Impedance analyzer questions

[otherguy]

I have a project which requires performing impedance measurements on a variety of twisted pair cables. I'm trying to evaluate whether the analog discovery2/3/pro & Waveforms are suitable for my needs.

Reviewing the documentation for Impedance Analyzer its not immediately clear to me how measurement of a transmission line (length of twisted pair cable) could be accomplished with the given topology arrangement options. It seems like the hardware should be capable of performing the task though.

 

How could one wire a transmission line to the Analog Discovery device for the purpose of running the Impedance Analyzer?

If this is not currently supported in Waveforms, would it be possible to perform this test using the Waveforms SDK/pydwf?

 

Thanks!

[JColvin]

Hi @otherguy,

The biggest caveat I can think of with regards to measuring a twisted wire pair with either the Impedance Analyzer Adapter or directly without the adapter (so you use your own reference resistor) is that you would only be able to analyze one wire at a time; with one end connected to the 1+ and the other end connected to the known resistor value.

It's not the most aesthetically pleasing setup, but this is how I would set it up with the Impedance Analyzer Adapter with one of the twisted pair cables that come with the Digital Discovery High Speed Adapter, with either end of the red wire in each of the prongs (I used a 1.5 cm male to male header to get the wire to attach).

The other catch I can think of would be what length of cable you have and what impedance you are expecting. The Adapter goes down to 10 Ohms (each reference resistor is 0.1% rated on the adapter), but if you are measuring a much longer cable or are expecting a super low impedance so that the 10 Ohm reference is comparatively a couple of orders magnitude larger than what you are measuring, I would be more likely to hedge my bets on how effective this would be.

Let me know if you have any questions.

Thanks,
JColvin

[otherguy]

It looks to me like you've left both ends of the black conductor disconnected in your circuit
I don't think this setup will correctly measure the characteristic impedance of the twisted pair.


In your setup would you not just be measuring resistance and ignoring the inductive and capacitive reactance of the transmission line?
 

[JColvin]

Hi @otherguy,

You're correct in that it would be ignoring any reactance/reflections from the other wire as it is unconnected/not under load.

I suppose in theory that the ADP3450 with its 4 analog inputs and pair of analog outputs would, at least on paper, be able to properly do impedance analysis on a twisted cable pair, but right now the Impedance Analyzer tool within WaveForms does not support it (WaveForms SDK would not be of any help either). The WaveForms developer is out of office this week, but I will ask them when they get back about the feasibility of adding this sort of feature.

Thanks,
JColvin

[otherguy]

Any luck engaging the WaveForms developer?

[JColvin]

I had thought Attila would be back yesterday, but learned from his manager that he is on vacation through the end of August. Realistically I hope to get some of his time for feedback next week as I doubt I will get a free cycle from him on Friday, though Attila does routinely dedicate some of his time to look and respond to different posts in the Test and Measurement section, so perhaps I'll be surprised.

Thanks,
JColvin

[attila]

Hi @otherguy

This is a measurement on a ~2m open-ended ethernet cable pair compared with a 100pF cap and the second with short at the end.

I'm not familiar with twisted wire measurements. Are these sufficient for you ?

[otherguy]

@attila thank you for getting back to me. Sorry for the delayed response, I must have missed the notification that you had replied.

The first screenshot (with open circuit UTP cable)makes sense to me.

I don't have experience running tests like this with a shorted cable, but I'd suspect at low frequencies, the inductive and capacitive components of the impedance to be negligible, and the resistance to be low (if shorted)... seems like the traces in the second screenshot illustrate that. I'm not familiar with the WaveForms user interface, so not sure what the difference is between the dotted and solid orange traces.

 

From your data peovided, it seems like it's worth a time and $ investment on my side to evaluate suitability further.

Could you describe the physical circuit configuration, any special breakout boards you're using, and what modes/settings you've used to performed these tests?

I see the graphic in the upper right of your screenshots, but I'm not sure how that compares to where you have physical components connected.

 

Thanks again!

Edited September 18, 2023 by otherguy

[JColvin]

Hi @otherguy,

I can't speak towards Attila's physical circuit setup outside of him having used the Impedance Adapter (https://digilent.com/reference/add-ons/impedance-analyzer/start, which uses 0.1% reference resistors) that was plugged into an Analog Discovery 2, but the dotted vs solid lines are there to differentiate which type of calculated measurement is being represented over the range of frequencies, with the different line patterns being shown underneath each toggle-able measurement type:

In terms of the software side setup, it's simply a matter of selecting the "Impedance" tool from the Welcome tab and then clicking on the various plots you want to see and adjusting the various values in the visible dropdowns. Additional explanations of each of the settings are available within the Help tab in the Impedance section.

If you wanted to try it out, the latest beta is freely available for download and has a demo modes for the different hardware options here:

Let me know if you have any questions.

Thanks,
JColvin

[attila]

Hi @otherguy

I used https://digilent.com/reference/add-ons/impedance-analyzer/start and connected one pair of twisted cable with open then whit short at the other end.

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