Books or resources on VNA fixturing and calibration? - Page 1
Forum section: electronics / general

author: Andrew_K
author_rank: Contributor
timestamp: June 26, 2022, 01:31:32 am
content: I wouldn't say I'm a complete noob when it comes to using VNAs, but I'm reaching a point where I want to care about precision.Does anyone have recommendations for books or articles on the theory side of how calibration works, and how to design good fixtures? Google isn't giving me anything that seems particularly useful.I'm trying to measure some rather unique components, and I'm realizing that alligator clips may not be adequate :pI would like to better understand how calibration works so I can make better decisions around fixture design and measurement process.

author: Bud
author_rank: Super Contributor
timestamp: June 26, 2022, 04:45:01 am
content: https://www.amazon.com/Handbook-Microwave-Component-Measurements-Techniques/dp/1119477131You can also google for Agilent VNA Application Notes, they are great publications.

author: virtualparticles
author_rank: Regular Contributor
timestamp: July 07, 2022, 07:48:55 pm
content: I wrote this about measurement accuracy and uncertainty. Hopefully this answers most questions.https://coppermountaintech.com/introduction-to-the-metrology-of-vna-measurement/Best,Brian

author: virtualparticles
author_rank: Regular Contributor
timestamp: July 26, 2022, 12:13:18 pm
content: I could add a few things to this topic. A VNA makes measurements in a 50 ohm (or 75 ohm) environment. It is expected that the connection between the ports and the Device Under Test (DUT) is a 50 ohm characteristic impedance coaxial interface. (Let's not get into waveguide at this time). Calibration is performed at the end of the coaxial cable by various means but always using standards which are well known and characterized so the VNA knows what the measurement of each piece should be. It is pretty much impossible to create open, short and load calibration standards that will work with alligator clips unless the frequency is limited to 10 MHz or so.To be really clear, if you calibrate to the ends of a pair of test cables and then attach a short "Pigtail" coax with the other end stripped to expose 0.1" of center conductor, you can use port extensions to fix the phase and delay error and move the calibration plane to the end of the pigtail. All well and good so far, but that 0.1" of exposed conductor is no longer surrounded by it's coaxial ground and is now an inductor equal to about 22 nH per inch. In this case, 2.2 nH for 0.1" of exposed conductor and another 0.1" to get back to the ground shield for a total of 4.4 nH. That will have a reactance of 50 ohms at 1.8 GHz, so that is the bandwidth of the measurement capability of the cable and pigtail arrangement.When I designed fixtures, I had an SMA connector which attached directly to a "Pogo pin" which itself was contained within a coaxial hole in a brass fixture. The ground surrounding the pogo pin was spaced to create a 50 ohm characteristic impedance WRT the pogo pin. The pin itself protruded from the opening somewhat but was compressed flush with the opening when pressed against the DUT interface. This maintained 50 ohms all the way to the DUT. It was then possible to calibrate to the ends of the test cables and use port extensions to the DUT interface by measuring a short in the fixture and adjusting the port extension delay until 180 degrees is seen in the S11 and S22 phase measurements.Hope this is helpful.Best,Brian

author: Andreas
author_rank: Super Contributor
timestamp: July 26, 2022, 08:15:47 pm
content: something like this?https://www.gigaparts.com/testboard-kit-for-nanovna-analyzers.htmlwith best regardsAndreas

author: IconicPCB
author_rank: Super Contributor
timestamp: July 27, 2022, 06:28:45 am
content: lookiehttp://www.doe.carleton.ca/~nagui/Appnotes/appnotes.html

author: virtualparticles
author_rank: Regular Contributor
timestamp: July 27, 2022, 12:53:54 pm
content: Quote from: Andreas on July 26, 2022, 08:15:47 pmThe 50 ohm tracks on the top and bottom with places to solder in components look pretty good, particularly for that price. The end-launch connectors might only be good to 3 GHz if they're the economy grade which I suspect they are. You could obtain better ones that would fit the board if necessary. To get the right phase, you'd need to use port extension but I don't know if the Nano has that capability. Nano phase measurements are suspect to begin with though due to the awful calibration kit that comes with it. Return loss measurements less than 10 dB have no accuracy at all due to the terrible load standard as well. That being said, the device is pretty handy for the amateur experimenter for the price.

author: virtualparticles
author_rank: Regular Contributor
timestamp: July 27, 2022, 01:18:52 pm
content: Quote from: IconicPCB on July 27, 2022, 06:28:45 amGreat list!There is also this:https://coppermountaintech.com/technical-library/andhttps://coppermountaintech.com/videos/andhttps://coppermountaintech.com/webinars/

author: kc0ngu
author_rank: Contributor
timestamp: September 06, 2022, 03:53:38 am
content: Not sure if you found what you needed in the resources already suggested, this maybe worth a look, although it is typically textbook expensive, I have it and have found it very useful so far:https://www.amazon.com/gp/product/1630816000?ref=ppx_pt2_dt_b_prod_imageHope it helps your search.Steve