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Author Topic: Smudge proposed NMR experiment replication.  (Read 28211 times)

Group: Professor
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First screenshot is with the toriod shield floating (not connected)
Second screenshot is with toroid shield connected to nanoVNA outer connectors.
You can recognize the capacitive coupling by the initial 90º phase-shift.

The difference between she shield connected and not, is stark at 4MHz: S21 Gain is -10.447dB with the shield floating and -39.252dB with the shield connected, which is almost a -30dB difference ...or 1000x less CM power reaching the toroidal coil.
Amplitudewise (│S21│ @ 4MHz) it is 0.3V/V vs. 0.011V/V which is a 27x smaller voltage amplitude when the shield is connected.

So the E-Shield is working but it could work better. Mine gives me -55dB CM attenuation @ 4MHz.  Your notch in the S21 Gain is surprising at approximately 2.6MHz. This notch is -77dB deep and should be investigated.
Maybe the coil leads or shield's grounding lead inductance is resonating in tandem with the CIW capacitance ?  Try to determine what factors affect the position of this notch.
   

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And i thought -30dB was very good.

Anyway, see picture below, when i change the 15cm long thick white litz wire going to the top of the shield with a 5cm copper strip, then this notch moves to ~5Mhz @ -75dB.

So i guess its the influence of this shield connection lead that is causing the notch.

With some tuning i might get this notch exactly on 4MHz.


Itsu
   

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I added the CMC-2 (1096uH each) to the output coil and toke 3 measurements;

SC1 floating shield   
SC2 shield connected using the 15cm litz wire 
SC3 shield connected using the 5cm copper strip.

Notice the notch is gone now.

Itsu
   

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I have minimized the lead from the nanoVNA to the shield to be a 2cm copper strip.

Now again without any CMC, tuner or output circuit, the below screenshot is the result.

Itsu
   

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Still with the minimized 2cm copper strip to shield, but now with cmc-1 on imput and cmc-2 on output:

Itsu
   

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So the CMCs give you almost 17dB more of common mode rejection.  That means that they are working but because the │S21│ curve is sloping up, it means that the capacitive transfer of energy overcomes the CMCs reactance at higher frequencies (just like in LC circuit). How do ferrite beads work for your CM rejection at higher frequencies ...or toroidal cores used like beads (½ turn) ?

P.S.
I took my computer apart and I was offline.
   

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I used 2 ferrite beads (the 6 holes ones) with 1 turn each at the output (adding to the cmc) and gained only -0.5dB or so @ 4MHz, no gain at higher frequency seen.

Then moved both to the input (adding to the cmc), and gained -1dB @ 4Mhz, and also no gain at higher frequency seen (-55dB @ 10MHz).

Also adding some small ferrite toroids (od 7mm) used like beads (½ turn) does not add anything.

SC shows besides the 2 cmc's on in- and output, also the 1 turn ferrite beads on input and the ½ turn toroids on output.


Itsu
« Last Edit: 2021-02-20, 20:42:11 by Itsu »
   

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One thing i did not test using verpies his proposed "S21 series measurement" test is the black and white little cmc's.

So the setup is now like the picture here: https://www.overunityresearch.com/index.php?topic=3924.msg87707#msg87707  but only with swapped cmc's (black at input, white at output).

Also these 2 cmc's together with the shield show a respectable CM attenuation (-61dB @ 4MHz).

The shield still connected to the nanoVNA using a 2cm copper strip.

Itsu   
   

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Also these 2 cmc's together with the shield show a respectable CM attenuation (-61dB @ 4MHz).
I think so, too. -61dB is more than million times power transfer attenuation from the input to the output.
So what is your differential S21 Gain when the shield is grounded in the same manner and these CMCs are in place?
   

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Not sure what you mean as the above situation IS with the 2 little cmc's installed and the shield grounded to the nanoVNA casing using the 2cm copper strip.

Itsu
   

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Not sure what you mean as the above situation IS with the 2 little cmc's installed and the shield grounded to the nanoVNA casing using the 2cm copper strip.
You are referring to common-mode S21 gain/loss.
I was referring to differential S21 gain/loss ...all other things being equal.
   

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OK,    i will test that tonight, but i expect it will be similar as the outcome seen in that link i posted above.


Itsu
   

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I will test that tonight, but i expect it will be similar as the outcome seen in that link i posted above.
I expect it to be very different because the pancakes' H-Field will transfer energy if it is not perfectly orthogonal to the toroidal coil and without baluns you need to connect one side to the RF ground like in Diag.3* or Diag.4* (whichever works best).


* Of course, CMCs can be put in lieu of Ferrite Beads ...or in addition to them. Also, notice their placement, i.e.: Diag.1 vs. 2 and Diag.3 vs. 4.


