understanding VNA impedance plot? (inappropriate impedance test errors) - Page 1
Forum section: electronics / rf-microwave

author: coppercone2
author_rank: Super Contributor
timestamp: January 22, 2022, 10:23:12 pm
content: So I was experimenting with that VNA fixture project I was working on for the E5100A and I get a very wavy outputThat is for a 0603 7.5kOhm resistor.I did the through cal on a BNC-BNC connector, and then I hooked up the fixture.I also soldered a 1/8 watt through hole 1Kohm resistor into a piece of brass tube with BNC connectors at both ends (so its just a series resistor between the two center pins of the connectors).. and it looks similar (I thought it had something to do with the pogo pin springs at first, but even without the fixture with a resistor in a tube it does similar. I am using a transmission reflection test set from HP.I thought maybe the problem is I need to do a proper calibration on it with a cal kit, that instrument has never seen a cal kit, I just have depended on the 'thru' method of calibration.So it seems the resistance measurement is in the ball park but its very wavy. I don't think its the test set because I previously made this out of a minicircuits directional coupler and a 3 way power splitter and the results were similar looking.When I tried to measure a tantalum SMD capacitor in series like that, to see ESR, I also got some wavy stuff (started at like 2 ohms and ended at 6 ohms).

author: Bud
author_rank: Super Contributor
timestamp: January 22, 2022, 10:52:11 pm
content: Beside that you must perform user calibration, VNAs are 50 Ohm devices and are not suitable for accurate high impedance measurements, they have large uncertainty outside of being close to 50 Ohm DUT value.

author: coppercone2
author_rank: Super Contributor
timestamp: January 22, 2022, 11:00:56 pm
content: Hmm I know, but I was looking at that document that I wrote a thread about and it seemed that the series measurement method may be accurate on higher resistances.Is the waveyness just a manifestation of the error? I tried several diff BNC-BNC connectors from diff manufacturers for the calibration and they all seem to agree with each other.I guess I did not figure the error would be sinusoidal, instead that it would 'trail off' in one direction or the other. The graph in that VNA series/shunt/shuntthru methods of measurement kinda gave me the feeling that the magnitude of error would just increase in one direction, not be sinusoidal.Now when I think about it, it does make sense, since the graph was error vs impedance not error vs freqeuncy, each impedance would need a separate graph to show error... I thought for some reason that the resistance/error graph would apply as error/frequency.https://base.imgix.net/files/base/ebm/mwrf/image/2019/06/mwrf_com_sites_mwrf.com_files_21J_Fig3.png?auto=format&fit=max&w=1440If anyone wants to post some inappropriate VNA impedance measurements that would be welcome... btw if you look at that on the dB scale, its around +-4dB

author: Bud
author_rank: Super Contributor
timestamp: January 22, 2022, 11:14:03 pm
content: Waviness would very much be because of impedance mismatch (causing reflections at both DUT ports) caused by the high impedance DUTs you are trying to measure. I'd say the practical limit for a VNA would be 1:10 SWR, i.e. 5...500 Ohm DUT impedance range.Edit: i can't see the picture in your original post

author: coppercone2
author_rank: Super Contributor
timestamp: January 22, 2022, 11:16:51 pm
content: https://+i.imgur.com/ZY4HPOq.jpegCan you see that if you copy paste it and get rid of the + sign?I will try to put like a 200 ohm resistor in the fixture and see what happens.

author: coppercone2
author_rank: Super Contributor
timestamp: January 22, 2022, 11:30:25 pm
content: well, it kind of looks the same on a 221 ohm 0603 resistorhttps://+i.imgur.com/94NJg7z.jpg

author: jwet
author_rank: Frequent Contributor
timestamp: January 22, 2022, 11:31:52 pm
content: Post a picture of your test setup and the reflection test set. You're doing a through (S12) type measurement. A reflection test set is generally used for S11, reflection measurements. Depending on how you have it hooked up to the ports of the VNA (this look similar to an HP4195), you can do through but I think you want a reflection measurement to get an idea of components. An RF network analyzer will have a hard time with SWR's over 10 or so, very little is reflected. Use an R or a reactance of 500 ohms or less.That looks like a long coax or something- those ramps on the phase trace are it wrapping around at 360 degrees several time. At 200 Mhz, a wavelength is 1.5 meters so this is like ten meters or something. The wavy line is similar- its sort of a standing wave display.You don't need lab grade cal standards. Use a terminator for a load, nothing for an open and short out a bnc for a short. At 200 Mhz, it won't matter much.Good luck,

author: coppercone2
author_rank: Super Contributor
timestamp: January 22, 2022, 11:35:25 pm
content: yeah there is a shit load of coaxial line because that analyzer has a test set on top of it and there are long leads that go on a work bench with the fixture. 25 feet round trip seems right. To get shorter cables I would need to build a little shelf that attaches to the rack that VNA is in.Can anything be adjusted to compensate for this? There is a electrical length or something parameter in the analyzer menu.I am using a HP 8502A reflection/transmission test set.The fixture is two pogo pins hidden inside of a coaxial cable that are re-shielded with a brass tube and both ends of brass tube are pressed in a copper block.. when I put a part in it, the pogo pins compress so there is only around 1mm of exposure (the heads of the pins) with the resistor between them. But I see the same wavyness with the fixture and the resistor-inside-tube series-only terminator.

author: jwet
author_rank: Frequent Contributor
timestamp: January 22, 2022, 11:43:07 pm
content: That's your problem. The electrical length can't cover that kind of delay- its designed to take out a few inches on a fixture or a board. Use some kind of cal standards at the end of the coax- that will be your"measurement plane". The VNA will take whatever it sees and correct the errors. Better to use as short as possible, but it will work. Like I said, you don't need a set of Agilent Standards in a jewelry box at 200 Mhz- just gin something up.

