VNA for cable characterization

[joeqsmith]

LiteVNA 64 was calibrated using the modified standards from the V2Plus4 (sorted ANNE terminator) from 300k-9GHz, 1kHz IFBW, ideal model.   The two Amphenol BNC SMA adapters (C & D) from:
https://www.eevblog.com/forum/rf-microwave/vna-for-cable-characterization/msg5602515/#msg5602515
were attached.  Note the Port1 cable was locked in the vice during calibration and other measurements.

Next I measured the insertion loss of a few cables.  Their lengths were also noted along with the connector brand (if known).   

***
Link for ProbeMaster cable:
https://probemaster.com/bnc-cables-2/

Also attached photo of the Pasternak RG-400/U.  Datasheet may be found:
https://www.pasternack.com/images/ProductPDF/RG400-U.pdf

At 1GHz its 14.7dB for 100' or 0.91dB for our 74inches.  Which is very close to what we measured with the added loss of our adapters and connectors.   

[Pinörkel]

Next I measured the insertion loss of a few cables.  Their lengths were also noted along with the connector brand (if known).   

At 1GHz its 14.7dB for 100' or 0.91dB for our 74inches.  Which is very close to what we measured with the added loss of our adapters and connectors.

So it seems like the liteVNA can produce some usable quantitative results.

I tried to reproduce your insertion loss measurement with my test setup. The liteVNA was warmed up one hour, and then calibrated with the supplied cables and standards from 100k-6.3G, 1kHz IFBW, ideal model. I did not go all the way up to 9G, because I stumbled upon a strange calibration artifact, which I will try to show later. Then, I connected one of my cheap SMA-M to BNC-F adapters to the double SMA-F adapter at the cable on port 1 and a SMA-F to BNC-F adapter to the cable at port 2. The DUT cables then went in-between the BNC-F adapters.



As cables, I had:
RG58 C/U MIL C17F, 107 cm
Datasheet: 1GHz  51.8 dB Attenuation dB/100m -> for 107 cm : 0.55426 dB

Belden H155, 91.7 cm
Datasheet: 1GHz  29.6 dB Attenuation dB/100m -> for 91.7 cm : 0.271432 dB

This is what I got:



The image shows two measurements for each cable in the 100k to 1G range. One measured with 801 points from 100k to 6.3G and one with 801 points from 100k to 1G. The one with the denser sampling showed a large peak at the beginning going up to about 5.2, a strange small disturbance at 100MHz (which stays there, even when varying the sweep parameters) and some periodic superimposed oscillation. Unfortunately, the measured insertion loss values are not even close to the theoretical values I listed above. Maybe I messed up something with my measurement setup or my SMA to BNC adapters are too bad. At the moment, I do not have sufficient materials to test the insertion loss of only the adapters.


I also got myself some PCB mount BNC-M connectors to experiment with building BNC calibration standards, like described here. It seems those are only available from china manufacturers and no known quality manufacturer makes those. The first try was a complete and perfect failure.  The springy outer conductor of the connectors is too small in diameter by about .3mm and does not make good contact to any mating BNC-F connector. So, the connectors themselves are really bad. Nonetheless, I created an open and short by filing down the center conductor and then soldering a small tinned copper disk for the short, just for the fun of it. The result was smith chart that looks like Indiana Jones whip after calibrating. For open, short and load (even below 1GHz) the smith chart does not even get close to where it is supposed to be and jumps all over the place when just looking at the connector. I really wonder how the guy from the website linked above managed to get anything out of these connectors, but it may also well be my soldering skills.

[joeqsmith]

I'm guessing much of that loss is the adapters and connectors.   I suspect the switch points have changed over time.  Because I don't run the VNA standalone and have no use for any features beyond getting the data into the PC,  its rare I would change the firmware.  Shown with the VNA uncalibrated and a thru attached at 801 and 1601 points.  Note the higher spikes at the start. 

If I increase the number of data points to 2001, calibrate  and measure the thru, it is within 0.04dB.  Now when I insert a 48" section of RG58U and BNC connectors/adapters, my LiteVNA64 does not exhibit the large transients you show. 

You could try taping the cables to the table to prevent them from moving during the calibrations then measurement.  You could also try running the same firmware just to remove that variable.

[joeqsmith]

You do not specifically state that you recal the VNA when you are changing the number of points and frequency range.   

While the software has a limited interpolation if you work inside the frequency range that the VNA was calibrated, changing the number of data points would lead to some pretty strange results.  Shown with the same setup but I have changed the number of data points.  Of course the software would tell you there is an problem (CalRange error is active) but by design, it is not going to prevent you from doing such things.   

