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Author Topic: Smudge proposed NMR experiment replication.  (Read 107396 times)

Group: Professor
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@Smudge

I see.
How would you model the CIT and the associated displacement current iIT in this type of pancake coil ?
...and the HIT* field generated by such iIT ?


* the HIT moniker describes the same field as H3 in previous messages.

Then my E field or displacement current arrows would have alternate directions and their H fields would cancel at distances from the surface greater than the turn spacings.  Clearly this would almost eliminate that HIT field at the toroidal coil.

Smudge 
   

Group: Professor
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Nice, so it goes from a high Q reactive environment to a low Q resistive one.
Yes, but │Z│ Peak#3 is affected ~2x more than │Z│ Peak#1.

When you consider the │Z│ ratio of (Peak#1 - Dip#2) / (Peak#3 - Dip#2) you can see that at 0Ω termination, the ratio of these peaks is 4.8 and at 200Ω termination the ratio of these peaks is 10.7.
This means that the │Z│ Peak#3 at 30MHz is most likely due to reflection since it decreases ~2x faster* than Peak#1 as the coil's termination** is matched better and better (from 0Ω to 200Ω).

* relative to Dip#2
** at the middle break
« Last Edit: 2020-07-28, 13:38:04 by verpies »
   

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Let me focus on the first method again for more accurate measurements.
Remember that warning which is printed on your i-probe.
Put a tape or heat shrink tubing on that part of the sensing loop which touches the i-probe. We want to maintain electric isolation and keep the capacitive coupling between i-probe and the sensing loop, to the minimum.  The thin enamel on the 1mm magnet wire is not much of a barrier.
   

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Clearly there is a potential difference between each coil’s start (let’s call this the hot end) and finish (let’s call this the cold end) connections,...
Fortunately, Itsu's and my pancake coils, do not have the "hot end" and "cold end" anymore.

...,and also from each coil’s hot end to the other coil’s cold end. 
However we still have this problem. Especially when the "front pancake" and "back pancake" are not driven symmetrically with respect to ground.
This is less of the problem then the previous one, though, because CIW is an order of magnitude smaller that CIT. Anyway, electric field between pancakes and the associated displacement current can by shunted to ground by a suitable conductive shield, but this shield cannot support eddy currents or it will attenuate the desirable high-frequency H1 field. ...and this means Litz cloth.

Nonetheless, I still don't see why an isolated loop (shown edge-on in red below) placed exactly between the bucking pancakes and parallel to them,  would have any current induced in it.
   

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Remember that warning which is printed on your i-probe.
Put a tape or heat shrink tubing on that part of the sensing loop which touches the i-probe. We want to maintain electric isolation and keep the capacitive coupling between i-probe and the sensing loop, to the minimum.  The thin enamel on the 1mm magnet wire is not much of a barrier.

Using the below diagram as how i measure.

I have a 11cm wide pvc pipe in the middle where the 1mm single loop wire is put around, so we only can have
the axial movement between the front and rear pancake coil.

There is a minimum (going through zero??) point, but its not in the middle between the 2 pancakes.
Its closer to the front pancake coil no matter how the FG red lead is connected (there is a strength difference though).

So we have an asymmetric field being picked up.

Video here:   https://www.youtube.com/watch?v=iy4itwUW-iI

Itsu
   

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If the pancake coils were driven by separate isolated current sources and there was a potential difference between those sources then your analogy would apply.   But that is not the case,
This sentence of yours gave me an idea.

Would you agree to alter your design and connect the pancake coils in parallel ?  Still bucking, of course.
This way, both of them would be at the same electric potential and the displacement current flowing through CIW would become non-issue.
The phase coherence would have to be maintained by keeping the interconnects equal length after the parallel T-splitter, but that is easy.
« Last Edit: 2020-08-06, 10:26:36 by verpies »
   

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This sentence of yours gave me an idea.

Would you agree to alter your design and connect the pancake coils in parallel ?  Still bucking, of course.
This way, both of them would be at the same electric potential and the displacement current flowing through CIW would become non-issue.
The phase coherence would have to be maintained by keeping the interconnects equal length after after the parallel T-splitter, but that is easy.
Of course I agree, and with the attention being paid to making both coils as identical as possible the two currents should be virtually identical.  Will the inductance change be of benefit?

Smudge
   

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So we have an asymmetric field being picked up.
Video here:   https://www.youtube.com/watch?v=iy4itwUW-iI
We sure do!
The asymmetry should not be so great when the generator is isolated and the sensing loop is isolated from the i-probe.
That is almost an order of magnitude greater induced current next to the back pancake compared to the front pancake.

P.S.
Warning - nit picking follows:  The small loop around the i-probe should have the smallest area possible.  The large sensing loop should be as flat as possible and parallel to the pancakes.
..but I doubt these three imperfections could cause such a large asymmetry.
   

