Percieved port 2 input impedance error on NanoVNA - calibration question - Page 1
Forum section: electronics / beginners

author: djnebs
author_rank: Contributor
timestamp: August 14, 2021, 11:55:53 pm
content: I got a question about VNA calibration.Say I completed the usual calibration procedure on my NanoVNA.If I connect the load that was used to do the cal on port 1, I see S11 magnitude is -90db across the band as expected. All is well and it shows that the instrument indeed learned the definition of what a load looks like.Now if I remove that load on port 1 and instead connect port 1 to port 2 using a through adapter, now I see S11 magnitude ranges around -40db to -60db across the band. Does this mean that looking into my port 2 cable, it is not a good 50ohm match?I understand that the short/load/open cal has brought the reference plane to the end of my port 1 cable and now I'm measuring beyond that reference plane. But how come there isn't a calibration procedure for the port 2 cable load match error?Does this also mean that any device I connect to port 2 (say when doing a S21 measurement through some device) is not being terminated into a good 50 ohm?On the other hand, the S11 smith chart shows a pretty good 50 ohm. Is it just the db scale on the magnitude S11 plot that is magnifying a small imperfection and giving me this perception?

author: gf
author_rank: Super Contributor
timestamp: August 15, 2021, 09:26:17 am
content: Quote from: djnebs on August 14, 2021, 11:55:53 pmThe NanoVNA is not a full 2-port VNA, but only a T/R VNA. This means that only port1 is equipped with a directional bridge, but port2 is not. Therefore it is not possible to do a SOL calibration on port2.Full error correction (12-term error model) is only possible for a full 2-port VNA.Through calibration (for S21 measuremens) of a T/R VNA is limited toResponse normalization - cannot correct source and load mismatchEnhanced response calibration - like (1), additionally corrects source mismatch, but not load mismatchOne-path two-port calibration - enables full error correction, and measurement of S22 and S12 with a T/R VNA, but the DUT needs to be reversed and measured twiceThe NanoVNA firmware suports (1), of course, normalizing the response of a flush through between port1 and port2 calibration plane to 0dB.NanoVNA V2 also supports Enhanced response calibration. I'm not sure if the firmware of the original NanoVNA supports it as well, does it?One-path two-port calibration is not supported by the firmware. It is also not granted in general that the DUT can be swapped (e.g. if input/output connectors are different).QuoteThe DUT is terminated with the impedance that happens to appear at the port2 calibration plane.The impedances of the ports and the cables will never be perfect, of course, and they are what the are.If the match is not good enough, you could improve it by adding a attenuator (a "good" one, which matches better than the port), but this comes at the cost of reducing the dynamic range.QuoteI would not consider -40...-60dB that bad (-40dB is 1%, and -60dB is only 0.1%). I guess that various other residual errors are larger.

author: virtualparticles
author_rank: Regular Contributor
timestamp: August 17, 2021, 07:13:59 pm
content: A couple of things to consider. When you put the awful, awful load back on the nano and measure it, you'll see a very low return loss (S11) because the VNA has measured it and assumed it is a perfect load. If you're using the load that comes with the nano, it's actually quite bad and has about 15 dB return loss at the highest frequency. Now when you measure the load match of port 2, you'll see a halfway decent return loss. It's really hard to say what to expect there. If you are measuring 30 or 40 dB then those measurements are quite meaningless. The uncertainty of S11 measurement is infinite at the actual return loss of the load, so all you can say is the return loss is better than 15 dB. The uncertainty is +/- 3.3 dB at an S11 of 5 dB in that case as well.The rule is this. For a load standard with 40 dB of return loss, the directivity error will cause the S11 uncertainty to be +/-1 at 20 dB and +/- 3.3 at 30 dB. Any measurement seen to be below 40 dB just means that the measured return loss might be better than 40 dB. Measurements of 60 or 70 don't really mean anything other than that.You would need to measure your load standard on a "real" VNA to know what the limits of your S11 uncertainty will be, at least for the first 5 minutes after calibration. Then drift will dominate the errors of the nano after that.The other calibration standards do also factor into the uncertainty of return loss measurments but the load uncertainty dominates.Here is a videohttps://coppermountaintech.com/reflection-vs-transmission-accuracy-in-vector-network-analyzer-measurement/Best,Brian