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Author Topic: Smudge proposed NMR experiment replication.  (Read 44161 times)

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Thanks,  found it:

Itsu
   

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Here a nanoVNA measurement of my new weave 1 coil (series measurement) with severall graphs.
The Smith chart is spiraling because the real part of the impedance is increasing with frequency due to skin effect, radiation resistance, proximity effect. This is normal.

I am puzzled why your S21 Gain (which is just the magnitude of the S21) shows values above 0dB. Positive dB values mean amplification.  Passive components always attenuate (in the ideal case the conduct the signal without losses) so the S21 measurement should always yield negative dB values for passive components.  If you were measuring an amplifier, then the S21 magnitudes would be positive (in dB).

...did a normalisation at the coil...
In VNA world they always call it "calibration".
There are several partial calibrations and two full ones: SOLT and TOSM.
   

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I see what you mean by the amplification, must have something to do with calibration.
I now did it NOT at the nanoVNA device itself, but at the SMA fixture.

Will redo it a few times.....
   

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I now did it NOT at the nanoVNA device itself, but at the SMA fixture.
You should always calibrate for the Measurement Planes.

Errors can also be caused by passive components with LOOONNG settling times (e.g. ringing LC networks).  The remedy is "Measurement delay" or long sweep times.
   
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FWIW, here are some test results of measurements between two parallel flat pancake coils that are connected in a bucking mode that is, the flux on the inside of each coil is going in the same direction as depicted in the drawing below.

Each coil measures 65.2uH and the aid inductance is 170.6uH and the buck inductance is 89.5uH so the calculated k = .31 .  These values are confirmed by Lbuck = ((1-k)*2)*(L1*L2)^1/2 = 89.9uH and
Laid = (1-((1-k)/2))*4*(L1*L2)^1/2 = 170.82uH.

The first pix and scope shot shows the position of the sense coil parallel to the flat coil with the resulting current measurement of 40.34ma rms at resonance.

The second pix and scope shot show the position of the sense coil 90 degrees to the flat coil with a current measurement of 1.235ma rms and is slightly off resonance.

My conclusion, which of course is subject to correction, is that the H field is on axis with the axis of the flat coils or at a right angle to the coil plane while the coil flux is bucking and in the same direction across the face
of each coil.

I am in no position to judge whether or not this will affect the operation of the device as intended.

Regards,
Pm
   

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Hi Pm,
You can't have flux and H field at right angles to each other.  Your flux is correct.  Flux is the B field (density) and the H field follows the same lines.  If your measurement is telling you otherwise then the measurement is suspect.
Smudge
   

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You should always calibrate for the Measurement Planes.

Errors can also be caused by passive components with LOOONNG settling times (e.g. ringing LC networks).  The remedy is "Measurement delay" or long sweep times.

Ok,   yes i tried that, but overdid it i think.

Now i used the calibration at the end of the S11 / S21 coax's, so close to the dut.

Below the result which is more accurate i think.

 
   

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FWIW, here are some test results of measurements between two parallel flat pancake coils that are connected in a bucking mode
Are these coils connected electrically in parallel or serial "bucking mode" ?

that is, the flux on the inside of each coil is going in the same direction as depicted in the drawing below.
But the flux produced by one coil is called the H-flux. The amount of flux lines penetrating a given area of a perpendicular 2D plane is the magnetic flux density - often just called the H-field or B-field* after interaction with a ferromagnetic.
The direction of the H-field is not parallel to the plane that serves to measure its density - it is perpendicular.
The vectors denoting the flux and flux density are parallel by definition so there is something wrong with your drawing.

Please, draw the direction of the electric current flowing in these coils and how they are connected (I cannot read that from the photo).
Also, please mark the printed side of the PCBs, so I can see from the edge-on view how they are oriented to each other. I assume that these two PCB coils are identical and single-sided. (pls confirm).

* In equations, the letter "B" is often used to denote magnetic flux density, regardless of the origin of that flux (i.e. coil, magnet).

