Masuratori in domeniul timp cu NanoVNA

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yo3mio
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Masuratori in domeniul timp cu NanoVNA

Mesajde yo3mio » Sâm Sep 18, 2021 8:09 pm

Salut,

In excelul atasat gasiti formulele cu care puteti determina rezolutia de masurare si distanta maxima a cablului in functie de frecventele alese (NanoVNA foloseste frequency domain reflectometry).

Sper sa fie de folos. Orice comentariu este binevenit si incurajat!
73 YO3MIO

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Notes:
(1) Number of points tool measures in a freq span.
By using NanoVNASaver, the number of sweep points may be increased. To get the number of points, take the segment setting and multiply by 100. However, be aware that by increasing the number of sweep points the measurements will take longer.
In NanoVNA-H4 (soft 1.0.45) you are able to choose the number of the points (51-401) directly on the device. The previous software version was limited to 101 points.
According to theory, increasing the sweep points will increase the maximum possible cable length. https://www.anritsu.com/en-us/test-meas ... e-to-fault (cap. Optimizing Frequency Range)
(2) Total measuring timeslot allowed for the "impulse" to travel forth and back.
It is improper to use the word "impulse". NanoVNA uses Frequency Domain Reflectometry (FDR) rather than Time DR. Considering this, a better word would be: the time allowed for the phase of the frequency to return to 0, two times (forth and back).
(3) Total measuring timeslot/2 - maximum time for the "impulse" to travel back.
(4) NanoVNA further halves the max time to receive the "impulse".
http://www.gunthard-kraus.de/fertig_Nan ... nglish.pdf - pag 45.
(5) Considering the comment at (4) the denominator is divided by 2 and not 4. I'm not sure it would be correct to see the further division by 2 made by NanoVNA as an improvement in resolution. According to the author of the document mentioned at point (4), the further division may be to avoid aliasing errors (do note that he questions himself related to this argument).
(6) For freqs > 300MHz NanoVNA relies its measurements on the 3rd harmonic. For freqs > 1500Mhz it relies on 5th harmonic.
Therefore, to ensure more accurate results, it is advisable to stay in the 50kHz - 295MHz band.
https://youtu.be/o1eLK4EMpEQ?t=1843 - min 30:43
(7) Minimum freq supported by the NanoVNA is 50kHz.


Measurement procedure for NanoVNA-H4 (soft 1.0.45):
1. Use this excel to determine the start and stop freqs, as well as number of sweep points, according to your needs
2. Set the determined start & stop freqs (Stimulus->Start/Stop) and sweep points (Config->Sweep Points) and do a SOL calibration. If you change any of those parameters, the calibration becomes invalid. However, you can return to the values without calibrating again. See NanoVNA documentation for further details.
3. Check the measured cable datasheet for propagation velocity and insert into Display -> Transform -> Velocity Factor (%)

If you want to measure the distance to fault (DTF):
4. Display -> Transform -> Low Pass Impulse
5. Display -> Transform -> Transform ON. This will set NanoVNA to time domain measurements.
6. Choose your desired CH0 (S11) to display LogMag. Display -> Format -> LogMag. You will now see on the VNA display the Return Loss (dB) versus distance(m) / time delay(ns). RL (dB) = 10*log10(Prefl / Ptrans)
The high peaks (>-20dB) in the chart will alert you about cable imperfections or impedance changes in the joints/connectors.
If the antenna is present at the end of the cable, you may see a peak higher than -14dB. This is normal especially if the resonance of the antenna is far from the start and stop freqs that you choose. Connecting a matching load (50ohm load for 50ohm cable) is a better option that will also reduce the harmonics/peaks that you see in the chart.
If you want to see sharper peaks choose Display -> Format -> More -> Linear. RL = Prefl / Ptrans
7. Adjust as per your needs or leave default
Display -> Scale -> Scale/Div - how many units the lines of the vertical axis represent (I use 10dB for LogMag)
Display -> Scale -> Reference Position - where 0 is. For the negative values such as LogMag, the 0 will be at the top. Vice-versa for positive values. (For negative I use 7).

If you want to see measurement along the transmission system (eg impedance along the cable to spot imperfections):
4. Display -> Transform -> Low Pass Step
5. Display -> Transform -> Transform ON. This will set NanoVNA to time domain measurements.
6. Choose your desired CH0 (S11) to display
- Display -> Format -> More -> Resistance. The chart will show the resistance along the transmission line.
Normally, you should see something around 50ohms. However, your cable(s) may not have perfect 50ohm impedance.
In a real world scenario, in which two cables are involved, you may see one segment corresponding to one cable at ~49ohm while the next one at ~53ohm. At the joint, you may see spikes (inductive distortions) or cuts (capacitive distortions) - the lower they are, the better. At the end of the cable, you may have a matched load (the line in the impedance chart will continue at around 50ohms), a short (impedance will go down) or an open (impedance will go up).
- Display -> Format -> LogMag. The chart will show attenuation along the cable.
7. Adjust as per your needs or leave default:
Display -> Scale -> Scale/Div - how many units the lines of the vertical axis represent
Display -> Scale -> Reference Position - where 0 is. For the negative values such as LogMag, the 0 will be at the top. Vice-versa for positive values.
Fişiere ataşate
VNA_FDR.xlsx
(4.76 MiB) Descărcat de 53 ori

YO3ITI
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Re: Masuratori in domeniul timp cu NanoVNA

Mesajde YO3ITI » Sâm Sep 18, 2021 9:10 pm

yo3mio scrie:Salut,

In excelul atasat gasiti formulele cu care puteti determina rezolutia de masurare si distanta maxima a cablului in functie de frecventele alese (NanoVNA foloseste frequency domain reflectometry).
.......


Foarte interesantă documentație. Mulțumiri.
73 de YO3ITI

yo3mio
Mesaje: 16
Membru din: Dum Ian 09, 2011 2:20 pm
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Re: Masuratori in domeniul timp cu NanoVNA

Mesajde yo3mio » Joi Sep 23, 2021 8:07 pm

Cu mult drag! Ma bucur ca am putut fi de folos



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