[joeqsmith]

Assuming you are calibrating the VNA for each setup, here is the LiteVNA with the two ports open after calibration using 2001 points, ideal model as before but with 100 averages.  Noise floor is about -100dB with mine.   I expect yours would be the same.   

I then attached two Coline 20dB attenuators in series and again averaged 100 times.  Note the min/max is enabled in both plots to show the peaks.   

The spec +/-0.2dB frp, DC to 1GHz with these where I measure about 1dB with the two. Again though, the VNA was calibrated with the SMAs and we have added these adapters which are another error source. 

[Pinörkel]

I'm guessing much of that loss is the adapters and connectors.   I suspect the switch points have changed over time.  Because I don't run the VNA standalone and have no use for any features beyond getting the data into the PC,  its rare I would change the firmware.  Shown with the VNA uncalibrated and a thru attached at 801 and 1601 points.  Note the higher spikes at the start.

If I increase the number of data points to 2001, calibrate  and measure the thru, it is within 0.04dB.  Now when I insert a 48" section of RG58U and BNC connectors/adapters, my LiteVNA64 does not exhibit the large transients you show. 

You could try taping the cables to the table to prevent them from moving during the calibrations then measurement.  You could also try running the same firmware just to remove that variable.

Looks like I get some better connectors then. Unfortunately, I can not really compare the ones from Telegärtner to another renowned brand, because Telegärtner seems to be kind of unknown outside of Germany. However, I have heard people say that their adapters might be similar in quality to same-priced Radiall, H&S and Pasternack devices.

You do not specifically state that you recal the VNA when you are changing the number of points and frequency range.   

While the software has a limited interpolation if you work inside the frequency range that the VNA was calibrated, changing the number of data points would lead to some pretty strange results.  Shown with the same setup but I have changed the number of data points.  Of course the software would tell you there is an problem (CalRange error is active) but by design, it is not going to prevent you from doing such things.   

Regarding the artifacts, I mentioned earlier, I already had a suspicion, that they might be interpolation errors. Though, I thought that I had recalibrated for the 100k to 1G range, I redid the measurement and the large spike at the start, the hiccup at 100M and the oscillations disappeared. The rest of the curve was very close to the two earlier measurements.

Assuming you are calibrating the VNA for each setup, here is the LiteVNA with the two ports open after calibration using 2001 points, ideal model as before but with 100 averages.  Noise floor is about -100dB with mine.   I expect yours would be the same.

This is what I get for the same parameters:



Did you enable averaging before or after calibrating? Since calibrating takes a lot longer with averaging on, it seems to make a difference.

[joeqsmith]

Did you enable averaging before or after calibrating? Since calibrating takes a lot longer with averaging on, it seems to make a difference.

After. 

[joeqsmith]

There are a few things about the LiteVNA you should be aware of that may cause you some problems.   When I showed the uncalibrated through measurement, you could see the switch points.  Normally these wouldn't cause a problem but if you are looking to measure losses in the 20dB or lower, these can be problematic.   The easiest way to see the problem is to cal the VNA for say 50k to 1M with say 800 points, then connect a step attenuator between the two ports.  As you add more loss, you will see a step in the response.  These happen at discrete frequencies.  The steps can be more than 2dB.   As you adjust the attenuator, the steps polarity can change.

It seems member Dislord had explained this in my large thread.  I think they provided the switch points and went into details of what was happening.  I added support to change some of the advanced settings of the LiteVNA to try and mitigate these problems.  There is no fix that I am aware of.  This was long after I had stopped updating the manual so the only place it is documented is in the forums. 

So when you see these spikes and shifts in your data, just be aware that the VNA can also attribute to them.

There are a few things that the original NanoVNA did much better than the LiteVNA and V2Plus.  I was surprised when I first bought the V2Plus4 how poor it was by comparison.  The designer of the V2Plus claimed that the LiteVNA was a ripoff of their design.   It certainly suffers from the same problems.    In some cases, you can work around it by using both the tools.   Of course, you still have that square wave drive.   I think when you get down to that level, it's time to think about getting a used high end system. 

[joeqsmith]

Going back a few years, here you can find the discussion switch points.   
https://www.eevblog.com/forum/rf-microwave/nanovna-custom-software/msg4136797/#msg4136797

After normalization, attached showing the 400kHz switch point with and without AGC when measuring a 30dB attenuator.

