NanoVNA V2+4 SMA Cal load - Page 1
Forum section: electronics / testgear

author: mawyatt
author_rank: Super Contributor
timestamp: December 06, 2020, 09:23:31 pm
content: Just received the mentioned V2+4 VNA and decided to check the supplied SMA 50 ohm load that comes with the cal kit. I measured 50.931 ohms with a new KS34465A, which is considerably higher than the type N load supplied with the SSA-2N version which is 49.634 ohms. Measurements were conducted with the supplied Short establishing the Zero ohm reference for the SMA, and using the type N short for the N Zero ohm reference.What are others finding with the cal load supplied with the various NanoVNA?Best,

author: joeqsmith
author_rank: Super Contributor
timestamp: December 06, 2020, 10:20:38 pm
content: https://www.eevblog.com/forum/rf-microwave/nanovna-custom-software/msg3323942/#msg3323942

author: mawyatt
author_rank: Super Contributor
timestamp: December 07, 2020, 05:00:49 pm
content: Any ohmic (DMM) measurements of Precision Loads?Best,

author: tautech
author_rank: Super Contributor
timestamp: December 07, 2020, 08:23:55 pm
content: Hey Mike, emails direct to you are bouncing back...dunno why.Couple of questions but nothing important.TIA

author: cdev
author_rank: Super Contributor
timestamp: December 07, 2020, 08:46:55 pm
content: Related question-I recently bought a RF resistor advertised as an RF termination.Not precision.I don't expect perfection.Its markedLogo which is a small v inside a larger ohm symbol"250NB"50 (ohms symbol)Does anybody have any experience with these parts or know where I can find more info or a datasheet?

author: mawyatt
author_rank: Super Contributor
timestamp: December 07, 2020, 11:38:34 pm
content: Quote from: tautech on December 07, 2020, 08:23:55 pmSorry, earthlink email server is down all over US.Best,

author: joeqsmith
author_rank: Super Contributor
timestamp: December 08, 2020, 12:16:50 am
content: Quote from: mawyatt on December 07, 2020, 05:00:49 pmTo measure 5 places as you claim to have, I would need to some sort of 4-wire arrangement. I would be concerned about touching the pins with anything other than a mating connector. Then again, I have little concern about their DC performance.Given the choice of a standard that measures 50.0000 ohms and was crap at a 200MHz or a 50.05 ohm that was good out to 6GHz, guess which one I would want to use.With the V2+4, working into the 4GHz, I am more concerned with the return loss which is what was shown.

author: joeqsmith
author_rank: Super Contributor
timestamp: December 08, 2020, 03:09:07 am
content: The following pictures of the standards I had made to demo the original Nano.Cal1: HP34401A with 2 wire, banana to BNC, BNC to SMA to my load.Cal2/3: Making a 4-wire measurement at the backside of the loads SMA.The DC resistance may far exceed the numbers you show but I can tell you this load is not going to be nearly as good at the parts supplied with my V2+, no matter how good their DC resistance is or isn't, at least not above a GHz.SOLT was a snapshot I took from a YT video where they are teaching a class on the nano. If the DCR was perfect on his loads, do you think it would be any good to use at a GHz?

author: mawyatt
author_rank: Super Contributor
timestamp: December 08, 2020, 03:18:33 am
content: Quote from: joeqsmith on December 08, 2020, 12:16:50 amNo risk to the load or need for 4 wire setup. I simply used a banana to BNC adapter, then BCN to N or SMA, and then the load. First replace load with the cal short and set DVM to null and null out residual, then replace short with load and make an accurate DCR load reading. No touching or fiddling with the delicate load pins.Agree 50.05 ohms would be fine, even 50.1 ohms, but starting with almost 51 ohms, then the VSWR minimum for a true 50 ohm measurement will start with a VSWR of 1.02, not 1.000, whereas even if 50.1 ohms were the case then the minimum VSWR would be 1.002.Just seems that starting out near 50 ohms is a better approach and really doesn't cost anything extra,unlessthis is to cover an instruments performance at higher frequencies.Best,

author: mawyatt
author_rank: Super Contributor
timestamp: December 08, 2020, 03:36:05 am
content: Quote from: joeqsmith on December 08, 2020, 03:09:07 amI previous posted just after this, See my notes above inblue.BTW your readings (49.866 and 49.969) are very close to 50 ohms, much closer than what I got with the SAA-2N (49.643) and V2+4 (50.931) cal kits loads. So it seems your loads are in fact starting out very close to 50 ohms, which makes sense, however my V2+4 is almost 51 ohms from the start which raises some questionsBest,

author: joeqsmith
author_rank: Super Contributor
timestamp: December 08, 2020, 04:10:32 am
content: If I could just null and get 3 places beyond the decimal.The problem I see with the short is the center pin rotates. Say you bought a new SMA adapter and start twisting that center pin around in your new part. Worse, you didn't inspect the short to make sure it had no burrs. Now you plug your new load into a connector you just damaged. The damage spreads like covid and soon you are left with a mess.You really want to know how well the standards behave at the frequencies you plan to use them. You won't get that answer with even an 8 place DMM.Even if the parts from my V2+/4 were 51 ohms DC, they would still far out perform that standard I show, except at very low frequencies. The meter is still on, so for fun here is some more DC resistances without nulling the meter:Custom PCB standard short: 0.015 This is the short shown in the previous picture.NanoVNA V2+ standard: 51.021Mini-circuits ANNE: 49.822 Used to replace the original part and shown in that graph I linked.Cheap unknown BNC Ethernet terminator: 49.921I would guess that BNC terminator is the worse of the four for RF work. I wouldn't be surprised to find an axial part inside if I cut it open. But again, if your goal is only to make measurements at very low frequencies, sure. Are you trying to measure SWR at 4MHz or 4GHz? You can get away with a lot below 100MHz. I made this RF circuit, which I am very proud of, to demonstrate this point:https://www.eevblog.com/forum/projects/20db-rf-attenuator-seeking-feedback-to-improve/msg2924286/#msg2924286Skip down to my next post and you can see it running at 300MHz with one small change..*****That picture I showed of the two gentlemen teaching a class on how to use the nano makes sense if the standards they show are for the HF bands. Still, very poor construction like the attenuator I show in that link. Not something I would show other than to make a point.Hope this all helps.

author: mawyatt
author_rank: Super Contributor
timestamp: December 08, 2020, 05:21:44 am
content: Quote from: joeqsmith on December 08, 2020, 04:10:32 amMy notes above inblue.With your V2+ measurement of 51.021 ohms this confirms what I was told why these are centered at ~51 ohms and not 50 ohms. So apparently the V2+ needs a 51 ohm cal to help with the RL at higher frequencies.So for reference if you use a cal load other than ~51 ohms (50 for example) with the V2+ then expect a slightly poorer RL at higher frequencies, and if you use a cal load of ~51 ohms expect a VSWR minimum of 1.02, not 1.000 for an ideal 50 ohm termination.Thanks for the measurements, now I knowwhythese V2+ cal loads are ~51 ohmsBest,

author: joeqsmith
author_rank: Super Contributor
timestamp: December 08, 2020, 12:58:33 pm
content: QuoteThe short of it, I don't. And get this, I don't care. For my home use, getting in the ballpark is good enough.QuoteFor the SMA, the center pin is locked to the cap. They turn together. There is going to be a lot of twisting going on.QuoteIf this was something someone had posted on a forum, post a link. I would like to read the whole discussion.QuoteRetired but still doing chip designs? I must have missed something. No matter.My early home experiments in the >2GHz may have been educational but that's about it. I would be pretending to suggest otherwise.Quote from: mawyatt on December 08, 2020, 05:21:44 amI would like to read this thread. Post a link.I don't know about the embedded firmware for the V2+ but I would expect if I cal the unit to 40ohms and install a 40ohm load, the SWR is going to be 1. You should be able to calibrate the VNA to any standards you come up with and those standards will be the new norm. That said, if the V2+ were designed for say 75 ohms and we are trying to use it with 50 ohms, sure that's a problem.

