Electronics > Projects, Designs, and Technical Stuff
[Solved] NanoVNA says impedance of my 3200R resistor is 542R-1250X at 50 MHz
hendorog:
--- Quote from: virtualparticles on October 17, 2019, 12:55:04 am ---au contraire!! One can make extremely acccurate impedance measurements with a VNA. See "Make Accurate Impedance Measurements Using a VNA", Microwaves and RF, June 21st, 2019 written by yours truly.
--- End quote ---
Thank you sir, that is what the crowd has come to see :)
Link here:
http://www.clarke.com.au/pdf/CMT_Accurate_Measurements_VNA.pdf
ogden:
--- Quote from: hendorog on October 17, 2019, 12:59:21 am ---
--- Quote from: virtualparticles on October 17, 2019, 12:55:04 am ---au contraire!! One can make extremely acccurate impedance measurements with a VNA. See "Make Accurate Impedance Measurements Using a VNA", Microwaves and RF, June 21st, 2019 written by yours truly.
--- End quote ---
Thank you sir, that is what the crowd has come to see :)
Link here:
http://www.clarke.com.au/pdf/CMT_Accurate_Measurements_VNA.pdf
--- End quote ---
Note that said accuracy can be achieved with CMT VNA that has whooping 140dB dynamic range and very good reflection measurement accuracy. NanoVNA is far from that. Actually it would be good to have similar impedance error chart for nanoVNA. Keysight impedance measurement handbook is another good paper to read for VNA beginners:
https://literature.cdn.keysight.com/litweb/pdf/5950-3000.pdf
hendorog:
--- Quote from: ogden on October 17, 2019, 04:53:45 am ---
--- Quote from: hendorog on October 17, 2019, 12:59:21 am ---
--- Quote from: virtualparticles on October 17, 2019, 12:55:04 am ---au contraire!! One can make extremely acccurate impedance measurements with a VNA. See "Make Accurate Impedance Measurements Using a VNA", Microwaves and RF, June 21st, 2019 written by yours truly.
--- End quote ---
Thank you sir, that is what the crowd has come to see :)
Link here:
http://www.clarke.com.au/pdf/CMT_Accurate_Measurements_VNA.pdf
--- End quote ---
Note that said accuracy can be achieved with CMT VNA that has whooping 140dB dynamic range and very good reflection measurement accuracy. NanoVNA is far from that. Actually it would be good to have similar impedance error chart for nanoVNA. Keysight impedance measurement handbook is another good paper to read for VNA beginners:
https://literature.cdn.keysight.com/litweb/pdf/5950-3000.pdf
--- End quote ---
I think the point of the document is that using the correct method makes an enormous difference to the accuracy which can be obtained - whatever VNA used.
I agree it would be good to get a similar chart for the nano, but I expect it is going to be very frequency dependent in the nano. So picking a limited frequency range will likely be needed.
No doubt a CMT VNA is way better than the Nano, but the paper is based on the CMT datasheet spec, which I assume is worst case over the entire span - all the way to 6.5GHz.
The Nano even in its fundamental range below 300MHz will still be worse, but maybe at or below 10 MHz it might not be terrible in comparison - as we are comparing it at 10MHz to the worst cast CMT VNA spec at 6.5GHz. No maths was harmed while writing in this sentence, so feel free...
There is still fixture removal maths and a common mode transformer required for series measurement. Also I assume the lack of 2 port calibration will be an issue. Maybe 2 port 1 path?
ogden:
--- Quote from: hendorog on October 17, 2019, 09:41:11 pm ---I agree it would be good to get a similar chart for the nano, but I expect it is going to be very frequency dependent in the nano. So picking a limited frequency range will likely be needed.
--- End quote ---
Indeed. NanoVNA have many ranges. The higher the range the worse performance: 1~100MHz, 100~300MHz, 300~450MHz, 450~900MHz.
--- Quote ---There is still fixture removal maths and a common mode transformer required for series measurement. Also I assume the lack of 2 port calibration will be an issue. Maybe 2 port 1 path?
--- End quote ---
Everything is there already. NanoVNA have both, fixture removal (as delay setting) and full SOLT calibration for S11 and S21. Note that NanoVNA do not have hardware for S22 and S12, accordingly no calibration for those.
hendorog:
--- Quote from: ogden on October 18, 2019, 05:18:48 am ---
--- Quote from: hendorog on October 17, 2019, 09:41:11 pm ---I agree it would be good to get a similar chart for the nano, but I expect it is going to be very frequency dependent in the nano. So picking a limited frequency range will likely be needed.
--- End quote ---
Indeed. NanoVNA have many ranges. The higher the range the worse performance: 1~100MHz, 100~300MHz, 300~450MHz, 450~900MHz.
--- Quote ---There is still fixture removal maths and a common mode transformer required for series measurement. Also I assume the lack of 2 port calibration will be an issue. Maybe 2 port 1 path?
--- End quote ---
Everything is there already. NanoVNA have both, fixture removal (as delay setting) and full SOLT calibration for S11 and S21. Note that NanoVNA do not have hardware for S22 and S12, accordingly no calibration for those.
--- End quote ---
I think there is a bit more to it:
Fixture removal is more than just fixing the delay - as described in the paper. Maybe that is good enough, but it will add to the error.
The need for the common mode transformer is also mentioned in the paper. I don't know how much error that will add either.
The SOLT calibration on the nano, and in the software I've seen for it, is just a response cal on the through. It doesn't correct for load match or source match. That will increase the error for series measurements. There is an improved cal type called enhanced response which improves this, but still doesn't correct for the load match of the CH1 port. You can use an attenuator to improve that match, and that will degrade the dynamic range.
http://literature.cdn.keysight.com/litweb/pdf/5965-7709E.pdf
There is another type of calibration which was implemented on the 8753 + T/R test set called 1 path 2 port, which requires the DUT be manually reversed. I think that cal type can be almost as good as a full 2 port cal if the switching hardware existed in the nano.
I'm not sure if I am up to working out the maths on these errors...
By pulling the data from the nano and using the calibrations available in scikit-rf or equivalent, the maths issues here could be resolved. Still need to build or buy a balun though.
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