Also shown is how the step can change location depending on attenuation.  The leading glitch at the low frequency starts showing up below 16dB.  You can toggle the attenuator between 16 and 17dB and watch the leading glitch come and go. 

I'm sure most of the people using the LiteVNA are aware of the shortcomings like this but there isn't a good source for all the little traps for people just getting started. 

So if you see something really odd that you can't explain, it may very well be inherent to the VNA. 

[DiSlord]

LiteVNA use 2 generators MS5351 (from min to 100M) and MAX2871 to 6.3 - 6.4GHz (and t ~8G use harmonic mode)
On low frequency range also switch IF
800Hz - 20k use 6k IF
20kHz - 400k use 12k IF
400kHz - 100MHz - use 60k IF and MS5351
> 100MHz use 60k IF and MAX2871

Exist 2 problems:
Different IF on OpAMP give little different gain and phase rotation (compensated on last firmwares but not ideal) also OpAmp gain not ideal linear (calibration allow fix only linear errors)

As result:
- difficult fix problems on switch 20k, 400k and 100MHz (errors small, but exist)

[joeqsmith]

Thanks for chiming in.   While its pretty easy to detect the switch points and I agree that for the most part, they really don't cause a problem.    I have always wondered about what appears to be a change in the switch points based on signal level.   Where the low frequency switch points seem to be very predictable, the 100MHz one appears to move.  Eventually locks in.   See the zoomed in area of the previous data set for a better view. 

You can see I am running very old firmware and maybe some of this has been improved.   That version I am running I think was when you opened up the harmonic mode for me to make it unlimited. 

[virtualparticles]

There are many different ways of measuring the impedance of cable, but if I had to choose a methodology that's a good compromise between accuracy, simplicity, and cost, I would use a VNA and the methodology I describe in this video:

This is how I would do the measurement as well. TDR measurements made with VNAs are helpful, but frankly, no VNA manufacturer provides metrology for this mode. The TDR method involves Time Domain inversion and requires aggregating a thousand or so frequency measurements. Determining uncertainties for this is impossible, as far as I know. VNA TDR has the advantage of making a series of measurements in a narrow bandwidth instead of a single measurement in a GHz or higher bandwidth, resulting in a 1000x better signal-to-noise performance. This virtual TDR will work over hundreds of feet of relatively low-loss cable.

See https://coppermountaintech.com/time-domain-processing/

[joeqsmith]

This is how I would do the measurement as well. TDR measurements made with VNAs are helpful, but frankly, no VNA manufacturer provides metrology for this mode. ...

As I understand it, the OP is not asking about a section of coax.  For their cable, they are considering the entire system.  We know Tektronix had made a custom connector to improve the return loss.   I don't see how you would use the method you linked to evaluate the effects of the connectors, adapters, workmanship...   Using the LiteVNA with TDR is not good enough to see a lot of detail but you can get a relative feel for which combinations are better.   

With that in mind, how would you recommend they measure these effects?

[virtualparticles]

Getting back to your questions,

One thing is the step and impulse diagrams in the time domain. As far as I understand it, those should be convertible into each other by means of integration and differentiation. ...

Beyond not having enough data points, looking at my software I have been using a trapezoidal integral rather than Simpon's which would cause some small errors.  I doubt you would see a difference but I have gone ahead and changed it. 

You can of course zero-pad in the frequency domain (before doing the IFFT) to interpolate in the time domain. With increasing resolution in the time domain, the integration method becomes less and less important.

EDIT: That results not only in a higher resulution in the time domain, but also in a proper sin(x)/x interpolation, while e.g. trapezoidal integration calculates the integral as if there was linear interpolation between the points, and Simpson is not equivalent to sin(x)/x either.

In practice, we "Zoom in" on time-domain ranges using the Inverse Chirp Z transform. Zero padding is feasible but increases computation time. See my write-up at https://coppermountaintech.com/inverse-chirp-z-transform-for-vna-time-domain-processing/ for more information.

[joeqsmith]

So you are suggesting TDR? 

***
While GF seemed to have missed my comment about zero padding, I have used that method for some time.  This is what the OP is currently using.

[tggzzz]

So you are suggesting TDR?

if you want to see how impedance varies as a function of distance, then TDR would be my first choice.

Examples: "wideband" digital signal integrity on PCBs, connector "suboptimal characteristics", faultfinding in inaccessible locations, e.g. inside aircraft fuselages.

OTOH, if you are interested in "narrowband" frequency domain behaviour, then a VNA would be my first choice.

As is usually the case, "there is more than one way to skin a cat".