Mini-teardown: Omicron B-WIC impedance test adapter - Page 1
Forum section: electronics / testgear

author: precaud
author_rank: Frequent Contributor
timestamp: December 30, 2017, 07:14:21 pm
content: Ever since I was exposed to the Omicron Bode 100 system by forum member Pitrsek, I've dreamed of one materializing on my bench. It appears to be the cat's meow for low-level impedance measurement up to ~30MHz. I want one. But the price new is too dear. And they're snapped up immediately when they appear used.Needless to say, a Bode 100 hasn't materialized. But a couple of their impedance test adapters did; the B-SMC and B-WIC. I was planning on making something like these, but these are better executed than I could make here. From what I've seen, they appear to be the best general-purpose impedance test fixtures you can buy. So the mission is, can I make them work with my Anritsu (or any other) VNA or FRA ?First up is the B-WIC, designed for thru-hole components. Nicely thought out, with gold-plated contacts, a strong actuating spring for good contact, and sitting in a machined teflon carrier.So what's inside, Johnnie? Five 1/4W metal film resistors. The circuit is attached below. I don't yet know the configuration of the Bode 100's inputs and output when using this adapter; I'm hoping Pitrsek can help us with that. It looks like they might all be 50 Ohms. What I do know is that the Bode 100 has an entirely separate mode when using the adapters, and requires full Open-Short-Load compensation sweeps be made before using them. So this is not a plug-and-play device as it sits.The CH 1 output (to the reference channel) is attenuated by ~27dB. Maximum input to the fixture is stated as 1VRMS, so I'm guessing the analyzer has to have good dynamic range. The reference impedance is formed by the two 4R7's in parallel at the CH 2 output, followed by a 47 Ohm buffer R. So after applying the OSL corrections to T/R, the magnitude vector * 2.35 would give the impedance in Ohms, correct?

author: petemate
author_rank: Regular Contributor
timestamp: December 30, 2017, 08:21:09 pm
content: Quote from: precaud on December 30, 2017, 07:14:21 pmYou can just download the software athttps://www.omicron-lab.com/.It has a graphical representation of how everything is connected in various modes, which makes it easy to understand. Its also nicely represented in the manual:https://www.omicron-lab.com/fileadmin/assets/manuals/Bode-100-User-Manual-ENU10060503.pdfCheck out page 37.

author: precaud
author_rank: Frequent Contributor
timestamp: December 30, 2017, 11:15:26 pm
content: Yes, I looked at the manual, the section on using the impedance adapters starts on pdf page 64. The internal configuration is not shown; they only say that it is "configured correctly".

author: petemate
author_rank: Regular Contributor
timestamp: December 30, 2017, 11:19:00 pm
content: Yeah, thats why you download the software, set up the measurement and then click on the hardware configuration to see how its actually configured.

author: Pitrsek
author_rank: Regular Contributor
timestamp: January 02, 2018, 07:02:08 am
content: When used with B-SMC/B-WIC, Bode inputs are configured to 1Meg,Output from the generator is 50Ohm

author: precaud
author_rank: Frequent Contributor
timestamp: January 02, 2018, 12:44:57 pm
content: Great, thanks. So it is basically two dividers in parallel, the DUT being a series element in one, the other one is just a 22.27X divider to better match signal levels for the analyzer. They can make up for it in the math, and probably use some preamp gain on both channels.I need to search for bridge formulas for this configuration. I've only worked with DUTs as shunt elements when measuring Z.Quote from: Pitrsek on January 02, 2018, 07:02:08 am

author: rx8pilot
author_rank: Super Contributor
timestamp: January 02, 2018, 10:04:19 pm
content: The Bode100 system is a really cool instrument - I just cannot justify the cost. I am sure they have plenty of customers that are not really price sensitive so that can charge what they want. Guessing the build cost is rather small and they are not interested in dealing with price shoppers like me.I saw one come up on eBay, but only for a moment. It was sold really fast.

author: precaud
author_rank: Frequent Contributor
timestamp: January 03, 2018, 03:08:00 pm
content: Quote from: rx8pilot on January 02, 2018, 10:04:19 pmSame here. But I do like their attention to detail, and willingness to think outside the box to optimize measurements. These fixtures are an example of that. In their defense, their support is excellent. That has to be built into the price.A question for Pitrsek: When you're using the Bode 100 in "Fixture" mode, do you run a "Through" before doing the Open-Short-Load sequence? If so, is the Through setup done with the fixture connected and with no DUT (same as Open) ?My previous assessment of the measurement setup was basically correct. It is a "Series-Thru" topology, with tweaks to optimize the dynamic range. The basic topology is discussed in good detail and compared to other impedance measurement topologies in this Agilent app note:https://www.keysight.com/upload/cmc_upload/All/ChallengesandsolutionsforImpedance.pdfThe Series-Thru circuit is shown on page 66 (the Gain-Phase one). They describe this technique as best used at mid-to-high impedances, from a few Ohms and up. So why the uncertainty at lower impedances? The problem mostly centers around the fact that it uses the 50 Ohm termination inside the analyzer as its reference impedance. That choice makes it generic and portable from one instrument to the next, but it also inserts a high (and inconsistent) amount of series R and L, and shunt C, from cables, connectors, relays, pcb traces, etc, between the DUT and reference R. Simply by moving the reference R next to the DUT where it belongs, optimizing its value, and changing the math to match it, you can extend the usable range of this technique down quite a bit, into the milliOhm range. That's what Omicron did. Simple, clever, and it appears to work.

