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Author Topic: Smudge proposed NMR experiment replication.  (Read 20667 times)
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Yes, around 20nH.

The transmitter's internal impedance and receiver's impedance in parallel are 25Ω.
Don't this impedance and inductance form a high pass RL filter ?
That is one way of looking at it, but the two 50 Ohm impedances are not in parallel, there is the transmission path in between.  The 10nH coil shunting the Tx 50 Ohms forms a high pass filter, then the other 10nH coil driving into the Rx 50 Ohms forms a low pass filter.  I think the two in series accounts for the linear phase progression over the very wide frequency range.

Quote
At higher frequencies it looks like this:


The phase wraps-around from -180º to +180º.

The second plot below was done with one H-Field probe physically flipped 180º.


This phase in the second plot also wraps-around from -180º to +180º.
Those phase flips are purely math artefacts (as anyone who has played with trig functions in Excel will tell you) and of no significance, the phase progression is continuous.

Smudge
   

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Those phase flips are purely math artefacts
Of course, that's what wrapping-around is.

So what phase relationships do you expect in this experiment ?
   
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Of course, that's what wrapping-around is.

So what phase relationships do you expect in this experiment ?
Sorry for the delay in replying but I have been busy dealing with family matters.  Going back to your data posted in reply 510 where you observed a resonance at 5.695MHz.  I misunderstood the set up so my reply 512 where I thought this would be a problem was wrong.  Your reply 518 showed the set up and (if we ignore the small phase delays along the coax cables) you had the pancake coils connected in parallel having an inductance that you quoted earlier as 4.9uH at the 16mm separation, and the signal was fed from transmitter to receiver through this inductance.  The coax connections added lumped capacitance across each coil, and you also had earlier measurements of intra-turn capacitance of 174.75pF for a single pancake coil, which is far greater than the coax capacitance.   Thus you had the signal being fed through a parallel LC circuit that calculates to have a resonance of 3.85MHz based on those values, whereas the actual resonance was that 5.695MHz.  That created the dip in transmission.  That is a good result since in the actual NMR experiment we want to use resonance to get maximum current into the coils, and we can easily lower the self resonance by adding capacitance.  The bad news is that we cannot use the input matching circuit I originally proposed, which puts a high value capacitance in series with the one producing the resonance.

Now answering your specific questions, your method of connecting the coils via the S21 fixture and the Tee is simply connecting the parallel LC circuit across the 50 Ohm signal source.  This is of course a complete mismatch at resonance and does not make use of the high Q that can be obtained with better matching.  So I would expect the signal induced into the H-field sensor to follow a low Q resonance peaking at 5.9MHz.  For the phase plot I would expect this be +90 degrees at the lowest frequency, falling as the frequency rises then rising again to pass through +90 degrees at that self resonant frequency, then rising towards +180 degrees.

To maximize the current into the coils we must use a high impedance source, and that requires a step-up transformer between the 50 ohm signal source and the coils.  I am sorry that this wasn't made clear at the start, but I did not envisage the complex interleaved winding arrangements that would create such huge self capacitance.

Smudge
   

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@Smudge

In this setup, do you expect the sensor orange loop's distance from the red line (when sliding along the blue line) to influence the plot of S21 Phase vs. f ?

   

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Your method of connecting the coils via the S21 fixture and the Tee is simply connecting the parallel LC circuit across the 50 Ohm signal source.  This is of course a complete mismatch at resonance and does not make use of the high Q that can be obtained with better matching. 
This is a measurement technique, not an optimal driving technique.

...I did not envisage the complex interleaved winding arrangements that would create such huge self capacitance.
With this winding arrangement, the turns of the coil are spaced the same distance, as with the Archimedean spiral winding.  The turns also have almost the same area of surface contact (a little less at crossings).
Seeing how much problems this inter-turn capacitance creates, now I am advocating putting an air gap between the turns (generated by removing a consistent spacer used during the winding process).

The bad news is that we cannot use the input matching circuit I originally proposed, which puts a high value capacitance in series with the one producing the resonance.
I don't understand.
From the point of view of these inductances (L1 and L2), the external capacitance CEXT and the inter-turn capacitances CIT1 and CIT2, are all in parallel.
   

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So I would expect the signal induced into the H-field sensor to follow a low Q resonance peaking at 5.9MHz.  For the phase plot I would expect this be +90 degrees at the lowest frequency, falling as the frequency rises then rising again to pass through +90 degrees at that self resonant frequency, then rising towards +180 degrees.
Did you account for the frequency characteristics of the H-field probe, being superimposed on this measurement ?  These characteristics were posted in post #510 and discussed in subsequent posts.

