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Author Topic: Smudge proposed NMR experiment replication.  (Read 20663 times)

Group: Professor
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A 510 Ohm smd resistor he!     I will take one from my stack of 510 Ohm smd resistors  :D
Yeah, like from this resistor kit
   

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470Ω will be fine, too.
   

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Can your VNA display phase angle?
Can you make a phase vs. frequency plot like this one ?
   

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I used a 560 (558 measured) Ohm metal film resistor instead for now.

First screenshot is with te saver PC program,
second screenshot from the nanoVNA itself like the earlier screenshots.

Itsu
   

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Can your VNA display phase angle?
Can you make a phase vs. frequency plot like this one ?

Yes it can,   from the coil you mean?

   

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Here the plots from S21 phase and Lin mag from the saver program (upper 2 graps) and the nanoVNA itself from the spaced coil (series measurement):

   

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I used a 560 (558 measured) Ohm metal film resistor instead for now.
Must be good enough

First screenshot is with the saver PC program,
The resistance (Series R) is 601.85Ω, which is in the general ballpark of 560Ω.
The capacitance (Parallel C) is small but not small enough - it should be close to zero for a resistor.
The inductance (Series L) is negative  :o   ...but that is nothing next to the units of Reactance (X), which are displayed as Farads ! :o :o :o

second screenshot from the nanoVNA itself like the earlier screenshots.
The reactance (X) is close to zero Ohms, as it should be for a good resistor  ...we also have an improvement in the Units of Reactance department.
But the real resistance (R) is displayed as 68.5Ω.  That's almost an order of magnitude smaller than the value displayed by the PC program !  ...and wrong, too.
« Last Edit: 2020-09-19, 00:58:10 by verpies »
   

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Here the plots from S21 phase and Lin mag from the saver program (upper 2 graps) and the nanoVNA itself from the spaced coil (series measurement):
This just gets better and better.

In this plot, the phase is negative( less than 0º ) from 0.1-10MHz - that's correct for an inductor (ask ELI the ICE man).
But wait!, in the previous plot the reactance was negative for the same coil from 0.1-10MHz, which means capacitive behavior.  Shouldn't the phase be greater than 0º for a capacitor ?    ...please check my homework !

You've got to plot the negative S21 reactance and negative S21 phase on the same screen and show it to the developers.
   

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I am posting S21 Reactance and S21 Phase plots of a real capacitor and a real inductor measured by a real VNA (using this fixture) in case anyone has any doubts how they are supposed to look like.
Note the opposite signs of the reactance and phase for both of them.

   

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I have to sit down and take a good look at my calibration setup as the more i fiddle around the more confused i get.

I will take that 560 Ohm resistor as a base and aim for a resulting value of around 560 Ohm not 68 is it shows now.

By the way, the series X and parallel X sometimes displays as Henries or Farad, so probably a bug.

Itsu
   

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Guess i have to skip those S21 measurements as they seem not reliable.

Will start tonight with some TDR measurements.


Itsu
   

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Using my picopulser, i see a reflection at 9.8ns which disappears when turning the pot.
It however disappears in the noise, so hard to know when the correct impedance match is there.
Also, when the pot is at 0 Ohm, the reflection pulse is positive instead of negative which it should be.


Anyway, 9.8ns for a round trip means 4.9ns for the cable/coil length.
Speed of light in vacuum is 299 792 458 m/s = 0.299792m/ns

0.299792 * 4.9 = 1.47m.

the cable/coil is 1.62m long, so velocity factor should be 0.90.

The resistance measured when the reflectionpuls disappears in the noise is around 175 Ohm, so close to the pots upper limit.

Guess something is not right here, so will recheck tomorrow.
   

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That velocity factor is plausible with the air dielectric.
Also, the characteristic impedance of 175Ω is not that far off from my coil's, which is not air gapped.

At 8.9ns round trip time the transmission line half-wave resonance (when the reflection can be 180º out o phase) would occur at 56MHz ...and 28MHz is half of that.

This still does not explain that peak in the H-field's amplitude at 5MHz.
Could your VNA's S21 measurement be that unreliable?  You can verify it with a long FG's sweep and scope.*



* Calibration of that method can be done by clamping the i-Probe on a piece of SHORT Litz wire shorting the FG and saving the result of this FM sweep as csv.  Later, the FM sweep of the coil with the H-probe can be normalized to that saved calibration csv.
   

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Here a video of the TDR measurement setup:  https://www.youtube.com/watch?v=q9PsTZWzdpQ

The H-field measurement i need to repeat as the "thru" calibration done was not correct.

I will use your calibration method mentioned above both for the nanoVNA as the FG/scope.
 
Not sure what you mean with saving as CSV file as the MDO3054 scope seems to have no save to CSV possibility (the SA does).

Itsu
   
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Here a video of the TDR measurement setup:  https://www.youtube.com/watch?v=q9PsTZWzdpQ

The H-field measurement i need to repeat as the "thru" calibration done was not correct.

I will use your calibration method mentioned above both for the nanoVNA as the FG/scope.
 
Not sure what you mean with saving as CSV file as the MDO3054 scope seems to have no save to CSV possibility (the SA does).

Itsu
Itsu,

The MDO series will save waveforms as a csv file in the following manner- First select the "Menu" button on the front panel and then press "Save Waveform".  Plug a USB drive into the front panel and then from the Save Waveforms menu, select which channel you wish to save with the "A" knob and then select the "File" destination with the "B" knob which will save the waveform in a .csv format on your USB drive.

