Hello,
I have V2_2 from OwoComm. Could anybody confirm my measurement (SOL calibratied after 18GHz short piece of semiflex cable) directly on the input port of the Nanovna V2? As far as the matching of the S21 port  is essential for measurement, I think this parameter helps for estimating one of the quality parameters. Maybe there could be a way to improve the matching to 3GHz by adding some discontinuity or modifiyng the input before the SMD attenuator.

That actually looks pretty good compared to mine (which is still in spec).

The large ripple looks bad though. 500MHz ripple corresponds to ~20cm of transmission line (with velocity factor 0.7), that sure looks like a calibration issue (mine is not great either).

To get comparable results, you have to calibrate AFTER the cable on the connector,  which will be then connected to RX port, to have the cable errors covered by the SOL calibration. Any poor cable or connector  will degrade the results.

That is what I have done. I have calibrated with a female cal kit at the end of the cable, then connected the cable directly to port 2. The residual ripple in my measurement may be from using the included cable, which may not be particularly phase stable.

I don't think there is anything after the port connector that is (electrically) long enough to produce 500MHz ripple, so it seems likely that it is a measurement artifact.

Hi Switchabl

First of all when you calibrate with a female cal kit did you then have s1p files for the kit to enter under calibration kit setting (suppose you use VNA-QT) else you have mismatch to the port2.

Secondly port 2 does not have a pure input impedance but a frequency dependent return loss. Thus you have reflection in the calibration plane (male test cable connection-point mating port2) and the test cable length 52cm with VF 0.7 causes ripple. Remember that VNA-QT dose not have 12 Term errror correction then these ripples are un-avoidable. If you insert a 20dB attenuation between test cable and port2 and calibrate thru with the attenuator fitted the Port2 input impedance is stabilized to be close to 50ohm and ripple reduced considerable.
If you do a thru calibration with male kit on port1 and a thru calibration with single test cable and fit the 20dB att directly at port1 with ideal thru calibration settings the look at s11 dB and S21 dB and you will see 0dB return loss and S21 0dB with 0 degree S21 phase shift (in this condition you use a virtual 0mm long male/female adaptor)

Kind regards

Kurt



Fra: NanoVNAV2@groups.io <NanoVNAV2@groups.io> På vegne af switchabl@mailbox.org
Sendt: 11. september 2020 17:13
Til: NanoVNAV2@groups.io
Emne: Re: [nanovnav2] V2 from OwoCom from Tindie -port s21 matching



That is what I have done. I have calibrated with a female cal kit at the end of the cable, then connected the cable directly to port 2. The residual ripple in my measurement may be from using the included cable, which may not be particularly phase stable.

I don't think there is anything after the port connector that is (electrically) long enough to produce 500MHz ripple, so it seems likely that it is a measurement artifact.

Kurt,

thank you for your comment. I think we are talking about different things here though. I apologize for maybe not making myself clear. First let me clarify that the point of this measurement was to characterize the load match of port 2, i.e. it is a single-port S11 measurement, port 2 is the DUT. The purpose of this was to be able to estimate the uncertainty in S21 measurement (without full 12-term correction) due to load match error.

For this purpose one of the included 30cm cables was connected to port 1, a SOL calibration was done using the open and short from the SDR-Kits female Rosenberger kit and a Suhner load (that is better than my particular Rosenberger one). I captured raw measurement values and applied calibration with scikit-rf and your model definitions (I have also generated S1P files for NanoVNA-Qt but I prefer to save the raw data for flexibility). So the reference plane was at the end of the test cable and this was directly connected to the DUT (port 2 of the NanoVNA).

The result is more or less as expected and within spec (load match is not that great on the V2). There is about 0.5dB of ripple at about 500MHz which as you say is what you would expect from a cable AFTER the reference plane and which you wouldn't expect to see with the reference plane at the end of the cable and a good calibration.

I am not quite sure what happened there. This is an old measurement I did when I first got and I should maybe repeat it with a better cable, especially since there is a bit of bending involved. Also, the directivity of the bridge is not great above 1.5GHz and if it has drifted significantly between the calibration measurement and this one, that might also be an explanation (there were some other ones done in between). But I am still looking into that.

I am also not completely sure if I can trust the Rosenberger parts (particularly the open) above 1.5 GHz. If you have ever measured these at higher frequencies, I would highly appreciate if you could share the touchstone files.

In any case, the main point I was trying to make is that the measurement by ok1vaw has similar ripple only a lot worse (several dB). So I suspect something went wrong with his measurement.

When I remember well, the SOL calibration on vector analyzer is able to correct all errors of 1port measurement with real HW, by use of inserting int model mathematic an error twoport, which has inside the imperfections of the measurement hardware. This is the way how it is done by sixport VNA  calibration and measurement or similar.

Hello,

I don't think the measure of ok1vaw is particularly bad. The positive peak value at 3.3 GHz (-17.5 dB) and the negative peak value at 2.9 GHz (-20.5 dB) turns in a Return Loss of port 2 of -18.9 dB (Good!) and a residual uncorrected port 1 mismatch of -34.2 dB that gives the peak error of about +/-1.5 dB. Not perfect but not awful. I guess you can hope to reach at that frequency with NanoVNA, and with a good calibration, a residual Return Loss of -40 dB on port 1, that would turn in a peak error of about +/- 0.8 dB instead of +/-1.5 dB.

Note also that the ok1vaw picture is much more zoomed (-10..-30 dB) than the @switchabl one (-5..-65 dB).

Best Regards, Marco.