Take a look at the measurement of the signal received by this H-field probe when it is placed exactly between the pancake coils at the radius of 50.75mm, when using these connections.
   

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I kept quiet the last weeks to give verpies and Smudge a chance to clear up some things and to avoid the heat here on the bench as it is unnatural hot here in the Netherlands at this time of the year having a heat wave the last 10 days with temperatures daily over 33°C.

What i did was testing the old pancake coils i have in parallel opposing mode sweeping the frequency in little steps both by FG as manual.

But in these 2 weeks or so i did not see anything out of the ordinary.

So either the NMR is not there or hidden underneath the crosstalk signals.

I will be waiting for my new litz wire to arrive so i can build 2 new pancake coils as proposed by verpies here: 
https://www.overunityresearch.com/index.php?topic=3924.msg83667#msg83667

which will be a challenge on its own.

Regards Itsu
   

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I kept quiet the last weeks to give verpies and Smudge a chance to clear up some things
I have more measurement data for Smudge when he replies.

...and to avoid the heat here on the bench as it is unnatural hot here in the Netherlands at this time of the year having a heat wave the last 10 days with temperatures daily over 33°C.
That ambient temperature is sick!

What i did was testing the old pancake coils i have in parallel opposing mode sweeping the frequency in little steps both by FG as manual.
But in these 2 weeks or so i did not see anything out of the ordinary.
So either the NMR is not there or hidden underneath the crosstalk signals.
Low electric and magnetic crosstalk* and the "DC" magnetic field uniformity from the magnets is the key to success.

I will be waiting for my new litz wire to arrive so i can build 2 new pancake coils as proposed by verpies...
which will be a challenge on its own.
Get yourself a good coil former, with an appropriate groove.


* achieved by spatial topology and shielding or by temporal separation
   
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I have more measurement data for Smudge when he replies.
I have not abandoned you.  My wife was rushed to hospital last Thursday with a serious blood infection.  Not covid related thank goodness but is cellulitis, infected from small ankle injury some two weeks previously.  She is on intravenous strong antibiotic drip so have to take her to hospital each day for this treatment.  Haven't had time to look into your latest data but will do so when these dratted hospital visits stop.

Smudge
   

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Wishing the best for you Smudge!  May your wife recover quickly
and completely.


---------------------------
"The best teachers are those who show you where to look, but don't tell you what to see." - Alexandra k. Trenfor.
   
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OK I am back and wife is now quite well thank you.  Getting back to the questions at hand, here is my interpretation of Verpies' measurement for the response of the two pancake coils in reply 510 using the connections shown with his S21 fixture and TEE.  Note that the outer casings of the Tee and the coax cables connecting to each coil are not at ground potential, so they contribute leakage capacitance to ground shunted across the output.  At low frequencies the phase delay along those coax cables is insignificant, but the cables do add capacitance across each coil that adds to the intraturn capacitance.  The connection diagram simplifies to the one shown below.  Thus one would expect a dip in the response at the LC resonant frequency as indeed occurs at about 5.7MHz in the measurement.

At somewhat higher frequencies the method of winding and feeding the pancake coils each using 3.2m of wire causes them to appear as a length of shorted transmission line of length 1.6m, and that feature has to be taken into consideration.  That would tend towards an open circuit at the first resonant frequency where the line is a quarter wavelength, a short circuit at the next frequency where the line is a half wavelength and another open circuit when the line is three-quarters wavelength and so on.  That half wavelength case is seen in the measurement as a peak at 18.7MHz, followed by the three-quarters wavelength dip at 29.5MHz.  The broad quarter wavelength dip at around 9MHz is overshadowed by the 5.7MHz lumped constant dip but does cause that to be assymmetric.  At even higher frequencies the coax cables lengths begin to take effect, but that is way outside our expected NMR freqencies.

Looking at the measurements using the H-field detector I have added the typical amplitude response for the LC resonance of the pancake coils at just above 5MHz.  Clearly this peak does not appear, we merely get an inflection of the rising curve expected for the half wavelength transmission line peaking near 18MHz. That tells us that at the 5MHz resonance the coil current sloshing back and forth between the L and the C is there but not enough to show a peak.  However note that here the Q of the LC circuit is low as the 50 Ohm input resistance is across it.  The coil current (hence the detected H field) can be increased if an appropriate input system that allows higher Q is applied.  The other detail at higher frequencies is really only of academic interest.  I have added detail along the top of the chart to indicate roughly regions where the LC approach, the transmission line effect and the coax cable lengths overlap.