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Will the inductance change be of benefit?
Yes, Itsu would love to have a higher self-resonance frequency.
...but I am afraid the parallel conected CIT will compensate for the smaller inductance.

Also, smaller inductance means lower impedance at higher frequencies and that is good.
   
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Yes, but note that the images only depict half of the pancake coil.  To represent the full pancake coil there should be a mirror image below it.  In the NMR experiment the two images would be well separated, in your pancake coil the images would be almost touching.

There is something wrong in that analogy.  If the pancake coils were driven by separate isolated current sources and there was a potential difference between those sources then your analogy would apply.   But that is not the case, the coils are connected and driven from a single source.  To establish the Ciw electric field you must examine where the potential difference arises (Verpies showed this in a post but I can’t find it now).  Clearly there is a potential difference between each coil’s start (let’s call this the hot end) and finish (let’s call this the cold end) connections, and also from each coil’s hot end to the other coil’s cold end.  Where these occur in space depend upon the winding directions, if each coil is CW or CCW then the series bucking connection puts the hot ends together (say on the outer radius) and the two cold ends together (on the inner radius).  That minimises Ciw.

Yes, but note the answer to your first question, the image shows only (say the top) half of the pancake coil.  Thus a small solenoid that is moved from top to bottom of the pancake coil could exhibit one reversal of polarity as it traverses the top half then another as it moves across the center and another as it traverses the bottom half.  But for your pancake coil the middle one might be seen as just a dip in magnitude and you would get just one reversal from top to bottom

Smudge

Smudge,

I respectively have to disagree with your above analysis based on the experimental results below.  I had worked up my proposal in schematic form but it doesn't exactly fit the test results when the flat coils are driven from the inner leads but I will post it anyway for reference.  It does seem to fit however when the coils are driven from the outside leads so more analysis is needed here.  Perhaps the phasing of the H1 Field is affecting the inner drive results.

Anyway, the first pix is of the test setup which consists of two identical wound solenoid coils placed on top of one another and centered as close as possible in the upper pcb foil traces.  The buck inductance measured 57.3uH and the Ciw measured 35pf at low signal levels with this assembly.

The first scope pix is with the inside leads of the bucking coils driven by the SG.  As can be seen, the open circuit voltages are in-phase but the inner solenoid coil CH2(blu) is considerably lower in amplitude than the outer solenoid coil CH3(pnk).

The second scope pix is with the outside leads of the bucking coils driven by the SG.  Here the voltages oc are near equal in amplitude and phase which is what I expected to see for the inner drive as well. 

Both drive conditions above were tuned for resonance and in off-resonance tuning, the phase and amplitude of the solenoid oc voltages would change.

Regards,
Pm

Edit: I have replaced both inner and outer drive scope pix as the previous outer drive data did not have grounds connected on the solenoid coils.  Now we see the situation is reversed as the inner solenoid CH2(blu) is higher in amplitude that the outer CH3(pnk).  The length of the pcb traces get shorter from the outside to the inside so basically the inner solenoid should not have as much influence from the directional H2 Field as the outer solenoid IMO.

 
« Last Edit: 2020-07-28, 21:05:44 by partzman »
   

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We sure do!
The asymmetry should not be so great when the generator is isolated and the sensing loop is isolated from the i-probe.
That is almost an order of magnitude greater induced current next to the back pancake compared to the front pancake.

P.S.
Warning - nit picking follows:  The small loop around the i-probe should have the smallest area possible.  The large sensing loop should be as flat as possible and parallel to the pancakes.
..but I doubt these three imperfections could cause such a large asymmetry.


I perfected the big and small loops so the small loop is similar in size as the H-probe loop used earlier and the big one is as flat as can be.

But no noticable difference was seen in the asymmetry shown earlier.

Itsu
   
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OK, upon more examination, I questioned why there was no differential when the ground was missing on the previous test so I ran another with a 1K load resistor placed across the outside solenoid coil and measured the differential voltage with two probes.  There was no differential voltage at any frequency as far as I could tell!

So, this means there is no H2 Field at least as I had proposed so I will end this posting of these results on this thread.

Sorry for all the distraction. :-[

Regards,
Pm
   

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I have made preliminary tests of my pancake coils connected in parallel. Their magnetic fields are opposing.

When they are 2mm apart (coaxial and parallel) their combined inductance is 2.6µH, at 8mm: 4.9µH, at 16mm: 6.3µH, at 23mm: 7.2µH and when they are far apart and perpendicular their combined inductance is 10µH. These inductances were measured with an LCR meter at 100kHz.

Below is their S21 │Z│ and Phase plot from 9kHz to 50MHz  at different separation distances:



...and a zoomed-in plot from 3MHz to 12MHz at different separation distances:


Notice, that in this configuration the coils exhibit very high Q.
The Q increases as the separation distance increases.
The self-resonance frequency decreases as the separation distance increases.

The phase of the signal transmitted THRU the coils stays at the textbook +90º (inductive) right up to the self-resonance peak, where it makes an abrupt transition to -88º (capacitive).  This is a very clean phase response because these coils act very inductively almost up to their self-resonance frequency.

