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Calibration coefficients - can one change them?
Dr. David Kirkby from Kirkby Microwave Ltd
7/02/19
#99
I've got nanoVNA on order, but don't have it yet. It's not due to at least
4th July, so that's no big surprise. Can anyone tell me if one can change the coefficients of the open/short/load kit? I just have done a test with 4 sorts of opens. The connector was a male N, so the calibration standard a female N. But anyway, here are the results at 902.625 MHz, which is the nearest frequency I have any data. (I had calibrated to 18 GHz for another job, so don't have data at exactly 900 MHz, but this is as close as makes no practical difference) *Male N plug just open. No female standard*. Phase = +4.64722 degrees @ 902.62500 Female open standard from HP 85032B open standard, used without any extender, as it is NOT supposed to be. Phase = +4.64014 degrees @ 902.62500 MHz Female open standard from HP 85032B calibration kit, with the extender, as it SHOULD be used -3.65161 degrees @ 902.62500 MHz Female Open standard from HP 85054B 18 GHz calibration kit. Phase = -18.18457 @ 902.62500 MHz IMPORTANT The male N plug was a metrology grade male-male adapter from an HP 85054B VNA calibration kit. Since the shape of the male pin is not well defined, and commercial grade connectors are recessed by various amounts, I would not expect this result to be reproducible with different connectors, but fairly substantial variations Anyway, the conclusion is that for the female N calibration standards, the phase varies from +4.6 to -18.2 degrees at 902 MHz. So a phase variation of 22.8 degrees depending on what calibration standard one uses. The variation would be smaller at lower frequencies, but depending on what the nanoVNA assumes about the calibration standard, one could get quite different results. Ideally, one needs to be able to enter the offset delay of the calibration standards as an *absolute minimum.* The fringe capacitance would be nice too, but could just about get away without that, as one could do a rough job of compensation via changing the offset delay. Better still would be a third order polynomial, which would make entering coefficients from a commercial calibration kit easy, without trying to work out any compensation values. If the firmware is dumb enough to assume the open standard is ideal (phase = 0 degrees), and the short is ideal (phase = 180 degrees), then it would cause significant errors. Likewise, if the firmware made assumptions about the SMA kit supplied, those assumptions would be wrong if a different SMA, 3.5 mm, N or APC cali kit was used. Even if you are not interested in phase measurements, be aware of the fact the point of the vector correction is to correct for amplitude variations too. Anyone that believes that they don't need to worry about vector correction, as they are only interested in amplitude measurements, is seriously mistaken. Dave -- Dr David Kirkby Ph.D C.Eng MIET Kirkby Microwave Ltd Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, CHELMSFORD, Essex, CM3 6DT, United Kingdom. Registered in England and Wales as company number 08914892 https://www.kirkbymicrowave.co.uk/ Tel 01621-680100 / +44 1621-680100 |
Dave, as far as I can tell to this point, there is no ability to enter your own polynomial coefficient of the standards. The unit comes with SOL 3.5 mm and they are described as fairly simple construction. Using independent calibration of a 3.5 mm cal kit on another VNA, I determined the quality of their standards. And based on that response I believe for HF applications the unit is quite acceptable.
Again, my earlier question are the standards they provide described within the firmware, don't know. My initial guess is they are taken as ideal, but I may be wrong. Alan ________________________________ From: nanovna-users@groups.io <nanovna-users@groups.io> on behalf of Dr. David Kirkby from Kirkby Microwave Ltd <drkirkby@...> Sent: Tuesday, July 2, 2019 12:06 PM To: nanovna-users Subject: [nanovna-users] Calibration coefficients - can one change them? I've got nanoVNA on order, but don't have it yet. It's not due to at least 4th July, so that's no big surprise. Can anyone tell me if one can change the coefficients of the open/short/load kit? I just have done a test with 4 sorts of opens. The connector was a male N, so the calibration standard a female N. But anyway, here are the results at 902.625 MHz, which is the nearest frequency I have any data. (I had calibrated to 18 GHz for another job, so don't have data at exactly 900 MHz, but this is as close as makes no practical difference) *Male N plug just open. No female standard*. Phase = +4.64722 degrees @ 902.62500 Female open standard from HP 85032B open standard, used without any extender, as it is NOT supposed to be. Phase = +4.64014 degrees @ 902.62500 MHz Female open standard from HP 85032B calibration kit, with the extender, as it SHOULD be used -3.65161 degrees @ 902.62500 MHz Female Open standard from HP 85054B 18 GHz calibration kit. Phase = -18.18457 @ 902.62500 MHz IMPORTANT The male N plug was a metrology grade male-male adapter from an HP 85054B VNA calibration kit. Since the shape of the male pin is not well defined, and commercial grade connectors are recessed by various amounts, I would not expect this result to be reproducible with different connectors, but fairly substantial variations Anyway, the conclusion is that for the female N calibration standards, the phase varies from +4.6 to -18.2 degrees at 902 MHz. So a phase variation of 22.8 degrees depending on what calibration standard one uses. The variation would be smaller at lower frequencies, but depending on what the nanoVNA assumes about the calibration standard, one could get quite different results. Ideally, one needs to be able to enter the offset delay of the calibration standards as an *absolute minimum.