@Marco
Yes, thank you! I should have done the math. Maybe I was also fooled a bit by the zoomed in plot. So the measurement from ok1vaw is maybe not ideal, but not as awful as I thought (and the port 2 return loss of his device is clearly quite good for a NanoVNA).

@ok1vaw
Yes, in principle with SOL calibration you have full correction of directivity, source match and tracking errors for 1-port measurements. In practice, there are factors that cannot be corrected, so residual errors remain:
- Calibration standards: the calibration is only as good as the standards. Or rather the characterization of the standards, they do not need to be perfect, but you need to know their parameters.
- Non-static effects: calibration is done at a single point in time and of course it cannot correct for anything that changes after that, like

* drift in your couplers (through temperature drift and relaxation processes)
* flexing your test port cables (and resulting phase/loss change)
* connector repeatability (the connectors of your DUT will be different from those of your calkit and connecting them at a different angle may change the results)
* in particular: bad cables/connectors

- Non-linear effects: the error model assumes that the errors can be described by a linear two-port, but there may be non-linear errors

* non-linearity of the receiver (e.g. the IF amp of the NanoVNA V2 has different gain settings and if the gain calibration for the different settings is not perfect, the resulting errors can not be corrected by a linear network)
* non-linearity of the source (this is normally not an issue because the source output is measured by the reference receiver through a power splitter/coupler, but the NanoVNA doesn't have one)

- "Actual" errors (as in, mistakes):

* software bugs (the 2-port calibration in NanoVNA-saver looks strange to me and I think it may be wrong, but have to look at it again in more detail; 1-port looks fine)
* user error (maybe the most common one :-)

If we wanted to have real confidence in the accuracy of the NanoVNA, in theory all these would have to be characterized.

You can tell that the ripple in our measurements is very likely due to residual errors if you consider the time domain. To get 400MHz ripple, you need two reflections with a delay of 2.5ns (roughly 2x30cm...), and it doesn't seem reasonable that the port 2 circuit can generate a reflection with that much delay. And indeed, looking at the impulse response calculated from my measurement, there is a second (small) reflection at 2.5ns negative (!) delay. Some of the signal is reflected before it even reaches the reference plane. So clearly this is a residual reflection at port 1 that has not been completely corrected through calibration.

Hi Switchabl

Sorry for jumping into an ongoing discussion, but as you say a better background info would have avoided the “lecturing” 😊

I fully understand your testing and I did some measurement you can play with:

I used NanoVNA-saver and measured with the Hugen SAA-2_2 300 points (3 segments) from 0.1 to 3000MHz uncalibrated data for the short open and load of my HP85033C female kit. Then also a 26.1cm UT141A line as well the port2 for the SAA-2_2. Included in the zip file is the calibration kit file which you can read by a test editor. I have done a calibration by help of VNWA facilities and found by comparing to the included s1p file for the UT141A line that the open delay (which is a two way delay should be reduced from 33.39ps to 31.29ps for the female open. This is simply due to the fact that the center conductor extender for the female open has a position dependent of the test cable male adaptor position/length/chest position of teflon surface for the male center pin. As the difference is and offset delay of 1.05ps is it corresponding to a physical offset of 0.3mm. Actually this mechanical offset should be divided between the female and male so female offset reduced by 0.525ps and short offset increased by 0.525ps, but it really does not matter so open only I would say is OK.
I have never used scikit-rf, as the VNWA software does the same job for uncalibrated data, but it could be fun if my measurement could be verified as I get a perfect match between the uncalibrated measurement of the UTR141line and the s1p reference file created by SimSmith based on the k0, k1 and k2 factor I have derived for my particular UT141A line

I have Rosenberger measurement which I will send by direct mail as it is a longer story

Kind regards

Kurt





Fra: NanoVNAV2@groups.io <NanoVNAV2@groups.io> På vegne af switchabl@mailbox.org
Sendt: 11. september 2020 23:44
Til: NanoVNAV2@groups.io
Emne: Re: [nanovnav2] V2 from OwoCom from Tindie -port s21 matching



Kurt,

thank you for your comment. I think we are talking about different things here though. I apologize for maybe not making myself clear. First let me clarify that the point of this measurement was to characterize the load match of port 2, i.e. it is a single-port S11 measurement, port 2 is the DUT. The purpose of this was to be able to estimate the uncertainty in S21 measurement (without full 12-term correction) due to load match error.

For this purpose one of the included 30cm cables was connected to port 1, a SOL calibration was done using the open and short from the SDR-Kits female Rosenberger kit and a Suhner load (that is better than my particular Rosenberger one). I captured raw measurement values and applied calibration with scikit-rf and your model definitions (I have also generated S1P files for NanoVNA-Qt but I prefer to save the raw data for flexibility). So the reference plane was at the end of the test cable and this was directly connected to the DUT (port 2 of the NanoVNA).

The result is more or less as expected and within spec (load match is not that great on the V2). There is about 0.5dB of ripple at about 500MHz which as you say is what you would expect from a cable AFTER the reference plane and which you wouldn't expect to see with the reference plane at the end of the cable and a good calibration.

I am not quite sure what happened there. This is an old measurement I did when I first got and I should maybe repeat it with a better cable, especially since there is a bit of bending involved. Also, the directivity of the bridge is not great above 1.5GHz and if it has drifted significantly between the calibration measurement and this one, that might also be an explanation (there were some other ones done in between). But I am still looking into that.

I am also not completely sure if I can trust the Rosenberger parts (particularly the open) above 1.5 GHz. If you have ever measured these at higher frequencies, I would highly appreciate if you could share the touchstone files.

In any case, the main point I was trying to make is that the measurement by ok1vaw has similar ripple only a lot worse (several dB). So I suspect something went wrong with his measurement.