Calculating correction data for an unknown VNA calibration kit - Page 1
Forum section: electronics / rf-microwave

author: TheSteve
author_rank: Unknown
timestamp: March 06, 2022, 07:11:59 am
content: A user on the forum asked for help measuring his Times Microwave OE5108 N male calibration kit that had no correction data.I have experimented with this a little in the past so I thought I'd give it a go.The process is as follows:Warm up my VNA for several hoursCalibrate the VNA with a known calibration kitMeasure S1P (touchstone) data for each of the unknown standards (open, short, load)Find best fit correction dataLoad the correction data into the VNA, perform a calibration and verify performanceEquipment used:VNA - Keysight FieldFox N9918A 26.5 GHz VNACable: Gore Phaseflex 3.5mm cableN adapter: Keysight precision 3.5mm to N female adapterI have three calibration kits on hand to compare against:Keysight 85032E 6 GHz mechanical O/S/LKeysight 85092-60005 9 GHz ECALMaury Microwave 8850P 18 GHz mechanical O/S/LThe DUT is a Time Microwave OE5108 18 GHz O/S/L N male kitThe target VNA the OE5108 kit is to be used on is a Keysight 8753C with 6 GHz option.The higher the frequency the harder it is to not only get good data but also calculate the best fit correction data. With this in mind I will be starting with a frequency range of 300 kHz to 6 GHz using the Keysight 85032E mechanical calibration kit.To perform the math required to calculate the best fit correction data I will be using METAS VNA Tools II.Attached are the following files/pictures:Measurement setupTouchstone(s1p) files with the measured data for the OE5108 cal kit pieces.Graphs of the measured data and calculated best fit correction curves.Calculated correction constantsThis is a first pass so far, I've not yet done any verification of the calculated values. I have let METAS VNA Tools optimize all parameters other than impedance for now.I'd like suggestions on the best way to verify the cal kit and anything in the process that may be improved. I can then post updated data and verification results. If 6 GHz goes well I will repeat the process for 18 GHz.Here are the current calculated correction values(also attached as a file):Agilent Definitions:Standard Type: Open (N male)Offset Z0 (Ohm): 50Offset Delay (ps): 117.251566Offset Loss (GOhm/s): 0.619522C0 (E-15 F): 338.152624C1 (E-27 F/Hz): 8100.514019C2 (E-36 F/Hz^2): -4468.405384C3 (E-45 F/Hz^3): 719.894636RMS Error Max Error0.004862 0.009313Standard Type: Short (N male)Offset Z0 (Ohm): 50Offset Delay (ps): 73.521902Offset Loss (GOhm/s): 0.206593L0 (E-12 H): 1065.970546L1 (E-24 H/Hz): 16965.387578L2 (E-33 H/Hz^2): -3271.896260L3 (E-42 H/Hz^3): 1787.549767RMS Error Max Error0.003897 0.006611Standard Type: Load (N male)Offset Z0 (Ohm): 50.000000Offset Delay (ps): 132.802668Offset Loss (GOhm/s): 0.000000RMS Error Max Error0.013200 0.021613

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: March 06, 2022, 07:21:27 am
content: The graphs show a yellow curve and a black curve. Can you explain what each curve is? (or maybe I missed it).

author: TheSteve
author_rank: Unknown
timestamp: March 06, 2022, 07:36:01 am
content: Yes, I forgot that. The black(magnitude) is the measured data, yellow is the "best fit" calculated curve. These are plots generated by METAS VNA Tools.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: March 06, 2022, 07:45:06 am
content: Quote from: TheSteve on March 06, 2022, 07:36:01 amThat's what I inferred. The deviation from ideal is 1.006 vs expected (just below 1), so that 0.006 error represents a -44 dB term which is almost certainly error in the calibration. Normally I would expect Ecal to be significantly better than a mechanical cal kit, unless you use a sliding load cal kit. In some ways the best fit kind-of "smears" the error and so likely gives a reasonable result, but for sure, any value greater than 1 must be an error (or you have a perpetual motion machine).

author: Bud
author_rank: Super Contributor
timestamp: March 06, 2022, 02:40:59 pm
content: Is it worth trying to mechanically fixate the N adapter of the cable to prevent cable movement ?

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: March 06, 2022, 04:24:55 pm
content: One more thing: this kit does not have a connection for the center pin of the open. Thus, the model of the open depends a bit on the exact nature of the SMA-to-N adapter that you use. The electrical-model of the open presumes a 6 slot socket (F) end, but most adapters use a 4 slot end, and thus have a slight lower capacitance than the 6 slot end (the 4 slot has longer slots and tapers to a smaller value). The gold adapter might be a precision adapter so it might have the proper socket (F) end. Without a center pin, the socket side impedance is a bit higher than expected, which is accounted for in the model. The Ecal of course will have a center pin that will will not be affected by the time of SMA-to-N adapter. I notice the Times kit also has a pin for the open standard.

author: Bud
author_rank: Super Contributor
timestamp: March 06, 2022, 07:16:01 pm
content: Quote from: Joel_Dunsmore on March 06, 2022, 04:24:55 pmYou mean the reference Keysight 85032E cal kit?