author: precaud
author_rank: Frequent Contributor
timestamp: January 04, 2018, 05:07:35 am
content: OK, I've modified my Z measurement software for the MS420 to accept the B-WIC. The results are mostly good, with one remaining issue.Here is a plot of three current sense R's. Each one is plotted twice; one with Open-Short-Load (OSL) compensation applied, and one without, so we can see what difference the OSL makes.The formula (polar coordinates, HTBasic syntax) to apply to the measured S21 magnitude at each freq to get correct impedance value is:RealS21 = 10^( (S21 - 26.956) / 20.) ! subtract the fixture's CH 1 divider from the MS420's dBr, and convert to a voltage ratioZmag_Ohms = ( ( 1. - RealS21) / RealS21) * 2.35 ! Series-thru impedance with 2.35 Ohm shunt RIt is worth noting that the phase term plays no part in these equations, and is carried through unchanged. All they're doing is scaling the magnitude vector.OSL corrections are applied after this. When they are being measured, they are also processed by the above before being stored.It should be noted; I do not have the standard short-load card for the B-WIC yet. One is on the way. I used a 3/4"-wide gold-plated shorting bar, and an 1/8W 0.1% 100 Ohm axial resistor for the OSL measurements. The difference between them and the factory-supplied standards could be impacting the results. I'll know in a few days when the standard one arrives.Some observations on the results:: The OSL correction is what makes it useful below 1 Ohm or so. Without it, the impedance accuracy is really poor down there.: My OSL-corrected values run about 2% high. This could be because I didn't include the 50 Ohm source R of the generator in the divider value. And/or the 1% tolerance parts used in the fixture. Easy to correct for.: Above a couple Ohms the accuracy becomes increasingly good enough to not use the OSL. This agrees with Agilent's findings with generic Series-Thru. At 10 Ohms the curves were identical up to the 10MHz plot limit.: Overall, I would agree that, with OSL, 20mOhm is the lower limit for this topology. Below that, the trace gets too noisy to be useful. It is already too noisy at 25mOhm for my needs.: These sweeps were done with a 10Hz IF bandwidth. To see if I could clean the trace up, I dropped the IF BW to 3Hz. The difference was minimal. Not enough to offset a tripling of sweep time. I then increased drive level by 10dB and dropped receiver sensitivity to match. No difference. The S21 short sweep is only 60dB down so this isn't a dynamic range problem.: Below a couple Ohms, the Z curves with OSL are not accurate at high freqs. The 25m and 100m resistors are axial current sense types and should show more inductance than they do.: The phase curves are not accurate above approx. 100kHz , with or without OSL, going capacitive while the Z was going inductive. The native sweeps on the analyzer showed it too, so it's not a math issue; it's coming out of the fixture that way. I did not run a "thru" sweep on the analyzer before doing these, the curves without OSL show a ~ -7º phase mismatch between the channels. This is not in the analyzer; it is coming out of the fixture. OSL corrects the mismatch but the high-freq error remains. Perhaps Omicron applies an inductive constant correction factor?I'll withhold further comment until the short-load pcb arrives.

author: Pitrsek
author_rank: Regular Contributor
timestamp: January 04, 2018, 10:37:11 am
content: Quote from: rx8pilot on January 02, 2018, 10:04:19 pmActually when we have been searching for the FRA/Impedance analyzer, the Bode100 was the most affordable one. AP300, FRA from Venable, Agilent... all are times more expensive. I agree tat for one man shops it is pricey, but I'd much rather have FRA/Impedance analyzer and average scope, than high end scope and no analyzer (my personal opinion with regard to stuff that I do, YMMV). The service and support is top notch.@precaud - with fixtures there is just OSL

author: precaud
author_rank: Frequent Contributor
timestamp: January 04, 2018, 02:07:32 pm
content: OK, thanks @Pitrsek. I agree with your assessment re: cost. The Bode 100 is less than half its competitors. Has much better software. And good accessories/fixtures. It looks expensive for us bottom-feeders, but for equivalent capability, it's a very good value proposition. (I didn't include Cleverscope FRA in this because its not a complete system, but it is very attractive too.)The mind was crunching away at this inaccuracy problem even while I slept. The thought is, that the phase error is caused by:a) my short reference is not good enough (I saw this with the shunt-thru), andb) the different ways that the two outputs to the VNA create their nominal 50 Ohms output.The Ref channel does it by the shunt R of the divider (which divides everything, not just the R component), while the test channel does it through a buildout R, which leaves reactive elements intact. Swamps them, yes, but they are still intact. So my R divider attenuation correction is valid at low freqs, but needs an inductive constant to be added back in to be accurate at high freqs.I would bet my boots that Omicron applies such a constant. (If I am wrong, I would like to keep my boots until winter is over, please...)When measuring Z/phase at high freqs, these little details make a huge difference...

author: BartSchroder
author_rank: Newbie
timestamp: January 04, 2018, 06:58:35 pm
content: Hi,As a comparison, I checked our CS328A-FRA with a 10 mOhm resistor. The frequency at which the impedance doubled (ie Zl = Zr) is 2.9 MHz. So that suggests the fixture series inductance is L= 10m/2 pi 2.9M = 0.54nH.Looking at the Bode 100 graph up above, witha 25 mOhm resistor, I guessed the doubling at about 1 MHz, so L= 25m/2 pi 1M = 4nH. However, as Precaud pointed out, the Bode plot is capacitive, which I would not have expected, so take this with a grain of salt.So some notes:- The CS328A-FRA test jig has about 7x lower series inductance than the Bode 100 one.- The CS328A-FRA response is much less noisy- The CS328A-FRA should be usable to quite a bit lower than 10 mOhm, all limited by the series inductance. With 0.54nH, the doubling for 1 mOhm would be F = X/2 pi L = 1m/2 pi 0.54n = 294 kHz. So you could use it with 1 mOhm up to about 300 kHz or so.- I did this with a bandwidth of 100 Hz. So 'd be guessing that it should complete way faster than the Bode 100.Attached is the plot.cheers, Bart