* The fringe capacitance would be nice too, but could just about get away without that, as one could do a rough job of compensation via changing the offset delay. Better still would be a third order polynomial, which would make entering coefficients from a commercial calibration kit easy, without trying to work out any compensation values. If the firmware is dumb enough to assume the open standard is ideal (phase = 0 degrees), and the short is ideal (phase = 180 degrees), then it would cause significant errors. Likewise, if the firmware made assumptions about the SMA kit supplied, those assumptions would be wrong if a different SMA, 3.5 mm, N or APC cali kit was used. Even if you are not interested in phase measurements, be aware of the fact the point of the vector correction is to correct for amplitude variations too. Anyone that believes that they don't need to worry about vector correction, as they are only interested in amplitude measurements, is seriously mistaken. Dave -- Dr David Kirkby Ph.D C.Eng MIET Kirkby Microwave Ltd Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, CHELMSFORD, Essex, CM3 6DT, United Kingdom. Registered in England and Wales as company number 08914892 https://www.kirkbymicrowave.co.uk/ Tel 01621-680100 / +44 1621-680100 |
Yes, calibration data can only be modified with electrical delay. For better accuracy, I customized the shortest and most accurate calibrations kits possible. Naturally, the accuracy of professional calibration parts cannot be achieved. Unfortunately, some clone makers do not understand the role of the calibration kit, providing a poor quality load to act as a calibration kit, which can result in worse measurement results.
hugen gen111.taobao.com |
Dr. David Kirkby from Kirkby Microwave Ltd
7/03/19
#109
On Wed, 3 Jul 2019 at 04:04, <hugen@...> wrote:
Yes, calibration data can only be modified with electrical delay. What exactly do you mean by that? Are you saying that the firmware has the ability to allow one to adjust the delay of the calibration standards? If so that’s good news. Just being able to enter a delay for the standards will be a vast improvement over having no control whatsoever. But someone else wrote ideal standards were assumed. For better accuracy, I customized the shortest and most accurate calibrations kits possible.There’s no need to make the shortest calibration standards possible, *IF* one can enter the delay of the standards. *The internet is full of people claiming incorrectly the need to have the shortest possible delays on calibration standards. * If you look at the delays on professional calibration kits, you will find they are *longer* on modern high-end kits than they were on obsolete kits! The obsolete HP/Agilent 85032B type-N calibration kit had delays of http://na.support.keysight.com/pna/caldefs/85032BE.htm Female Short = 0.000 ps Female Open = 0.093 ps Those are the shortest possible delays one can make on female N. Now if you compare those figures to the current $20,690 high-end 18 GHz 85054B kit, you will find the delays are much longer in the modern kit. http://na.support.keysight.com/pna/caldefs/85054B.htm Female short = 27.990 ps Female open = 22.905 ps For the lower cost ($2410) 9 GHz 85033F 9 GHz kit, the delays on the female parts are: http://na.support.keysight.com/pna/caldefs/85032F.htm Female Short = 45.995 ps Female Open = 41.170 ps *So very approximately, for female N calibration standards:* Obsolete 6 GHz kit - delays around 0 ps Current $2410 9 GHz kit - delays around 43 ps Current $20,690 18 GHz kit - delays around 24 ps Clearly on female N parts, it’s possible to make delays close to 0, but Keysight don’t use them. The same general principle is true for the male N standards, but in that case it is impossible to make a zero delay. If you check the links I gave above, you will find the following. *Very approximately, for male N calibration standards:* Obsolete 6 GHz kit - delays around 17 ps Current 9 GHz kit - delays around 43 ps Current 18 GHz kit - delays around 60 ps For obsolete HP 3.5 mm kits, the delays were from memory were around 17 ps, but any of the modern kits (85033E, 85052D or 85052B) all have delays of about 30 ps. Naturally, the accuracy of professional calibration parts cannot be achieved. That statement is a bit ambiguous. If you are saying that a high end VNA is best used with a professional calibration kit, I would agree with you. However, if the NanoVNA firmware always assumes idealised parts with a delay of zero, then things would change *dramatically*. One could easily make more suitable opens and shorts than would would have if one spent $2410 on an 85032F. Spending even more, buying the $20,690 85054B would give you even less suitable open and short standards. The loads from the $20,690 kit would always be the best loads, but of course it would be crazy to use loads that probably cost $1500 each on a VNA costing less than $100. Unfortunately, some clone makers do not understand the role of the calibration kit, providing a poor quality load to act as a calibration kit, Yes, clone makers often don’t appreciate or care what crap they turn out. But a lot of self-proclaimed experts, writing web pages about how to make calibration kits don’t understand what they are doing. My own company, Kirkby Microwave, does care and we have enough knowledge to understand most of the intricacies. I cringe at some of the stuff I see written on the internet about VNA calibration kits. hugen Dr. David Kirkby Kirkby Microwave Ltd. https://www.kirkbymicrowave.co.uk/ <http://gen111.taobao.com> -- Dr. David Kirkby, |