author: mawyatt
author_rank: Super Contributor
timestamp: December 08, 2020, 03:24:22 pm
content: Quote from: joeqsmith on December 08, 2020, 12:58:33 pmNotes above inblue.BTW I agree that just playing around with this stuff at any frequency at home is fun. Most of my later career was advanced research into SOTA system and chip designs, so was spent doing research, writing papers (mostly proprietary), giving presentations (also proprietary), and doing simulations to acquire funding. The lab was generally off limits since my perceived value added was higher doing the above (and mentoring) and not in the lab. I really missed the "hands on" lab stuff so jumped at the opportunity to get some decent test equipment and play around at home when IretiredBest,

author: dougg
author_rank: Regular Contributor
timestamp: December 08, 2020, 03:31:26 pm
content: Quote from: mawyatt on December 06, 2020, 09:23:31 pmI have the same model. Measured with LCR Pro1 Plus tweezers at 100 kHz and 1 Volt rms, I get 51.08 Ohms on the 50 ohm terminator that came with the nanoVNA. Similar numbers at lower frequencies. The LCR specs claim 0.5% accuracy in the 100 Ohm resistance range @ 1 Volt rms, 0.2% accuracy at lower frequencies. The LCR shows a phase on resistance (between voltage and current I assume) and with the 51.08 Ohm measurement it was showing 0.12 degrees.

author: mawyatt
author_rank: Super Contributor
timestamp: December 08, 2020, 03:48:57 pm
content: Quote from: dougg on December 08, 2020, 03:31:26 pmThese seem to jive with what Joe and I have measured. Also adds credibility to this 51 ohms is intentional and not a sloppy 50 ohms, confirming what was said about helping the V2+ with better higher frequency RL.Thanks for the measurements,Best,

author: joeqsmith
author_rank: Super Contributor
timestamp: December 08, 2020, 06:37:13 pm
content: Quote from: mawyatt on December 08, 2020, 03:24:22 pmMine doesn't have enough of a nub to hold it securely with my fingers or tweezers. I could maybe solder a nut to the backside to hold it with a wrench. At least with the new load, the end is long enough to grab hold of.Quote from: mawyatt on December 08, 2020, 03:24:22 pmCan you provide a link, or just post their response with your original question? There is something curious about the need for it in the first place and then the presented justification. If the Nano was actually designed to with with slight miss match from 50 ohms, that's really odd.You seem to have an interest in SWR, or at least mention it a lot, down to three places. Is it really this big of a deal for you to try and measure SWR this many places out? If so, why?For me, 51-49 good enough IF the part is well behaved over the range I want to use it. If I still played with radios, that bit of error in an SWR reading would be of no concern to me.

author: cdev
author_rank: Super Contributor
timestamp: December 08, 2020, 06:48:53 pm
content: Are there any RF-derived calculations you can remember where an small error ends up multiplying, so ends up being important? With the caveat that there is a chance I am thinking about something else, I am around, say 80-85% sure that at some point a few weeks ago, after I got my Nanovna 2 - while reading about VNAs, I read that that minimal (SWR?) error was highly desirable with some VNA based calculations, (come to think of it I suspect that it was in one of those QEX articles I mentioned previously) because the errors multiplied and would totally throw some (infrequently done?) measurements way off.It might have been in a discussion about the precision available with different hardware means of implementing VNAs or some such? I don't remember!Arrgh.. I hate getting old..I made a mental note to myself to remember the specifics but now I cant remember what thereasonis. That's almost worse than nothing, I'm sorry.

author: mawyatt
author_rank: Super Contributor
timestamp: December 08, 2020, 08:03:54 pm
content: Quote from: joeqsmith on December 08, 2020, 06:37:13 pmSome responses in blueabove.Agree, generally this 49-51 range is fine but I wanted to understand the root reason for the 51 ohms, was this a sloppy load or was it intentional. Now I know the answer. Leaving things "out in the bush" has a habit of allowing Murphy in at some point, and he will bite and bite hard, I know I have "teeth" marks to prove itSo having a cal load of 51 ohms could have some impact on precision measurements at some time, and it's better to know the reasoning behind the decision rather than sweep in under the rug and open one's self up for a possible issue later. Sure I wouldn't rely on the V2+ for any precision work, but might use it to verify another measurement from a precision instrument as a sanity check. When you come from the IC design world you learn to make sure you understand everything possible about what you are doing and what you are using, because a single simple mistake or mis-calculation, or mis-measurement can cost millions of $. Unlike the PCB world where a respin only cost a few $, and takes only a week or so, an IC fab respin can cost millions (10 years ago a 45nm SOI CMOS mask set cost over $4M, think what a SOTA 5, 7 or 10nm CMOS mask set cost today!!) and take many many months to process, so you do your best to keep Murphy out from the get goBest

author: joeqsmith
author_rank: Super Contributor
timestamp: December 08, 2020, 11:28:15 pm
content: Quote from: mawyatt on December 08, 2020, 08:03:54 pmYes, I question your explanation as to why the supplied load resistor would be 51 ohms. There are no facts associated with it. I have no details from the source and you are reluctant to supply it. My guess is they found some 51 ohm parts with a better return loss over the working range of the Nano than 50 ohm parts. I find it far more believable the return loss is better because of the quality of the part they found rather than its DC resistance.Quote from: mawyatt on December 08, 2020, 08:03:54 pmRight, lots of maybes. Little facts. This is why I question your comments.QuoteI find swagging to 1 place beyond the decimal easier than sorting out 3 places. SWR to 3 places out makes no sense to me.QuoteTrue, it's good to understand any tools you use. But we still do not know why the 51 ohm was chosen. I doubt is has anything at all to do with the V2+'s hardware but just the quality of the parts they found.I was thinking about that short. So I made up an end cap and soldered to the back side. Fits a standard 5/16" wrench.

author: mawyatt
author_rank: Super Contributor
timestamp: December 09, 2020, 01:05:55 am
content: Quote from: joeqsmith on December 08, 2020, 11:28:15 pmNotes above inpurple.Best,

author: joeqsmith
author_rank: Super Contributor
timestamp: December 09, 2020, 01:39:35 am
content: Quote from: mawyatt on December 09, 2020, 01:05:55 amI didn't start the thread asking the question and suggesting to know the reason why. No need to get upset because you don't have data or that I question your thoughts on the matter. I acknowledged I didn't know when I wrote " Myguessis they found ..." I made no claims otherwise. Why would I contact anyone. I've stated, I don't have a need to measure to such accuracy.If you are not sure what the designer have explained to you in your private conversation, then why ask in the public forums. Just ask them for further explanation.I am not sure what your picture is showing. Is this what was supplied with your V2+ for a short?? If so, that not even close to what they supplied with mine.Attached looking at SWR for three different resistors. If you think that yellow is the best part, you would be wrong. Red is actually the best of the three. Because SI=SO.

author: xrunner
author_rank: Super Contributor
timestamp: December 09, 2020, 01:47:38 am
content: Quote from: joeqsmith on December 08, 2020, 07:01:33 pmI'm a ham and I wouldn't care. Nothing I doin ham radiowould need to be any more accurate.

author: joeqsmith
author_rank: Super Contributor
timestamp: December 09, 2020, 02:09:24 am
content: Thanks for confirming. I have still have my old Mars SWR meter and I don't think I could use it to read 1.1 squinting my eyes.Capture 2, red trace is the terminator that had the best DC resistance of the four.Capture 3, Zooming in the red and yellow converge at 1.001. The two worse parts of the four. Red is the Ethernet terminator. yellow is my home made PCB standards. V2+ was calibrated using my PCB standard.I thought that the Kirkby video was a straight to the point demonstration. Wonder why so many down votes.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 09, 2020, 02:47:02 am
content: Quote from: mawyatt on December 07, 2020, 05:00:49 pmin VNA world i guess this is mute. you can measure precision in DC (not everybody have your precision kelvin setup) if you want and have the standard swings like aloha dance in GHz range.