« Last Edit: 2021-02-22, 12:00:00 by verpies »
   

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For the Common mode measurements, i used the Diag.2 setup.

Now for the Differential Measurement, i used the Diag. 4 setup as it is the most convenient, but the nanoVNA does not like that as i got:
- WARNING - Got a non plausible data value: (25.612972260 -2.380255222)

then terminated the measurement after 10 warnings

It does gave a solution which worked: 
You can disable data validation on the device settings screen.

which i did, but probably invalidated the data received.

Anyway, this is what it recorded then, see screenshot below.

Will try the Diag. 3 setup....

Itsu
   

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I used the Diag. 4 setup as it is the most convenient, but the nanoVNA does not like that as i got:
I'll wait for Gyula's opinion about not including the ground in the CMC.  I suspect he will have a lot to say about it...
   
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Hi Verpies,

Thanks for placing confidence in me,  unfortunately I have not done such sophisticated common mode or differential mode measurements.
Anyway, I think that it would be better to include the CMCs and use Diag. 3,  this way any ground loop between the sig. generator and the oscilloscope would see a relatively high impedance hence ground separation.

Gyula
   

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It is worth remembering that the grounds are connected at the VNA (or scope) anyway.
   

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The scope and FG are, but the nanoVNA is not grounded to the house earth system.

Itsu
   

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I used the Diag.3 setup, which compromised the short 2cm copper strip from VNA to shield, and again the nanaVNA (saver programm) still did not like it:
- WARNING - Got a non plausible data value: (33.164096832 -3.170856953)

So again i had to "disable data validation on the device settings screen".

The below screenshot shows the resulting graphs.

Itsu
   

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- WARNING - Got a non plausible data value: (33.164096832 -3.170856953)
So again i had to "disable data validation on the device settings screen".
You have positive voltage gain because of the turns ratio between the pancakes and the toroidal coil. Of course that would not matter if the H-Field generated by the pancakes was completely orthogonal to the toroidal coil.
Q @Smudge: "Why is Itsu's toroidal coil picking up so much magnetic flux variations from the pancakes ?"   From the previous experiments we know, that his E-filed coupling is attenuated by more than million times, so it must be H-field coupling.  The downward slope of the S21 Gain supports that conclusion.

Dave Jones made a video titled "How NOT To Blow Up Your Oscilloscope", but he forgot to make an analogous one about VNAs, ...TLDR: Inline coaxial attenuators.

   
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Q @Smudge: "Why is Itsu's toroidal coil picking up so much magnetic flux variations from the pancakes ?"   From the previous experiments we know, that his E-filed coupling is attenuated by more than million times, so it must be H-field coupling.  The downward slope of the S21 Gain supports that conclusion
Could it be th feed to the center of the pancake coils?  If there is a small feed along the axis that will create a circular H field even though it doesn't pass through the ring.
Smudge
   
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Itsu has previously described that the pancake coils are connected in a a parallel bucking mode.  If true, then IMO you are experiencing what I had experimentally discovered as described in my post # 548 quote-

FWIW, I ran an experiment with my two flat pcb coils in parallel but wired in bucking mode such that the voltage differential between opposite coil turns is zero.  In this mode, I could measure appreciable voltage/current in an H field sense coil of any shape with differential probes.  The problem is, the equivalent inductance is now ~1/2 each coil thus raising the resonance frequency.

A possible solution may be a current multiplier as described in the following link-

https://www.accelinstruments.com/Applications/WaveformAmp/Magnetic-Field-Generator.html

Here, the resonance could be controlled with Cs and Cp.  Just a thot.

Regards,
Pm


The toroid coil meets the criteria of an "H field sense coil of any shape" that I describe.

Pm
   

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Could it be the feed to the center of the pancake coils?  If there is a small feed along the axis that will create a circular H field even though it doesn't pass through the ring.
Which feeds do you have in mind?

Please refer to the photo below:
The red ones are the feedlines to the pancake coils.
The green ones are the outputs out of the toroidal coil.
The blue one is the grounding for the E-shield between the pancakes and toroidal coil (the shield is covered in yellow-green tape).
   

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...I ran an experiment with my two flat pcb coils in parallel but wired in bucking mode such that the voltage differential between opposite coil turns is zero.  In this mode, I could measure appreciable voltage/current in an H field sense coil of any shape with differential probes. 
How much of it was E-field coupling and how much H-field coupling ?
   
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How much of it was E-field coupling and how much H-field coupling ?

I would have to re-visit the experiment but at time I logically concluded from my testing that it was predominately H-field coupling.  I can't remember exactly why I came to that conclusion though!

Pm 
   
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