author: Bud
author_rank: Super Contributor
timestamp: January 22, 2022, 11:43:29 pm
content: I can see the screenshots in the browser as you suggested. No they are not the same, if you look at scale per division, you will notice the swing of both magnitude and phase is much less with 200 Ohm.You should perform a calibration with the coax in place in order to calibrate the coax out. You will connect calibration stsndards to the coax during calibration process, so the coax DYT ends will become your "calibration plane" after the calibration is complete.You have to recalibrate at least each time you change the coax , make this a habbit.

author: coppercone2
author_rank: Super Contributor
timestamp: January 22, 2022, 11:53:45 pm
content: I see, I think I will get some cleco rivet clips so I can temporarily and quickly "bolt" a shelf to the test equipment rack and try this experiment with out long bendy cables going every which way. Starting to remind me of warhammer emperors throne as it is, or maybe shodan.I can drill a few holes and have a adjustable and very strong shelf that way.It kind of looks like the shape of the phase response shows you how bad the impedance mismatch is also, it looks 'neater' on the lower resistance.If someone really needed to measure the impedance far away though, what would be a (impractical) solution? Not that I am too interested because I defiantly was looking for an excuse to order some clecos Can you put a extra long cable into the reference input (from the power splitter) that matches the length of the cable going to the DUT (i.e. 20 foot spool of coax)?

author: Bud
author_rank: Super Contributor
timestamp: January 23, 2022, 12:02:05 am
content: You can measure impedance many meters away from the VNA, the key is to first perform calibration at the end of the coax to remove its effect ( you may need to be inventive how to control the VNA when you are at the far end of the coax, lol)People routingly measure antenna impedances at the feedpoint , tens of meters away from the VNA.You pretty much cant use a VNA without doing calibrations.

author: coppercone2
author_rank: Super Contributor
timestamp: January 23, 2022, 12:11:07 am
content: I did do the 'through' calibration from the menu at the end of the coax though. Maybe jwet is saying that its just too long regardless of calibration for this instrument?

author: G0HZU
author_rank: Super Contributor
timestamp: January 23, 2022, 12:28:07 am
content: It should be possible to make fairly good measurements using a basic s11 measurement with a VNA as long as you avoid using long cables (measure direct at the VNA port 1 is best).To get the best results for a s11 measurement of something like a 10k resistor up into to VHF I'd recommend making up a decent cal kit and test fixture. The cal kit would ideally need to be corrected for delays in the short and open down to about 1ps to get the best from the VNA. If you don't do this you will be throwing away the potential of the VNA even down at 50MHz.

author: coppercone2
author_rank: Super Contributor
timestamp: January 23, 2022, 12:40:47 am
content: Ok I have the Output, R, A, B, C.I have output connecting to the ref/trans kit Input. I have the power splitter inside of the kit going to R (reference). Then I have the reflection output of the kit going to A. Then the output of the thing is going to the test fixture and the other end of the test fixture is connected to B.So to get transmission I do B/R and reflection would be A/RWhen I do the thru-cal, I just connect a double ended BNC connector to the ends of the cable and press thru, that is what the picture are with.But the instrument also has a option for a 3 point calibration, using Open/Short/Load.I will make some BNC open/short/loads up.The manual does not show a diagram on how to connect Open/Short/Load (this manual really sucks),...This is the test set:https://i.ebayimg.com/images/g/~uwAAOSwtGlZJKAf/s-l1600.jpgWould I put the open/short/load on the end of the cable plugged into the port that says Output in the picture? Does the instrument care whats going on with the "b" input on the analyzer, since its floating? Thru-cal has a connector between the test set output and the B port on the analyzer. But I am not sure what is going on with the B input when I try the 3-cal method.this is the test set block diagramhttps://www.surplussales.com/Images/Equipment/TestEquipment/Misc/eqp-hp8502a/eqp-hp8502a_4.jpgBut that does not account for the long cable from the test fixture to port B (that analyzer in the diagram does not have a port B). I guess it just assumes that the connection there is really good?I will get back to this thread when I make a rackmount shelf addon and the short/open/load..

author: Bud
author_rank: Super Contributor
timestamp: January 23, 2022, 06:16:13 am
content: Either you do not operate the VNA properly or it may be defective. It apears that the Thru calibration that you perform does not apply to the subsequent measurements you are doing after the cal. I based this on the puctures of the resistors scans that you posted.As a sanity check, do a Thru cal and then right after do a scan with the Thru still connected. You should get a flat zero degree S21 phase line and zero dB magnitude line. In case you get waves, the cal was not properly saved or applied.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: January 23, 2022, 07:38:22 am
content: There was some mention of this already, and I did have a bit of trouble following your setup but I guess you are putting a 221 or 600 ohm resistor from the port 1 (test) bridge to the B port but you display Z:T. Try showing S21. Likely you will see a a lot of loss with a little ripple. The ripple is because your source and load impedances are not 50 ohms and the 50 ohm cables you use to connect to the DUT transforms the not-50-ohm source and load to different not-50-ohm values over the frequency sweep. Suppose your source is 40 ohms and your load is 60 ohm and you put a 220 ohm between them. Since you don't do a full 2-port cal (with a switching test set) your through is in error because you are normalizing the loss of a 40 ohm source to a 60 ohm load (remember 10 ohm offset from 50 ohms is about a 1.2 VSWR or 20 dB return loss). but over frequency the 40 ohm source is transformed to 60 ohms and the 60 ohm load is transform to 40 ohm through the cable length between the actual source or load and the DUT. Now when you insert your DUT the phasing changes again (it is 200 ohms in-line not zero ohms) so the mismatch effect is different and it gives you ripple at a rate according to your test lead lengths. If S21 has ripple then the transform to Z will have ripple. And as noted, for most case we say VNA is good for impedances from 10 ohms to 250 ohms (5:1 ratio). With a very good cal you might get 5 ohms to 500 ohms. But you cannot get a good cal, you can only do thru normalization, so your task if forlorn.

