Hi Carlos,
like Herb said, the latest version of NanoVNA-Saver was due to include
averaging - it does now :-) I saw what seemed like significant improvements
in readings from data point to data point at the high end of the
measurement range, but I don't have the background to be able to quantify
it. The software includes the option of throwing away a number of the
"worst looking" samples. I've certainly had times where my NanoVNA returned
completely bogus data on a single out of several readings, so I decided it
was probably good to implement a truncated mean function.
I hope you find some use for it. Do give me feedback on it if you try it :-)
--
Rune / 5Q5R
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On Sun, 22 Sep 2019 at 16:23, Carlos Cabezas <eb4fbz@...> wrote:
On professional VNAs, stimulus source is calibrated for flatness on
factory with a bolometer. On nanoVNA you can't control stimulus output
power, so we should probably calibrate the audio codec gain to get most of
it's dynamic range at each frequency, or at least for each band. However
multiple mixing products, others than the desired one, could be present at
audio frequencies, so this must be done with care.
This could also lead to a debate about needed headroom to measure DUTs
with gain. IMHO, it would be better to sacrifice having a lot of headroom
and just insert an attenuator if an amplifier S21 is to be measured.
I have not taken a look to the code in much detail yet, maybe this is
being done, but probably not. I have read somewhere that people have
measured their LNA's gain, so current code must be setting the codec gain
too low if it has so much headroom and the S21 70-80dB noise floor we are
seeing could be in fact being limited by the codec dynamic range.
I also miss an averaging mode, as some measurements are quite noisy, and
it would help for duplexer adjustments with notches near the VNA noise
floor. Maybe just a moving average low pass filter with just 2 taps to
avoid increasing memory requirements (new=0.9*last+0.1*meas). David
mentions using averaged measurements for isolation, and in fact most
professional VNAs use averaging when performing calibration.
Carlos
