.   

[joeqsmith]

Looking at the Beatty standard on the Smith chart, zoomed in 20X, not sure this method would work well.   Using a real Beatty standard would be another good addition for Pdenisowski's video.   

[vk6zgo]

Except for its bulk, a coax line with a (mainly) air dielectric using a separate outer tube, with the inner supported inside it, could be designed &/or "tweaked" to be a very precise 50 , & would be unlikely to change to any significant degree over time.

The larger diameter 31/8inch, or the like in air dielectric coax lines would probably be the easiest to configure, but, at the same time the bulkiest.

As an aside, back in the day Marconi preferred 51.5 coax & the 1959 vintage TV transmitter setup at the ABW2 site, duly used that coax standard for the 31/8inch rigid cable sections involved in all the "plumbing" involved in getting the TV signal "up the spout".

These early installations used discrete Vision & Sound transmitters, the outputs of which were combined in "filterplexers", which as well as combining V & S, carried out a respectable amount of the sideband shaping of the VSB signal.
This particular installation used 2 Vision &  Sound Transmitter pairs, with the output of the appropriate filterplexers being combined in a further device called a combining unit.

For the colour modifications in 1974, all of the equipment between the transmitters & the antenna feeder had to be replaced by tighter spec stuff, which was duly supplied by Amalgamated Wireless Australasia Ltd.

All the new stuff was  50 , so AWA also supplied, at great expense, a number of 1/4 wavelength impedance matching sections sections of 50.7445 .

Of course, being curious, we opened one up, & found that the weird impedance was achieved simply by turning the centre conductor down in a lathe!

[joeqsmith]

Good luck with that one on the bench and getting it adapted down.   

[vk6zgo]

Good luck with that one on the bench and getting it adapted down.   

The length would definitely need a large bench as the Velocity Factor would be close to that in free space.! 

On the other hand, 31/8 inch to "N" adaptors are a standard part from Bird, RFS & others.
To SMA might be a tad more difficult, though! 

[joeqsmith]

What's the cost of the adapters?

[Pinörkel]

Comparing the same three cables measured with Agilent E8357A, different cables, adapters, standards, but using the ideal model like I had done with the LiteVNA.  I would call it a wash.

Thank you for taking additional measurements. Would you mind detailing that assessment of yours? Looking at your two measurements, and ignoring the values before 0.1m, my inexperienced eyes would see comparatively consistent impedances of the three cables, also maintaining their relative values. The only exception is the Pasternak cable between 0.1m and 0.4m which reads about 1.5Ω higher on the Agilent.  Considering the Agilent device has at least a 110dB noise floor(if I got the datasheet right) and thus should be much better than the liteVNA, I would conclude that the liteVNA measurements seem to be comparatively accurate keeping in mind that it is a low cost device. However, your comment suggests that some part of the conclusions your second measurement allows when combined with the first one, are rather disappointing.

[joeqsmith]

If we really wanted to improve the measurement, to start I would want some sort of reference standard (along with better cal standards).  Then take the time to use the correct coefficients and torque the connectors.  Maybe then we could get some idea.    Right now, there are too many variables (including the cable).   

I could at least try and lay the cables flat, torque them, then overlay the data.   May be better off with the homemade Beatty standard as that should be more consistent than the cables.   Not sure if it would be helpful but I am willing to run it if you like.

What I was attempting to show is that depending on the cable, the method shown in the video may not be a good choice and that TDR can indeed be used to measure the impedance.  I am hoping pdenisowski has access to proper standards and can actually run a good test for us as it sounded like they were going to make a video on the subject anyway.   

[vk6zgo]

What's the cost of the adapters?

Probably a King's Ransom!
People who installed TV transmitters & the like could quite happily buy them as they could be used & reused over decades.
Such things do turn up surplus, at a much lower cost.

Obviously, smaller diameter fully rigid line could be bought back in the day.
I have a short section of "smallish" diameter 50 rigid coax out of an old Siemens TV transmitter, but it is terminated in old style "Spinner" connectors, which I don't think even "Spinner" makes these days.

Semi-flexible "Heliax" is probably a good 50 , too, with the advantage of reasonably usable sizes & better availability of connectors/adaptors.

[joeqsmith]

Are you thinking that the OP would source the airline and pay a kings ransom to replace a $100 cable?  Or are you suggesting a standard could be made from it that would out perform a Beatty standard from a name brand?

I think they still have the problem of how to verify their results.  Then, I am still not sure that what ever they come up with could be integrated with what ever test equipment they have, and aligned to meet their requirements.   

In OP long post, they fail to mention anything about the actual test equipment,  only the need for a Tektronix 012-0482-00 cable.  Was the SG503 Tektronix's only use case for it?  Sounds like a higher performance cable may not be better for this application.  They setup the generator for that one specific cable, what ever it is. 

That paper they posted seems like a good starting point to making your own but without any real references, I don't see how they would verify it.