VNA calibration kit effect on measurement accuracy - Page 1
Forum section: electronics / rf-microwave

author: prutser
author_rank: Regular Contributor
timestamp: July 06, 2023, 02:10:01 pm
content: I am wondering what the effect of an 'arbitrary' VNA calibration kit is on measurement accuracy. Assuming we measure only to let's say 6GHz. Is the effect of the open/short and load in an SMA connector that much that we get big measurement errors ?e.g. what error to expect.Anyone experience what the effect is on using an arbitrary calibration kit and ignoring any characterization data ?

author: radiolistener
author_rank: Super Contributor
timestamp: July 06, 2023, 05:11:35 pm
content: Quote from: prutser on July 06, 2023, 02:10:01 pmMeasurement error will depends on calibration kit error. For example if your calibration kit has 200% error at 6 GHz, then your measurement will be about 200% error or even worse.VNA just compare your DUT with calibration kit.

author: nctnico
author_rank: Super Contributor
timestamp: July 06, 2023, 05:38:10 pm
content: Quote from: radiolistener on July 06, 2023, 05:11:35 pmAgreed. AFAIK the most critical component from a calibration kit is the 50 Ohm terminator. I have seen some really poor behaviour at a few GHz from cheap 50 Ohm terminators. So a quick check for a calibration kit is to compare the 50 Ohm (load) to a high quality 50 Ohm terminator from an A-brand bought from a reputable seller.

author: switchabl
author_rank: Frequent Contributor
timestamp: July 06, 2023, 06:43:13 pm
content: Yes, in a proper SOL kit, the load is almost always the limiting factor. But if we are really talking about random, uncharacterized parts, then all bets are off.E.g. I have an Anritsu open with 17.83mm offset. If I were to combine that with a flush short (instead of the matching offset short), I could actually getunboundederror. Even if you knew the proper coefficients, the combination wouldn't work up to 6 GHz (because at some point the open will look the same as the short).So you need to have at least some idea what the offsets of the open and short are. Otherwise, even with high quality parts taken from an expensive cal kit, it is entirely possible to get a calibration that isn't even approximately correct.If your VNA has a proper base calibration, you might be better off using that (+ port extension).

author: joeqsmith
author_rank: Super Contributor
timestamp: July 07, 2023, 12:31:58 pm
content: Quote from: prutser on July 06, 2023, 02:10:01 pmCalibration can only correct systematic errors. If your using say the LiteVNA to measure to 6GHz and you have poor cables, connectors..... even the best standards are not going to be of much help.Flip side, more often than not I am just looking for some idea how things are improving or wanting to get some ball park idea how things behave. If I calibrate the VNA, it's normally the last thing I do. Even the LiteVNA can put up usable information without calibration. Not suggesting you can get away with that in all cases.

author: pdenisowski
author_rank: Super Contributor
timestamp: July 07, 2023, 03:36:39 pm
content: Quote from: joeqsmith on July 07, 2023, 12:31:58 pmAgreed. There are plenty of times when I just need a rough idea about something -- like the resonant frequency of a "mystery" antenna or the pass band of a "mystery" filter, or checking if a cable is still good or not.That said, I have also been burned on multiple occasions when I failed to properly cal my setup and got results that were misleading or wrong.I sometimes joke that the third troubleshooting question (after "Is it plugged in?" and "Is it turned on?") should be "Did you run a cal?"

author: joeqsmith
author_rank: Super Contributor
timestamp: July 07, 2023, 05:14:08 pm
content: Normally I have some idea what I am expecting to see. I think part of it too is going to be how well you know your equipment. For example, I designed an amplifier one time for the sole purpose of going unstable when excited with a square wave like these low cost VNAs produce. You could use expensive cables, metrology standards, torque everything, still if you try and make any measurements of that circuit, the data will be bogus.

author: virtualparticles
author_rank: Regular Contributor
timestamp: August 03, 2023, 11:32:45 am
content: My article in the link is fairly informative about calibration and residual errors.https://coppermountaintech.com/introduction-to-the-metrology-of-vna-measurement/The quality of the load will determine the "Floor" for all reflection measurements. Errors on the delay of the open and short cause ripples in the measurement. For decent measurements the VNA has to know the "Actual" reflections from the cal kit standards.

author: G0HZU
author_rank: Super Contributor
timestamp: August 03, 2023, 01:58:05 pm
content: When I got my first proper VNA many years ago (HP 8714B) I spend quite a bit of time making an SMA cal kit for it. This was probably about 20 years ago.I still have and still use the same cal kit the most out of any of my cal kits. In recent years I improved the cal coefficients and used the maths engine in Eagleware Genesys to work out the Cx and Lx correction coefficients. This extended the usefulness beyond 2GHz to about 6GHz for all but the most critical measurements.If I turn off the corrections then the performance degrades because the VNA can't define the reference plane. It is possible to explore the implications of this (by doing various comparison tests) but I would argue that the time is better spent exploring ways to improve any external test fixtures you might also have. When measuring components, the component won't be at the reference plane defined by the cal kit. So more work is needed to accurately shift the reference plane to where the device under test sits in the external test fixture.It shouldn't be that difficult to achieve this fairly quickly if the aim is to do testing up the VHF or so. Things become a bit more 'nerdy' if you want to make really precise measurements or if you want to start working up towards 6GHz. Then it isn't a case of comparing an 'arbitrary' cal against a corrected measurement, it's more of a case of abandoning the arbitrary cal and trying to improve the existing corrections used in the corrected measurement. Otherwise, the results can be misleading. Up at 6GHz, it's usually fairly obvious when the VNA hasn't been calibrated and corrected properly.Do a few tests of your own if have access to a VNA with devices you know the results for. This could be something from Minicircuits for example. They often provide s-parameter data for their products.

