Questions about VNA Calibration - Page 1
Forum section: electronics / rf-microwave

author: M3K33L
author_rank: Contributor
timestamp: June 11, 2025, 02:03:24 pm
content: Hello! I've gotten to the point where I'm calling in the experts. At work, we recently purchased a quality Siglent VNA as a big upgrade from the LibreVNA. I am having an issue where my calibrations produce loops around the smith chart for open and short. I know from reading some of Dr. Kirkby's posts that a short arc is reasonable, but multiple loops seems excessive. In my head, the ideal outcome is a single point at short, open, and load. Using the same adapters, cables, etc. on the LibreVNA gets me that single point. My question is: what am I doing wrong?I believe my issue lies in calibration coefficients, but I'm not sure where to implement those. My path from the VNA to my DUT is: Ntype to SMA adapter -> SMA to SMA cable -> SMA F to F adapter -> SMA to SMP cable -> PCB with SMP connectors -> Gold wire bond to DUT chip. Because I want my reference plane to be on the chip, I've designed TRL standards for the substrate we're using.However, I'm confused as to where I should be inputting calibration coefficients. Should I get an SMA cal kit with known coefficients and calibrate at the end of my SMA cable, then port extend to the end of my SMP cable, then de-embed to get the reference plane down to the DUT? Should I just be calibrating straight away at the DUT despite not having exact coefficients? (I can get some from simulation if I need to) If the VNA is already analyzing the delay and amplitude change, why are calibration coefficients even needed?Sorry if I'm rambling, but this has been haunting me.A bit more detail: Even when I put a generic SMA cal kit (the one you get with a LiteVNA) on the Ntype to SMA adapter, I still get the smith chart loops. I'm including images of these loops and of some of my setup. Also, everything has been torque wrenched together with 8 N-m.

author: tszaboo
author_rank: Super Contributor
timestamp: June 11, 2025, 02:14:37 pm
content: You want the calibration to happen as close to the DUT as possible. If that's not possible then do a port extension.If you have loops that tells me that you didn't enter port extension correctly. Which is not that easy to do, unless you measure your cable, or it comes with a calibration.When I do antenna matching on a PCB, I order extra PCBs and solder my cal kit directly on that PCB.Also I'm just going to assume you meant 0.8Nm...

author: M3K33L
author_rank: Contributor
timestamp: June 11, 2025, 02:20:04 pm
content: In that case, would you recommend doing the SMA cal at the end of my longer cable and port extending with a known length of my SMP cable? The SMP cal kits are crazy expensive. Regarding soldering the cal kit on PCB - what cal kits do you get that can be soldered like that? I am curious. Also, I'm still unsure about why a calibration wouldn't perform well with the standard right at the VNA port.Whoops, yes, I meant 8 in-lbs = 0.9 N-m.

author: tszaboo
author_rank: Super Contributor
timestamp: June 11, 2025, 03:27:46 pm
content: Quote from: M3K33L on June 11, 2025, 02:20:04 pmThe end of the SMA cable would be better. On the PCB it would be even better.On the PCB, I use Vishay FC series resistors. I tried regular 0402 in the past but I got really inconsistent results at my frequency of interest, which is 1.5, 2.4 and 3ish GHz. Open is no component, short is copper tape soldered in place. I basically make the layout to have a test port, and it's within like a mm from the matching network of the antenna, everything kept at 50 Ohm. But I've seen others solder semi rigid coax in place.I prefer the detachable cable method, because I can just grab 3 PCBs and turn them into a calibration kit once, and I don't have to touch the same PCB twice.I don't know what frequencies you use though. You might need to use special SMA connectors that are screwed in place, and not soldered. Maybe have a look at those.

author: M3K33L
author_rank: Contributor
timestamp: June 11, 2025, 03:35:25 pm
content: That is similar to what I've already tried. In fact, if you look at the third image I posted, you can see a PCB that I used as a calibration PCB using an RF resistor. There must be something that I'm missing. Could you walk me through your calibration steps when using a PCB as your cal kit?Thank you, by the way. I appreciate your quick responses.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 11, 2025, 04:11:55 pm
content: Sorry but not owning any Siglent equipment, I don't think I can be of much help.That said, the only way you are going to get your perfect dots you describe is if you use the ideal model for the standards. I often use this model with the low cost VNAs. To improve the measurements, the standards have to be characterized. This data is then entered into the VNA. When you perform a calibration with those coefficients, you will no longer see a perfect dot.QuoteI assume that you designed those to fit with what ever the VNA supports and you had a way to characterize them?I am curious, if you just use a standard SOL rather than TRL and use the ideal model, does it work as expected? It should.