author: gf
author_rank: Super Contributor
timestamp: January 23, 2022, 09:16:27 am
content: Assuming that the thru standard is reciprocal and symmetric, and given that a series resistor DUT is likely reciprocal and symmetric as well (at least approximately, at the considered frequencies up to 300MHz only), shoudn't it be possible to compensate both, source and load mismatch for the S21 calculation (after unidirectional 1.5 port SOLT calibration, and doing a "fake flip" when measuring the DUT)? A different question is of course whether the VNA supports this calculation out of the box.

author: coppercone2
author_rank: Super Contributor
timestamp: January 23, 2022, 07:22:13 pm
content: with the thru still connected after you do the cal after you autoscale it is a little bit of ripple, some number of miliohms+-. I have alot of adapters in the system so I thought it might just have to do with contacts drifting around. In the dB mode, it shows +-4dB ripple, but I don't have a picture right now.the phase line is ugly though.It seems pretty stable but the higher frequency range (200-300 MHz) starts dancing around a little when you move the fixture at the end of the wire around.I will try some more tests today after I make the correct terminators. glad to finally get to testing this machine, its been forever.

author: cdev
author_rank: Super Contributor
timestamp: January 23, 2022, 07:41:19 pm
content: Anything thats conductive you connect to RF is going to have a phase trace that relates to its electrical size (when acting as an antenna)The only conductive thing that is not going to show something like that at some frequency is a literally perfect 50 ohm dummy load. Try it, its fun!Phase plot will illustrate how hard it is to get something like that right and why.Its also super useful when making resonant coaxial filters to null out or peak certain frequencies, which can be used in antennas as traps.

author: coppercone2
author_rank: Super Contributor
timestamp: January 23, 2022, 08:48:49 pm
content: Ok during the course of my investigations an N connector fell off and 2 bnc cables I made failed the 'tug test' while looking at the analyzer screen (they were wiggling the value, replacement cable was not wiggling the value).I bought the N connector cable and made the BNC cables but it looks like there might be serious cable problems afoot. This is gonna take a while to test.I think I will make a strain relief that you can attach to a BNC connector some how.I tested these cables before, but I was using them for other experiments over the past year.. perhaps they got damaged. I was also using a caliper to make some of them, but I wanted to get everything done in a hurry when i was assembling them and maybe there was strip length measurement problems.Looks like I need to order some N connectors, thankfully I decided to buy the 100pack of amphenol connectors.. almost thought it was a stupid move until this happened.I feel like I have the worst luck with coaxial cable integrity. This was a problem for my audio work last year.Time to totally clean up the work benches and slowly/meticulously assemble new cables

author: jwet
author_rank: Frequent Contributor
timestamp: January 23, 2022, 08:55:13 pm
content: You might have misunderstood. You can cal out 25 feet of cable with a VNA- its not recommended but it will do it. You can probably even add port extensions to your cal to take out 25 feet but its far from optimal. Light travels at a nS per foor or about 2/3 of this in coax. When a full cycle is 5 nS (200 Mhz), a foot of coax looks like a lot of phase. VNA are trying to make very careful complex number calc and a complex number modified by a few degrees of phase shift is a different number.If you calibrate the VNA with the standards at the end of a long cable, this is fine, the VNA will look at what it sees and if the lengths are somewhat matched, it will do fine. It isn't optimal but it will work. You're basically throwing away dynamic range by having loss and delay but a good VNA can handle it.The port extension is a different topic- its purpose is to add or subtract a bit of extra line length- in VNA speak, move the measurement plane after doing a calibration. Example, you want to measure the input impedance (~S11) of a filter on a board. You have a BNC cable on your test set. You put open, load and shorts on it and do a use cal. You then mount a BNC connector on your board right at the filter. This BNC is really about 3 cm away from the filter's input. At 200 MHz, lambda is 150 cm, so this 3 cm will cause an extra delay that will cause the whole smith chart to be rotated 360*3/150 degrees- about 7 degrees- you could make this correction your self. Get the values in polar form and then rotate them 7 degrees or the VNA will do this for you. Put 3 cm in the port extension and the VNA will crank this in. This port extension stuff kind of degrades the calibration. I looked at my VNA and it will take large values of Port Extension- out to kilometers even! This isn't a great way to do it. What I do if I want to use a port extension, I do a good user cal at the instrument input (the DUT port on you test port). I would then add the length of cable that I wanted to use and put a short on it- on a board, this could be a solder blob right at the filter as in the example above. I would then look at the smith chart display and add port extension until the do was a the far left of the display (0 + j0). I've kind of a done a one poiint cal correction for the extra wire. I would then make my measurement and the value would be right at the fiter.All this "measurement plane stuff gets more important as frequency goes up. At 3Ghz for example, a wavelenght is only .1 m or 10 cm. If you make a measurement an inch (2.5 cm) away for you calibrated measurement plane, you'd be off 90 degrees on a smith chart. This could make an inductive load look capacitive etc. At 200 Mhz, its all much more forgiving since lambda is 1.5m (150 cm).Those $3000 APC-7 Cal Standards (85032x) in the jewelry boxes are accurate to a fraction of a millimeter which when you to get even higher in frequency really matter. You can make a set of quick and dirty standards- open, short and load with some connectors without a lot of care that are probably +-3mm. This is less than a degree of rotation of the smith chart at 200 Mhz (1.5 meters). If you're careful and think about it some, you might be able to get +-1 mm, this is the point of diminishing returns for a 200 Mhz instruments. Most VNA's will also let you put in actual values for your standards and might even have a library of different cal kits with different values.Those $50 Mini VNA's, have a "cal kit" with some sma loads that are probably not bad up to a few hundred Mhz. Its amazing how well they work. If you have access to a good RF lab or an RF engineer friend, you can do a careful and complete cal on a good analyzer (8753x or better) using an 85032 kit and then measure your standards carefully. This is a good exercise. Write these values down and enter them into your VNA if you can. I made some standards with SMA's on little PCB's and was very careful and they didn't have anything really odd up to about 500 MHz. They'll start to make little loops and bends on a smith charts that are strange little resonance modes that they can get into.Welcome to the strange world of RF where wires aren't wires, caps aren't caps always and L's aren't L's anymore. HP has some great App notes about using a VNA, its an eye opener to play with this stuff. I finally figured out why a lot of the VHF stuff I put together as a teenager never worked. You put an standard L and C together, keep the leads short and think you know its resonance, etc- the L has become a cap and C has become an L and the resonant is super far off and the transistor is borking the whole thing with some parasitic reactance of feedback. Life gets pretty weird at 10 Mhz with conventional construction (Manhattan style) and 100 Mhz with SMT construction.Take care and have fun.