author: precaud
author_rank: Frequent Contributor
timestamp: January 04, 2018, 07:46:26 pm
content: Thanks for weighing in, Bart. I guess I should make clear:: I'm not evaluating or comparing the performance of impedance measurements systems.: The plots I posted were not made with a Bode 100 system.: The B-WIC is a fixture for thru-hole components, so the resistors being tested are axial- and radial-lead types, which have higher self-inductance than SMD types.The purpose of this thread is to evaluate the B-WIC fixture and figure out how to use it with other VNA's and FRA's (including yours). It is a work-in-progress, and the results to this point are not final.I'll be inviting the Omicron guys to chime in (if they wish) once I get their standard short-load pcb and update the measurements to reflect its use.

author: precaud
author_rank: Frequent Contributor
timestamp: January 05, 2018, 01:05:28 am
content: I think I found the problem. See attached graph. It shows the freq response of the CH1 and CH2 outputs separately, with DUT terminals open and shorted.The CH2 output (yellow) behaves as one would expect; a flat response becoming increasingly inductive when open (capacitive leakage through the fixture, I would guess).The freq response of the CH1 output (cyan), the Reference channel, the channel with the ~27dB L-pad attenuator, varies as a function of the DUT's impedance. It is flat only when the DUT's impedance is near the output resistance of the L-pad, around 45 Ohms. For DUT's below 45 Ohms, this "reference" sweep becomes increasingly inductive. When the reference is inductive, the phase of the DUT is going to appear capacitive whether it is or not.You can correct for this if you have access to the raw data for each channel before doing the T/R division. I couldn't get it to work by doing it after the fact. I tried adding inductance and removing capacitance from the measurement to get the phase to match the impedance curve. For a 100mOhm resistor it took adding 40nH of inductance to get the phase to match the Z curve at 1MHz. But there was still a "capacitive" rolloff starting at 5MHz.The logical thing to do it put the CH1 into a 50 Ohm load, change the L-pad attenuation so the impedance values are correct, and try again.Then again, it could just be the inductance of the long leads of the 1k0 series R in the divider causing all of this.This has turned into more than I bargained for...EDIT: Changing CH1 to work into 50 Ohms made not difference.

author: precaud
author_rank: Frequent Contributor
timestamp: January 05, 2018, 04:42:10 am
content: I tried the fixture with another analyzer (an old AP 102B) and I'm not seeing nearly as much of the odd phase behavior with it. So either the R channel on my Anritsu has an issue (I just cal'ed it last month and it was fine...), or else its floating inputs are a problem with this topology. The 102B has common ground for inputs and output and the phase looks better for CH 1's magnitude curve, though the transfer function with shorted DUT still does show negative phase shift. I'm temped to delete the posts with the errant measurements until I get this sorted out. My apologies...

author: r0d3z1
author_rank: Regular Contributor
timestamp: January 12, 2018, 01:21:57 pm
content: Quote from: precaud on January 04, 2018, 05:07:35 amI am not sure to have understand well this 2 equation, but are you sure that Omicron is using T/R measurement, couldn't that be a I-V measurement ?

author: precaud
author_rank: Frequent Contributor
timestamp: January 12, 2018, 02:00:49 pm
content: Quote from: r0d3z1 on January 12, 2018, 01:21:57 pmThe math doesn't work for any I-V method I tried. Those measure across a standard resistor, with the DUT shunted to ground. If you look at the circuit diagram in the first post, and the Keysight paper I referenced, you'll see it is a "classic" Series-Thru setup. The only difference is, Omicron adds the ~27dB divider in the Ref channel. This equalizes the output levels for each channel which optimizes levels for the ADC that follows. It's the same as what Test channel sees when DUT is a short (50 / 2.35 = 27dB). That's how they get an impedance measurement range that is better-than-usual from this setup. It's a simple and clever solution. But it seems to fall apart with large capacitors. Choosing a smaller ref resistor would extend the range but throws away even more level for low impedances and you'd need more make-up gain in the preamp. It's something I may experiment with.I'm still waiting for the short-load card to arrive to verify my results. It is supposedly in the mail...

author: Pitrsek
author_rank: Regular Contributor
timestamp: January 12, 2018, 02:32:08 pm
content: I believe that r0d3z1 is onto something... With series trough, you have 50Ohms to ground, with B-WIC the 50 on the right side is just termination and leads to 1Meg input. There is 4R7//4R7 to ground. Basically just sampling current. I've attached measurements of 1000 and 2200uF capacitors. The wobble in phase is probably result of low signal magnitude/low measured impedance.

author: precaud
author_rank: Frequent Contributor
timestamp: January 12, 2018, 03:11:02 pm
content: Quote from: Pitrsek on January 12, 2018, 02:32:08 pmYes, with "classic" Series-Thru you have 50 Ohms to ground. But I think that is only because most VNA's have 50 Ohm inputs. So it's only a change from 50 Ohms current sense to 2.35 Ohms current sense. That's how they can measure below 1 Ohm.If anyone sees it different and can flesh it out, I'm certainly open to it. But as of now, I see no reason to think it is other than a "tweaked" Series-Thru setup.Pitrsek, thanks for posting your measurements for 1000 and 2200uF caps. They are in the range one would expect. So no issues with large capacitances....