author: ywara
author_rank: Contributor
timestamp: August 05, 2023, 12:40:40 am
content: I'm not the expert that many around here are, but I spent a long time in the microwave world.On a VNA your calibration is everything. Yes, dynamic range and stability matter but even on a $150K VNA your measurement on Tuesday will not be very good with your Monday calibration, even with the expensive HP/Anritsu test cables and no changes to your setup. I can't speak to exact uncertainties, but ignoring drift your measurement accuracy is nearly 100% dependent on your calibration, with the remainder due to measurement noise from your equipment (which can be compensated for).6 GHz is not a particularly challenging frequency for most hardware. We always joked that everything below 1 GHz is DC. This is a braindead boast, but it's not too hard to achieve good performance to that point. Even COTS through-hole resistors can be pretty flat to 1 GHz.At 6 GHz in a low-precision interface like SMA, I'd expect that knowing the electrical length of your offset and short is good enough. The polynomial correction coefficients would be nice to know, but without them you'd probably do pretty well.Your load will still be critical. You could try to make a good load that was <1.05 to 6 GHz, but this is best left to the experts. Unless your time is free and you have a large amount of resources, you will not make a nice flat 50 Ohm load without having a good instrument to easily measure it - basically a VNA with a good cal kit and a TDR function. VNAs usually have correction coefficients for the open and short, but the load will only have an electrical length (if that). It is assumed to be a perfect 50 Ohms. You want your load to be nearly perfect. This will be essential for any reflection measurement.

author: jwet
author_rank: Frequent Contributor
timestamp: August 05, 2023, 03:42:25 am
content: I'm also not an expert but have logged a lot of time in front of VNA's of all types including these amazing little $50 VNA's. The short answer to your original question is- the CAL is EVERYTHING. VNA's are basically ratiometric and depend completely on the calibration/compensation to get below 10% kind of absolute accuracy- often absolute accuracy is not the goal. The mini VNA type devices are a <30 dB or so directivity type instrument. This creates a floor of uncertainty in all measurements that are in this same range. This is in the 5% kind of measurment accuracy with calibration. There are a lot of vagaries and unknown/uncontrolled parasitics at high frequencies so this is really pretty good. I attached a graph of uncertainty from an Anritsu app note- the rule of thumb (from Anritsu) is that a directivity that is 20 dB lower than the DUT leads to uncertainties of around 1 dB. Below is a link to the Anritsu App Note. HP and others have similar notes.https://www.anritsu.com/en-US/test-measurement/solutions/en-us/understanding-directivity

author: pdenisowski
author_rank: Super Contributor
timestamp: August 06, 2023, 04:44:58 pm
content: Quote from: ywara on August 05, 2023, 12:40:40 amQuote from: ywara on August 05, 2023, 12:40:40 amExactly. When people ask why a "proper" VNA cal kit costs so much, they often aren't considering the time and resources (vs. simple material cost) needed to produce that cal kit.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 06, 2023, 10:41:55 pm
content: Quote from: ywara on August 05, 2023, 12:40:40 amSee attached for Agilent load example.

author: ywara
author_rank: Contributor
timestamp: August 07, 2023, 12:18:42 am
content: Yes, I went through the entire cal kit definitions document from Keysight and loads are either perfect loads (delay does not even make sense for this) or a data-based calibration.My thinker is faulty.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 07, 2023, 12:38:27 am
content: Attached from Agilent's documentation, reasons why you may want to modify your coefficients. In some cases, you may not even be working with a 50ohm system. For example, I have been playing a bit with waveguides where the Zo is considered 1.Even the software I wrote supports using something other than 50 ohms for the load. I don't support the load loss or delay but have them in there for a place holder. Future expansion.

author: G0HZU
author_rank: Super Contributor
timestamp: August 07, 2023, 12:52:01 am
content: In case it is of any interest, the plot below is an old plot of my homebrew SMA 50R load that I made when I first got my 3GHz HP 8714B VNA. This load wasn't expected to work to 6GHz as the 8714B VNA only works to 3GHz, but you can see how well it performed up to 3GHz. All I did was solder two 100R SMD resistors to a decent SMA end launch connector. I did select on test the 100R resistors with a decent ohmmeter to get really close to 50.00R though. This is why the return loss is so good at lower frequencies. I think I posted the same plot to Eevblog quite a few years ago, but it shows how well a quick and dirty homebrew 50R load can perform.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 07, 2023, 01:08:19 am
content: How did you cal the VNA to measure the home made standard?What package size did you use?

author: G0HZU
author_rank: Super Contributor
timestamp: August 07, 2023, 01:42:34 pm
content: The resistors I used for the cal kit were 0805 SMD 100R. I most likely used my E5071 VNA and an N4431B Ecal to make that plot and this would have been several years ago. The resistive load in this homebrew cal kit itself is much older than this and is probably about 20 years old.