VNA Cal Kit Coefficients  - Page 1
Forum section: electronics / rf-microwave

author: SparkFly
author_rank: Regular Contributor
timestamp: August 16, 2017, 12:08:19 pm
content: Dear all.I have recently managed to purchase a VNA (HP8753D), however it did not come with a cal kit. I have access to some higher spec VNAs and thought it would be fun to experiment with making my own cal kit. However, I am quite puzzled by what the exact cal kit coefficients are. I have tried downloading a few files from Keysight, but I am certainly doing something wrong as when I inspect them with something like notepad++ I get a list of NULLS. I cannot seem to find any more detailed information regarding how to go about measuring the pieces and then creating my own cal files.Does anyone have any experience in this matter and would be willing to offer some pointers? Or any ideas where I should otherwise be looking for information?Many thanks for your time.SF

author: w2aew
author_rank: Super Contributor
timestamp: August 16, 2017, 12:26:24 pm
content: Cal kit coefficients are polynomial coefficients that model the fringing effects (capacitance and inductance) that describe the effects of these parasitics vs. frequency. They are not easily determined. They're often derived via 3D electromagnetic simulation or with painstakingly precise measurements. Keysight does have a cal kit tool that allows you to open the cal kit files and examine the coefficients.

author: SparkFly
author_rank: Regular Contributor
timestamp: August 16, 2017, 02:06:14 pm
content: Many thanks for your reply W2AEW. I am a very big fan of your videos.How precise are we talking with these measurements? I doubt I'd be able to accurately 3D model my built opens/shorts/loads, so if any means is open to determining the coefficients it would have to be through measurements I think. Would an instrument such as a PNA series suffice?I will search for the keysight cal kit tool, many thanks for the heads up on that.

author: rfeecs
author_rank: Frequent Contributor
timestamp: August 16, 2017, 04:58:25 pm
content: The procedure would be:Measure the S-parameters of your standards.Make a circuit model for the cal standard and fit the simulated S-parameters to measured S-parameters.Then mathematically derive the coefficients by doing a curve fit.So your accuracy of the coefficients will depend on how accurately you measure the S-parameters. This in part will depend on the high quality cal kit you use. There are various levels of cal kits. Cheap and dirty is open/short/load. Next level is a high quality airline sliding load kit. Then TRL, which only depends on airline impedance. Nowadays you can throw in electronic cal kits like Keysight eCal, which may be the best of both worlds, fast and accurate.Here are a few references from Keysight:"Specifying Calibration Standards and Kits for Keysight Vector Network Analyzers"The appendices have derivations of the cal coefficient models.http://literature.cdn.keysight.com/litweb/pdf/5989-4840EN.pdf"Electronic vs. Mechanical Calibration Kits: Calibration Methods and Accuracy"http://literature.cdn.keysight.com/litweb/pdf/5988-9477EN.pdf

author: SparkFly
author_rank: Regular Contributor
timestamp: August 16, 2017, 05:20:21 pm
content: Wow thanks alot rfeecs, brilliant post. The references are great too, exactly what I'm after! I'll digest them slowly. :]many thanks again.

author: G0HZU
author_rank: Super Contributor
timestamp: August 16, 2017, 07:56:44 pm
content: If you don't mind buying something then also consider the book below:https://www.amazon.co.uk/Handbook-Microwave-Component-Measurements-Techniques/dp/1119979552Dr. Joel Dunsmore designed a lot of the stuff in HP/Agilent VNAs and I think there is a section in the book on how to make your own VNA cal kit on a budget. I assume this will be a SOLT kit but I haven't read the book. I keep meaning to buy it though...I made my own SOLT kit many years ago for my old HP8714B using SMA connectors and it worked very well for me. However, it only performs well up to about 3GHz.

author: SparkFly
author_rank: Regular Contributor
timestamp: August 16, 2017, 08:25:55 pm
content: Thanks G0HZU, I've managed to find a copy online (legally) and that looks to be a very very useful book. It really does provide a nice example with a pcb cal setup.Appreciate the heads up on that.

author: Bud
author_rank: Super Contributor
timestamp: August 17, 2017, 01:25:18 am
content: Quote from: rfeecs on August 16, 2017, 04:58:25 pmIn order to measure an unknown cal standard he needs a known cal standard which he does not have.QuoteThey are not cheap at all. And not dirty at all. I do not understand your statement.QuoteI do not think 8753D supports eCal.

author: SparkFly
author_rank: Regular Contributor
timestamp: August 17, 2017, 12:34:53 pm
content: I have been a bit unclear. I have access to an ecal kit, along with the PNA series analyser. I also believe rfeecs was talking in relative terms about cheap and dirty. After a bit of googling I found that a typical ecal will cost around 10 times a 'cheap' o/s/l kit. Dirty is also somewhat relative. :] Let's not be too harsh on one another.Regards,SF

author: hendorog
author_rank: Super Contributor
timestamp: August 19, 2017, 12:23:24 am
content: Since you can access a good (e)Cal kit then you could try using this procedure to derive the coefficients:http://www.qsl.net/in3otd/electronics/VNA_calkit/calkit.htmlThe key point being the script which uses an optimisation library to fit the S-param file which is extracted for each of your diy/purchased standards from the calibrated VNA.I've been thinking that this procedure could be used to transfer VNA calibrations - i.e. ' VNA Calibration Club'.Would require good quality connectors to be purchased of course.OSL brand name connectors are reasonably cheap and just need to be measured on a calibrated VNA for a full set of coefficients to be determined.