author: TheSteve
author_rank: Unknown
timestamp: March 06, 2022, 08:58:57 pm
content: The 3.5mm to N adapter I am using is the 6 slot end version. I will keep experimenting with the various cal kits I have here but so far the results look most optimal with the 85032E kit. I am also testing things on a VNA with native N female connectors so no adapters of any kind are used.

author: hendorog
author_rank: Super Contributor
timestamp: March 07, 2022, 12:41:43 am
content: At a more basic level, I've had better luck in the past with DIY cal kits by chopping the freq range into two segments:e.g.300kHz to 3.5GHz3.5GHz to 6GHzLess convenient to use of course but might be worth a crack.

author: TheSteve
author_rank: Unknown
timestamp: March 07, 2022, 12:53:04 am
content: I'm frustrated so far with the results I'm seeing with the various N cal kits I have. When I have done this before with 3.5mm gear I've had some amazing results(and some so/so results). I'm not giving up though.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: March 07, 2022, 01:10:39 am
content: Quote from: Bud on March 06, 2022, 07:16:01 pmYes, so the length of the (F) socket can affect the calibration, and it should be gauged. If you had a (M) Ecal, I would think it should give the best results if the Ecal connector isn't damaged and isn't too old (the Ecal can drift over time, but this mostly affects the S21 thru, not so much the OSL definitions.It might be the Times kit has an offset impedance from 50 ohms; if so it can give you that odd shape as well; but most likely it is an error in the cal. For example, the short should be very near 1.000 as it is a total reflection, regardless of anything else. The fact that it shows a ripple is almost for sure a consequence of the initial type-N calibration. Maybe try calibrating with Ecal and then measuring both 85032E standards and the Times standards. See if anything becomes apparent.

author: TheSteve
author_rank: Unknown
timestamp: March 07, 2022, 01:55:28 am
content: I've tried the ECAL and it performs a cal but it certainly isn't any better. It likely needs a cal itself. It is a real struggle to find really good N ECAL units for hobby level pricing. I have excellent 3.5mm ECAL's though

author: Bud
author_rank: Super Contributor
timestamp: March 07, 2022, 02:28:43 am
content: Quote from: hendorog on March 07, 2022, 12:41:43 amCertainly is less convenient, but better than nothing.@TheSteve - can we just do 0.3-3GHz and 3-6 GHz sets of coefficients?

author: TheSteve
author_rank: Unknown
timestamp: March 07, 2022, 04:47:27 am
content: We can absolutely do that, but I haven't given up on a 6 GHz set yet.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: March 07, 2022, 04:55:46 am
content: Quote from: TheSteve on March 07, 2022, 01:55:28 amWhen you say isn't any better, can you plot the Ecal calibrated results (of the Times kit measurement) and the 85032E results on the same plot. It would be interesting to see if the results are both bad in similar ways. And, just for fun, measure the 85032E standards using the calibration from the Ecal.The question in my mind is whether the Times kit is a bit odd, thus having a response that is not well modeled by the simple polynomial model, or if it all relates back to the reference calkit. But, since I don't see a ripple associated with the cable length (1 m should give a ripple on the order of a couple hundred MHz), then it means the raw source match and directivity of the Fieldfox is being removed.On the topic of splitting the cal standards: it won't do much good to get a "better match" if the reference data isn't correct. So the thing to determine is if it the Times standards that are unusual, or if it's the reference cal kit. The 3 GHz ripple pattern points to something like a 5 cm length, which I wonder if it is consistent with the actual length of Times kit. If it is, it might be the nominal center conductor impedance is off and so the delay term should have a non-50 ohm result. The fact that the values you obtained has the impedance asexactly50 ohms seems to indicate to me that the chance that the center-conductor to outer-conductor impedance error is not being considered in the fitting. Otherwise I would expect numbers like 50.00315 or 49.9990; given that you have 9 significant digits on the delay, for example. Maybe you have to fool around a bit with the offset delay value.

author: Bud
author_rank: Super Contributor
timestamp: March 07, 2022, 01:18:42 pm
content: The serial number of the Times kit is very low: # 0019. Could it be a prototype ?

author: G0HZU
author_rank: Super Contributor
timestamp: March 07, 2022, 08:43:58 pm
content: I don't know if this helps but a few years ago at work I checked out the health of an old 85032E N type cal kit and I managed to find the project file for it today.See below for a Genesys simulation after using the 85032E to initially calibrate an Agilent E5071C VNA up to 6GHz. The 85032E open and short was measured after a calibration and I then applied the C0 C1 C2 C3 corrections and the offset delay corrections via Genesys. The results are as below. I didn't include the Offset Zo or Offset Loss corrections so the plots of the reflection coefficient are what the VNA measures if presented with the 85032E after calibration. You can see that the phase response gives a flat line if I apply the official C0 C1 C2 C3 corrections and the offset delay correction via Genesys. The magnitude of the reflection coeffcient is shown for both the open and the short.Genesys also plots the fringing capacitance vs frequency for the combined C0-C3 corrections which may be of interest.I've also included the info in the E5071B user manual for the 85032B/E cal kit. I have an old E5071B VNA here but these tests were done several years ago at work using a works E5071C VNA.I've never had great results when doing critical stuff with N type cal kits or N type test fixtures so I'm interested to see how this thread progresses. I have a home made N cal kit here (male and female) and I'd be keen to try and improve it. So far I've generated C0-C3 and L0-L3 coefficients for it using Genesys but I think it would be better to use the META VNA Tools for this stuff.It's worth checking out the way your VNA treats the gender of the cal kit descriptions. On the E5071 VNA an (f) means female for the analyser connector and the cal kit is fitted with a MALE connector. I'm not sure why Agilent did this but it can lead to confusion.