Each coil measures 65.2uH and the aid inductance is 170.6uH and the buck inductance is 89.5uH so the calculated k = .31 .  These values are confirmed by Lbuck = ((1-k)*2)*(L1*L2)^1/2 = 89.9uH and
Laid = (1-((1-k)/2))*4*(L1*L2)^1/2 = 170.82uH.
That makes sense but I still do not know how these coils are connected electrically.

The first pix and scope shot shows the position of the sense coil parallel to the flat coil with the resulting current measurement of 40.34ma rms at resonance.
The second pix and scope shot show the position of the sense coil 90 degrees to the flat coil with a current measurement of 1.235ma rms and is slightly off resonance.
Please decrease the frequency an order of magnitude, so the measurement is not deceived by capacitive coupling.

My conclusion, which of course is subject to correction, is that the H field is on axis with the axis of the flat coils or at a right angle to the coil plane while the coil flux is bucking
It's an interesting challenge, then !

[H-field] ...and in the same direction across the face of each coil.
Measured when the coils are together or separate ?
Also, in this context, does "across the face" mean parallel to the plane of the coil ?


« Last Edit: 2020-07-24, 21:52:47 by verpies »
   
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Hi Pm,
You can't have flux and H field at right angles to each other.  Your flux is correct.  Flux is the B field (density) and the H field follows the same lines.  If your measurement is telling you otherwise then the measurement is suspect.
Smudge

Hi Smudge,

OK I understand but my measurements are what is shown.  I will perhaps wind some new coils and run more tests to determine exactly what field is producing the parallel sense coil currents.

Regards,
Pm
   

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Below the result which is more accurate i think.
Yes it is much better now.

To make it easier to read, please keep the reference line at 0dB and at:
- the bottom of the plot for real values of S21 and Impedance and Admittance in linear units.
- the middle of the plot for all imaginary values
- the top of the plot for S21 real values and magnitudes in dB units  (your "S21 Gain" plot already has the reference line positioned correctly).
   
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Are these coils connected electrically in parallel or serial "bucking mode" ?

They are electrically connected in a serial bucking mode.

Quote
But the flux produced by one coil is called the H-flux. The amount of flux lines penetrating a given area of a perpendicular 2D plane is the magnetic flux density - often just called the H-field.
The direction of the H-field is not parallel to the plane that serves to measure its density - it is perpendicular.
The vectors denoting the flux and flux density are parallel by definition so there is something wrong with your drawing.

My depiction of the coil 'flux' may be incorrect but the coils are in a bucking mode and there is a measurement of some field creating current in the parallel sense coil.

Quote
Please, draw the direction of the electric current flowing in these coils and how they are connected (I cannot read that from the photo).
Also, please mark the printed side of the PCBs, so I can see from the edge-on view how they are oriented to each other. I assume that these two PCBs are identical and single-sided. (pls confirm).

The pcbs are double sided and the coil traces from one side to the other are exactly on top of each other so there would be maximum coupling and capacitance between each side in the original application.  I used only the inside set of coils for this test with the outside coils left open.  In rechecking my physical coil positions, I see that I complicated the situation by having the inside coils wound in opposite directions.  This should not make any difference in the overall outcome as I checked for electrical buck and aid connections and used the buck connections for the test.  However, I will rerun the test with the inside coils running the same direction as they face each other and then the electrical connections will be easy to see and analyze. 

Quote
That makes sense but I still do not know how these coils are connected electrically.
Please decrease the frequency an order of magnitude, so the measurement is not deceived by capacitive coupling.
It's an interesting challenge, then !

Yes, I will do this in the test rerun.

Quote
Measured when the coils are together or separate ?

All coil measurements were taken with the coils positioned as shown with a spacing of 21.9mm.

Quote
Also, in this context, does "across the face" mean parallel to the plane of the coil ?

Yes.

Regards,
Pm





   

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My depiction of the coil 'flux' may be incorrect but the coils are in a bucking mode and there is a measurement of some field creating current in the parallel sense coil.
I see only four possibilities that can explain this measurement:
  • The coils and their interconnects have large unbalanced radial currents.
  • Your probe is also sensitive to E-fields or to H-fields outside of the sensing loop.
  • You made a mistake with coils' orientation/polarity.
  • Smudge, Biot, Savart and I are wrong.
   