Regards,
Pm
   

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PM,   O0   thanks,     

had to rename the .csv file to .rar to have it uploaded here.
After download, rename to .csv and it should work

Its just a test, so the waveform is from my picopulse test screen.


EDIT: i renamed the file to .pdf to avoid truncating

Itsu
« Last Edit: 2020-09-20, 19:27:18 by Itsu »
   

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Used the above suggested H-field probe thru calibration and made a nanoVNA sweep, see screenshot:

Itsu
   

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Here a video of the TDR measurement setup:  https://www.youtube.com/watch?v=q9PsTZWzdpQ
Yes, the polarity of the reflected pulse seems to be off. That would explain why the peak was at 28MHz instead of 56MHz.  This is weird and I'd like others to verify this conclusion.

Also, the amplitude ratio of the incident picopulse to the reflected one appears smaller than in my TDR experiments here and here and here. I used a scope with 50Ω inputs to do the first two measurements and I think you are using HighZ inputs that are not compensated by a scope probe.

Not sure what you mean with saving as CSV file as the MDO3054 scope seems to have no save to CSV possibility (the SA does).
I did not know it cannot save the acquired data.  I guess I will have to make Imagemagick count the pixels to obtain the calibration curve...


Click on the images below, to see them animate:
« Last Edit: 2020-09-20, 18:47:16 by verpies »
   

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Yes, the amplitude of the reflected pulse seems low and has the wrong polarity somehow.

I have meanwhile changed the coils SMA connector to a BNC one to ommit the sma2bnc adapter, i used both the scope probes 50 Ohm and 1MOhm setting, but no significant change was seen,
so i guess the before measured 0.90 velocity factor and cable impedance of 175 Ohm holds.

This video is from the H-Field probe test mentioned 2 posts above:
https://www.youtube.com/watch?v=tuDoNOm8ZMA

Itsu
   

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FG sweeps of the pancake coil when pot at 0 Ohm:

1st 10KHz to 100MHz  (resonances at 8, 42 and 96MHz)
2th 10KHz to 50MHz   (resonances at 8 and 42MHz)
3th 10KHz to 10MHz.  (resonance at 7.9MHz)

FG has one big turn 5cm away from the pancake coil, scope probe at coils connection.

.CSV files attached (rename them to .csv after downloading).
   

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FG sweeps of the pancake coil when pot at 0 Ohm:
Apparently, it is an X amplitude vs. frequency relationship of the pancake coil.
What is X ?

FG has one big turn 5cm away from the pancake coil, ...
Is it an inductive turn that is supposed to be a transmitter of an H-field ?
If "yes" - do you know its inductance and the transmitted H-field's amplitude vs. frequency relationship ?

...scope probe at coils connection.
What is the capacitance of the probe ?  Is it a HighZ probe ?
   

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I have the feeling is misunderstood you with this FG sweep setup, but nevertheless, here some answers:


Quote
Apparently, it is an X amplitude vs. frequency relationship of the pancake coil.
What is X ?


X at the 10MHz sweep is 52V, see screenshot.


Quote
Is it an inductive turn that is supposed to be a transmitter of an H-field ?
If "yes" - do you know its inductance and the transmitted H-field's amplitude vs. frequency relationship ?

Its more a capacitive coupled loop i think. Induction measured is 0.6uH @ 100KHz


Quote
What is the capacitance of the probe ?  Is it a HighZ probe ?

3.9pF and 10Mohm.
   

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Quote
I have the feeling is misunderstood you with this FG sweep setup, but nevertheless, here some answers:

I now used the H-field probe as with the nanoVNA to measure the H-Field with the scope.

See setup in picture.

Calibration was done with small single loop litz directly on the FG, see screenshot 1
Measurement was done with a 50MHz sweep, see screenshot 2.

We see a peak at 17Mhz and 32MHz.
   

Group: Professor
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I now used the H-field probe as with the nanoVNA to measure the H-Field with the scope.

See setup in picture.

Calibration was done with small single loop litz directly on the FG, see screenshot 1
Measurement was done with a 50MHz sweep, see screenshot 2.

We see a peak at 17Mhz and 32MHz.
The hookups are OK, but there is some nasty aliasing of the RF signal with the scope's sample rate, which makes it difficult to process the csv files numerically.
Please test for the aliasing first with the FG connected directly to the scope. The signal should not disappear like the attached one at 3.10E-02s due to aliasing.

10s sweep time is fine, but please repeat it with an integration time set below the half of scope's sampling rate... that setting might be near the "bandwidth limiting" settings or sample averaging in acquisition menu.

For example my scope has this RMS mode in which up to 16M samples can be squared and added up inside the FPGA on the fly and then only their average (sqr actually) is written to scope's memory.  I have seen other scopes having a similar acquisition mode where the absolute values of the samples are averaged arithmetically instead, and then only their average is written into scope's memory*.  If your scope has a similar mode - please use it.

With such memory saving acquisition modes, e.g. 10G samples do not need to be written into scope's memory when the scope samples at e.g. 1Gsps for 10s and the high (1GSps) sampling rate prevents aliasing of all signals below 500MHz.

Also, some i-Probe amplifiers have an RMS mode and their output in this mode only needs to be low pass filtered at kHz frequencies to avoid aliasing.

* In analog world this is equivalent to a high-frequency rectifier followed by an RC integrator.
« Last Edit: 2020-09-22, 01:47:06 by verpies »
   

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OK,  will have to do some investigation into this,  thanks.
   
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