My conclusion is that the lumped constant LC approach is satisfactory at the expected NMR frequency, but my suggested input circuit is not suitable due to the high self capacitance of the special pancake coils.  It should be relatively easy to create an input circuit that allows much higher Q and hence greater coil current.  The transmission line effect in the special pancake coils, while seriously affecting the coil current at high frequencies that are not of interest, has a small effect at our expected NMR frequency that should be even smaller when the LC circuit obtains higher Q.

Smudge   
   

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@Smudge

I am still analyzing your latest reply but I noticed that it did not consider this question.


P.S.
I hope that you (plural) did not catch the virus in the hospital. Are you in the U.S., where the only effective prophylactics have been politicized?
   
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@Smudge

I am still analyzing your latest reply but I noticed that it did not consider this question.

As the H field is radial within the pancake coils I would expect it to diminish with increasing radial distance from the axis.  Thus I would expect maximum H field value with the sensor close to the minimum radius of the pancake coils, reducing in value as the sensor moves to the outer radius, and of course dropping drastically as it moves away from the coils conductors.  That peak could be exaggerated by the quarter wavelength transmission line effect in the pancake coils where maximum current occurs at the shorted end.

Quote
P.S.
I hope that you (plural) did not catch the virus in the hospital. Are you in the U.S., where the only effective prophylactics have been politicized?
My wife got the infection in strong wind when the shed door blew against her lower leg causing her skin to be penetrated and a wooden splinter to enter.  It developed as cellulitis.  She was tested for Covid 19 on entry to the hospital and that came back negative.  I have not developed any Covid 19 symptoms so far.  I am in the UK where there is no evidence of effective prophylactics against Covid 19 being deliberately politicized, but as is normal in life the stringent protocols needed to ensure any new regimes are absolutely safe is holding back any introductions.  Of course that is seen by some as being politically motivated by big pharma.

Smudge
   

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As the H field is radial within the pancake coils I would expect it to diminish with increasing radial distance from the axis.  Thus I would expect maximum H field value with the sensor close to the minimum radius of the pancake coils, reducing in value as the sensor moves to the outer radius, and of course dropping drastically as it moves away from the coils conductors. 
Does that meet your expectations?

https://i.imgur.com/5gmW1RP.gif
Smudge proposed NMR experiment replication.


Click on the image to magnify it.
This animation packs a lot of information so viewing it frame by frame, might give you more time to analyze it as it changes.
The red numbers at the bottom indicate the radial distance of the H-field sensor loop in mm from the major axis (we recently discussed the frequency response of this sensor loop).

That peak could be exaggerated by the quarter wavelength transmission line effect in the pancake coils where maximum current occurs at the shorted end.
Yeah, about that...
Could the displacement currents and standing waves in that coiled "transmission line" explain the spatial H-field distribution vs. f, depicted in this animation ?
   
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Does that meet your expectations?
There is a lot of information in that animation much of which is of little significance wrt our NMR experiment.  Those wild variations in the S21 value at high frequencies well above the expected NMR frequency is only of academic interest.  I should point out to others watching this thread that we are interested in maximising the H value and the S21 value does not give us H directly.  I think we can ignore those wild peaks and troughs as they do not relate to our work.  I have neither the software, the time or the inclination to fully analyze all that information.  I have eyeballed the animation and crudely extracted the S21 value at a fixed frequency of 5MHz which is close to the expected NMR frequency.  I get the data plotted below.  The S21 value is a maximum when the H probe is at a radius of about 53 cm which conveniently places it nicely within the pancake coil region that would be occupied by the water sample.

Quote
Yeah, about that...
Could the displacement currents and standing waves in that coiled "transmission line" explain the spatial H-field distribution vs. f, depicted in this animation ?
Probably, but things are much simpler in our frequency region of interest.

Smudge
   

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...we are interested in maximising the H value and the S21 value does not give us H directly. 
Why not?
S21 is a ratio of two voltage amplitudes.  These two amplitudes can represent anything.
In this case it is a ratio of the voltage amplitude applied to the coils and the voltage amplitude induced in the H-field sensor's loop*.

Doesn't the amplitude of the voltage induced in the H-field sensor's loop give us the amplitude of the H-field directly ?


loaded by the 50Ω resistance of the receiver.
   
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Doesn't the amplitude of the voltage induced in the H-field sensor's loop give us the amplitude of the H-field directly ?