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OK, this is a retest with new pcbs that each have both coils on each side connected in parallel with the winding direction identical.  The pcb coil pairs are connected in a buck mode as can be seen in the photos below.  The outside winding points are connected together and each inside winding point is used for the input and common respectively.  The series connected bucking flat coils are operating at their respective SRF with the probe and loop inserted.  The overall sensitivity is very high.

The first photo is of the parallel single turn sense winding as before and the first scope pix is the current measured.

With no change in the bucking flat coils, the second photo is an ~80T single layer coil positioned as shown or midway between the upper coil traces which somewhat represents the NMR application in discussion.  The second and third scope pix show the open circuit voltage and the current respectively of this coil. 

I have done lower frequency measurements as well and will follow up with those later on.

Regards,
Pm

Edit: I don't mean to derail or otherwise impede progress here so you may ignore any of my last posts if so desired.
   

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Edit: I don't mean to derail or otherwise impede progress here so you may ignore any of my last posts if so desired.
No, no. Your input is great.  A sound principle cannot be derailed...and if it can - it is not sound.
It helps us identify any unwanted effects. For example radial currents or currents flowing through the intrawinding capacitance.
Your 80T coil resembles a fragment of a toroidal coil. Although it does not curve and does not contain its flux entirely inside, it gives us insight what the toroidal coil is subjected to.

Whatever you probe feels, will be felt by the other component of the system, too, e.g. protons.
   

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What is the difference between the single sensing loop (in the i-probe) placed parallel vs. perpendicular to the planes of the coils ?

The first photo is of the parallel single turn sense winding as before and the first scope pix is the current measured.
In that photo, the probing loop is placed at the 12 o'clock position (in coil's polar coordinates). Is there a difference in the sensed amplitude, when you place it at the 3 o'clock position away from the interconnects ? 
Please move the pink wire to the 9 o'clock position for this measurement.
   

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Now i used the calibration at the end of the S11 / S21 coax's, so close to the dut.
Look how similar your S21 plot is to mine:
I put all 3 traces of S21 on one plot, in order to have only one screenshot to post.
   

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Yes,   thats amazing  O0

Itsu
   

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These are plots of my existing 2 pancake coils (none litz) only, in bucking mode (series measurement).

I see a resonance point at 12.3Mhz (red marker 1) in the S21 gain (Magnitude) plot.
I also see in the S21 real/imaginary plot the red marker 1 at the zero crossing meaning Xc and Xl are canceling (resonance).

But why do i see in the S11 Z plot the green 2 and blue 3 markers NOT at resonance?
Should in parallel resonance Impedance not be max. at resonance?  Why 2 peaks?

Itsu
   

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I see a resonance point at 12.3Mhz (red marker 1) in the S21 gain (Magnitude) plot.
I also see in the S21 real/imaginary plot the red marker 1 at the zero crossing meaning Xc and Xl are canceling (resonance).
Yes

But why do i see in the S11 Z plot the green 2 and blue 3 markers NOT at resonance?
Should in parallel resonance Impedance not be max. at resonance?  Why 2 peaks?
Because of this. You have 3.7kΩ/div there !  That's ~100x over the limit of usability!

Remember that S11 │Z│ is a calculated parameter - it is not measured by the VNA directly (it is calculated from the raw S11 measurement* in this case). 
That calculation has a 10% error at only 220Ω. Extrapolate that error to 2.2kΩ ...and above  :o

Also, radiation resistance is a factor at the dip between markers.

* Take a look at the raw │S11│ measurement in [dB] ( this is the "magnitude of S11" but your VNA proboably calls it "S11 Gain (dB)" ) and you will see what the VNA sees at Port 1.  The S11 │Z│ is calculated from that...
   The details of this calculation are in this PDF.
   

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Quote
Because of this. You have 3.7kΩ/div there !  That's ~100x over the limit of usability!

But those settings (3.7kΩ/div) cannot be changed, they are adjusted according to the presented min / max. data.

Quote
Remember that S11 │Z│ is a calculated parameter - it is not measured by the VNA directly (it is calculated from the raw S11 measurement* in this case).
That calculation has a 10% error at only 220Ω. Extrapolate that to 2.2kΩ ...and above  :o

Also, radiation resistance is a factor at the dip between markers.