author: TheSteve
author_rank: Unknown
timestamp: March 07, 2022, 10:16:10 pm
content: Thanks for posting the info. Newer VNA's now base everything off of the "DUT" connector/gender. They even show a picture of the connector you've selected and gender.As I said above I've found this much more challenging with N connectors so far vs 3.5mm. In the past I calibrated using a known good 3.5mm ECAL and then measured the female Keysight 85052B set. I cheated and used the official impedance, delay and loss figures and let METAS VNA calculate the rest. The values it came up with were extremely close to the official Keysight values, shockingly close even. It is hard to reproduce to that level though, each connector mating is critical. If cables are used they can't be moved at all etc. This was also to the full 26.5 GHz.

author: G0HZU
author_rank: Super Contributor
timestamp: March 07, 2022, 11:45:48 pm
content: Thanks. I can't remember now but I think I may have calibrated the E5071C using a 85032F cal kit when I did the above tests of the 85032E. There's a project file for the 85032F cal kit in the next folder and this was the full male and female cal kit. At work I think the production area now has a N type Ecal so I could try measuring my homebrew N cal kit with that at some point and I can re-check the 85032E if I can find it at work.I'm planning on early retirement so I won't have access to new calibrated equipment in the near future... I have a decent 3.5mm Ecal module here at home but nothing decent for N connectors apart from a fairly dodgy Anritsu T cal kit.

author: TheSteve
author_rank: Unknown
timestamp: March 08, 2022, 03:53:39 am
content: Performed a cal using the 85032E kit, then saved S1P data for both the 85032E and the OE5108. This is using a different VNA with slightly different parameters, 2 MHz to 6 GHz.Attached are the graphs from the measured 85032 open/short and the 5108 open/short. The S1P files are also attached.So far to me the measured data looks pretty reasonable. Black again is measured data, orange is the best fit correction curve METAS VNA Tools has calculated.I didn't attach plots but I also checked the Maury open/short after the 85032 cal and the lines were very reasonable(very similar to the 85032 kit), no big curves or dips. It seems only the OE5108 kit has them. Perhaps this kit can't be well characterized by the simple polynomial model as Joel mentioned.

author: TheSteve
author_rank: Unknown
timestamp: March 08, 2022, 05:23:01 am
content: As mentioned in my previous post I've been experimenting with correction values and then performing a calibration using the EO5108 cal kit.Here are best results so far:We have the return loss of the open female N connector on the VNA (0.2 dB/div) - this should ideally be a flat line.Phase of the open female N connectorPhase with the Keysight 85032 open connected(looks 99.9% the same as with nothing connected which is correct).Phase with the Keysight 85032 short connected.I'd like to see flatter lines, but at least it is a result - Joel what are your thoughts?

author: KE5FX
author_rank: Super Contributor
timestamp: March 08, 2022, 05:31:25 am
content: You've seen Claudio, IN3OTD'sOctave script, right?

author: TheSteve
author_rank: Unknown
timestamp: March 08, 2022, 06:14:41 am
content: I've experimented with his software as well in the past. I found METAS VNA easier to use.

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 08, 2022, 04:43:39 pm
content: Quote from: TheSteve on March 06, 2022, 07:11:59 ambest fit for 6GHz may not be the best fit for 18GHz and vice versa... source: me (or i've read or heard from other sources but have forgotten from where)... so you are free to reject the idea... cheers.

author: TheSteve
author_rank: Unknown
timestamp: March 08, 2022, 05:04:57 pm
content: Quote from: Mechatrommer on March 08, 2022, 04:43:39 pmReject what idea?

author: TheSteve
author_rank: Unknown
timestamp: March 08, 2022, 07:04:07 pm
content: Quote from: Mechatrommer on March 08, 2022, 06:47:53 pmOf course the correction values will not be the same. The polynomial model is really only suited to a single curve, not a line with multiple curves. So measuring the kit to 6 GHz should get a better fit with the standard correction model vs going to 18 GHz.In an ideal world with an ideal cal kit the error would be small across the entire range to 18 GHz and it wouldn't matter. If it was a nice curve with negative magnitude the correction would be quite simple. But it seems nothing is ideal