author: mawyatt
author_rank: Super Contributor
timestamp: December 09, 2020, 03:05:57 am
content: Quote from: joeqsmith on December 09, 2020, 01:39:35 amNotes above inpurple.Best,

author: joeqsmith
author_rank: Super Contributor
timestamp: December 09, 2020, 03:34:59 am
content: Quote from: mawyatt on December 09, 2020, 03:05:57 amNo doubt the parts are 51 ohms, they offer a better return loss at GHz + compared with the 50 ohm standard supplied with my Nano or that you can use 50 ohms. I don't think we learned much from any of that. My guess is they used the 51 ohm part because the part had acceptable return loss and met their cost goal, nothing to do with the V2+ hardware. Maybe they told you otherwise but I don't know.QuoteJust to be clear, that's the short supplied with my V2+, no home made. I just machined up that nut from some hex stock and soldered it to the back.Here's a relic. 47 ohm load connected to my old SWR meter and home made amp. The needle is hardly off 1:1. I don't remember my antennas ever being tuned this good.

author: radiolistener
author_rank: Super Contributor
timestamp: December 09, 2020, 03:53:39 am
content: Quote from: cdev on December 07, 2020, 08:46:55 pmI bought two used RF resistors on aliexpress. They are used, but looks good. They have the following marking:- 32A1213F FLORIDA RF C0C, measured with multimeter as 49.79 Ohm- G150N 50W4B, measured with multimeter as 50.27 Ohm, VSWR=1.03 @ 200 MHz, VSWR=1.13 at 500 MHzThe price is just 1 USD for resistor. Seller wrote that they are 150W. But I'm skeptic about that, I suspect they cannot survive at 150W.Both have pretty good VSWR. One with FLORIDA RF marking has a little better VSWR, but not much (lost my measurement, so cannot provide exact values at the moment, something like VSWR=1.01 @ 200 MHz).

author: mawyatt
author_rank: Super Contributor
timestamp: December 09, 2020, 03:58:15 am
content: Quote from: Mechatrommer on December 09, 2020, 02:47:02 amIf you follow the thread you will find I wanted some DCR measurements to question the V2+ supplied 51 ohm load, I was concerned this was an out-of-spec load and wanted to convey this to the OEM. The OEM response was this is specified as 51 ohms to improve the RL at higher frequencies. I know that a DCR reading isn't going to hold up at 1GHz, or 100MHz, but likely to have an influence below 10MHz. At 1MHz 1 ohm of inductive reactance is ~160nH, that's a lot of unaccounted for inductance, 2.4K shunt capacitive reactance is ~66pF, that's a lot of unaccounted for capacitance, these are the values to skew the 50 ohms load away from 50 by ~ +-1 ohm. This was the point to start out at near 50 ohms so the LF measurements are more in line and referenced to 50 ohms, not 51 ohms.However, the OEM explanation was that 51 ohms load reference is specified and gives a better HF RL. You can still use the standard 50 ohms loads but maybe with a slightly inferior HF RL.Now that I know this I can make the appropriate load selection when doing a cal based upon what the intended measurements are. So for GP work it doesn't matter, for HF work I'll use the 51 ohm supplied load, for precision LF work I'll use a 50 ohm load with a good DCR (read not +- 1ohm!) that behaves well in the LF area.Anyway, this was the reason for the initial question about the load DCR readings....wasn't sure if the V2+ supplied load was in spec, evidently according to the OEM it was almost spot on, as were the others measured.Best,

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 09, 2020, 06:30:11 am
content: Quote from: mawyatt on December 09, 2020, 03:58:15 ami'm not sure where you get the 160nH and 66pF figure. you are implying 2wL = 1ohm reactance @ 1MHz. thats not what i figured when 1 ohm deviation Load is simulated, assuming perfect zero stray capacitance (1st picture). 1 ohm reactance only achieved past 10GHz assuming on a perfect 50 ohm transmission line medium and about 1mm distance from measurement plane. increasing offset length will make it aloha jump around that 1ohm.j the worst and VSWR is invisibly changed from 1 (flat at maybe 1.05 from dc to daylight). we'll need larger deviation from 50 ohm to make reactance worsen. otoh 66pF shunt stray capacitance however will make it pretty useless as Load standard past few KHz even if perfect 50 ohm resistance and 0ps offset, which is not what we figured out from our provided CAL kit from NanoVNA, even the crappiest Load that i can make.

author: mawyatt
author_rank: Super Contributor
timestamp: December 09, 2020, 02:58:11 pm
content: Quote from: Mechatrommer on December 09, 2020, 06:30:11 amThis is for a measured load, a 50 ohm resistance and a 160nH series inductance should produce a measurement of 50 +j1 ohm result @ 1MHz. A 50 ohm resistance and parallel 66pF capacitance should produce a 50 -j1 ohm result at 1MHz. My point is if I had calibrated to 51 ohms instead of 50 ohms, these measurements would be more inaccurate at this low frequency. 160nH is a large residual inductance and possible with a leaded resistor (~130mm long), 66pF is a large residual capacitance and likely more than stray capacitance of a leaded resistor or test fixture.Anyway I got the answer I wanted about the 51 ohm load (thought this was an out-of-spec part, but now know it is the OEM specified load with the V2+). The V2+ SMA load has a rather large rear can (other SMA load is a typical short flat stubby can), which could indicate other effects (large physical R size?) helping the HF RL, don't know but do know it's spot on 51 ohms from the start.Best,

author: virtualparticles
author_rank: Regular Contributor
timestamp: December 09, 2020, 03:01:26 pm
content: I measured the supplied cal kit using a real VNA in the lab and found the load to have a return loss of about 22 dB. Pretty much unusable. You'll have 3.3 dB of return loss error at 12 dB.

author: virtualparticles
author_rank: Regular Contributor
timestamp: December 09, 2020, 03:09:45 pm
content: This video is preposterous. You CANNOT get good results using a bad calibration kit. Notice how the narrator never actually chose a calibration kit from the VNA menu. Basically the mathematics will force anything you apply to the spigot to match the chosen calibration kit definitions. The uncertainty of the measurements, particularly reflection measurements are ENTIRELY a function of the accuracy of the calibration kit. See the video below for more information.https://coppermountaintech.com/reflection-vs-transmission-accuracy-in-vector-network-analyzer-measurement/Also this first webinar in the "Master's Series"https://coppermountaintech.com/webinar-series/

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 09, 2020, 04:32:44 pm
content: Quote from: virtualparticles on December 09, 2020, 03:01:26 pmi remember chatting with Mr Kirkby that measurement uncertainty is quite high when try measuring Load accurately due to very little reflection signal coming back to VNA. i dont entirely understand what he meant though, or maybe i just explained it wrongly. but i guess... (continue below)Quote from: virtualparticles on December 09, 2020, 03:09:45 pmwhat i understand so far, real care must be taken on characterizing Open and Short standard (especially Open), thats why (older) HP "real" VNA have to support fringing capacitances effect of the Open. where Inductances effect (polynomial terms) is neglected for the Short and esp the Load, there is "none" model for it, not even the actual DC resistance nor offset delay. Kirkby CAL kit also doesnt provide Inductance terms (coefficients) for the Short, only its offset delay/length probably i guess the VNA Kirkby is using (20GHz calibrated HP VNA) doesnt support it. from this i concluded, the accuracy of the Load is lesser critical compared to Short and esp the Open. today's VNA may support Inductance effect for the Short, but i'm yet to see a common practice (in literatures at least) try to model the Load with some fancy stray/virtual elements model. Probably now its called "Arbitrary" load. the most complex model for the Load i can see so far is in Anritsu literature. and reflected in the above snapshots i attached. even that its just a simple shunt capacitance and series inductance, i have to make my own "hybrid" model by combining model for both Open and Short into the Load since some Load, ie diy and cheap version are not ideal 50 ohm at HF, so i need to model them as well even though some VNA (esp cheaper one) may not support it. i guess this is how modern VNA model the "Arbitrary" standard (if without "data-based" measured data) but i'm just imagining, i have no established literature backing me on this.Luckily for Kirkby kit, he provided the S11/S21 plot (data-based) for all of his kit (SOLT) including the Load (not ideal as well past few GHz wrt HP CAL Kit he's using) so i can later reconstruct their coefficients when necessary when a VNA can support the model. having said this, there are countless calibration methods more out there, one of fanciest name is "unknown through" and less fancy is "tee" method, but lets not get into that i havent read about those methods. all of them have math and matrices behind them as a "backing proof".