* Take a look at the raw │S11│ measurement in [dB] (your VNA proboably calls it "S11 Gain (dB)") and you will see what the VNA sees at Port 1.  The S11 │Z│ is calculated from that...
   The details of this calculation are in this PDF.


Below screenshot is the S11/S21 (no separate S11) gain (LogMag) plot (left bottom).
S11 is indeed kind of "none telling"


By the way, that PDF is hard to read as its kind of mingled.
Guess i have to start reading from this (highlighted) Configuration 3 part:

Itsu
   

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But those settings (3.7kΩ/div) cannot be changed, they are adjusted according to the presented min / max. data.
It is not a matter of setting the Ω/div scaling on the screen.  It is a matter of having such high impedances in the plot that require such scaling. These impedances are the result of the S11 -> S11│Z│ conversion.

By the way, that PDF is hard to read as it's kind of mangled.
Maybe this forum's code is mangling attachments again.
Remind me what your email or FTP server is, so I can send it to you directly.  This file is small enough for an email attachment.
   

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Below screenshot is the S11/S21 (no separate S11) gain (LogMag) plot (left bottom).
S11 is indeed kind of "none telling"
That dip at the lowest frequency (10kHz ?) causes the autoscaling to obscure details at higher frequencies.
Try to get the │S11│measurement displayed with a separate scaling or in a separate plot.  There are details to be seen in it.

   

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OK,  i put up S11 (right top) and adjusted the scaling to be more narrow (max 1.1    min 0.9)

EDIT,  added a second plot with the S21 /S11 LogMag (dB) scale adjusted (+ - 0.5dB).
« Last Edit: 2020-07-25, 15:29:57 by Itsu »
   

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Below is a Linear plot of the │S21│ transmission/attenuation with phase information superimposed. Both are displayed  Linearly.

It can be seen, that from the start (9kHz) to the 1st marker, the transmission of the signal THRU the coil decreases.  This is typical of an inductor that blocks high frequencies.
However, from the 1st marker to the 2nd marker, the transmission THRU the coil increases - like through a capacitor  :o

This is evidenced by the phase of the signal transmitted THRU the coil between the Start (9kHz) and the 1st marker, where the received voltage (which is proportional to Rx current) lags behind the applied voltage (like in an inductor) - this manifests itself as being below the 0º reference line (it never quite reaches the textbook -90º, though).
From the 1st marker to the 2nd marker, the received voltage (proportional to Rx current) leads the applied voltage (like in a capacitor).

It is important to remember that the current transmitted THRU our coil (modeled as a parallel LC circuit) is not the same as the reactive current circulating IN it.

The question you should be asking yourself is WHAT PATH the current takes between the 1st and the 2nd marker, and what magnetic field is generated by this current* ?


* BTW: This is the reason why I asked Partzman to decrease his test frequency.
« Last Edit: 2020-07-26, 02:03:50 by verpies »
   

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Below is a plot of the │S21│ transmission/attenuation with phase information superimposed.

It can be seen, that from the start (9kHz) to the 1st marker, the transmission of the signal THRU the coil decreases.  This is typical of an inductor that blocks high frequencies.
However, from the 1st marker to the 2nd marker, the transmission THRU the coil increases - like through a capacitor  :o

This is evidenced by the phase of the signal transmitted THRU the coil between the Start (9kHz) and the 1st marker, where the received voltage (which is proportional to Rx current) lags behind the applied voltage (like in an inductor) - this manifests itself as being below the 0º reference line (it never reaches the textbook -90º, though).
From the 1st marker to the 2nd marker, the received voltage (proportional to Rx current) leads the applied voltage (like in an capacitor).

The question you should be asking yourself is WHAT PATH the current takes between the 1st and the 2nd marker, and what magnetic field is generated by this current*.


* BTW: This is the reason why I asked Partzman to decrease his test frequency.


Well,  if this a naked coil, than that path should be the intrawiring capacitance (parallel capacitance).
If any magnetic field is generated by this current i doubt it as thats normally done by inductance.


   
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