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 08, 2022, 10:23:28 pm
content: Quote from: TheSteve on March 08, 2022, 07:04:07 pmi mean, you can measure once up to the maximum range your VNA can provide from the beginning (and how far your standard CAL kit is characterized).. because measuring up to 6GHz vs measuring the entire range (18GHz or anything your VNA can provide), you'll get the same measurement result, except beyond that 6GHz. once the technique/method is established, its just a matter of feeding the curvefit function with any subset, ie a particular portion from the universal set (entire range measurement) to get different curve fit coefficients. i'm sorry if i'm not clear enough, its quite hard to explain it textually... (edit: except.. my point will be invalid if you want to argue about poorer resolution when measuring the entire range, ymmv cheers)

author: TheSteve
author_rank: Unknown
timestamp: March 09, 2022, 03:30:51 am
content: OK, I understand what you are saying now. I would only calibrate with a 6 GHz max as it makes better use of the number of sample points. If I used the same number and went to 18 GHz I'd only have one third of the sample points to 6 GHz. And yes I could just add more points but that slows things down.The sample image you listed is the load, so I wouldn't worry about the graph to much. As for the the opens/shorts, they can only match the measured trace if it can be expressed by the polynomial equation used to generate the calibration correction values. If it can't match it perfectly then it just does the best it can.

author: gf
author_rank: Super Contributor
timestamp: March 09, 2022, 07:14:32 am
content: Quote from: Mechatrommer on March 08, 2022, 10:23:28 pmQuoteI wonder in particular whether the shape of these yellow curves can be the result of a 3-parameter load model as given in the OPQuotefor any (real) values of the model parameters Z0, Delay and Loss?

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: March 09, 2022, 09:32:36 pm
content: Quote from: Mechatrommer on March 08, 2022, 10:23:28 pmFor sure, with a load element that has either some net inductance or capacitance, you cannot get a good fit if the delay impedance is fixed to 50 ohms. I am not familiar with METAS tool for fitting, but if it does not allow a change of delay impedance, it will have issues fitting some responses I think.

author: Bud
author_rank: Super Contributor
timestamp: March 09, 2022, 10:55:19 pm
content: Sorry what is "delay impedance" ? Is it the impedance of the delay element of the Load model?

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: March 10, 2022, 05:59:43 am
content: Quote from: Bud on March 09, 2022, 10:55:19 pmYes, it is the impedance of the delay element and it can be used to compensate for first order parasitic L or C. I discuss it in some detail in pages 738-741 of the 2nd edition of my book.

author: TheSteve
author_rank: Unknown
timestamp: March 11, 2022, 08:55:26 pm
content: METAS VNA does allow impedances other than 50 ohms. I had fixed it to 50 for my initial run of calculations(you can fix any parameter you want). I have since changed this for the open and short, I can also try it for the load. I hope to have more time to experiment this weekend(supposed to rain, so heli flying is out).Quote from: Joel_Dunsmore on March 09, 2022, 09:32:36 pm

author: Marsupilami
author_rank: Frequent Contributor
timestamp: March 12, 2022, 06:04:17 am
content: This thread is getting long and I wasn't following it so forgive me if it was said and/or I'm missing some detail but do I get it right that you're trying to characterize an N cal kit with 3 other N cal kits, none of which you're sure about they're accurate?I'm wondering if you'd be better off cal'ing at the end of your 3.5mm cable with your known good 3.5mm ECal. Adding the 3.5-N adapter will still add 3 unknowns but maybe you can make better educated guesses about those than the who knows what's wrong with them N cal standards.I'm not very versed in the traditional cal standard models, I've always been using data based standards (always meaning for the 5 minutes I've been working with VNAs) but maybe you would still be able to try to curve fit through the adapter, in which case your parameters would be obviously off but potentially show smaller error compared to the measurement result, proving that the your N calibration was at fault.

author: TheSteve
author_rank: Unknown
timestamp: March 13, 2022, 07:05:23 am
content: After experimenting I am very pleased with the calibration the 85032E is providing. I have gathered the S1P data for the Times Microwave kit many times now. It is pretty repeatable. I have calculated correction constants to 6 GHz. From 300 kHz to 5.5 GHz the maximum magnitude error after calibrating with the Times Microwave kit is 0.2 dB or less. From 5.5 to 6 GHz it increases to 0.3 dB. Phase error is also looking reasonable to 5 GHz with some increasing error after that, but still usable.For reference the magnitude error from my ECAL is also around 0.2 dB - should be lower but the male connector appears to be quite worn. The magnitude error from my Maury Microwave kit is around 0.1 dB.I've attached pictures of the measured Times Microwave OSL data. Black is measured data, orange is best fit correction.The load picture has extra colors as it is also calculating best fit parameters for Anritsu and R&S equipment and they aren't the same for the load.The open and short phase pictures are after calibrating with the Times Microwave kit and then measuring the Keysight 85032E open and short.The zip file has the S1P data, graphs, correction values and files for various Keysight VNA's to make loading the constants easy.Final data values:Agilent DefinitionStandard Type: OpenOffset Z0 (Ohm): 50.151807Offset Delay (ps): 110.016566Offset Loss (GOhm/s): 1.265273C0 (E-15 F): 505.266674C1 (E-27 F/Hz): -13751.991616C2 (E-36 F/Hz^2): 2877.240920C3 (E-45 F/Hz^3): 628.099576RMS Error: 0.002981Max Error: 0.007227Standard Type: ShortOffset Z0 (Ohm): 50.474695Offset Delay (ps): 73.979612Offset Loss (GOhm/s): 1.250892L0 (E-12 H): 1001.065848L1 (E-24 H/Hz): 28754.821253L2 (E-33 H/Hz^2): 4696.023892L3 (E-42 H/Hz^3): 254.983229RMS Error: 0.002161Max Error: 0.004829Standard Type: LoadOffset Z0 (Ohm): 49.587352Offset Delay (ps): 159.997260Offset Loss (GOhm/s): 2.795148RMS Error: 0.011838Max Error: 0.023109Workflow was to calibrate with the 85032E cal kit. Save S1P files for the open/short/load of the OE5108 kit and then load those files in METAS VNA to find the best fit correction data. The correction data was then loaded in the Agilent cal kit editor and saved. From there it is transferred into my VNA so I can perform a test calibration with the new kit defs.I'm hoping to repeat this process with an unknown R&S 26.5 GHz 3.5mm kit in the near future. it will be referenced from a 26.5 GHz 3.5mm ECAL unit. I will start a thread for that one as well.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: March 13, 2022, 07:21:36 am
content: This looks quite reasonable. I do notice that this set of coefficients as the delay impedance as not exactly 50 ohms; did you do something different to get the curve fitting to accept non-50 ohm for this parameter. I'm guessing the variation you saw was associated with the non-50ohm section. I also not some earlier plots had S11 greater than zero (which for sure indicated a calibration issue). Did you do something different in the calibration to get the better results for the measurement of Times open and short?