author: szoftveres
author_rank: Regular Contributor
timestamp: June 11, 2025, 04:31:24 pm
content: On this particular model, entering zero coefficients for an "ideal model" produces no trace, likely there's a division by zero somewhere in the calculations. The workaround is simple, just enter some negligibly small numbers instead of zero (0.001pH, 0.001fF, etc..). Haven't heard about a fix yet.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 11, 2025, 04:44:02 pm
content: Quote from: szoftveres on June 11, 2025, 04:31:24 pmI ran into that when the hams would try and use the software I wrote for the low cost VNAs. They did not understand the math and by word of mouth of what their buddy did with some other VNA, would start plugging in zeros. You could point out the polynomials to them but I think that goes beyond high school mathTo avoid it, I also added an offset as there was no way to correct the root problem and I grew tired of them asking.Lucky for me, few people have claimed their were problems when using waveguides.

author: M3K33L
author_rank: Contributor
timestamp: June 11, 2025, 05:19:15 pm
content: Quote from: joeqsmith on June 11, 2025, 04:44:02 pmSo, for the sake of me getting decent measurements, it sounds like I need to:1. Get a nicer SMA kit and input the parameters, or input low values and treat my cheapo ones as "ideal".2. Calibrate at the end of my SMA cable with the standards.3. Characterize my PCB cal kit with my VNA acting like a "golden" VNA.a. Extract the S1P file and get delay, coefficients via excel or software like METAS.4. Use the PCB cal kit as my ultimate cal kit from then on - in other words, begin my cal with all cables hooked up and my SMPs plugged into my PCBs.Am I on the right track?

author: joeqsmith
author_rank: Super Contributor
timestamp: June 11, 2025, 05:51:44 pm
content: Quote from: M3K33L on June 11, 2025, 05:19:15 pm1. As a general rule, better standards are not going to hurt.2. Always cal to what ever you are going to use for a reference plane. If you can't, there is port extension (fudge it in math)3. This may be a shit in = shit out. I did something similar with the low cost VNAs. Borrowed a set of metrology grade standard for a reference. I used METAS to get the coefficients. It all worked well but in the end, the cheap VNA was the limitation, not the standards.1-3, I am not sure if you can buy a good TRL standards (PCB, not airline) and how you would use that to characterize your home made PCB TRL standards.Again, I would just start with a simple SOL and make sure that works. I made up some sections of coax to play with my Agilent's TRL. Maybe something like this would help cement the basics before trying to move to your setup.Not having a known reference could be a problem, depending what you are after. Does Siglent offer any kind of TRL demo board with artwork that you could use as a starting point? If not, you could check with some of the other manufactures. Doing a quick search...https://www.renesas.com/en/document/apn/874-using-trl-calibration-board-f0480-matched-broadband-rf-vgahttps://coppermountaintech.com/wp-content/uploads/2018/05/Design-and-Fabrication-of-a-TRL-Calibration-Kit.pdf

author: M3K33L
author_rank: Contributor
timestamp: June 11, 2025, 06:01:48 pm
content: Regarding the TRL that I'm ending up at: I based my designs off of the second link you provided. They are shown in the images of this response. I'm going to test out a simple SOL with ideal coeffs and see how that goes. Thank you again.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 11, 2025, 06:56:27 pm
content: When I attempted my poor man's coax (not air line) TRL standards, after, I measured some known devices. It was good enough to see the math worked but not really useful beyond that. Your screen shots show sweeping from 10MHz to 8.5GHz. Off the cuff, I was no where near as low as you are showing. And, for the low frequency that coax was very long compared to what you are showing. See the following link:https://www.eevblog.com/forum/testgear/agilent-e835xa-pna-vna-notes/msg3604844/#msg360484410MHz, I would expect a few feet? I certainly could be wrong but I think your calculations for your TRL standards are off for what you are trying to do. Would you be comfortable going over your calculations here in detail?***Don't you dare laugh at my poor man's standards!!!

author: G0HZU
author_rank: Super Contributor
timestamp: June 11, 2025, 07:50:31 pm
content: There's obviously something wrong in terms of how you have defined the reference plane or maybe how you have tried to calibrate it.QuoteIf you just want to get reasonable results, then maybe consider using the port extension feature to improve things. However, looking at your test setup, using a port extension to make up for your SMP cable and adaptors is risky for various reasons. I've been using VNAs all my career and it's a long time since I used the port extension feature. It's OK for fairly short extensions but a modern VNA like yours should be able to offer a better alternative.I would always use the fixture simulator feature on a modern VNA like yours for example. But maybe you should try and get close with the (ancient and crude) port extension method first. Then I would recommend you abandon the port extension method and look at the modern alternatives your VNA can offer.