author: r0d3z1
author_rank: Regular Contributor
timestamp: January 12, 2018, 03:13:46 pm
content: Quote from: precaud on January 12, 2018, 02:00:49 pmZmag = ( (V1/V2) - 1) * 2.35 with V1 and V2 RMS value, the phase could be measured as delay between source voltage and current. This is like what you do with signal generator and scope, isn't it ?However, precaud could you please explain your formula in a more clear way, I have few experience with Scattering parameters and VNA.

author: precaud
author_rank: Frequent Contributor
timestamp: January 12, 2018, 04:32:55 pm
content: Quote from: r0d3z1 on January 12, 2018, 03:13:46 pmYes, but that is a different topology, with series reference R and the DUT to ground. This topology is different; the DUT is in series and the reference R is to ground.I refer to the Agilent paper because it shows all the topologies and gives the basic formulas for each one:https://www.keysight.com/upload/cmc_upload/All/ChallengesandsolutionsforImpedance.pdfThe V/I topology you refer to is shown on p. 26. The formula for Zmag is the same as the one you cited.QuoteThe "Series-Thru" method is shown on p. 66 . It is the same as the Omicron fixture's topology, except: 1) it shows the VNA's 50 Ohm input as the reference R, where Omicron uses 2.35 Ohms; and 2) Omicron adds a 22.276X L-pad in the V1 channel. The formula is given as:Zmag = (50 x 2) x ((1 - S21) / S21) where S21 = V2/V1 (or Vt/Vr) (I wonder if they made a typo, and it should be (50 || 50) ?)In the formula, I replace the 50x2 Ohms with 2.35 Ohms.Then we must adjust the V1 magnitude to correct for the L-pad divider in the fixture (1kOhms into 47 Ohms = -22.276 or -26.956dBr). The Anritsu gives V2/V1 in dBr, so I subtract 26.956 from itSo the formula becomes two steps:The first step, X = 10^ ((S21db - 26.956) / 20.) gives correct V2/V1 as a voltage ratio.Then Zmag = 2.35 * ((1 - X) / X) calculates Series-thru impedance scaled to the 2.35 Ohm reference R.When combined with open-short-load compensation (which the Omicron also uses), this gives correct results within tolerances for components (except for large capacitors at low freqs...).The only thing I have not accounted for is the 50 Ohms source impedance. Maybe the first number should be (50 || 2.35) = 2.244 instead of 2.35 ?I welcome your comments.

author: Pitrsek
author_rank: Regular Contributor
timestamp: January 13, 2018, 06:56:30 am
content: If I go from the schematic in 1st post:U1=Uref*Att (attenuation factor of the divider)U2=Uref*2,35/(DUT+2,35)Uref=U1/AttUref=(DUT+2,35)*U2/2,35U1/Att=(DUT+2,35)*U2/2,35U1*2,35/(U2*Att)=DUT+2,35DUT=(U1/U2)*(2,35/Att)-2,35Att=47/(1000+47)DUT=(U1/U2)*2,35/(47/1047)-2,35DUT=(U1/U2)*52,35-2,35What about this one?EDIT:Since the voltage is sampled at same node for both DUT and U1, I disregard the internal impedance of generator. Since both inputs are 1Meg Ohm, I disregard input currents as well.What are your results without calibration? If you had a wrong formula, and make a calibration, wouldn't the calibration fix your results to a certain degree(slopes, offset)? So it might work, till you run out of signal gain/precision?....Just a speculation on my part. I'll take a look at the S parameter definition...

author: Pitrsek
author_rank: Regular Contributor
timestamp: January 13, 2018, 07:32:39 am
content: work in progressEDIT:I'd like to mention that speaking about S parameters in systems that are not 50Ohms might make some people grumpy and may be not technically correct... but for the sake of augmenter let's live with it.From the agilent paper:Z=(50 x 2) x ((1 - S21) / S21)Where 50 is the system impedance, ie the impedance on which you are measuring the reflected power/sampling the voltage that is proportional to current. This is an assumption on my end.And S21=U2/U1 - in linear scale, NOT dB!!Z=(2*Zref) * ((1 - S21) / S21)Z=(2*Zref - S21*2*Zref) / S21)Z=((2*Zref)/S21 - 2*Zref)Since there is attenuator in U1 path, we need toboostfix S21 by 1047/47lets say our Zref is 2.35Z=((2*Zref)*(1047/47)/S21 - 2*Zref)Z=104.7/S21-4.7 - if we divide by twoZ/2=52.35/S21-2.35So our impedance is precisely half the previous result, what a coincidence...My guess is that it is result of the 1Meg inputs, ie. the original formula is compensating 50R/50R divider somewhere on the way. But it's a wild guess. I need to dig a little bit deeper...Any input?

author: Pitrsek
author_rank: Regular Contributor
timestamp: January 13, 2018, 08:36:28 am
content: Quote from: precaud on January 12, 2018, 04:32:55 pmWhy did you that? What was the reasoning?

author: Pitrsek
author_rank: Regular Contributor
timestamp: January 13, 2018, 09:06:30 am
content: Quote from: precaud on January 05, 2018, 01:05:28 amYou can't get Anritsu to spit out separate data for each channel(you have only S21 available)?Not even for the 50Ohm inputs?