author: G0HZU
author_rank: Super Contributor
timestamp: March 13, 2022, 09:54:46 pm
content: To see what my old homebrew N type cal kit can do up at 6GHz I brought it into work on Friday and measured it with an E5071C VNA and an 85032F cal kit. See below for the phase and s11 response after I computed the correction coefficients using Genesys for the male and female parts of the kit. The complete cal kit is made using male and female N type bulkhead connectors. These are very good quality connectors (expensive!) but I've been lazy and left the open 'open' as it has no end cap so it has some radiation losses up towards 6GHz. However, the results were quite good. When I first made this cal kit I only expected to use it up to about 3GHz. It's tempting to find a couple more bulkhead connectors and try fitting end caps to the male and female OPENs. It might work better up towards 6GHz if I do this.The second set of plots are for the female half of the kit. These results seem a little better up at 6GHz in terms of s11. Note that the s11 response is raw/uncorrected as the results below are obtained in Genesys after optimising just C0 to C3 and L0 to L3 and the delay offset.It is quite rewarding to manually tweak the delay and C0 through C3 (and L0 and L3) to optimise the response and I get the best results by then swapping across to the Genesys optimisation tool where I ask the optimiser to tune for minimum phase difference. However, it would be nice to try the IN3OTD and the METAS Tools to see what I can really achieve with this cal kit but I've tried both and they aren't intuitive to set up and use.

author: G0HZU
author_rank: Super Contributor
timestamp: March 13, 2022, 10:18:01 pm
content: Note that these weren't ordinary N bulkhead connectors that I used in my homebrew cal kit. They were salvaged from some old pre-production prototype notch filters I designed at work 12-15 years ago. The filter had to have ultra low insertion loss to 6GHz and these bulkhead connectors were very expensive and have 8 slots in the female centre. They are physically very short and provide a direct launch into a PCB.Here's an old plot of the notch filter. It had about a 60dB notch around 1500MHz and what made this filter 'special' was that the insertion loss was very low on the high side of the notch all the way up to about 8GHz. I originally designed this notch filter using Sonnet as a multilayer structure. Since then thousands have been made and they can be used on the output of various wideband transmitters.

author: TheSteve
author_rank: Unknown
timestamp: March 13, 2022, 11:31:18 pm
content: Quote from: Joel_Dunsmore on March 13, 2022, 07:21:36 amI switched to another VNA with native precision N connectors on it. I don't trust my 3.5mm to N adapter to be 100%. I then allowed METAS VNA to optimize all parameters including impedance. Adapters, cables etc all look good until you set things to 0.1 dB/div.

author: TheSteve
author_rank: Unknown
timestamp: March 13, 2022, 11:35:19 pm
content: Quote from: G0HZU on March 13, 2022, 09:54:46 pmMETAS VNA tools is quite easy to use for correction if you provide it the S1P files manually. I'm not sure all versions handle it though, I am using version 2.0.Do you have the S1P files from your cal kit as measured after performing the cal with the 85032 that you can share? I'll pop them into METAS and see what it can determine.

author: G0HZU
author_rank: Super Contributor
timestamp: March 14, 2022, 12:32:31 am
content: Thanks. See below for the female OPEN and SHORT part of the homebrew N kit. I'm going to remake the male OPEN as this connector has seen previous use and was soldered to a test enclosure at one point. This might explain why it seems worse than the others in terms of s11 loss.This cal kit has been very useful over the years and it's probably good enough for the stuff I use it for. However, it would be interesting to see if it can be improved. I still feel that the various uncertainties associated with the limitations of the E5071C VNA and the 85032F cal kit mean that there isn't much point in trying too hard to fine tune these corrections. At some point the overall uncertainty dominates.I downloaded METAS VNA Tools and I think it is version 2.58. It could be that I've downloaded the wrong version because I couldn't work out how to make it generate these corrections.Note that the s1p files below cover up to 8.5GHz but I think it would be best to limit the optimisation to just 6GHz. I struggled to get good results above 6GHz when I tried.