author: coppercone2
author_rank: Super Contributor
timestamp: January 23, 2022, 08:57:10 pm
content: yeah, the 'phase noise' problem I was seeing was related to one of the cables. When I pull on the BNC cable lightly while its inserted into the connector, the phase response profile totally changed and dropped down by ALOT. My "good" cables do not show any response to a light yank, but I guess the incorrectly assembled ones are strain-sensitive.But the bigger problem is that I don't have any N connectors to repair the connection on the fixture, need to order it.I think I will make a little perch for the module that fixtures it correctly and make some hardline cables to connect the VNA to the trans/ref box, the braid-shield connector is not worth a damn with the 'screw on' cables IMO. I want a soldered pipe. This was my plan from the beginning but I thought I could at least get preliminary work using my cables that I made... guess that is gonna be more trouble then its worth. I am going to make a screw-on shelf with some 'guide rails' to center the transmission/reflection kit and provide a 'back stop' for it, then make a piece that slides in front of the box after its connected so if you yank on the front cables it does not transfer strain to the VNA/testbox assembly. Like a tank bunker or something. Maybe I will put some carpenters clamps on the shelf so once the VNA box is connected you can press it down from the top so it has a hard connection to the perch/shelf. Or maybe I will make so the test fixture is bolted to the top of the analyzer.

author: TimFox
author_rank: Super Contributor
timestamp: January 23, 2022, 09:04:20 pm
content: Years ago, before retirement, we needed to measure relatively high impedance values (roughly 10 k, >> 50) with an -hp- 8753 VNA and reflection bridge at roughly 10 MHz. We successfully used inexpensive transformers from MCL with nominal secondary impedance up to 13 k (iirc), fabricating jigs with N connectors on the 50 ohm primary and simple connections with 0.1 or 0.2 inch spacing connectors on the high Z side. We calibrated the S11 measurements with matching open, short, and terminated male header connectors, where the termination was an appropriate 1206 resistor. Since the 8753 firmware was not designed for 10 k impedance, we lied to it and set the nominal Z0 to a smaller value (i.e. 100 for an actual 10 k calibration).

author: coppercone2
author_rank: Super Contributor
timestamp: January 23, 2022, 09:21:16 pm
content: I am going to raise the VNA on four legs like a big IC and then mount the trans/ref box under it with hardline jumpers that snake around, that is the only way that makes sense I think. Everything else is going to end up breaking. Having the box in front is obnoxious. Having the cables loop around the keypad is also obnoxious. I can make a pipe clamp style thing to anchor the 141. Then it might be considered usable test equipment. I might as well make a chassis extension.I just bought a rosebud torch tip so it will be fun to bend some 'skis' that go under the instrument to give the box clearance.

author: G0HZU
author_rank: Super Contributor
timestamp: January 23, 2022, 09:25:39 pm
content: Here's an old VNA s11 measurement of a 32.4k 0603 SMD resistor up to 100MHz. This was taken using a homebrew mechanical cal kit and a carefully corrected test fixture. I corrected the delays in the short and open in the cal kit down to sub 1ps.You can see the plot is a bit noisy but the VNA did quite well here I think. I'm not sure what level of performance you are hoping to get but this is close to the limit of what a VNA can achieve. To get performance like this I didn't use the default settings of the VNA as I'm measuring a reflection coefficient very close to 1.The plot also shows the parallel capacitance of the little SMD resistor out to 100MHz. My test fixture adds about 0.05pF fringing capacitance so the real Cp of the resistor is closer to 0.05pF rather than 0.1pF.

author: jwet
author_rank: Frequent Contributor
timestamp: January 23, 2022, 09:27:25 pm
content: Yes Tim- I've had similar experiences working with IC's that have differential inputs for their RF ports. (even the NE-602 mixer has Diff I/O). You end up having to have baluns and transformers in the system that have all of their own transfer functions. After you got a a measurement, you'd have to "de-embed" all this fixture stuff to tell what is really happening. I had basic programs written in HP "Rocky Mountain" Basic that could do all this stuff an old HP-87 lab computer and GPIB controller (mid 80's). I later used excel spreadsheets but I kind of miss those old HP computers. Its gets to be kind of a mess and its easy to forget where you are and what it is you're trying to do. I've never done much work about about 200 MHz, I've helped customer as a Maxim FAE at ridiculous frequencies but I was basically passing info between them and our factory experts. At 1 Ghz, when you start getting into non lumped components, strip lines, C's and L's on Rogers boards with fancy EM simulation, its gets impossible. You really need a lot of knowledge, nice equipment and expensive software. These guys who do this every day are some smart and well funded guys.