Hello David,
I mentioned in a much earlier post, I have no idea what is in the firmware definition for the standards supplied with the nanoVNA. In addition, based on the physical description of the standards provided, i.e. the 50 ohm load a pair of 100 ohm chip resistors soldered in parallel, that they may not represent the state of the art! However, if there were a way to measure and characterize them as a entry to the unit, such as a classification table, that would be helpful, I did measure their 50 ohm load after an independent 3.5 mm SOLT cal on a hp 8753 VNA and found their load standard at 900 MHz was at best 17 dB return loss. Another measurement of a second independent 50 ohm load from Weinschel on the 8753 at 900 MHz demonstrated a significantly better return loss. However, on the nanoVNA the Weinschel showed the same 17 dB return loss. Is there an issue here? YES! I might add, folks who use this instrument should consider obtaining SMA connector savers. It will not be long before the poor SMA connectors on the nanoVNA are tainted. Alan |
I should add, that after a calibration with their standards, the resulting short, open and load are PERFECTLY located on the chart. Really! Wow, I have to pray that a set of APC-7 or my sophisticated cal standards could do so well. That said, this is a neat box and applying a little care to the measurement and compensation for the short comings can rectify some of these issues.
|
Dr. David Kirkby from Kirkby Microwave Ltd
7/03/19
#115
On Wed, 3 Jul 2019 at 13:19, alan victor <avictor73@...> wrote:
I should add, that after a calibration with their standards, the resultingI'm afraid to say you are stepping into the error many people make. They first calibrate with a set of standards, then see those standards look perfect on the Smith Chart. I have hayfever at the moment, but made my first YouTube video yesterday to show the problem. https://youtu.be/9HEchzY0Gvw Please ignore the sniffles I have and the fact I only used a mobile phone. https://www.kirkbymicrowave.co.uk/ Tel 01621-680100 / +44 1621-680100 |
I am quite familiar with the error. Please read my post carefully. I point this measurement observation out as this is a STRONG HINT that something is NOT correct!
In any case, I measured their "standards" independent and I found the 50 ohm load at 58.5 + j 11.5 ohm at 900 MHz. The short and open are 412 pH and 190 fF respective. I suspect no polynomial L or C coefficients are loaded to a table in the nanoVNA to reflect these deviations. |
Dr. David Kirkby from Kirkby Microwave Ltd
7/03/19
#122
Okay, I get what you are saying now.
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That problem should I guess be resolvable in the firmware, although it will take a very careful look at the firmware to see how to resolve it. But the problem should be resolvable. Dave On Wed, 3 Jul 2019 at 14:31, alan victor <avictor73@...> wrote:
I guess to make the point clear, 3.5 mm SMA standards DO NOT APPEAR as --
Dr David Kirkby Ph.D C.Eng MIET Kirkby Microwave Ltd Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, CHELMSFORD, Essex, CM3 6DT, United Kingdom. Registered in England and Wales as company number 08914892 https://www.kirkbymicrowave.co.uk/ Tel 01621-680100 / +44 1621-680100 |
Spot on Dave, I agree. Addressing this process would provide a nice value added capability.
Alan ________________________________ From: nanovna-users@groups.io <nanovna-users@groups.io> on behalf of Dr. David Kirkby from Kirkby Microwave Ltd <drkirkby@...> Sent: Wednesday, July 3, 2019 2:57 PM To: nanovna-users Subject: Re: [nanovna-users] Calibration coefficients - can one change them? Okay, I get what you are saying now. That problem should I guess be resolvable in the firmware, although it will take a very careful look at the firmware to see how to resolve it. But the problem should be resolvable. Dave On Wed, 3 Jul 2019 at 14:31, alan victor <avictor73@...> wrote: I guess to make the point clear, 3.5 mm SMA standards DO NOT APPEAR as-- Dr David Kirkby Ph.D C.Eng MIET Kirkby Microwave Ltd Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, CHELMSFORD, Essex, CM3 6DT, United Kingdom. Registered in England and Wales as company number 08914892 https://www.kirkbymicrowave.co.uk/ Tel 01621-680100 / +44 1621-680100 |
Hello, Dr. David.
You can set the electrical delay with "DISPLAY, SCALE , ELECTRICALDELAY". Yes, commercial calibrations are not required to be as short as possible, they can provide accurate correction data. But at a limited cost, it is easier for manufacturing to be as short and accurate as possible than to provide more accurate correction data. At the same time for HAMs with no VNA experience, it is easier to provide calibrations that no longer require additional data to be entered, although this is not precise enough, but the measurement below 1GHz is sufficient. BTW,please forgive me for my poor English expression. Thank you! hugen gen111.taobao.com |
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