author: joeqsmith
author_rank: Super Contributor
timestamp: December 09, 2020, 05:16:42 pm
content: Quote from: Mechatrommer on December 09, 2020, 04:32:44 pmGood to know. I support all the HPAK terms except offset loss and delay for both the thru and load with the V2+. He should work with our local friend who helped out with their MatLab scripts to characterize them. No big deal and I think the have added some documentation after I went through it.Quote from: virtualparticles on December 09, 2020, 03:09:45 pmI'm pretty sure that was the whole point. You may think you're getting useful data but yeah, no. Shit in equals shit out. This pretty much sums up my early attempts of playing at higher frequencies. I was up and down converting the VNA, using a home made cal kit with an ideal model. I was impressed with myself, until I learned some basics and how foolish I was.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 09, 2020, 06:07:32 pm
content: Quote from: joeqsmith on December 09, 2020, 05:16:42 pmyou (and VNA) will need to support offset loss term if we want to improve model matching to measured data at high frequency, imho from playing with my tuning tool.Quote from: joeqsmith on December 09, 2020, 05:16:42 pmhis coefficients for Open is quite acceptable (good) up to rated BW, and his measured S11 plot is good for twice the rated BW i paid him. similar to Short with only delay coefficient. and he also provided offset loss parm/value too. maybe he intentionally left the Short coefficients out due to unsupported by most brand name VNA. i'm not sure what tuning tool he is using, but his C coefficients values are satisfactory. he does support and produces cal files (text) format for major brand names like anritsu/HP/Copper Mountain/Tek/R&S but it can be easily created by the VNA owner from his master file. i dont see any one of the brand names supporting L parm in their text format file.but playing with my new tuning tool, without L/C coefficients (leaving them to zeros) ie by adjusting offset delay alone can match measured data closely quite satisfactory up to certain GHz BW, better with inclusion of offset loss adjustment. thats why i guess SDR-kit/Rosenberger came up with tuning tool for offset delay alone. but i guess we'll deviate from true 1 way trip offset delay/length in order to compensate for zero L/C terms ideality (from non-ideality). next step is to see this "virtual"/"elongated" delay effect on VNA measurement accuracy. btw, modelling the Kirkby Load or any other "sloppy diy" standard is several magnitude more difficult than modelling a good (Kirkby's) Open or Short. and with no existence or care about the model of the Load by VNA manufaturers probably imply, building a good Load should be easier than characterizing an Open or Short, or small/slight deviation on the Load from ideal 0 ohm will have less impact on VNA measurement accuracy, or teh said difficulty/uncertainty at measuring accurate S11 of the Load, either one or both , or either i'm wrong at any of these

author: mawyatt
author_rank: Super Contributor
timestamp: December 09, 2020, 06:10:12 pm
content: Thanks virtualparticles for the posts above!!!My point about starting from a good known 50 ohms, even at DC, is better than starting a something other than 50 ohms if you intend to make lower frequency measurements as shown with these measurements.Test setup:NanoVNA SAA2N with SMA Cal kit (load DCR 50.931 ohms)NanoVNA V2+4 with type N cal kit (load DCR 49.643 ohms)Siglent SAA3021X Plus expanded to 3.2GHz and VNA enabled.Another SMA Cal kit (load 49.687 ohms, short stubby type).SMA test devices (SMA loads with long rear extensions) Green 50.882 ohms, Black 51.395 ohms.All DCR measured with new KS34465A and fixture residual R nulled out.SAA2N calibrated with supplied type N cal kitV2+4 calibrated with supplied SMA cal kitexcept when 49.687 ohm SMA used for calibrationSiglent SSA VNA calibrated with SAA2N type N cal kit.Measurements at 10MHz utilizing averaging.Short stubby SMA test load (DCR 49.687 ohms):V2+4 48.7 ohms with RL of -37.92dB delta of -0.987 ohmsSAA2N 50.0 ohms with RL of -60.24dB delta of 0.313 ohmsSSA VNA 50.06 ohms with RL of -60.23dB delta of 0.373 ohmsSMA Green test load (DCR 50.882 ohms):V2+4 49.9 ohms with RL of -54.04dB delta of -0.982SAA2N 51.2 ohms with RL of -38.14dB delta of 0.318SSA VNA 51.27 ohms with RL of -37.85dB delta of 0.388V2+4 51.2 ohms with RL of -38.27dB delta of 0.318 (cal with 49.687 ohm SMA)SAM Black test load (DCR 51.395 ohms):V2+4 50.4 ohms with RL of 46.51dB delta of -0.995SAA2N 51.7 ohms with RL of -35.87dB delta of 0.305SSA VNA 51.84 ohms with RL of -34.66dB delta of 0.455V2+4 51.7 ohms with RL of -35.14dB delta of 0.305 (cal with 49.687 ohm SMA)A test at 100MHz with SMA test load (DCR 49.687 ohms):V2+4 48.7 ohms with RL of 38.21dB delta of -0.987SAA2N 50.0 ohms with RL of -41.69dB delta of 0.313SSA VNA 50.19 ohms with RL of -41.03dB delta of 0.503Note how the SAA2N and SSA VNA readings agree with each other and are more inline with the DUT load DCR value, much better than the V2+4 when calibrated with the supplied 51 ohm load. However theV2+4 also agrees well with the SAA2N and SSA VNA when calibrated with a load that is nearer 50 ohms as shown in blue.Note how the delta between the DUT DCR and the Cal DCR readings track. So it seems that starting from a reference near 50 ohms, the nearer the better, does indeed yield a superior measurement result at lower frequenciesBest,

author: virtualparticles
author_rank: Regular Contributor
timestamp: December 09, 2020, 07:09:36 pm
content: Interesting measurements. I would say this though, any return loss measurements below 35 dB are not terribly meaningful. The uncertainty of reflection measurement is entirely dependent on the return loss of the load standard. A load with 45 dB of return loss would allow VNA measurements of 35 dB with +/- 3.3 dB of accuracy and +/- 1dB at 25 dB. The best load in the world is at NIST and it's 52 dB. To get a better calibration one would have to use TRL where it is possible to get the directivity uncertainty down to 60 dB with a pristine air line.

author: joeqsmith
author_rank: Super Contributor
timestamp: December 09, 2020, 07:11:26 pm
content: Quote from: Mechatrommer on December 09, 2020, 06:07:32 pmThe V2+ is basically in dumb mode when running headless. It's up to the software to handle all of the corrections. There's no need for the VNA to support it when using software to control it. That's true with all my old VNAs.Quote from: Mechatrommer on December 09, 2020, 06:07:32 pmKeysight has their calibration database software you can download for free and play with. There you will find all of the coefficients they support for their various kits. My vintage VNA doesn't support their latest format but the software has conversions built in. The right thing to do for the V2+ would be to add support for all of the coefficients to my model. Even the load delay.I think if you look, they left that one zero for every standard.Yeah, I won't pretend to be able to make a load that will work at 4GHz. My early attempts at an open and short were so poor, Mario's scripts were having problems with them.