author: G0HZU
author_rank: Super Contributor
timestamp: January 23, 2022, 09:34:29 pm
content: A modern n-port VNA can measure differential inputs quite well without needing external baluns so the results can be quite good in my experience. Some VNAs have a 'balun' mode to make all this stuff a lot easier nowadays.

author: G0HZU
author_rank: Super Contributor
timestamp: January 23, 2022, 09:54:08 pm
content: Here's a quick measurement of a 9.94k SMD resistor using an original Hugen 2.8" nanovnaH purchased in 2019 for just £35 delivered.The plot only covers up to 50MHz because this old/early nanovna quickly loses performance above about 50MHz.The result looks quite good to me considering this is a basic 1 port s11 measurement...Note this is my first go with the latest DisLord firmware so I haven't had time to evaluate this firmware very much.

author: coppercone2
author_rank: Super Contributor
timestamp: January 23, 2022, 10:23:22 pm
content: Ok, I did do some more testing anyway.. it looks like when you try to calibrate it WHILE its set to the impedance mode (not the dB mode), when you try to calibrate it, after its done calibration, there is huge phase noise on the blue trace. Total jumble. But when I calibrate it on the dB mode (response), both traces are smooth. It was smooth to where it rescaled the axis was in mili-degrees. But whenever I try to calirbate the trans/ref mode, I get 50 degree scale (not milidegrees). Just need to solder this short standard together after I make tea to see what happens on the 3 standard mode.Should I put a 50 ohm terminator on the unused port during the cal?I also think I will go through the service manual for a while to see what I can test, I have a bad feelin about this instrument, its pissing me off pretty bad

author: coppercone2
author_rank: Super Contributor
timestamp: January 23, 2022, 11:28:16 pm
content: Ok, so I did a SLO-T calibration. First I pressed the buttons to do SLO and then after that was done I hit the menu button for T and I did it. I had trans-BR selected on the menu. This is for a 1kOhm resistor inside a brass tube between two coaxial connector (75 ohm kind). It was meant for audio work. If you want to make one, you solder a resistor to 1 coaxial connector (I used 1/6 Watt), then you put a brass tube over it with another brass tube over it with a notch so you can solder the other coaxial connector once its inserted with a thin iron. Then you solder the center pin, you solder the connector body, and then you slide the brass tube that is nested on the other tube over to cover up your solder hole. If you use 75 ohm PCB upright connectors, they wedge pretty good inside of a brass tube with their legs straddling it so you can solder it on 4 corners.Anyway:https+://i.imgur.com/TqZSdhD.jpgThis is looking alot better right? I can't test my fixture anymore until I replace the connector on it unfortunately.

author: jwet
author_rank: Frequent Contributor
timestamp: January 23, 2022, 11:28:41 pm
content: Copper cone, don't get discouraged. Your VNA is completely working- its a steep learning curve to learn how to use one. I can tell from your pics that its working perfectly.When you're doing the reflection cal, you don't have to terminate the second B input- its not active.

author: coppercone2
author_rank: Super Contributor
timestamp: January 23, 2022, 11:31:45 pm
content: also, the fixture is 500KHz minimum, while the instrument is 10KHz, so the early part of the sweep is bogus. I did not set the bounds in the analyzer right.I think the analyzer wants you to do both SLO and T calibration, but it looked from the menu like it wants either SLO or T. I think its combining them. When I watch youtube for VNA/IA it looks like the later models have a better 'explanation' in the menu about how to set them up. I think maybe there was some 'bullshit' SLO data in the memory that was being used, because I thought it would totally disregard that so long the thru calibration was done (the only one mentioned in the menu).. but it looks like SLO is necessary too.BTW I see a menu that you can modify the calibration standard.. like you can select R/L/C and stuff. What is that for? Like hard core standards that come with a sheet that tells you what numbers to type in (the non-idealities are listed)?Also what comes to mind is if you get a forming tap I wonder if you can thread the right size tube for accepting a screw in coaxial connector. A forming tap is a type of tap that deforms the material rather then cutting it, so its suitable for thin stuff. There is always a problem with sealing the standard chassis mount easy available coaxial connectors with solder.. if you had a laser welder you can stitch over the junction but who has that?Also, does anyone have a more advanced manual for the E5100A? Its totally gutted. They tell you like two test examples. I suppose maybe the repair/service manual has some tips but regardless, the base E5100A manual is really wimpy considering what its representing.Is there maybe a sister unit that has the same control scheme but slightly different RF parameters that has a better manual?

author: jwet
author_rank: Frequent Contributor
timestamp: January 24, 2022, 02:49:15 pm
content: That trace looks pretty good and what I would expect mostly. Looking into the input- It would look like 1050 ohms which is pretty close to what you have. The bit of ripple is normal and might be the cleanliness of the brass tube or little dents, etc. These are very sensitive measurementsThe phase delta is about 160 degrees if i'm reading the screen right- this is about 2 feet of tubing? If this is off then you added or some coax since you did you thru cal. It all shows up.You're getting there. VNA's are really learning machines. Once you get things settle down, you can learn so much by interpreting what you're seeing.Your other question about entering standards- read my long response yesterday. If you have know standards, you can put their parameters there and they will give you a better cal. If you don't, it will assume your standards are perfect 50 ohm, 0 ohms and infintity with no reactance. If you can get your hands on a good VNA and some good standards, you can characterize yours and enter them.HP/Agilent/Keysight has generously put all their manuals, new and old on line. Check there first.Look at reflection measurments, smith charts and S11, very powerful.It look like you are on your way.