author: precaud
author_rank: Frequent Contributor
timestamp: January 13, 2018, 03:57:28 pm
content: Your linear equation solving skills are fresh. Thanks for giving me a kick to brush up on mineQuote from: Pitrsek on January 13, 2018, 06:56:30 amOKQuoteYup.QuoteYou're rockin'QuoteIsn't Att = 1 / (((50 + 1000) / 47) + 1) , which = .0428... ?QuoteOr DUT = (U1/U2 * .0428) -2.35 is fine for the computerI'll try it today.QuoteAs I wrote in earlier post, without OSL, the results were accurate above an Ohm or so. This is consistent with the findings of others with this method using 50 Ohms Rref. It seems the Rref choice sets a "resolution baseline" for the measurement. Which makes sense, if you think about it.QuoteExactly. And the further the DUT number is below the Rref, the worse the precision becomes. For the classic Rref=50 Ohms, it's not good at all. For Rref=2.35 Ohms, it's much better. If you lower Rref even more, then you need more preamp makeup gain, which adds its own problems (noise, imprecision, bandwidth limiting...). So all things considered, this seems like an intelligent choice.

author: precaud
author_rank: Frequent Contributor
timestamp: January 13, 2018, 04:07:14 pm
content: Quote from: Pitrsek on January 13, 2018, 08:36:28 amI assumed that the 50x2 was really 50 + 50, combining the source and terminating impedances.As you noted, since the inputs are 1Meg, I chose to ignore the 50 Ohms source for now, and just use the 2.35 Rref.QuoteI know, that's why I had to convert it. No big deal - you work with what you are given...QuoteYes, but it takes separate sweeps to get each, and of course there's no phase data... only level.

author: Pitrsek
author_rank: Regular Contributor
timestamp: January 13, 2018, 05:20:27 pm
content: Quote from: precaud on January 13, 2018, 04:07:14 pmActually this might not be a problem, as long as you are measuring passive RLCs, or any other minimum phase system, you can use Hilbert transform to calculate phase from magnitude. Depending on your math libraries, there might be some gotchas:https://www.dsprelated.com/thread/2670/calculating-the-minimum-phase-of-a-given-magnitude-responseI can understand that with Hilbert transform and two cal sweeps we are way past "nice test fixture that will be really useful, let me just connect it with 3 cables to ma VNA... " and two checkpoints in "not what I've bargained for" and "oh boy, why did I bought this" territory.

author: precaud
author_rank: Frequent Contributor
timestamp: January 13, 2018, 06:14:07 pm
content: Quote from: Pitrsek on January 13, 2018, 05:20:27 pmYes, I understand. I wrote a Hilbert routine decades ago to do that. And I stopped using it. Too many assumptions being made. Especially, assuming minimum-phase behavior is not a good idea when you are still sorting out the test system itself!(Besides, why bother having a VNA if you're not going to look at phase?!?!)QuoteYeah, that too. I think we can get to the bottom of this without resorting to such measures. But I like how you think.

author: precaud
author_rank: Frequent Contributor
timestamp: January 13, 2018, 09:26:07 pm
content: Ok, here's a worked example using the three methods. The DUT is a 0.1 Ohm resistor at 1kHz. The analyzer returns V1/V2 as 26.49dB, which = 21.11For DUT=(U1/U2)*52,35-2,35 ( Pitrsek's version) : 21.11 * 52.35 - 2.35 = 1,102.76 (way off)But if we use 1/21.11 instead, we get 0.1299 which is in the ballpark. (We'll call it Modified Pitrsek).For DUT = (U1/U2 * .0428) -2.35 (my edit of ^) : 21.11 *.0428 - 2.35 = -1.4465 (impossible, of course)For the Series-Thru method I first derived: X=10^( (26.49 - 26.956) / 20.) = .94776. Then ( ( 1. -X) / X) * 2.35 = 0.1295 OhmsIt's nearly 30% error but that is without OSL. Very close to Modified Pitrsek.Let's try a 1 Ohm DUT. The analyzer returns V1/V2 as 23.63dB .X=10^( (23.63 - 26.956) / 20.) = .68187. Then ( ( 1. -X) / X) * 2.35 = 1.096 Ohms Which is better at about 10% error.Modified Pitrsek = 1/15.188 * 52.35 - 2.35 = 1.097 VERY close again.And now let's see what a 10 Ohm DUT looks like (10.03 actually). The analyzer returns V1/V2 as 14.58dB .X=10^( (14.58 - 26.956) / 20.) = .24055. Then ( ( 1. -X) / X) * 2.35 = 7.420 Ohms Which is total stink.Modified Pitrsek = 1/5.358 * 52.35 - 2.35 = 7.420 Same stink, and same as Series-thru again.And just to see where the trend goes, let's look at 100 Ohm DUT (99.98 actual). The analyzer returns V1/V2 as -4.03dB .X=10^( (-4.03 - 26.956) / 20.) = .02823. Then ( ( 1. -X) / X) * 2.35 = 80.89 Ohms.Modified Pitrsek = 1 / .6288 * 52.35 - 2.35 = 80.91 .It appears we haven't found the correct formula yet. Or it totally relies on OSL to pull it into line.I just got the Omicron short/load card in the mail. It won't change these results, because we're not using OSL...

author: r0d3z1
author_rank: Regular Contributor
timestamp: January 15, 2018, 11:38:42 am
content: Quote from: precaud on January 13, 2018, 09:26:07 pmBut we are moving near to the right result.I have checked Pitrsek calc, and they look right. However we are still missing something.