author: mawyatt
author_rank: Super Contributor
timestamp: December 09, 2020, 07:34:18 pm
content: Quote from: virtualparticles on December 09, 2020, 07:09:36 pmAfter the holidays hopefully I'll be able to take these devices and cal kits over to the university and work with a real VNA and cal kits. One of my former grad students has offered to help, so likely I'll take him up on thatIt's fun playing around with these things and trying squeeze out more, amazing little devices considering the cost, so hat's off to the designersBest,

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 09, 2020, 07:58:59 pm
content: Quote from: joeqsmith on December 09, 2020, 07:11:26 pmyes this is a good thing.Quote from: joeqsmith on December 09, 2020, 07:11:26 pmor even better if they support "data-based" method, like VNA View SW, we dont have to enter coefficients one by one, just provide s1p files. its wonderfull xoxoxo the original programmer provided it in the first place, its really eye opening to the "next level" since i asked how to use it properly in another thread. this is why Mr Kirkby will not recommend any VNA that doesnt support cal kit coefficients built-in, this means the NanoVNA, and "was" the Deepace KC901V, now Deepace responded to that by providing L and C coefficients input in their latest FW. for NanoVNA, the good thing is it spits raw/unprocessed S11 back to VNA View for post processing, by itself its just a "toy VNA" or another fancy VSWR meter that can do more, if without "near ideal" cal kit.

author: switchabl
author_rank: Frequent Contributor
timestamp: December 09, 2020, 09:10:51 pm
content: Quote from: virtualparticles on December 09, 2020, 07:09:36 pmIt is maybe useful to add that this is usually true for good-quality, characterized calibration kits, as well as generally for low-reflection DUTs. For some of the improvised/home-made calibration kits discussed in relation with the NanoVNA, the residual source match error (relevant with more reflective DUTs) is likely to be set by the phase error on the open/short, as well the return loss of the load.But it is true that, while you can get away with surprisingly much for non-critical measurements, a bad load standard is a no-go. The return loss on a calibration standard really needs to bea lotbetter than the one you are trying to measure. And consequently, if you want toevaluatea calibration-grade load properly, you do need a TRL or equivalent calibration or compare to a precision air line directly.Incidentally, this is in my eyes also the main problem with the cheap Kirkby calibration kits. The load standard is only specified to >32dB RL and even if they managed to source better ones cheap, they might not be able to verify that with their measurement setup.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 09, 2020, 09:43:36 pm
content: Quote from: switchabl on December 09, 2020, 09:10:51 pmfwiw, this is RL derived from my Kirky's Load's profile. for 3GHz NanoV2 usage, it will be better than 40dB. will be glad to see other/Nano's Load's profile. of course this is measured wrt a much precise Cal Standard Kit of his (HP)...

author: mawyatt
author_rank: Super Contributor
timestamp: December 09, 2020, 11:36:38 pm
content: Quote from: switchabl on December 09, 2020, 09:10:51 pmAbsolutely agree. While attending meeting at Keysight's Santa Rosa headquarters just before the terrible fires, we had a discussion with the CTO and the main designer behind the key semiconductor components in the latest high performance SA and VNAs, although we were discussing a new >100dB SFDR >20GSPS DAC and what to "expect" in the spectral results. They said that you need an instrument at least 10dB better than what you are measuring, which requires semiconductor components at least 10dB better than the instrument. They couldn't find chips good enough to meet this criteria, and thus had to design thier own in many cases, and why many of the critical ICs in these advanced instruments are custom designed and fabricated at Keysight. They even have their own special semiconductor fabs for Indium Phosphide (I've design in this >600GHz process), GaAs and other special processes in Santa Rosa. Even had to design special connectors and cables because they couldn't achieve the isolation required (>150dB I recall). All this makes sense as the measurements usually follow a Root-Sum-Square type and you are pushing the SOTA in dynamic range and frequency, so the ICs are also pushing SOTA.BTW earlier version of the mentioned DAC had a measured phase noise better than -168dBm/Hz (output at 0dBm) at 1KHz offset at >1GHz, I know I saw it in 2009!! And this was dual DACs with Dual DDS on a single chip!! There is some really sophisticated stuff in these advanced instruments that most never see or even know about. The clever use of the AD and TI chips in these NanoVNAs is also impressive IMO, especially considered to cost.Best,

author: switchabl
author_rank: Frequent Contributor
timestamp: December 10, 2020, 12:00:17 am
content: Quote from: Mechatrommer on December 09, 2020, 09:43:36 pmWell, that is the point isn't it. On the website they mention they are using an Agilent 85052B cal kit. The included broadband load is >44dB @3GHz. Now that is quite respectable, but not nearly enough to know how good your load really is. Just going by the plot, yours might be 45dB up to 3GHz, quite good isn't it? But what is the uncertainty here? Pretty large actually, the real value might be as low as 38.5dB!Not to say this load will not usually be good enough @3GHz. If you are going to use it @6GHz though, you definitely need to keep the limitations in mind.

author: joeqsmith
author_rank: Super Contributor
timestamp: December 10, 2020, 01:28:22 am
content: Quote from: Mechatrommer on December 09, 2020, 07:58:59 pmCertainly doable on the PC side but I'm not sure how you would get a Touchstone file loaded into a stand alone V2+. I think before I do that, I would want to add interpolation for the calibration as it's something I would actually use.Using Google to search for VNA View software and it comes back with a LabView program. Not thinking this is what you are referencing.http://wb9jps.com/Gary_Johnson/VNAView.htmlPost a link and I'll have a look.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 10, 2020, 04:00:45 am
content: Quote from: switchabl on December 10, 2020, 12:00:17 amif Agilent's Load is only 38-44dB, what do you expect from $4 kit? or even a $500 Kirkby's Kit? 60dB? (dividing equally from pieces i got, the female Load the plot i attached is about $50 price incl characterizing service)Quote from: switchabl on December 10, 2020, 12:00:17 amif the VNA supports data-based Load/Arbitrary standard, and if the math implementation is right and leakage and error terms are taken cared of, measurement should be good, theoritically up to 12GHz (the Kirkby's s1p plot) plus minus whatever the uncertainty is. otherwise if the VNA doesnt support any kind of reasonable Load model, i believe my measurement is as good as any others budget/Deepace/NanoVNA + the provided cal kit. i will compare Kirkby's vs NanoVNA's vs my diy best/worst Load's profile later, for now i have some other tasks slipping in. and i will be outstationed tomorrow.Quote from: joeqsmith on December 10, 2020, 01:28:22 amno! i thought i've seen you used it, and i believe its linked in NanoVNA website (from where i got it), here..https://github.com/nanovna/NanoVNA-QT/releasesfromhttps://nanorfe.com/nanovna-v2.htmlopps i think my bad, not sure its official name from the website, i guess its the NanoVNA-QT? but the program's windows' caption is "VNA View"..so may i ask you a favor in return... which link you are refering to the Agilent's cal data in below comment.. so i can have a lookQuote from: joeqsmith on December 09, 2020, 07:11:26 pmsometime my google-fu is failing esp with lack of sleep and time, and brain cells resources used up during past few weeks typing codes.

author: switchabl
author_rank: Frequent Contributor
timestamp: December 10, 2020, 10:27:09 am
content: Quote from: Mechatrommer on December 10, 2020, 04:00:45 amThat is not what I am trying to say. I am trying to illustrate how hard it is to characterize a calibration standard. The Agilent load is >44dB, which is quite respectable. But the uncertainty in the measurement you posted is still so high it can only show that yours is >38.5dB (below ~3GHz).For what it's worth, I think the Kirkby kit is pretty good value. It's just worth keeping in mind that the load standards are not real calibration-grade loads (that would increase the price _a lot_) and in particular for >3GHz that may introduce significant measurement uncertainties.If you can do data-based calibration, you are in a better place because then you are depending on the stability of the VNA and the load instead, which will be quite a bit better. But unless you are using a modern lab-grade VNA, that likely means offline processing on a PC.