author: coppercone2
author_rank: Super Contributor
timestamp: January 26, 2022, 02:28:01 am
content: Hmm.. your two foot estimation is a concern, the lenght of the brass tube with the resistor is approx 2 inches total, including the 75 ohm connectors.But, it almost might be because the standard I whipped up is pretty bootleg. I used a single 50 ohm resistor (think its 5% carbon) soldered inside of a BNC cable crimp/screw type connector (solder to center pin and solder to the threaded bit), for the open I used just a female to female BNC and for the short I used a chassis mount BNC with braid soldered to the center pin and the little ring lug that you would solder a wire to on a chassis mount connection... so its HIGHLY bootleg.I need to make the high quality standards.. I just wanted to make sure the instrument was not broken. i think maybe all these errors might just make it give a false impression of length.When I make my mods to hookup the test box I can get rid of a few feet of coaxial cable and a whole bunch of adapters (at least 10).

author: jwet
author_rank: Frequent Contributor
timestamp: January 26, 2022, 04:19:51 pm
content: Keep up the momentum. I only say 2 feet because of all the phase shift. It could be that the DUT is causing that. This is the problem with through measurements- they're more complicated in some ways and tell you less often. I would start with reflection measurements- there is only one reference plane and with a good cal, all you see is the component.I dug out some of my cal standards and took some photos- annotated below. Its all kind of a mish mash of stuff I made and acquired over the years. A lot of folks don't know what these are when the buy a big lot of surplus and will let them go pretty cheap. I probably have a few hundred bucks invested here but its worth a few grand I'm sure. I took them to my company's lab a long time ago and characterized them all with a good RF engineer looking over my shoulder.NB- I have a HP4195 (500 MHz) and a HP4396B (1.8GHz) analyzer. I've had both for a long time. The 4195 is kind of redundant but its not worth enough to sell so I'll hang on to it for now. My heirs can sort it out I guess...Annotation of photos- this is about 1/4 of my terminators but are most relevant.I use my shop made standards for most work and use the nice standards occasionally for finnicky work or to verify stuff.1- Shop made BNC's. I started with a BNC T or Angle and cut it have with a hack saw or a dremel wheel. I then filed them all flat or belt sanded them. The open I added nothing. The short I added some solder and term I added 2 100 ohm R's. (it common to use more than 1 to make things symmetrical- you could use 4 200 ohm eg.). These are good to about 500 MHz. The idea is to get a consistent "plane". BNC are not considered a precision connector- they're all over the place at higher frequencies and really can't be used.2 and 3- These are little Male and Female SMA Standards. HP used to make little triangles like this that I used in school. The patterns on the board are all the same and I added SOL stuff. You can see the traces on the open. I deliberately made these with a little length because I often use a connector to go onto a board and I wanted it be close. These don't get squirelly until about 1 GHz. This is an easy evening project- work up the cad, send the artwork off to china, buy some connectors and you've pretty good to go. I did some these in BNC too but I can't find them.4- This is a set of Maury Microwave SMA (actually they call them precision 3.5 mm). These are good to 6 GHz. I bought a coffee can full of RF connector adapter stuff at a Ham Fest for $50 and these and a bunch more were in there in a baggy. These 3 new are worth a couple of grand.5. This a set of APC-7 Standards that came with my impedance test set I believe. APC-7 is a hermaphrodite (no males/females- they all mate) connector with a well defined internal plane. I think they're good to 3 GHz IIRC. This connector is used on a lot of precision cables and test sets- its expensive but very repeatable with proper maintanance and storage. Keeping them in a coffee can is not recommended.6. This is an Narda N Set with a double ended SO standard and good termination. N connectors are much more repeatable than BNC due to the threads but not as good as APC-7. Medium frequency test sets often use N. I got these on ebay IIRC.Keep your eyes open on Ebay or setup a search. Generally these things are pretty robust if they've been stored well. If they look good (no scatches or dings), they'll generally work well. Its unlikely that you'll get cal sheets. You can call the company and they'll give you a general idea of where they run or find a friend with a good VNA and cal kit to characterize.Note- I couldn't upload all pics so I made a separate post with the last 2

author: jwet
author_rank: Frequent Contributor
timestamp: January 26, 2022, 04:22:04 pm
content: Two final pics attached - hit the 4M limit.5- APC-7 Set6- Narda N setBest Regards

author: coppercone2
author_rank: Super Contributor
timestamp: January 27, 2022, 03:43:37 pm
content: I do recall some thread somewhere years ago where a person compared 4 resistors vs 2 resistors in a home made terminator/load.. and the 2 100 ohm resistor load had better properties then a load made with 4 resistors to get 50 ohms. It probobly has to do with the resistor type, but it seemed like a potential error area. Apparently they don't scale as well.

author: Bud
author_rank: Super Contributor
timestamp: January 27, 2022, 04:19:22 pm
content: Not resistor type, resistors' parallel capacitance.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: January 27, 2022, 05:08:18 pm
content: it all comes down to parasitic inductance and capacitance, and cancelling each. I find two 100 ohms to ground at 45 degrees worked well, but still a little excess capacitance. You can compensate a bit by relieving the ground just under the resistor. but very slight tweaks in position will have an effect at frequencies above 3 GHz. Should be able to get 40 db return loss to 1 GHz if the connector can support it.

author: gf
author_rank: Super Contributor
timestamp: January 27, 2022, 05:37:06 pm
content: Quote from: Joel_Dunsmore on January 27, 2022, 05:08:18 pmHow important is it achtually that the short/open/load are close to ideal, versus having just well-known (measured) S11?