author: r0d3z1
author_rank: Regular Contributor
timestamp: January 15, 2018, 12:55:51 pm
content: hoping this will be useful, here is a brief explanation of ApInstruments about impedance measurementhttp://www.apinstruments.com/techImpedance.html

author: precaud
author_rank: Frequent Contributor
timestamp: January 15, 2018, 01:55:21 pm
content: Yes, that is the "Series-R, shunt DUT" method. Pitrsek's first equation gives right answers for that method. But for "Series-DUT, shunt-R" we have to change it to:52.35 / (V1/V2) - 2.35I agree, we're heading in the right direction; both approaches are giving identical answers. But we're still missing something in the analysis.The main problem I see with this Series-DUT method is in the computational model. For a perfect short, neither V1 or V2 approach zero; in particular, V2 is the reference R (2.35) divided by the output R of the generator (47.5 here), or about .0495 . That is the smallest number the divisor can be. That is the main reason why all the papers on Series-DUT method says it is not useful at low impedances. It runs out of numerical accuracy. And that is why OSL (especially for Short) is absolutely necessary. The results have to be re-mapped to the real-world. Without OSL, the lowest this method can go is about 125mOhm. So that becomes zero.Here's another good paper that compares Series-Thru to other methods.http://electronics.etfbl.net/journal/Vol18No1/xPaper_05.pdfIt appears to have very careful analysis of the parasitics. The math for Series-thru with parasitics is on bottom of p.3, with a general case on p.4. It is the same formula that the Keysight paper gave. Like all previous analysis, it assumes 50 Ohms Rsource and Rref. But maybe we can get some clues from it. (My son is better at this math than I am, I can ask him when he returns tomorrow).(One problem I see with their measurement is the ground connection between their type-N connectors, it has significant inductance not accounted for in their model...)

author: r0d3z1
author_rank: Regular Contributor
timestamp: January 15, 2018, 03:03:42 pm
content: I'Quote from: precaud on January 15, 2018, 01:55:21 pmI am also reading it, at the end of page 9 in section "ANALYSIS OF THE MEASUREMENT RESULTS AND MEASUREMENT ERRORS", they said that shunt-through measurement report a big error when you try to measure DUT with impedance quite high. So in quite normal that the measurement of 100ohm report some strange result but it doesn't means that the equation is wrong for low impedance DUT.There is another importanr concept that i have understand from this ANhttp://literature.cdn.keysight.com/litweb/pdf/5989-5935EN.pdfIf I am right, the source of measurement error is the ratio between Transmitted/Reflected wave. So, if the impedance is too low, the Reflected wave is small and difficult to measure.

author: precaud
author_rank: Frequent Contributor
timestamp: January 15, 2018, 03:21:22 pm
content: Quote from: r0d3z1 on January 15, 2018, 03:03:42 pmYes, but this is not "Shunt-thru" we are using, it is "Series-thru".QuoteYes, it is true. That is a good paper also, but it only looks at Shunt-thru method... it is not what this fixture uses.

author: rx8pilot
author_rank: Super Contributor
timestamp: January 15, 2018, 06:26:45 pm
content: Quote from: r0d3z1 on January 15, 2018, 03:03:42 pmThis is a good one. Thanks!

author: precaud
author_rank: Frequent Contributor
timestamp: January 17, 2018, 01:41:08 pm
content: This graphic was culled from the paper I referenced a few posts earlier. The authors lay out the 50 Ohm source and load series-thru measurement in quite some detail, along with cable and connector (fixture) parasitics. The complete formula is in the upper right corner. The circuit diagram shows which specific item in the physical layout relate to which parasitics. And in the lower-left corner, they show the familiar Series-thru formula (if parasitics are ignored) as also was given in the Agilent app notes.It seems to me our case lies somewhere between these extremes. Some of the parasitcs can be ignored - they will be picked up by the OSL compensation. (But maybe some need to be included in this form?). And some need to be modified to reflect the conditions of the B-WIC. For instance, the resistance Rp1 can represent the total resistance of the 1k/47 divider. And of course the 50 Ohm R at VNA port 2 will be replaced by the 2.35 Ohm Rref in the B-WIC.I don't have any results yet, I'm just sharing this as my next possible approach to find an improved formula.

author: precaud
author_rank: Frequent Contributor
timestamp: January 20, 2018, 09:45:29 pm
content: A couple days ago I shared my math with the boys at Omicron and kindly asked if they would review it and correct any errors.They respectfully declined. Not surprisingly, they don't want to give away the company jewels.So I decided first to satisfy myself that, if everything were measured correctly, then is the technique I'm pursuing capable of giving the right answers?I wrote a simple program to calculate the impedances and voltages at each node, and then run them through the math to see if the answers are correct.First attachment is the circuit diagram with the important nodes and components labeled. Voltage points are in orange, resistances in red.Constant values:Rs=50. ! Generator source resistanceRref=2.35 ! B-WIC Reference ResistorVs=1. ! Assume a 1 volt source at some low freq so the parasitics can be ignoredVdiv = 1000 / 47 ! Voltage divider in the Ref channel outputRdiv = 1047 ! Total shunt resistance of the dividerSome key variables:V1 = the voltage present at the source end of the DUTVt = " " " load " " "Rdut = the resistance value of the Device Under Test.For each DUT, calculate:V1 as:X = 1 / (1 / Rdiv + (1 / (Rdut+2.35)) ! actual shunt impedance at the V1 node, including the dividerthen V1 = X / (Rs + X) ! voltage presented to DUT, including generator output resistanceVref = V1 / Vdiv ! the value presented to VNA Ch1 or R channel. Later, multiply it by Vdiv to restore the correct valuethen Vt as:Vt = V1 * (Rref / (Rdut + Rref)) ! voltage at DUT, already corrected for Rs and Rdiv, goes through Rdut / Rref dividerand then S21 ( or Vt / V1):S21 = Vt / (Vref * Vdiv) ! Corrected output of the VNA, from which the Z is calculatedand then the impedance, using the equation we started with :Zv = 2.35 * ((1 - S21) / S21)The second attachment is a chart showing each element calculated for each DUT value. It's interesting to see which ones change very little. The voltage at the T channel changes very little. Most of the movement is on the source side of the DUT.The impedances at each node are shown for interest's sake.So, as you can see, yes, it can be done. If the voltages are calculated correctly, and all factors accounted for, then this equation [ Zmag = 2.35 * ((1 - S21) / S21) ] gives the correct results. To solve for DUT from the measurements, the effects of the source resistance and the R channel divider have to be precisely removed from V1 and Vt for the result to be correct. That is what we haven't done yet, and what we have to figure out.The boys at Omicron are indeed very clever.The question now is: Can Rs and Rdiv effects be accurately removed from the numbers that the VNA put out (Vt/V1)? Or do we need access to V1 and Vt separately?Time for a hot tub.

