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Kirkby calibration kit alternatives?
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G0HZU:
I don't have any precision 3dB attenuators here at home but I have measured a basic SMA 3dB attenuator on my VNA and the data is below. This is after a full 2 port calibration with an Ecal module. The scale is 0.2dB/div and I used a BW of 500Hz. Note that this is after an Ecal calibration and not after a calibration with my DIY cal kit. A plot after calibrating with my DIY cal kit would have ripple on it up at frequencies above 3GHz.

The attenuator isn't very accurate and is more like a 2.8dB attenuator at LF but it is rated to a high frequency.

But you can see how low the ripple and noise is after a full 2port calibration for S11 S21 S12 S22. Note that I'm using Gore cables here that would have cost a 4 figure sum each here and a $13k Ecal module and a fairly decent VNA.

However, a 3dB attenuator test isn't exactly a demanding proof of the integrity of a VNA calibration. There are much tougher tests than this that will show up any sloppiness in the cal kit corrections.
Even an error of 0.2ps in a cal kit delay correction can be highlighted with some tests.

G0HZU:
I entered a polynomial for the Lx corrections for my DIY cal kit SHORT and got the result below on the simulator after a quick manual tweak. It could be better than this but I'm not really that bothered to polish it any further.

I then did a full 2 port calibration using the DIY cal kit (rather than the Ecal) and remeasured the 2.8dB attenuator. So this was calibrated using my first attempt at some decent cal kit correction factors for my DIY kit. Note that this cal kit also includes my Cx corrections to get a similar flat response on the OPEN to 6GHz.

See below for the result. This is actually better than I expected. If I hadn't done the Lx corrections the trace would have a LOT of ripple on it by 5GHz.

G0HZU:
Here's a plot of what my DIY kit looks like with only crude corrections in it. There are no Lx corrections for the SHORT and only a basic C0 correction for the OPEN. This would still be good to 3GHz as long as the various cal kit correction factors for DELAY and LOSS were correct.

That's why I think anyone could make a decent DIY cal kit to 3GHz assuming they are prepared to tweak/optimise their cal kit after some tests that help to optimise it.

I could put together a cal kit file with this data in it and show you the 3dB test if that helps? It would have some obvious ripple above 3GHz. However the 3dB test is a really soft test anyway.
Mechatrommer:

--- Quote from: rfspezi on February 22, 2018, 11:30:03 am ---By the way... can anybody explain how the "reflection bridge" of the KC901V exactly works?
I guess it is the structure that looks like a wilkinson power divider?

--- End quote ---
its a power splitter as described by TheSignalPath. there must be detailed explanation, concept and math behind it. i will read them up when i want to make my own diy VNA ;) or if i figure out my VNA is broken and need a tear down. fwiw i guess this type of topology that makes KC901V gives false reading esp impedance value at very low end of frequency 5KHz - 1MHz or so.


--- Quote from: G0HZU on February 22, 2018, 12:11:19 pm ---
--- Quote ---fwiw, among other things, attached is attenuator plot provided from Kirkby to me. the result i got on my VNA is much noisier, so i know something went wrong. i'm ordering stuffs to further verify my VNA condition if its fits for measurement etc later. i hope they will not get lost during CNY...
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Is that attenuator (reference) plot taken with the 8720D VNA and your cal kit? Or is it a 'best' reference taken of the attenuator with the 8720 and a Keysight/Agilent cal kit?

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he claimed he tested all the cal kit sold to me by his own hand. incl the attenuator. so the plot you see should be unique only to my attenuator. i expect other kirkby cal kit owner should have different plot. it took long time (about a week or more) for the calibration to be made, he claimed he do this process in some quantities, to quicken his process. going to lab with my cal kit alone to do the process will take him even longer time, i take and understand this as his economical point of view. furthermore he was not well when i made the purchase i almost cancel the order because it took so long for him that i thought he gave less interest in my order. but i'm glad to support him.


