Calibration service for VNA? - Page 1
Forum section: electronics / rf-microwave

author: mtwieg
author_rank: Frequent Contributor
timestamp: January 17, 2025, 01:31:57 pm
content: At my work we have a Keysight VNA (E5061B) and an eCal module (N7550A), which are used in a couple formal verification procedures. Those procedures specifically call out that both the eCal module and the VNA itself must be calibrated. That is, both get sent to Keysight every year and receive updated cal certificates.This doesn't make sense to me. If we have a calibrated eCal module and use it, shouldn't sending the VNA itself for calibration be unnecessary? The only thing I can think of which wouldn't be covered by the eCal unit is maybe drift in the VNA's frequency reference (which isn't a concern for us in the first place).In fact, I'm wondering what the VNA calibration service actually does. I dug around on Keysight's website and cannot find any info on what actually gets checked/changed. Basically everything redirects me to information on their calibration kits (which makes sense!).The procedures insisting on calibrating both devices came from engineers no longer with the company, so nobody can give me an explanationedit: maybe I should have put this in the metrology board?

author: stretchyman
author_rank: Regular Contributor
timestamp: January 17, 2025, 01:41:07 pm
content: Never made any sense to me either. You have to cal the VNA before you use it.Just another way to make money I'm afraid.If you have to get an accredited cal then pay up otherwise I wouldn't bother.Ive got a 40 year old VNA and have checked with my 'Calibrated' standards with it and its fine.Mind you its below 2GHz anything above you have to take extra special care of those connectors.J.

author: joeqsmith
author_rank: Super Contributor
timestamp: January 17, 2025, 02:26:26 pm
content: Doesn't your company maintain the reports from Keysight? I would pull the last one as they would contain every test ran. The purpose is to make sure the instrument meets the manufactures specifications. It is not to run a simple SOLT. I would expect as a minimum they would check....Gage all connectors and replace defective partsupdate any softwareclean / battery replace (or any other periodic maintenance)basic functional test of keys/encoders/display/portsinput matchlinearitydistortionsource levelattenuatorsWith my old PNA, they would run the alignment using a power meter. I was hoping to have that system checked when I purchased it but they had stopped all support by then.I suspect this is a foreign concept with the current generation, but we have something called a telephone. You could actually call Keysight and talk (not text) with them about the services they offer. No AI but an actual conversation with a human.

author: Bud
author_rank: Super Contributor
timestamp: January 17, 2025, 03:23:21 pm
content: User calibration only calibrates out (removes) systemic measurement errors, e.g. caused by connections between the VNA port and DUT (in very simple terms). It does not verify the VNA's noise performance, frequency range, attenuator accuracy, clock source parameters, etc etc etc. Those things are verified against manufacturer's specifications at the calibration lab, as joesmith said.

author: mtwieg
author_rank: Frequent Contributor
timestamp: January 17, 2025, 09:21:00 pm
content: Quote from: joeqsmith on January 17, 2025, 02:26:26 pmI certainly hope so. I've asked my test team, waiting on replies...QuoteGood points, in our case we're characterizing passive filters/couplers so level/attenuator accuracy isn't an issue. But for a PA or mixer those would be more significant.I'd be surprised if they actually replace defective components though (except maybe giving a fresh cmos battery).QuoteTried that, they were asking for account numbers and POs, I guess to prove I was a customer, but I don't have access to that info because I'm just a lowly engineer.

author: joeqsmith
author_rank: Super Contributor
timestamp: January 18, 2025, 12:37:15 am
content: I pulled a report from Keysight for one of our old 8753 VNAs. About 21 pages of test data.Typically it seems repairs are part of calibration (assuming OEM, no some third party). Or the company will flag it not repairable when they no longer have parts to service it. I have an old ECAL that was flagged that it could not be calibrated after one of the connectors had been damaged. I suspect Keysight had no more stock to replace it.

author: Pezikon
author_rank: Newbie
timestamp: March 09, 2025, 11:46:37 pm
content: Yeah, you probably should have posted this over in the Metrology section where calibration people roam. I work in the calibration industry and have experience calibrating VNAs and sending them to Keysight for repair and calibration. Keysight calibrates their modern VNAs using their N7800A software. The documentation is freely available online, so you can get an idea what the calibration entails.https://www.keysight.com/us/en/lib/resources/software-releases/n7800a-test-management-environment-software.htmlhttps://helpfiles.keysight.com/calsoft/N7842A/Help/N7842AWebHelp.htmhttps://helpfiles.keysight.com/calsoft/N7843A/Help/N7843AWebHelp.htmhttps://helpfiles.keysight.com/calsoft/N7840A/Help/N7840AWebHelp.htmA calibration procedure verifies warranted specifications. Look at the datasheet for a network analyzer, such as your E5061B, and you will see warranted specs like:Frequency AccuracyDynamic AccuracyOutput Level AccuracyOutput Level LinearityNoise FloorCrosstalkOutput Power RangeTrace NoiseUncorrected System Performance (raw directivity, raw source match, raw load match, reflection tracking, transmission tracking)A cal lab will verify all these specs. They will not verify specs identified as "typical" - these are not warranted specs. You might wonder how important these warranted specs are considering your e-cal module is doing the business - error correcting. The only thing an e-cal module can do is compensate for Uncorrected System Performance. And even then, the worse off the uncorrected system performance is, the less effective cal kit error correction is. That is, poor uncorrected system performance results in poor measurement uncertainty. In fact, worse anything, worse dynamic accuracy, worse trace noise, worse frequency accuracy .. all this can contribute to worse measurements uncertainty.If frequency accuracy is bad, the VNA will associate cal kit coefficients with the wrong frequency (however minor).If dynamic accuracy is bad, insertion loss and gain measurements are bad.If noise floor is bad, you don't have as much dynamic range as you thought you did.If output level linearity is bad, your power sweep is bad.If trace noise is bad, then you have noise in your error correction and noise in your measurements. You'll have to use more averaging to control it which increases sweep time.What's been described is sometimes called "instrument calibration" or "instrument verification". This is only to differentiate it from "system verification". System verification involves the use a verification kit with four passive components that try to challenge the VNAs measurement capabilities. A long time ago, system verification was the only way calibration was done (see model 8510). Then later, routine calibration included a combination of instrument and system verification (see model 8722). Anritsu still calibrates their VNAs this way. Then later, Keysight's routine calibration transitioned into being only instrument verification. System verification is still an option - it's described in Keysight service manuals and accessible from the Keysight VNA firmware service menus. But Keysight cal labs don't perform system verification anymore.Lastly, not all calibration labs are created equal. Some lesser cal labs will only verify some of these specifications. Maybe because they lack the equipment, the expertise, the time, or access to automation. For example, Keysight will only sell a license to use their calibration software under certain circumstances. A cal lab might just do system verification - after all, there's precident for it.