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: January 27, 2022, 06:06:23 pm
content: Quote from: gf on January 27, 2022, 05:37:06 pmDepends on the VNA capability. For Keysight VNAs it doesn't matter at all as long as the impedances don't overlap at the same frequency (making the solution degenerate). We can use "database" standards that can be anything. Older VNAs (like HP8753) used polynomial+delay models for open and short and you can map to some significant errors with just that. The load model is not so flexible but you can adjust the delay element to account for first order capacitance or inductance (see chapter 11 in 2nd edition of my book, or chapter 9 in first). The delay element is set either very high (to model an inductive load) or very low (to model a capacitive load). Should do a reasonable job up to 3ish GHz.

author: gf
author_rank: Super Contributor
timestamp: January 27, 2022, 07:29:21 pm
content: Quote from: Joel_Dunsmore on January 27, 2022, 06:06:23 pmI understand that three known standards are basically sufficient (as long at they don't lead to a degenerate solution), and I also understand that the known S11(f) of the standards can either come from a polynomial model, or from an interpolated table.I rather wanted to ask - given that the VNA is noisy and the dynamic range is limited - does it make a differencefor the uncertaintyof the derived error terms, whether the known load standard has (say) 55 Ohm, or whether it is closer to ideal, like say 49.9 Ohm?

author: coppercone2
author_rank: Super Contributor
timestamp: January 27, 2022, 07:41:18 pm
content: I also suggest if you make you own standards (which I am researching now), you try to find things made with teflon since when you solder to the body, its way harder to mess up the connector. When I tried to do mods on BNC connectors, I often get melting, and if it has a gasket that will get deformed and make the connector lock poorly.

author: G0HZU
author_rank: Super Contributor
timestamp: January 27, 2022, 08:28:33 pm
content: Here's an old plot of my homebrew SMA 50R load. This load is really old now. I made it for my first VNA and it has seen a lot of use over the years. I used the best SMA end launch connector I could find after evaluating several contenders for the end launcher. I also tried several manufacturers for the 100R chip resistors and chose the best one.This was made using a pair of specially selected 100R chip resistors in parallel and I tried to get as close to a resistive 50R as possible down at DC. The RF performance to 1GHz is very good as you can see in the plot below. This was independently measured with an Agilent Ecal module and the result is shown below. It is still 'very good' even by 3GHz and just manages to achieve 40dB return loss at 3GHz (according to the Ecal module that is).

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: February 12, 2022, 07:27:43 pm
content: Quote from: gf on January 27, 2022, 07:29:21 pmThe difference between 55 ohms (26 dB return loss) and 49.9 ohms (60 dB), matters almost not at all; Especially if your open and short are well separated as discussed in section 2.2.4.3 about 1+gamma test-setswww.tinyurl.com/JoelsMicrowaveBookfor reference) . For most VNAs, the noise is floor is independent of the reflection and as long as the reflections from at least 2 standards are well separated (open'ish and short'ish), the position of third standard (load'ish) doesn't matter very much. I suppose you could knock together a monte-carlo analysis including noise and standard position and sort out the exact effect, but I suspect it will be small. On the other hand, do what we do with the Ecal and just use a 4th standard (well, we actually use 7 standards above 26 GHz) and then you can have a 55 ohm and 45 ohm and almost for sure be better than just a single 49.9 ohm load standard.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: February 12, 2022, 07:39:42 pm
content: Quote from: G0HZU on January 27, 2022, 08:28:33 pmI think the typical Ecal should have 58 dB directivity (load uncertainty) out the door across most of the range. Our very best airlines are on the order of 65 dB directivity, and I would have to say our very best airlines are better than any other airlines in the world (or we would stop using them and start using the better ones). METAS might question who has the better air-gage, I guess they're pretty good too (air gauge is how you characterize airlines; we don't care what the impedance is really, just that we know what the impedance is). But typical connector repeatability is in the order of 65 dB as well so you start to run into physics of the connection at that point.

author: jjoonathan
author_rank: Frequent Contributor
timestamp: February 16, 2022, 07:50:31 pm
content: Speaking of airlines, do you have any favorite swiss shops for prototyping?I just ordered a bunch of 1.85mm airline center conductors from Swissomation -- but I really had to shop around to find someone willing to do a job this small (5 lengths, 125 pieces), let alone someone who was willing to chase tenths in after-plate tolerances. In the end, I had to relax the tolerances even slightly beyond the standard for what would go in a GPC connector. As you note, that's not so much a problem with airlines, because you characterize & compensate anyway, but going forward it's something to figure out.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: February 17, 2022, 07:15:44 am
content: Sure, I walk downstairs to guys in precision turning, and ask them to make me one. Seriously, we have our own Swiss turning machines and gun-lathes to make all our own airlines and plate them in our Santa Rosa plating shop. Years ago, when everything was off-shoring, that was one bit of technology that we didn't think we could successfully move. So it stayed in Santa Rosa, where it has been since about 1978. In fact I think we made specialty parts for METAS on contract. As I recall it was something like a set of offset-short lines, we made them, sent them to METAS, they measured the bits, sent them back, we put them together, and sent the off to live out their days in Bern.