--- Quote from: G0HZU on February 22, 2018, 12:11:19 pm ---If a calibration kit is made using SMA F-F bullets and end caps there are a few things that will limit its performance. There will be some (extremely tiny) added inductance at the end of the SHORT and also the SMA bullet won't have a perfect 50R Zo. The SMA bullet is about 42ps in terms of delay and this is longer than a regular 85033 cal kit and so this can make any phase variation (due to imperfect 50R Zo) appear slightly worse. The 85033 cal kit uses precision 3.5mm connectors and an air dielectric and it is much better suited for use up at 7GHz.

It doesn't take much to get 4 degrees of phase error over 6GHz in a DIY cal kit. If you get several degrees of phase error and it isn't corrected for by the cal kit corrections then you can expect to see that reference plot degrade a lot by 5GHz and it will get quite noisy by 7GHz. Also, an inline test of a 3dB attenuator isn't a very critical test.

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i'm not sure how to reply to this, just as how i'm going to reply to Kirkby in my last test, i'm not knowledgable much that i need to gain some info, do some test (or even need to have access to much expensive gears) to be able to comprehend some of the meaning. i only can hope or rely on some expert to do the testing and give some promise that it should work.

btw attached (1st file) is my translation of the parameters extracted from kirkby's data relevant to my kit and VNA. for open standard, he provided varying offset delays (that i translated from offset delay ps metric in his file into mm). its varying in sub ps range or max deviation (not std deviation, maybe variance) of 0.162 mm throughout the BW. so i just prefer the mean value of 17.357 mm @ 3.5GHz (57-58 ps) to punch in the VNA if i want to do quick testing. fringing capacitance effects should already be lumped in the figures during calculation or modelling). as for short he only provided a single offset delay so i suspect the variance due to inductance effect should be negligible? i never heard of fringing inductance effect for short standard ;D he should provide the complete data if this is a concern. anyway this only based on trust to the expertise because i'm not at his location during the modelling. if he lied, then i lied ;)

the 2nd attachment is the zipped s1p for my SOL kit, i dont know how to read this and not sure if its relevant to this discussion, i need time to gain on this. 3rd pdf file is my latest test report to him to seek advice. you can see S11 and S22 plot of the terminated attenuator (after calibration with his SOL kit) are quite spot on except at the area of notches which he claimed of high uncertainty even if measured with his 20GHz VNA since attenuator is terminated hence reflecting very little signal for processing. his arguments are quite rational though... and i can accept a $2K VNA will be of no match to the $17K VNA. the concern in the report is S21 you can see the noises i was talking about. he stated the attenuator should not jump up and down 5dB variance in very short BW range. i'm yet be able to reply on this before i can do more testing..


--- Quote from: G0HZU on February 22, 2018, 12:11:19 pm ---However, I see this process as polishing a turd once you get up to 7GHz. Much better to have a decent 85033 cal kit here.

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that Agilent cal kit if well within spec and in full complete data, should be about 2-3X the cost of my VNA. i can see used price is $2K already without any data. i would not give my money betting on that. ps: i'm not sure why Kirkby called his cal kit as 85033E similar to agilent naming. maybe its tracable to the agilent cal kit he owned? and his cal kit is modelled/compared after that? i'm not sure.
Mechatrommer:

--- Quote from: G0HZU on February 22, 2018, 01:01:11 pm ---I entered a polynomial for the Lx corrections for my DIY cal kit SHORT and got the result below...

--- End quote ---
what i concluded from your reply #35 and #36 is, are you suggesting that using less quality cal kit in the same VNA will result in more ripple? if that is the case, there is possibility that kirkby did the attenuator report using agilent cal kit as his calibration standard? i'll need to confirm this back to him.

anyway i'm doing the test with cheap crap china sma connection and 50 ohm coax cable. maybe those also contributes to the rippling effect, i'm not sure. what i'm sure is the cabling still give consistent result even if i move them around a little bit... fwiw...
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