author: joeqsmith
author_rank: Super Contributor
timestamp: December 10, 2020, 12:36:46 pm
content: Quote from: Mechatrommer on December 10, 2020, 04:00:45 amI missed you were talking about the OEMs software. I thought they had a load and save cal was all. It's a pain to run on my PC. I'll fire up the old laptop and have a closer look at what this database is that you referred to.You may find the Keysight software here:http://na.support.keysight.com/pna/apps/applications.htmlQuoteI learned that lesson with the V2+, you need to account for leakage (crosstalk) terms or it falls apart above 3GHz.

author: virtualparticles
author_rank: Regular Contributor
timestamp: December 10, 2020, 02:42:46 pm
content: Quote from: switchabl on December 09, 2020, 09:10:51 pmExcellent point

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 10, 2020, 03:08:52 pm
content: Quote from: joeqsmith on December 10, 2020, 12:36:46 pmyou need to have your cal kit's database ready though. ie their profile plot in s1p files from very well calibrated VNA and Cal Standard. supposedly these profile will be used as transfer standard (if i term it correctly) so your measurement on other VNA calibrated with data-based CAL kit (lets call it secondary or tertiary transfer standard) will be similar to what is measured by the source VNA that generated the data-based files in the first place. if you have say Agilent Cal Kit with their s1p profiles, you may play with that NanoVNA-QT SW. ymmv cheers.Quote from: joeqsmith on December 10, 2020, 12:36:46 pmthats why the good thing that it spits raw data back to PC, so at least we can do/study something about it in PC and math... should be a good learning process given we have time for that. afaik my Deepace only saved out processed data, so i'm not sure how to cancel back its internal systematic error/leakage. maybe later i can play further with it by disabling calibration data altogether so maybe i can get raw measurement data. fwiw.

author: joeqsmith
author_rank: Super Contributor
timestamp: December 10, 2020, 10:44:17 pm
content: Quote from: mawyatt on December 09, 2020, 01:05:55 am"The 51 ohm load just happaned to be the best load we can find in this cost range, and it is not specific to the V2 or V2Plus4."A good book for those who haven't read it.https://www.amazon.com/Death-Expertise-Campaign-Established-Knowledge/dp/0190469412

author: switchabl
author_rank: Frequent Contributor
timestamp: December 10, 2020, 11:08:13 pm
content: I think the answer is probably simple and maybe slightly disappointing: if you take it apart/cut it open, you will likely find a single resistor (hopefully SMT) inside. 51 ohm is a E24 value and widely available. 50 ohm RF resistors exist but are more expensive.Btw if you open up a quality 50 ohm load rated for higher frequencies, you will instead find a tantalum nitride (TaN) or similar thin-film, possibly laser-trimmed, deposited on an RF substrate.

author: joeqsmith
author_rank: Super Contributor
timestamp: December 10, 2020, 11:20:49 pm
content: I had been following this thread which was an interesting read:https://www.eevblog.com/forum/testgear/kirkby-calibration-kit-alternatives/msg1599934/#msg1599934Quote from: Mechatrommer on December 10, 2020, 03:08:52 pmI had seen that it had support for Touchstone format. Is this what you are calling a database? I will do a search and see if I can find some cal files. Anything I have done is based on that Agilent format.Quote from: Mechatrommer on December 10, 2020, 03:08:52 pmI like the original Nano in that I can view and use the touch screen while it's running. Normally, I just clear the cal and pull the data. I heard they had some fudge factors coded into it but it hasn't caused me any problems. Added support for interpretation but need to test it.Occam's razor strikes again. Not sure why that was even a question as it seemed obvious. Oh well.

author: mawyatt
author_rank: Super Contributor
timestamp: December 11, 2020, 12:22:36 am
content: Quote from: switchabl on December 10, 2020, 11:08:13 pmYes 51 and 47 are the standard 5% values round 50, it may even be a leaded resistor since the back has a tall "cap" that covers whatever resistive element they are using1% 49.9 ohm SMD resistors are not expensive, but they chose 51 instead. The 3 independent DCR measurements show this is 51 ohms spot on, so likely they are not using a 5% resistor.I would expect a quality thin film resistor deposited on an alumina substrate in a quality load, but not at this price and performance level. Would also expect this to have a DCR of very close to 50 ohms.I'll have known good references after the holidays when I go to university lab, then I'll be able to verifyIFthe 51 ohms actually does improve the HF RL on the V2+4 like the OEM suggests.Best,

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 11, 2020, 07:00:35 am
content: @joe yes afaik from literature reading, the data-based means using measured data of call kit s11 for calibration. I cant think of anything other than the s1p file... back then, vnas were using model based ie their C polynomials coefficients because digital storage was a premium. Now storing thousands of measured data points is cheap...any modern vna should be able to do it. Its more real representation. Polynomials implementation today is only following the legacy method.@mawyatt, 2x 100ohm smd is cheap

author: Hydron
author_rank: Super Contributor
timestamp: December 11, 2020, 08:27:02 am
content: Mine was also 50.9something, measured using adaptors relative to the short as a zero reference.

author: switchabl
author_rank: Frequent Contributor
timestamp: December 11, 2020, 01:19:13 pm
content: I mean, in principle it is possible to use the parasitics as a matching network so that 51 ohms ends up close to 50+0j ohms at high frequency. But even if you managed to pull that off, it would be quite sensitive, so you might well end up having to fine-tune every load by hand. At 3GHz just building something that has lower parasitics in the first place really looks like a much simpler option.But, it turns out I actually measured the supplied load when I experimented with the NanoVNA v2 (without +) a few months ago. So let's look at some data. The reference was my working load standard, which was selected from a bag of Huber&Suhner 18GHz SMA loads. So not ideal, but should be good enough to get a qualitative picture (incidentally, that one measures almost exactly 50.00ohms DCR on my 34401A).There is always a chance they changed suppliers in the mean-time, so yours may differ. In any case, at low frequencies, it is 50.8 Ohms, so seems to be 51 Ohm as well. Overall performance is however dominated by the reactive part. To get an idea if the 51 Ohm are actually beneficial, I fit a model consisting of a piece of transmission line, terminated by R || C. Now we can compare to, say, R=49.9 Ohm: therealpart of the impedance is indeed closer to 50 Ohm at higher frequencies, but that is not really helpful as the return loss is still worse. Needless to say, the low frequency performance is degraded with 51 Ohm.

author: mawyatt
author_rank: Super Contributor
timestamp: December 11, 2020, 01:36:24 pm
content: Quote from: Mechatrommer on December 11, 2020, 07:00:35 amYes I know, probably not even $0.01 each for 1% SMD resistors. Recall someone did some home-brew loads and found that two 100 ohm SMD performed the best overall with 1, 2 or 3 parallel resistors in these moderate frequency ranges.Best,

author: mawyatt
author_rank: Super Contributor
timestamp: December 11, 2020, 03:56:34 pm
content: Quote from: switchabl on December 11, 2020, 01:19:13 pmUsing the parasitics as a means to achieve a certain goal seems like a recipe for disaster IMO, especially if these parasitics are what measurements are based from since parasitics aren't generally well controlled!Nice plots that show the VNA readings monotonically move away from the DCR value as parasitics come into play as the frequency increases and at very low frequencies look like the DCR value.As a quick experiment I just calibrated with the V2+4 supplied 51 ohm load (DCR 50.931 ohms). Then measured SMA load (DCR 49.687), the VNA reports 48.7 ohms @ 1MHz & 10MHz. This VNA reading agrees with simple Ratio (50/50.931)*49.687= 48.78 ohms and VNA reading*(50.931/50) = 49.61 close to the DCR value. Did a few more values below.DCR NanoVNA 51 ohm cal {Ratio 50.931/50*Reading} NanoVNA reading & {Ratio (50/50.931)*DCR}49.687 48.7 {49.61} 48.7 {48.78}51.395 50.4 {51.34} 50.4 {50.46}50.882 49.8 {50.73} 49.8 {49.95}50.182 49.2 {50.12} 49.2 {49.26}49.643 48.6 {49.50} 48.6 {48.74}So it seems that using a load cal of something closer to an ideal 50 ohms for LF work might improve the measurements around 50 ohms, or at least provide a "hint" at what the DUT might really be.Of course the best solution is just use a quality 50 load and be done with it and not have to worry about all this!!Best,