IMHO, not necessarily.  In my experiments with the rectangular pcb flat coils spaced apart at a fixed distance and run in a resonant bucking mode, circular, triangular and even oval shaped H field sense coils yielded no output when measured differentially.  However, a square shaped sense coil that fit up to the inside edges of the flat coils yielded output.  My conclusion was the E field was the source but this is my opinion at this point.

Regards,
Pm 
   
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Why not?
S21 is a ratio of two voltage amplitudes.  These two amplitudes can represent anything.
In this case it is a ratio of the voltage amplitude applied to the coils and the voltage amplitude induced in the H-field sensor's loop*.

Doesn't the amplitude of the voltage induced in the H-field sensor's loop give us the amplitude of the H-field directly ?


loaded by the 50Ω resistance of the receiver.
Not directly as the voltage is proportional to frequency, hence in the S21 plots against frequency the higher frequency values are over emphasised.

Edit.  Forget that statement, the voltage on the pancake coils is also proportional to frequency so you are right.

Smudge
   

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My conclusion was the E field was the source but this is my opinion at this point.
I agree that E-field plays a role with series connected pancake coils and unshielded sensing loop, but I do not think, that this is the case, with the pancake coils connected in parallel, because both coils are at the same electric potential. Also the H-field sensing loop is small and electrically shielded in my system.

Furthermore, the 90º rotation of the H-Field sensor's loop with respect to the pancake coils influences the voltage appearing across it greatly and 180º rotation shifts the phase of the voltage appearing across it by 180º, too. That is an additional evidence that the voltage appearing across it is the result of Faraday's induction.
   

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There is a lot of information in that animation much of which is of little significance wrt our NMR experiment.  Those wild variations in the S21 value at high frequencies well above the expected NMR frequency is only of academic interest.  I should point out to others watching this thread that we are interested in maximising the H value and the S21 value does not give us H directly.
I think there is nothing wrong with trying to understand what is going on because a deeper understanding can lead to further optimization methods.

For example, take a look at the the H-field's amplitude 58mm away from the major axis.  It is 3.3x higher at 23MHz than at 4.3MHz.  That is over 11x more power - nothing to sneeze at.
A better understanding of this phenomenon would allow us to generate an order of magnitude more powerful H-fields at the desired NMR frequency, instead of the undesired 23MHz ...or better.
Also, a better understanding would allow us to have a better control over the spatial distribution of the H-field's amplitudes in the radial direction.

   

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I have neither the software, the time or the inclination to fully analyze all that information.
It took me an entire day to build a robotic H sensor positioner and make 182 of these H-field measurements for you.
Ask Itsu to provide you with the access to the video of this experiment, if you are curious how it was performed.
   
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I think there is nothing wrong with trying to understand what is going on because a deeper understanding can lead to further optimization methods.

For example, take a look at the the H-field's amplitude 58mm away from the major axis.  It is 3.3x higher at 23MHz than at 4.3MHz.  That is over 11x more power - nothing to sneeze at.
A better understanding of this phenomenon would allow us to generate an order of magnitude more powerful H-fields at the desired NMR frequency, instead of the undesired 23MHz ...or better.
Also, a better understanding would allow us to have a better control over the spatial distribution of the H-field's amplitudes in the radial direction.
Yes, I see that 11x more power at a resonant frequency of the shorted transmission line inside the pancake coils, which is offering a high Q.  To get that effect at our NMR frequency would require pancake coils wound with longer length fine wire of more turns, which with their special winding technique may not be practical.  On the other hand I see the LC resonance as low Q which could be improved greatly with the right input circuit, and that could be a more practical way of achieving the same effect.  However, being just an armchair observer, I am not in a position to dictate what is the best way forward, that must come from you guys doing the work.

Quote
It took me an entire day to build a robotic H sensor positioner and make 182 of these H-field measurements for you.

I do appreciate all that you and Itsu are doing, and I am sorry if my response offended you.  I am frustrated that I can't get hands-on experience to tell us what to do next and rebelled against being led down a more complicated route.  Please accept my apology.

Smudge
   

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Still stuck waiting for my litz wire.

I have one former ready for a 10 turns weave 1 type coil with 1mm nylon separation between turns.

So i hope to make some measurements soon.

Regards Itsu
   
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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

   

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The article does not mention whether the inter-turn capacitance CIT is the parallel capacitance CP depicted on the schematic diagram below:



This is important because any current flowing through the CIT distorts the direction of the magnetic field generated by the turns of the coil.  A current flowing in an external parallel capacitance, does not do that.
   
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