author: jjoonathan
author_rank: Frequent Contributor
timestamp: February 17, 2022, 06:22:56 pm
content: Niiiiceeee!That explains a lot. Every time I go to measure parts (used parts -- I can't really speak for new stuff) the typical situation is that HPAK parts are not only within tolerance but bang-center in the middle of their specified tolerance band, or a couple of tenths away from a desirable edge of the tolerance band. Almost like someone with mechanical aptitude fussed over their constructionMeanwhile, no-name parts tend to be all over the tolerance band, and Anritsu is consistently dire. In the non-damaging direction, thankfully, but I can throw together a little simulation to see what 6 thou of pin gap does to return loss, and, well, that's where a lot of my interest in DIY comes from.Plating, too. The mirror-finish on new gold-plated Keysight parts is really something -- your plating shop knows their brighteners, and I bet if I were to investigate further I'd find other interesting things, like a nickel-free diffusion barrier and really tight tolerances.I was going to say: based on the prices and the tolerances I found on the open job-shop market, it would make sense surprisingly quickly to in-source it, buy a swiss lathe, and git gud. It looks like HPAK did the same calculation and reached the same conclusion.

author: coppercone2
author_rank: Super Contributor
timestamp: February 17, 2022, 06:45:15 pm
content: this sounds kind of like the air bearing lathe that dan gelbert madeHow do you do the tubes ID? special hones?I spent alot of time thinking about precision finishing of shapes but it does not become useful until this frequency range we are talking about.Tubing is pretty interesting to machine, just putting some brass tube on my milling machine with a rotary attachment I managed to get a 'paper thin' section of brass by milling out the interior by spinning it around (entrance to rabbit hole)The interior of thin tubes seems like its in the domain of laps, hones. I don't suppose they have a double connected spaghetti grinding stone made on a special dual collet jig grinder do they? I.e. unlike a normal jig grinder that has the stone clamped on only one side, this one I am imagining would have a diamond covered metal rod (like spaghetti) clamped by two collets with the tube inserted over it, fixtured in concentric collets and have a mechanism that connects the top and bottom so they move harmoniously (for some reason on a 2 foot long stone i imagine if you just spin it with it clamped on only one side it would end up bending alot, but if you tighten it like a hacksaw and have it supported on top and bottom maybe it could work?That tool I am imagining would likely have a scary cost associated with it. Jig grinder back to back

author: jjoonathan
author_rank: Frequent Contributor
timestamp: February 17, 2022, 06:54:08 pm
content: Could be, but I suspect they can get pretty close with a gundrill + ream. If plating is in-house, they could air-gauge and plate to tolerance too. Fun to think about!

author: coppercone2
author_rank: Super Contributor
timestamp: February 17, 2022, 06:57:22 pm
content: Hmm I figured it would also be lapped but just experimenting with bore lapping for my VNA fixture I can tell this would not be easy. That's so specialized they probobly can figure out how to tune the electropolishing to work better, my electropolishing experiments have been a disaster. black crap

author: jjoonathan
author_rank: Frequent Contributor
timestamp: February 17, 2022, 07:07:16 pm
content: Right, it costs enough that I suspect it would be trouble even at the low scale of the T&M market. I bet they do grind hardened tools, though -- like the gundrills and the reamsSpeaking of interesting machining, are those little center pin grabby finger things ("collettes" in the documentation -- I love it) EDM'd? At first I thought they were stamped or laser cut and folded or something, but I bought one with a missing tooth the other day, poked at it a bit, and it seemed pretty rigid.

author: coppercone2
author_rank: Super Contributor
timestamp: February 17, 2022, 07:09:39 pm
content: the center pin receptical of a coaxial connector?I figure that is like a machine pin, micromachined with a really small endmill from a piece of stockhttps://www.ns-tool.com/en/products/product_information/micro/I think they go smaller then 0.01 mmI watched a youtube video about micromachining from a swiss shop and he had a tray of screws that were conventionally machined that could be mistaken for sandend of this video shows the ridiculously tiny screws built with what looks to be conventional machinery5 minutes 16 seconds in. I would vacuum those screws (at least I think so, could be something else) up because they look like iron filings lol, i wonder how that screw driver for them looks like

author: jjoonathan
author_rank: Frequent Contributor
timestamp: February 17, 2022, 07:58:18 pm
content: It's not so much the size that makes me suspect EDM over micromachining, it's the long floppy features. A clever machinist can path around a lot of it, but still only so much.

author: coppercone2
author_rank: Super Contributor
timestamp: February 17, 2022, 08:20:24 pm
content: the way they can make EDM things nest seamlessly makes me think you are right

author: jjoonathan
author_rank: Frequent Contributor
timestamp: February 17, 2022, 08:42:47 pm
content: But hey, I'd love to be surprised! If it's micromachining, it means that someone did a really impressive job with paths + supports.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: February 17, 2022, 09:43:19 pm
content: Quote from: jjoonathan on February 17, 2022, 06:54:08 pmI believe that's correct. You start with oversize solid cylinder, gun drill the hole (and the gun drill might not be dead on the center of the bore), then you mount the cylinder with the hole between centers on the lath and machine the outside to match the hole on the inside, then off to precision plating to finalize the dimension. At least that's what I remember. I have to talk to the machine shop guys soon so I'll ask if my memory is correct.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: February 17, 2022, 09:48:35 pm
content: Quote from: coppercone2 on February 17, 2022, 07:09:39 pmFor center conductor Pin (male) I guess it is machined directly. BeCu is common to use and it should be stiff enough.For the center conductor socket (female) end, remember at Keysight we don't use slotted F-sockets, we use slotless connectors on all these parts (https://na.support.keysight.com/pna/connectorcare/Connector_Structure.htm). The inside bit I think is a chem-milled and formed part.

author: coppercone2
author_rank: Super Contributor
timestamp: February 17, 2022, 10:10:40 pm
content: I kind of wonder if there would be a use for that with some kind of collet inside of the coaxial cable... makes me think about those hidden magnetically driven ultra expensive gimmick screws