author: joeqsmith
author_rank: Super Contributor
timestamp: December 12, 2020, 07:48:19 pm
content: Quote from: joeqsmith on December 11, 2020, 01:52:43 pmDigging into it a bit I see both the Cooper Mountain and Keysight manuals refer to it as a database. It looks some of Keysights PNAs support it. Mine is too old.https://coppermountaintech.com/wp-content/uploads/2018/05/Using-a-Databased-SOLT-Calibration-Kit.pdfhttps://www.rosenberger.com/fileadmin/content/headquarter/News/Application_Notes/AN002.pdfI didn't find many details about it. I would guess that there isn't a standard and companies may supply different resolution files. I would guess then that the software has to interpolate the data for the range you want to use. Maybe one kit has data every 10MHz where the other is ever KHz. If you find any detailed documents on it, post a link and I will have a look.For the fun of it, I have added support for both interpolation and offset loss. I doubt it would make any difference but in case you want to attach your $15,000 standards and cables to your Nano, you have the option to enter all the coefficients.

author: mawyatt
author_rank: Super Contributor
timestamp: December 13, 2020, 01:50:56 pm
content: @ joeqsmithIt seems the latest NanoVNA-H4 version is also using a different cal load that is above 50 ohms (50.78). Here's what nano-users Hugin (OEM for the H4) says. Same answer I got from V2 Plus4 OEM HCXQS.https://groups.io/g/nanovna-users/topic/nanovna_h4_calibration_loads/78920711?p=,,,100,0,0,0::recentpostdate%2Fsticky,,,100,2,0,78920711From Hugin,The load attached to the H4 has been re-matched to perform better at higher frequencies. The 49.9 ohm load used earlier performed quite badly above 2GHz, but the new and improved load can get S11 below -30dB at 6GHz. For measurements that require higher frequencies, the new load better. For UHF measurement, I suggest you use the load included with H4 for calibration.BTW the H4 (50.78 ohms) load pictured in the link above does not look like the load supplied with the V2+4 (50.93 ohms) we have, they are not the same SMA loads.The V2+4 loads are supplied from,https://item.taobao.com/item.htm?id=607518859769So it seems the NanoVNA OEM Hugin is recommending the same approach as we hinted, use a cal load closer to 50 ohms for lower frequency work and the supplied higher than 50 ohms cal load for higher frequency work.Best,Edit: Very interesting reference "About the Extra Ohm in 50 Ohm Calibration Loads" from Avian's Blog. Shows the ~51 ohms cal load selection is to offset the lower HF cal impedance and meet an overall better worst case RL over the entire frequency span, but sacrifices the lower frequency RL.https://www.tablix.org/~avian/blog/

author: switchabl
author_rank: Frequent Contributor
timestamp: December 13, 2020, 05:22:34 pm
content: Quote from: mawyatt on December 13, 2020, 01:50:56 pmA likely explanation is that they found one from a different manufacturer with better RF performance and that one just happens to be 51 ohms. Maybe they already had a lot of those or they found a cheap resistor model that works better at higher frequencies but is not available as 49.9/50 ohms (or would cost a few cents more). Hard to know for sure.I would very much assume that if they could get the exact same construction but with a 50 ohm resistor, performance would not only be better at low frequencies, but at the very least no worse at higher frequencies.Quote from: mawyatt on December 13, 2020, 01:50:56 pmThat makes a lot of (questionable) assumptions. If you look at my data, you can also see that 51 ohms results in therealpart of the impedance shifted closer to 50 ohms. What it doesn't do is improve the return loss since that is totally dominated by imaginary part. Again, you could imagine the parasitic C and L forming an L-match to actually get (close) to 50 + 0j ohms, but I don't expect that to be practical and would be very surprised if they could pull it off consistently. It certainly doesn't work that way for the one I have.Quote from: mawyatt on December 13, 2020, 01:50:56 pmThere is the concept of a low-band load, that has very, very good return loss but only at low frequencies (say <2 GHz) and may be supplemented e.g. with a sliding load that only works for higher frequencies. But in this case you would rather look for something that is better across the whole range of interest. I mean it's not like the included one is particularly good at higher frequencies either (at least mine is not).Quote from: joeqsmith on December 13, 2020, 03:25:03 pmI am not entirely sure what you are trying to say. If it is "don't believe something just because a chinese seller or a random blogger says so", I concur.On the more immediate subject matter, the "Handbook of Microwave Component Measurements" by Joel Dunsmore is something I have found quite helpful.

author: virtualparticles
author_rank: Regular Contributor
timestamp: December 14, 2020, 06:58:53 pm
content: Quote from: joeqsmith on December 12, 2020, 07:48:19 pmTo add a "database" for a calibration piece, one just needs the s1p file for the open, short or load, or s2p for the thru. Using the calibration kit menu, one simply defines each of the pieces by these files. Of course these data files must be created by a carefully calibrated and accurate VNA. It doesn't matter how many frequency points are used, but the more the merrier of course. The VNA will interpolate as necessary. As long as the calibration pieces themselves are mechanically/thermally stable with repeatable results, this method gives very good results.One can implement 12 term calibration in Python or MatLab without too much trouble. The mathematics for it is in this slide set:http://emlab.uiuc.edu/ece451/appnotes/Rytting_NAModels.pdf

author: joeqsmith
author_rank: Super Contributor
timestamp: December 14, 2020, 07:48:09 pm
content: Quote from: virtualparticles on December 14, 2020, 06:58:53 pmI could make a Touchstone file with ten points or thousands. I could have one point every GHz or one every 10Hz. I have to imagine the companies making these low cost standards use a common resolution. I would also assume, say it's a 100KHz to 100GHz standard and you want to measure 1MHz to 1.1MHz. You use what ever few points from the touchstone file and maybe fit it. I'll read the paper and see if it covers some of the basic details. I have had no luck with my searches. I was thinking to write Kirkby as I assumed he would use touchstone but I couldn't find where he supports it.I have not seen were anyone is using touchstone for the metrology grade standards, only for the low cost references.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 14, 2020, 09:18:52 pm
content: Quote from: joeqsmith on December 14, 2020, 07:48:09 pmi'm not sure what you are trying to search. even though the VNA Kirkby is using may not support databased cal data, it may produce touchstone (s1p) data for measured DUT i assume. or he may has a tool in PC to do format conversion.Quote from: joeqsmith on December 14, 2020, 07:48:09 pmprobably due to metrology grade VNAs are the old boat anchors that only support polynomials coefficients? if you have friend with Keysight FieldFox unit (more modern VNA), you may ask how they do calibration, using s1p files or still using coefficients? the drawback with databased (s1p touchstone) method is if we are measuring on noisy VNA, such as Nano and Deepace VNA. the noise/error will be reflected in the s1p file and will be brought to other VNA thats using the s1p file for calibration. one of the trick will be to smoothen/averaged the database/s1p/touchstone file with hope to attenuate the noise/systematic error in the measurement.

author: virtualparticles
author_rank: Regular Contributor
timestamp: December 14, 2020, 09:21:32 pm
content: 1601 points is pretty common the the data base file. As long as the cal piece isn't changing quickly then interpolation works well. Rytting is the go-to guy for all things In VNA calibration.