author: G0HZU
author_rank: Super Contributor
timestamp: March 14, 2022, 12:44:33 am
content: If you need the 50R load data for the homebrew kit then I kind of cheat here because I used a male Anritsu 50R N type termination in series with a decent F-F N adaptor. This adaptor is very high quality and the female centres have 8 slots. I've attached it below if you need it. I do keep meaning to try and make a homebrew 50R N termination but at the moment I use the Anritsu termination.

author: TheSteve
author_rank: Unknown
timestamp: March 14, 2022, 12:48:01 am
content: I processed the open and short.The black trace is the measured data, red trace is the best fit Agilent correction.The error looks very low, interesting how it finds the ideal parameters are quite a bit off from 50 ohms.Here are the suggest cal parameters:Agilent DefinitionStandard Type: OpenOffset Z0 (Ohm): 54.323655Offset Delay (ps): 52.659936Offset Loss (GOhm/s): 2.376092C0 (E-15 F): -51.559719C1 (E-27 F/Hz): 7194.698536C2 (E-36 F/Hz^2): -7632.558772C3 (E-45 F/Hz^3): 660.074834Agilent Fit ErrorRMS Error: 0.000823Max Error: 0.001326Standard Type: ShortOffset Z0 (Ohm): 34.781713Offset Delay (ps): 18.304420Offset Loss (GOhm/s): 2.913411L0 (E-12 H): 1545.445695L1 (E-24 H/Hz): 9342.640691L2 (E-33 H/Hz^2): -26590.509714L3 (E-42 H/Hz^3): 3071.521470Agilent Fit ErrorRMS Error: 0.000578Max Error: 0.001237

author: G0HZU
author_rank: Super Contributor
timestamp: March 14, 2022, 01:02:04 am
content: Thanks! The numbers for Z0 do seem a bit strange and I'll have to do some studying to make sure I understand what the results mean.When I made this kit I really didn't expect it to work that well despite the huge cost of the connectors. In order to provide IP sealing the last couple of mm of the connector has a PTFE dielectric so there is a transition from air to PTFE right at the point it launches to a PCB. Presumably Suhner will have tried quite hard to optimise this transition but maybe this partly explains the strange results for Z0.

author: TheSteve
author_rank: Unknown
timestamp: March 14, 2022, 01:07:30 am
content: If I fix the impedance to 50 ohms the error doesn't change that much. Anyway, maybe give the values a try and see how it performs.

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 14, 2022, 06:32:55 am
content: Quote from: G0HZU on March 14, 2022, 01:02:04 amits explained in Agilent app note AN 8510-5B as transmission line's impedance after calibration plane. so iassume:1) its inside the cal kit, and it was assumed Zc (transmission line's impedance before calibration plane) = Zo = 50, when older VNAs dont support modeling it.2) all offsets... Zo, delay, loss are physical, they can be measured with proper tools, the problem is there is no easy "proper tools", so now with advance computing, they are included in the curve fit modelling.3) Steve's VNA support it, so thats why he included the parameter in the modelling..sometime i wonder... sometime we can leave fringing capacitances parameters to all 0s and tune the offset delay (or with offset loss) alone to get "good enough" model, or set the delay to0some other value and Z0 = simply 50 ohm... and tune the C and/or L 0,1,2,3 for Open/Short to get similarly good enough model. so i guess whats important is the model parameters calculated by curve fit function. the actual physical offset value (Zo, delay, loss) is less important/impact/preferable (due to the difficulty of actually measuring them), as long as the model function used in the VNA (given the parameters calculated by the curve fit function) can give very close to the actual measured S11. with data-based VNA, curve fit is unecessary, only interpolation. ymmv.

author: TheSteve
author_rank: Unknown
timestamp: March 14, 2022, 03:55:41 pm
content: Quote from: Mechatrommer on March 14, 2022, 06:32:55 amYou can certainly exclude any parameter you like. METAS VNA will indicate what the expected error is when it has calculated the best fit model. Of course I've never done anything to verify the math or modeling they use, but I have no reason to doubt it.The Keysight N 85032B/E doesn't include any inductance values for the short and uses only delay, loss and impedance.

