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Hi David

I got permission from R&S to publish their T-Check software
and

But that was in 2012 and now you just in the VNWA software in a custom trace right click the Expression field and the T-Check formula is filled in. Enable a trace with linear magnitude (s parameter linear) and with 0.01 per division (1%) and reference 1 followed by a press on the F2 key and the VNWA performs a full 12Term measurement of T-Check.
Of course the VNWA shall in advance be SOLT calibrated and the T-Adaptor fitted.
If a s2p file imported with full s11/s22/s21/s12 measurement The VNWA software is totally unique (and free to use) and I have several times "banged you on the head" to overcome you HP bias and fixation and get on board your wonderful VNWA hardware/Software sitting collecting dust on the shelf.
? ? ?

Kind regards

Kurt



-----Oprindelig meddelelse-----
Fra: [email protected] <[email protected]> P? vegne af Dr. David Kirkby from Kirkby Microwave Ltd
Sendt: 24. september 2019 10:17
Til: [email protected]
Emne: Re: [nanovna-users] on the comparisons



On Mon, 23 Sep 2019 at 21:04, alan victor < <mailto:avictor73@...> avictor73@...> wrote:

Hello Reg,

I brought up this question of uncertainty in measurements several

posts ago. Although the calculation is not complicated, obtaining the

parameters to find the uncertainty boundaries is a task.

The easiest one to address is S11 and there are papers published by

NIST and the Automatic Radio Frequency Test Group (IEEE) that have

addressed the exact calculations. I'll see what I can find and can post.

I was unclear what the gentleman in this thread were requesting, but I

believe it is the same information required to calculate the

uncertainty error for any VNA measurement. For S11 this would include

the directivity errors, the reflection tracking errors and the source

match errors. These are exactly the three of the five elements posted

by the NanoVNA after a cal is complete for S11 only. Additional ones come into play for S21 cal.

Hence the measurement uncertainty in S11 is a function of these three

LINEAR values which we must obtain (somehow) from the NanoVNA

architecture; i.e. the bridge and the mixers.

Once these values are in hand, it is possible to find the difference

(error) between the measured S11 and the actual.

For S11 we would find that the difference between the measured and the

actual S11 or (S11M-S11A) is given by DELTA(S11) as follows:

DELTA(S11)=(S11M-S11A ) ~ D + TR * S11A + MS * S11A^2

D is the directivity errors, TR is the reflection tracking errors, MS

is the source match errors.

Hence, devices with small reflection coefficient, the D value is the

source of the major error. While the devices with large reflection

coefficient, source match is a most significant error.

This is a very terse answer to a subject that is well documented but

not easy to answer in a brief email.

Hope this sheds some light.

Alan

I don’t have the mathematical abilities for this, but I don’t think this is an easy problem to address, as finding out the magnitude of the different errors (eg source match, directivity errors etc) is not trivial without some items that will cost more than a NanoVNA.



However, I do know some of the techniques used, the institutions doing work in this area, as well as the name of a couple of helpful people who maybe able to give advice. I will list these in the order I think of them, rather than any more logical order.??



1) I am aware that the Swiss standards laboratory METAS is doing a lot of work on VNA measurement uncertainty. They have a tool for this



<>



Their algorithms are purchased. Google



VNA uncertainty METAS



2) There is also a lot of work on this area at the National Physical Laboratory (NPL) In the UK.



Google



VNA uncertainty NPL



3) Although the first paper I found on this topic is about waveguide calibration, so is not particularly relevant here, it does contain the email address of the person leading this work at NPL, who is *Nick Ridler. *



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I spoke to Nick at one of the Keysight meetings on THz measurements. He was very helpful about what I asked him about. (As a side note, I recall being quite impressed to see a piece of rectangular waveguide Nick bought along, about 100 mm long, where the cross section of the waveguide is so small that I thought it was a spec of dust. ???)



4) Precision airlines, which are lengths of transmission line, very close to 50 ohms, are usually used for determining the error terms. The techniques for this are well documented.



5) Airlines for 3.5 mm, which is physically compatible with SMA, are readily available on eBay , (I have some myself as part of an HP 85053B VNA verification kit). The 3.5 mm airlines tend to be quite short in length, limiting their use to high frequencies.



5.5) In my opinion, for one interested in obtaining the best performance at modest cost, it is best to test with APC7 connectors. They are very high precision connectors, better than N or SMA, yet since they are not used much nowadays, they can be found quite quite cheap.



6) 7 mm (APC7) airlines are readily available at modest prices on eBay.

They can easily be obtained at up to about 300 mm in length, but they still cost more than a NanoVNA. Prices of these tend to vary dramatically, but if one waits, one will find them for a few tens of dollars.



6.5) The best APC7 airlines don't have connectors on them, but are supported by a test port at one end a short with a hole at the other end.

The centre conductor is stored in a via. These are often sold as “tested”

on eBay, despite they have no centre conductor included, so are of no use.



Slightly inferior airlines have connectors on them and the centre conductor can not be removed or lost. I would personally buy these, as the other type are difficult to handle. I have them in one of my VNA verification kits,



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These can be found at reasonable price on eBay, but often are missing a floppy disk.l That makes them totally useless for their intended purpose, as does the S/N of any part being different from that on the floppy disk.

If you see such a kit, you can tell the seller that, and can potentially pick up a couple of airlines very cheaply.



7) A technique known as the T-checker,



<>



can be used to verify the calibration accuracy of 2-port measurements. This makes use of a mathematical property that relates the 9 S-parameters of a lossless 3-port network. One of the ports is terminated in an arbitrary impedance, the measurements made at the other two points, then a simple scalar parameter is computed. *This is a very cheap technique to use*, requiring nothing more than a coaxial T piece, but it only verifies the calibration - it tells you nothing about the sources of error.



I am unaware of any standalone programs that can take a Touchstone file around generate the T- check formula, but it would not be rocket science to write one. M



8) Kurt Poulsen, who is a member of this group, has done a lot of work on calibration kits using the software designed for use with the VNWA. Perhaps Kurt can provide links to the relevant documents he had written.



9) Unfortunately the VNWA software is closed source, but there are open source versions of tools like optimisers in high school



10) There’s going to be some discontinuity between SMA & APC7 adapters due to the change in diameter of the lines. That will be less of an issue with N to APC7, as there is little change in the diameter of connectors.



11) The components for APC7 calibration kits are quite inexpensive, but still higher than a NanoVNA. APC7 loads with return losses of almost 50 dB can be found for a few tens of dollars.



12) APC7 open standards fail into 2 types

* The simplest is just a tube that acts as a waveguide beyond cutoff. The basic problem with this design is that the fringe capacitance varies depending on how far the collet protrudes beyond the reference plane which is not well defined.

* More sophisticated opens have a piece of plastic that pushes the collet flush with the reference plane. The plastic adds capacitance, but at least one then has a well defined geometry.



That’s all I can think of just now.

--

Dr. David Kirkby,

Kirkby Microwave Ltd,

<mailto:drkirkby@...> drkirkby@...

<>

Telephone 01621-680100./ +44 1621 680100



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