author: precaud
author_rank: Frequent Contributor
timestamp: January 21, 2018, 04:22:46 am
content: So we know the equation works for Real numbers, i.e. resistances.So why, when I measured a 1,000uF capacitor, was the impedance and phase way low below a few hundred Hz?Look at the diagram of the fixture in the previous post. Let's say the DUT is a 1,000uF capacitor with 20 milliOhms ESR. Some calculations are attached below.In mag/phase terms, 1000uF @ 20 mOhm at 100Hz equates to 1.5917 Ohms at -89.28 deg phase.In resistance/reactance terms (real/imag), it is .02 Real Ohms and -1.5915 Imag Ohms, or (.02,-1.5915). Almost all reactance.Now we're going to try to measure it in the fixture. The "reference resistor" is 2.35 Ohms (larger than the DUT). In real/imag, that is (2.35, 0) .And you know what happens when you put a resistance in series with a reactance. They add. (.02,-1.5915) + (2.35, 0) = (2.37,-1.5915). Which in mag/phase is 2.84 Ohms @ -33.9 deg phase. What happened?From the V1 point of view (which connects to the Reference input of the VNA), the DUT looks like a sh!tty 1,000uF capacitor with 2.37 Ohms of ESR. Ooops.This is stinky stuff. It leads me to think that, when using its fixtures, it is quite possible that the Bode 100 isn't actually measuring phase. Perhaps it converts the measurement immediately to magnitude and, as Pitrsek suggested, it calculates and plots a Hilbert transform of the magnitude for the phase. Either that, or else it is not measuring in "Series-thru" configuration. It has the ability to sample the source signal before the 50 Ohm resistor, so it could be some sort of hybrid technique.I need another hot tub. Wish I had one...

author: precaud
author_rank: Frequent Contributor
timestamp: January 22, 2018, 12:35:38 am
content: Perserverence furthers. By golly, it works! The ruminations of the last day or two helped identify three errors I was making.One of the main problems was errors caused by the mixing of real and complex math operations in my code. The equations themselves were fine; but a couple were being executed wrongly. Some operations, like correcting for the R channel L-pad, need to be done on the magnitude vector, in polar. But others, like (especially) scaling the data to the 2.35 Ohm reference R,mustbe done as a complex number in rectangular coords, which leaves the imaginary term intact, or else the phase gets demolished, and rectances measure wrong.Another issue is the fixture's sensitivity to the BNC cables' shield resistance. One of my big beefs with BNC cables is that they don't solder the ground braid to the connector shell. So with some of 'em you get variable ground impedance with each cable. Just move it a tad and it changes. Well this fixture needs short cables with as-low-as-possible shield impedance.And lastly, when using Open-Short-Load compensation, doing a Thru sweep on the analyzer is unnecessary and will cause major errors. It turns out, the channel imbalances which the Thru quantifies are taken care of by the OSL routines.Having corrected these things, I'm getting good results, all the way up to 10MHZ and down to about 20-ish milliOhms. Including large capacitors. See the attached plot. Three current sense R's; a 1 Ohm Caddock radial-lead, a 100mOhm axial, and a 25mOhm axial. And a 1,000uF lytic which measures 44mOhm ESR on the Wayne Kerr meter.As you can see, the traces start getting noisy at say 50mOhm and below. This isn't caused by a measurement dynamic range problem, per se; there was plenty of dynamic range to spare on both channels, and very little difference between sweeps with a 30Hz IF and 10Hz IF. it's a numeric precision issue with the Anritsu analyzer. It rounds and stores its magnitude data with 0.01dB precision. Normally, that would be plenty good. But with this measurement, at low impedances, .01dB in the raw measurement converts to linear voltage ratio and then impedance at 2-4 milliOhms per hundredth of a dB! The lower the impedance value, the coarser the conversion, hence what looks like "noise". That coarseness is also transferred into the OSL routines. So an analyzer which stores its data with more precision will give much smoother traces (and better data) with this method.Anyway, I'm a happy camper. Being able to measure the impedance of thru-hole parts up into the MHZ region was one of my major goals. Once I get the noise issue solved (I have a couple ideas), I can live with a 20mOhm lower limit over a 10Hz - 10MHz range.I also have Omicron's B-SMC fixture, which uses the same network inside. So it should be plug-and-play and give the same results with SMD parts.BTW, for anyone making their own fixture, this approach is worth considering. If anyone wants details on the corrected math routines, let me know.

author: Pitrsek
author_rank: Regular Contributor
timestamp: January 22, 2018, 12:06:59 pm
content: Nice job

author: precaud
author_rank: Frequent Contributor
timestamp: January 22, 2018, 02:36:31 pm
content: Quote from: Pitrsek on January 22, 2018, 12:06:59 pmThanks Pitrsek, I couldn't have done it without your help!