author: switchabl
author_rank: Frequent Contributor
timestamp: December 14, 2020, 10:31:21 pm
content: The most common application of data based calibration is probably e-cals. But you usually don't see the data because it is stored on the unit and loaded automatically. Common mechanical SOLT cal-kits at reasonable frequencies are well-behaved enough that coefficient based characterization is normally fine. Once you get beyond 20-30GHz or so, there is more benefit from data based standard definitions. Mind you, that data doesn't have to be measured data, it often comes from CEM modeling.Touchstone files are the standard for exchanging S-parameter data. VNA manufacturers have their own proprietary calkit files that contain values for several standards along with extra info on type of standards, frequency range, maybe uncertainty data etc. But you should always be able to import individual touchstone files into the cal-kit editor on your VNA if you need to define a custom one.The minimum number of data points will depend on how smooth the data is. And well, there must of course be enough to avoid phase ambiguities (but that is a low bar). There are different ways to go about the interpolation, if you interpolate real and complex parts separately, that is fine but may need more data points than interpolating in polar coordinates. Evaluate by doing a measurement at higher resolution and compare to the lower point-count interpolated version.You should also do multiple measurements and average them.

author: joeqsmith
author_rank: Super Contributor
timestamp: December 15, 2020, 01:21:47 am
content: Quote from: Mechatrommer on December 14, 2020, 09:18:52 pmMin data points, resolution... all the things I've mentioned. Just the basics. For Kirkby, I just didn't see where he offered anything beyond the standard coefficients. If he were doing this as a normal service, I would have asked about the format.Quote from: virtualparticles on December 14, 2020, 09:21:32 pmGood info. I really had no idea. So the thought is what ever the calibration standards are, you would always have 1600 points? Is this documented somewhere or is it just typical of what you have seen?From what you have found, do they commonly provide you with data outside of what the kit is rated for? For exampled 900K to 10.1MHz for a 1 to 10MHz standard?I read the paper but again, they don't provide any details about what the companies who are making the standards are doing, which is really what I was after.Quote from: switchabl on December 14, 2020, 10:31:21 pmThe two papers I linked above led me to believe the primary reason was to reduce cost. Each kit would have it's own baby papers to ensure accuracy.It makes sense that as you go up, this technique would provide better performance.Anything for work I would imagine is e-cals now. I doubt many places are using mechanical standards on their 4-port + VNAs.Quote from: switchabl on December 14, 2020, 10:31:21 pmI am not surprised to hear they would have some sort of custom format.Anyway, interesting topic. I would like to know more about it. If people find materials, please post them up. Know of a good book that covers it, ISBN would be great.Thanks.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 15, 2020, 03:27:16 am
content: Quote from: joeqsmith on December 15, 2020, 01:21:47 amKirkby provided s1p touchstone format spanning from 50MHz to 12GHz (6GHz rated cal kit), besides a master (text) file for coefficients and other text files compatible with various VNA's brands derived from master file. see attached as a teaser, i've provided RL plot for the Load earlier from s1p measured data. whats not shown is countless literatures he included in usb drive (mostly HPAK's that you can easily download (i've downloaded most) from the net) that i've backupped in my external drive. for your other questions, let the persons you replied to answer them. short answer... Kirkby provided 1601 points of what i believe as measured data (averaged maybe) but not derived from modelled data, if they are, my little tool will plot exactly matched between measured and modelled.

author: virtualparticles
author_rank: Regular Contributor
timestamp: December 15, 2020, 02:55:23 pm
content: Quote from: joeqsmith on December 15, 2020, 01:21:47 amOur Automatic Calibration Modules (ACMs, same as eCal) use a 1601 point data base over the operating frequency. Data is only taken over the operating frequency range and it is actually measured with a golden VNA. There are more than 4 calibration artifacts used internally as you can see by the block diagrams. A least squares approach is used to perform calibration in the over-determined system of equations. This is helpful as it improves the accuracy of the calibration somewhat with some random variation in the measurements.

author: mawyatt
author_rank: Super Contributor
timestamp: December 15, 2020, 03:56:06 pm
content: Quote from: switchabl on December 13, 2020, 05:22:34 pmIf you do a little research you will find thechinese selleryou refer to, Hugin is actually one of the original developers of the NanoVNA and also the OEM of various versions of the NanoVNA. The otherchinese sellerHCXQS the designer and OEM of the NanoVNA V2 Plus and Plus4. Therandom bloggeryou refer to is Avian's Blog, I certainly find his blog interesting and informative, maybe not as a reference material likeHandbookreference, but still worthwhile.BTW Dr Joel Dunsmore is a brilliant creative engineer/scientist that has published some excellent references and has numerous patents. I've personally known Joel since 1995 when we both were on the initial advisory board helping create the Center for Wireless and Microwave Information Systems (WAMI) at University of South Florida (USF) where I later was an adjunct. Joel represented HP/Agilent with equipment for the lab, along with Tektronix and Mini-Circuits supplied additional equipment and components respectively, and I represented Honeywell and supplied significant "$ endowments" to help finance the initial lab setup. We worked closely with the WAMI founders Drs Larry Dunleavy and Tom Weller, which later founded Modelithics at USFs research center. WAMI has been a huge success over the years and a credit to Larry, Tom, Joel and other supporters!http://wami.eng.usf.edu/about/mission.htmBest,

author: joeqsmith
author_rank: Super Contributor
timestamp: December 16, 2020, 12:37:21 am
content: Quote from: virtualparticles on December 15, 2020, 02:55:23 pmThanks for the information. I read the document and am a little unclear. Maybe you can help. The document lists a Max number of characterization points at 1601 but is seems to support between 2 and 1601. They also state "The user characterization option is provided for saving new S-parameters of the Module after connecting adapters to its ports.". Does these two statements suggest the user may create a 2 point characterization but the factory data is always 1601?In your particular case, is your process to setup the standard on one of your VNAs (the gold standard) and to use it to measure the ecal with what ever adapters you have attached to it, sweep it just over the range you plan to use it, and store that back into the ecal? Then the ecal is then used on the other non-gold standard VNAs?When you collect the data from the ecal over your range of interest, you perform the least squares fit to the raw data and that fit is what you write back?When you perform the least squares, are you still using interpolation to get back to 1601 points over your range of interest?I did try and contact Kirkby Microwave today to see if they could provide any further insight as well.Thanks again.

author: virtualparticles
author_rank: Regular Contributor
timestamp: December 16, 2020, 02:55:20 pm
content: Quote from: joeqsmith on December 16, 2020, 12:37:21 amthe "Golden" VNA is calibrated using primary standards in a temperature controlled dungeon. The ACM is then characterized and 1601 data points are stored internally for each calibration artifact. for a 100 kHz to 9 GHz ACM, point 1 would be 100 kHz and point 1601 would be 9 GHz."User Characterization" allows one to re-characterize the ACM with an added adapter. This requires a set of mechanical calibration standards with type and gender that match the other side of the adapter. The result is only as good as the mechanical standards which is typically not at all as good as the original ACM performance. I think the user can choose the number of points but 1601 would be advised since more is better. 2 points would obviously not be very good.The Least Squares method is applied during the calibration process. The complex reflection coefficient of each calibration artifact is cubic spline interpolated at each user frequency point against entries in the 1601 point stored table. The set of known vs measured values are used to set up the over-determined system of equations and the 12 error terms are calculated and stored for each frequency point in the sweep.I hope this answers your questions!

author: joeqsmith
author_rank: Super Contributor
timestamp: December 16, 2020, 06:41:09 pm
content: Thank you for the write up.QuoteProblem is I just have more questions.Based on the little bit I had found on primary standards for VNAs, I think Dave needs to interview you about your work as I have a hunch there is some interesting things going on.https://www.euramet.org/Media/news/I-CAL-GUI-012_Calibration_Guide_No._12.web.pdfQuote

author: virtualparticles
author_rank: Regular Contributor
timestamp: December 16, 2020, 06:52:18 pm
content: Quote from: joeqsmith on December 16, 2020, 06:41:09 pmEuromet is the Swiss standards outfit. We generally use NIST over here. The primary standards that we have here in our ISO17025 lab are accredited through NIST, which in practical terms means that we occasionally send them over to NIST for a really expensive checkup. We purchase them and then have them accredited if memory serves. I'm sure it is possible to roll your own standards with precision machining but we don't do that.