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 15, 2022, 01:53:26 am
content: yeah i got a chance to read some of their app notes this morning making me realize how sloppy i am still... their setup up to 100+ GHz with narrow window of uncertainty is something beyond my capability. i've downloaded the setup file and app notes a while a go but dont have a chance to really use them,,, anyway i got to install and play with their VNA tool, i'm not sure if i'm doing it right but it seems too simple to use... just go to Cal Std Model Fit -> Load Data -> Start Optimization and thats it... yeah some parameters can be left fixed (uncheck Mask).. attached are 2 optimizations, all parameters optimized (1) vs only fring. Cs optimized with offsets fixed (2)... s1p used is provided by Steve (5108-open-85032-6ghz.s1p) fwiw... edit: but unfortunately when i tried to fit my diy (imperfect) Load kit (more specifically... Arbitrary Standard.. ie near but not exactly 50 ohm termination), META tool fitting still has some limitation there...

author: joeqsmith
author_rank: Super Contributor
timestamp: September 07, 2024, 11:57:25 pm
content: Quote from: TheSteve on March 06, 2022, 07:11:59 amQuote from: alan.bain on February 08, 2024, 11:26:31 pmWhile not mentioned, I would assume IFBW was set very low and the number of data points was maximized. Outside of this, is there anything else (tips, tricks or otherwise) I should be aware of?Quote from: TheSteve on February 10, 2024, 07:47:16 amI assume you just load these into the far left Settings, Start Agilent Type (see attached). If you had other supplied parameters, is there a way to select them?

author: joeqsmith
author_rank: Super Contributor
timestamp: September 09, 2024, 02:49:16 pm
content: To start, I measured the same standards I used for calibration. Leaving all of the parameters unconstrained, it never converges, or at least I stopped it once the offset Zo reached 100kohms. I also noticed that when I aborted the run, it doesn't stop gracefully.Is there a way to place limits on the various coefficients rather than hard coding them?What are the common causes of the software being unable to converge?

author: TheSteve
author_rank: Unknown
timestamp: September 09, 2024, 03:58:15 pm
content: If it goes to crazy values I'd say it doesn't like the data its working with too much. I'm not aware of anyway to give it any sanity limits. Ideally I think it is best to see if you can get it to calculate values that seem reasonable for a known good cal kit that you measure with respect to another known good kit. That allows you to experiment a little to see the best ways to get it to work and to know your process is working, then you can experiment with an unknown kit.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 09, 2024, 04:33:05 pm
content: Thanks for the feedback. I wonder what it's not liking. I would have thought even with everything being bad, as long as it was repeatable, it would converge.

author: hendorog
author_rank: Super Contributor
timestamp: September 09, 2024, 07:53:53 pm
content: Could you post the s1p files?We can run them too and see if it does the same thing.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 09, 2024, 09:58:21 pm
content: Sure. These were created using the LiteVNA64, provided standards, ideal model, 801 points, 1kHz IFBW. After SOL, Short and Open were remeasured. I don't believe these will converge. I would expect it to show coefficients similar to the ideal model. Maybe that thinking is flawed.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 09, 2024, 10:26:30 pm
content: Same setup but using a torque wrench and the Agilent 85033E coefficients. Again, I don't believe it will converge without adding constraints.

author: hendorog
author_rank: Super Contributor
timestamp: September 09, 2024, 11:00:00 pm
content: FWIW:I got it to converge by unticking the Z0 option and leaving the others ticked.That produces what looks like an OK fit.I also played around with removing bits of the frequency range (low end and high end where it looks a bit suspect), and it did then converge on the SHORT with Z0 ticked.But not on the OPEN.That was seeding it with the default values - i.e. 50 ohms and all zeros.

author: hendorog
author_rank: Super Contributor
timestamp: September 10, 2024, 12:52:37 am
content: Hang on, looking at these files they are showing a higher reflection than the incident signal.That puts them outside the Smith chart boundary doesn't it?

author: joeqsmith
author_rank: Super Contributor
timestamp: September 10, 2024, 04:13:24 am
content: Quote from: hendorog on September 09, 2024, 11:00:00 pmSure, we can force the constraints like was mentioned.Quote from: joeqsmith on September 09, 2024, 02:49:16 pmQuote from: hendorog on September 10, 2024, 12:52:37 amIt sure does and we have seen this in the majority of the data shown in this thread and there was never a mention of it causing a convergence problem. So lets coerce the data to force it into range. For me, it does not converge.

author: hendorog
author_rank: Super Contributor
timestamp: September 10, 2024, 05:20:25 am
content: QuoteCool, let us know what you find.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 10, 2024, 11:10:51 am
content: Rather than limiting the gain, I forced it to the outer edge of the Smith chart. A perfect circle if you will. This is in file 2. METAS will not converge. Problem appears to be the phase. I then created a perfectly linear phase from the min to the max. It will converge. This is in file 3.

author: TheSteve
author_rank: Unknown
timestamp: September 10, 2024, 05:37:05 pm
content: Quote from: joeqsmith on September 09, 2024, 10:26:30 pmI found using a torque wrench was absolutely required to get repeatable results with 3.5mm.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 10, 2024, 06:09:18 pm
content: Quote from: TheSteve on September 10, 2024, 05:37:05 pmIf you're looking for repeatable results with any setup, torque wrenches are a must.Just in case it was not clear, I am using the 3.5mm coefficients with the supplied low cost standards. I would never attach such a standard to any of the parts supplied with these low cost VNAs. The only reason to change the coefficients was to see if the ideal model was causing a problem with the convergence.For now I am just trying to get some understanding of the METAS software. With that last set of data being 100% synthesized, the torque wrench can stay on the table.