author: M3K33L
author_rank: Contributor
timestamp: June 11, 2025, 08:03:12 pm
content: joeqsmith, I'm impressed by the homemade standards. Don't mind the 10MHz, that was just because I was being rapid. The TRL standard shown in the images is made for 1-8GHz. With Keff=5.8276, I get a 20 degree phase shift at 1GHz and a 160 degree phase shift at 8GHz with a Line of length 6904um. My devices are on silicon with a silicon oxide layer, patterned with aluminum. I used the MW101 excel sheet for these values... I'm going to check them in Ansys ASAP. I also used a reflect standard that is less than lambda/20, so I used a 640um short. I suppose I could have used 1.2mm as well by that metric. My thru was as short as possible given my chip size, and that came out to be 4700um.If any of this seems wrong, please let me know!G0HZU, I'll try and read up on the fixture removal features. Thank you.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 11, 2025, 09:12:50 pm
content: Ok, 1G, not 10M. That makes much more sense. I also used that spreadsheet, so no problem there. I think I may have used 50 - 500MHz to minimize other errors. That's why my sections are so long. My experience in the 8GHz region is very limited. Seems that MW101 had a good article about the TRL that went along with their spreadsheet. You may find some gold nuggets in there.

author: M3K33L
author_rank: Contributor
timestamp: June 12, 2025, 01:42:32 pm
content: So, I tried adding an "ideal" cal kit into my VNA and it gave me the dots that agree with the cheaper VNA. Of course, this is likely not an accurate calibration, so I feel the need to continue investigating further.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 12, 2025, 02:01:56 pm
content: Right, but at least you know the VNA is fine.Next, lets assume for now that your home made standards are good. How did you plug the data from the MW101 spreadsheet into the Siglent?My vintage Agilent has what they call the cal kit editor. I can use that to create a new calibration kit or modify an existing one. See attached. I assume the Siglent has something similar. Did you double check to make sure you entered the data correctly? This is something I would screw up! lol.***My Agilent also allows you to edit/create the calibration standards within the VNA itself. Instead I find using the PC and uploading them is easier.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 12, 2025, 02:09:49 pm
content: I doubt helpful but something like this:https://helpfiles.keysight.com/csg/pxivna/S3_Cals/Multiline_TRL_Calibration.htm

author: M3K33L
author_rank: Contributor
timestamp: June 12, 2025, 02:31:11 pm
content: Ah, I still need to fabricate the cal standards that match my updated DUT, so I'll hopefully do that soon. Siglent does have a cal kit manager. I do have a question though: for the Line and Thru standards, the delay is supposed to be from port 1 to port 2. But, on the Reflect standard, the delay should be the time for the wave to reach the end of the waveguide and return, rather than just the delay to reach the end, right? It sounds like a silly question, but I want to make sure.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 12, 2025, 03:03:07 pm
content: If they support the Keysight standard, you could try comparing Keysight's published data against what Siglent shows. If Siglent offers a TRL demo board, you could maybe scale the photos. If you just want a guess, I would think round trip.I think any specific questions you have about the VNA, firmware, software, you will be far better served asking Siglent directly. I assume they have some sort of support line or email.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 12, 2025, 03:20:47 pm
content: Doing a quick Google search for Siglent, I came up with this. I did not read it but may be of some help.https://www.siglenteu.com/wp-content/uploads/2023/04/RF-measurments-using-VNA-Rev-8.pdf

author: tautech
author_rank: Super Contributor
timestamp: June 29, 2025, 04:06:28 am
content: Quote from: szoftveres on June 11, 2025, 04:31:24 pmInsert is to add another or unlisted Cal kit and Edit is for tweaking it.Add any name you like for later reference to find it.Here's some S11 Cal screenshots quickly done with the 4.5 GHz Siglent F603FE Cal kit at the end of a 600mm N-SMA cable and done on the same VNA as yours.One screenshot shows a much shorter Smith sweep with the Open removed vs it attached.As far as tweaking the port extensions and such the SNA5000A is pretty powerful yet above my paygrade.I'd put member hendorog onto this thread but he's overseas for a couple weeks and he can give good advice as he's very familiar with these models and has this unit a lot of the time.If you want a better Cal kit you might like to check out his shop:https://vnalab.net/

author: khach
author_rank: Contributor
timestamp: June 29, 2025, 10:31:23 am
content: Probe station VNA calibration substrate like thishttps://www.mpi-corporation.com/ast/mpi-rf-probes-accessories/mpi-rf-calibration-substrates/Check probe pitch before ordering!

author: Coppo76
author_rank: Newbie
timestamp: September 18, 2025, 12:40:42 am
content: If you see a dot on the Smith chart after calibration, it means the correct correction data has not been entered. The number of turns on the chart depends on the length of the standard and the capacity of the aperture.

author: M3K33L
author_rank: Contributor
timestamp: September 18, 2025, 01:29:06 pm
content: Hello everyone, following up this post with my results.I purchased some of user hendorog's VNALab SMA standards, which have been working well. I also have dialed in my probe standard coefficients and calibration procedure - those work beautifully now. Still dialing in the PCB TRL, which I've created in two versions:1. TRL by directly connecting the PCB traces to each other. Should be much more straight-forward since no gold wire bonding is involved.2. TRL by wirebonding my TRL chips into PCBs then using those as the calibration standards. So far, the probe data for these TRL chips is promising.Thank you all for your help. It's been very useful.