author: Pitrsek
author_rank: Regular Contributor
timestamp: January 25, 2018, 10:04:05 am
content: Glad to help

author: precaud
author_rank: Frequent Contributor
timestamp: January 25, 2018, 03:41:57 pm
content: There is no question that the B-WIC fixture is well-conceived and executed, and easy to use. It's limitations are more a consequence of the Series-Thru method. Above about 1/2 an Ohm or so, it is quite reliable and gives good results. But it still has problems at lower impedances, even with the clever modified topology Omicron came up with. These problems can be clearly seen in the graph a few posts up, and in Pitrsek's measurement of two low-ESR 'lytics posted on p.1 of this thread. (I am concentrating on the low-impedance performance because that's where I'll use them the most.)There are basically two problems:1. Raggedness in the measurement in the 1MHz-10MHz range, due to impedance discontinuities in the input stage of the VNA. My Anritsu has it. The Bode 100 has it too. An AP 102B is much better. And an HP 3577A is excellent and has no sign of it.2. Numeric precision limitations that come into play from about 50mOhm and below. Both magnitude and phase are significantly impacted by it.And so I wondered, how does the B-WIC compare at low impedances to my homemade Series-R fixture?The attached plot uses Series-R method with 10 Ohm series R, and the same OSL (Open-Short-Load) compensation math used for the B-WIC. For consistency, I used a 100 Ohm load reference just like the B-WIC. The parts measured are the same ones I've been using.As you can see, the Series-R method is much more accurate and usable at low impedances. With greater numeric precision data to work with, the OSL routines do a better job of removing the discontinuities above 1MHz, and are way better below 100mOhms.I also compared the results using these two fixtures, to the same parts measured in several dedicated 4-wire impedance meters (only up to 300kHz) from HP and Wayne Kerr. The Series-R results are more accurate.As a result, I am seriously considering removing the B-WIC's resistor network and converting it to Series-R topology.

author: precaud
author_rank: Frequent Contributor
timestamp: February 06, 2018, 03:14:23 pm
content: Before converting the B-WIC to Series-R, I decided to try a different VNA with it, one that does not have the .01dB resolution limit that causes the "noise" at low impedances (as shown in the post and plot in Reply #39 above). I have an AP Instruments 102B VNA that I've been wanting to interface with my program and this gave me the reason to do it. It also flung me into unknown territory; the only way to gain access to its measurement data is via Windows DDE, or Direct Data Exchange, the method Windows has provided (since inception) for applications to share data. It's fast and actually more straightforward than GPIB programming,IFyour language of choice has a good DDE interface driver. The one HTBasic provides (with all versions) works but has a bug. It has a maximum size limit of 256 bytes per transfer. Which is useless when you want to fetch say 200 points of Magnitude and Phase each with full 48-bit Real precision. It wasn't until yesterday that they sent me a workaround that actually works.Anyway, enough grumbling, on to the measurements. I first wanted to see what an uncompensated Short looked like, since that's the problem area for this Series-Thru method, and it basically sets the usable low-impedance limit. It's the blue trace in the first plot below, labelled "Direct". The curve is indeed smoother than it was with the Anritsu, confirming that its .01dB resolution limit was the cause of the rough traces. But now we see the infamous "braid error" rear its ugly head. (Braid error is a fancy term for a ground loop between the fixture and the analyzer.) Like most VNA's, the shell of the 102B's input and output connectors are all referenced to its chassis and circuit ground, hence the ground loop. (I have been spoiled by the Anritsu's isolated inputs, which I have come to really appreciate...)Having seen that the reference divider and reference resistor in the fixture connect right at the B-WIC's R channel output, I reasoned that would be the most important ground reference to maintain. So I inserted a wideband isolation transformer (North Hills 0017 CC) to isolate the source. See the yellow trace labelled "xfmr". Ground loop gone.So now we can see the low-impedance limit of the Series-Thru technique with a VNA that has good numeric precision, but grounded inputs. Without OSL compensation, zero ohms measures at about 23 milliOhms (before the expected inductive rise). (The Anritsu measures this at around 2 or 3 mOhms, nearly 10X better.) This high Short impedance impacts the accuracy below 100mOhms, as we'll see.I did the same series of measurements using OSL, using the same parts as in post #39. Traces are much smoother with the better numeric precision, but less accurate below 100mOhms due to the high Short impedance. The 25mOhm R measures closer to 20mOhm. The ESR of the capacitor measures closer to 33mOhms than its actual of 38. At around 100mOhms and up its fine.So changing VNA's solved one problem and created another. Tradeoffs.I've seen three ways to deal with the ground loop problem. 1. A large common mode choke on the T channel output. (not very practical in this case) 2. Keysight lifts shell of the Source and T channel connectors off ground by 30 Ohms (not an option here). 3. Source isolation transformer, as I've done here.I'm not sure what the next move is. Cogitate a bit. Any feedback is welcomed.

author: Pitrsek
author_rank: Regular Contributor
timestamp: February 18, 2018, 05:00:42 pm
content: Or you can go for differential pre-amps for inputs. And toss in some gain, while you're at it.

author: precaud
author_rank: Frequent Contributor
timestamp: February 19, 2018, 04:19:07 pm
content: Quote from: Pitrsek on February 18, 2018, 05:00:42 pmWell I agree, in the sense that isolated or diff inputs is the way to go, better than isolating the output...With isolated-ground inputs, you can forget about braid error, ground loops, etc. which impact all low-level measurements, not just impedance...