author: hendorog
author_rank: Super Contributor
timestamp: September 10, 2024, 06:22:10 pm
content: Quote from: joeqsmith on September 10, 2024, 11:10:51 amThat is another difference in your standards to others. They are point and have no length.Do they have a Z0 if there is no transmission line?Perhaps it makes no sense trying to model Z0 for these standards? I don't know.Edit: Easy to test this theory, attach a thru to them and sweep them again.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 10, 2024, 07:32:18 pm
content: Quote from: hendorog on September 10, 2024, 06:22:10 pmThat's due the ideal model. The other Touchstone files used the 85033E coefficients to add the typical rotation.Interesting enough, I have never used anything besides the ideal model with the low cost VNAs, except when playing with waveguides.

author: hendorog
author_rank: Super Contributor
timestamp: September 10, 2024, 07:40:31 pm
content: Quote from: joeqsmith on September 10, 2024, 07:32:18 pmOk I am now seeing what I missed. You did say it, sorry I missed it.You are measuring the same standards as you used for the calibration. And you told the VNA those were ideal.So there isn't really any point in modelling those. They are ideal by definition and the residual error is not in the standards, it is connections, drift, repeatability etc.So you're trying to fit a model to drift error.Attach a thru to them. You've now created a new standard. Then sweep those and try and fit a model to them.Lets see if that works.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 10, 2024, 07:58:09 pm
content: QuoteCorrect. That is where I started. Errors were then applied to create the test files. Last file was fully synthesized. Nothing was measured. Just a perfect circle and linear phase.METAS only knows the one file it has to work with. So adding the typical rotation manually, I wouldn't expect to see a problem.I will try it with the ideal model and add the thru. I expect a bit of loss (if the VNA can detect it below the noise) but should be similar. Just a different degree of rotation.***When using Mario's script, he would import both the reference standard as well as the unknown.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 10, 2024, 09:26:20 pm
content: Ideal model used to calibrate, remeasure open with a thru, not torqued. Raw data falls outside the Smith. Limited as well to 1. Both will not converge without constraints.So far, the only test that has worked was the perfect synthesized data. I wonder with the previous data that was posted, OP locked the 50 ohm. Maybe they ran into the same problem and had to constrain it. There is no mention of that though.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 10, 2024, 09:52:58 pm
content: I repeated the above test with the added thru but this time locking the cable down, torquing, and lowing the IFBW to 800Hz. Both the open and short fall outside the Smith chart. Both will converge without any constraints.I searched a bit to see if they had published anything about troubleshooting convergence problems. No luck there so I have attempted to join their group.

author: hendorog
author_rank: Super Contributor
timestamp: September 10, 2024, 11:59:58 pm
content: Quote from: joeqsmith on September 10, 2024, 09:52:58 pmGreat to know. So in summary it needs an offset for Z0 to converge.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 11, 2024, 12:01:51 pm
content: Quote from: hendorog on September 10, 2024, 11:59:58 pmAt this time all I have is the data I've presented, which isn't enough to tell me what the problems are. Most the cases shown have an offset and did not converge. If you like guesses, I think it has more to do with the instability, or discontinuity of the data. I plan to write a Touchstone generator that will allow me to easily run various conditions. Hopefully between this and opening a dialog with the creator, I can get a better understanding of what is required for convergence.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 11, 2024, 05:34:27 pm
content: I wrote a simple Touchstone file generator that allows me to add loss and noise. It seems that METAS has a memory leak. Running it over and over, it burns up memory and slows to a crawl. My plan was to use 16000 data points but this really causes problems with the leak.Convergence seems to be tied to a combination of things. The phase doesn't appear to matter much but it seems to want pristine data to converge. So very low noise, low loss, levels <= 1, and low number of samples.

author: hendorog
author_rank: Super Contributor
timestamp: September 11, 2024, 06:39:45 pm
content: Quote from: joeqsmith on September 11, 2024, 05:34:27 pmExcellent, will you report that to them?

author: joeqsmith
author_rank: Super Contributor
timestamp: September 11, 2024, 07:34:23 pm
content: Quote from: hendorog on September 11, 2024, 06:39:45 pmAs of today, they have not approved my account. Once they allow me access, I hope to open a dialog with them and will go over my findings.***Just to demonstrate that it can converge without constraint with basically no rotation and even a bit outside the Smith chart. So far I have not introduced any phase noise into my test program. So that is always perfect for now.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 11, 2024, 09:39:07 pm
content: I've added phase noise and it appears that the convergence is less sensitive to this than gain.I think the next step is to feed METASs' results back into my software to see what the effects are.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 18, 2024, 08:26:46 pm
content: I simulated a short and open with some delay and let METAS fit the data. These coefficients were then entered back into my simulator and a perfect open was measured using the new correction factors. The two plots overlay as expected so their fitter appears to work as I would expect.The data was very low noise and small number of data points just to make sure METAS would converge.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 19, 2024, 01:41:53 pm
content: I synthesized a 40ish ohm load for a standard. METAS determines it is a load but when fitting, it appears to pass the data through.Does the software not account for any errors in the load?

author: joeqsmith
author_rank: Super Contributor
timestamp: September 19, 2024, 01:54:37 pm
content: Using something closer to 50 ohms with a bit of noise and it attempts to fit the data.