P.S.
My interconnect to the coils after the Tee are not equal length (one is 4cm longer). This is because I did not have short SMA pigtails of equal length today.
« Last Edit: 2020-07-28, 22:56:44 by verpies »
   

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Anyway, the first pix is of the test setup which consists of two identical wound solenoid coils
I'd like to notice that these coils have an odd number of layers and are susceptible to longitudinal induction.  They will form a 1 turn loop with the test leads.
   

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I perfected the big and small loops so the small loop is similar in size as the H-probe loop used earlier and the big one is as flat as can be.
But no noticeable difference was seen in the asymmetry shown earlier.
This is not good! 
I am starting to suspect, there is something wrong with one of your coils, i.e. superficially tinned ends of the Litz wires, or a short somewhere .

...or some kind of disturbance from your workbench - try rotating the entire apparatus or moving it to a different place.
   

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This is not good! 
I am starting to suspect, there is something wrong with one of your coils, i.e. superficially tinned ends of the Litz wires, or a short somewhere .

...or some kind of disturbance from your workbench - try rotating the entire apparatus or moving it to a different place.

I remeasured the both pancake coils and both are solid around 17.8uH @ 100Khz.
I retinned the litz connections and moved the setup 90° and to a different place.
I reversed the i-probe, but the offset to center of the dip (going through zero?) stays.

I do notice that when in dip position, the series connection between the pancake coils is very sensitive to touch and squeezing.
The harder i squeeze the litz, the greater the signal amplitude becomes.


Will look into paralleling the coils later today.


Itsu
   

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I do notice that when in dip position, the series connection between the pancake coils is very sensitive to touch and squeezing.
The harder i squeeze the litz, the greater the signal amplitude becomes.
Even if you squeeze it with a plastic tool ?
   

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No,  then it stays in its dip
   

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So if it is very sensitive only to your hands then it means that this junction is very "hot", i.e. it has a high voltage amplitude with respect to ground.
Does it remain "hot" if you solder this junction instead of using the connector with screws?

Does a 1:1 symmetryzing balun decrease this sensitivity ?
   

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A quick test using my old pancake coils with the similar setup as above, shows a much higher resonance (10Mhz), but also shows NOT this asymmetric behaviour in that there is a linear tapering off of the amplitude going from FG red lead connected coil (max) to FG black lead connected coil (min).

No dip in the center nor close to either pancake.


Will do your suggested tests later today,  have to run now.


Itsu
   

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Of course I agree, and with the attention being paid to making both coils as identical as possible the two currents should be virtually identical.
@Itsu

Keeping the quote above in mind, it would be a good idea to compare these two coils with your VNA.

This can be done with these two measurements from 100kHz-50MHz and 100kHz-10MHz (in total, 4 measurements per coil):
  • │S21│ also known as "S21 Magnitude", but I think your VNA calls it "S21 Gain", with the Reference Value = 1U and set to be displayed at the top of the plot.
  • S21 Phase, with the Reference Value = 0º and set to be displayed in the middle of the plot.

The │S21│and Phase can be on the same plot.

These measurements should be done using a good RF test fixture soldered to the coil, similar to this one:




Of course, do a TOSM or SOLT calibration first with the RF test fixture attached and the coil disconnected.  Later solder the fixture to the coil and keep it away from any conductive stuff, when measuring it.
   

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So if it is very sensitive only to your hands then it means that this junction is very "hot", i.e. it has a high voltage amplitude with respect to ground.
Does it remain "hot" if you solder this junction instead of using the connector with screws?

Does a 1:1 symmetryzing balun decrease this sensitivity ?

It remains "hot" when soldering the series connection and no change there when using a 1:1 balun.


Will do your suggested VNA tests later,    what does reference value "1U" mean? 


Itsu
   

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What does reference value "1U" mean? 
I assume you already know what a "reference value" is in SA and VNA.

The "1U" means "1 unit"  This is because the │S21│ is a ratio of 2 voltages* and a ratio is dimensionless.
For passive devices the │S21│ ratio will vary between 1 and 0. ( 1U meaning full THRU transmission and 0U - full THRU attenuation, this way 500mU would mean 50% attenuation of the voltage amplitude ). These are linear measures.

You can also set the display to dB. For passive devices the │S21│ratio will vary between 0 dB and -∞ dB. ( 0dB meaning full THRU transmission and -∞dB meaning full THRU attenuation, this way -6dB would mean 50% attenuation of the voltage amplitude ). These are logarithmic measures.

* the voltage amplitude of the incident signal (at Port1) and the received signed (at Port2). Your VNA might number its ports A & B instead of 1 & 2.
   
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Itsu,

When it is convenient for you, I'm curious if you get any output from your toroid coil when positioned properly in between the driven bucking flat coils when using just two scope probes for measurement?  IOW, no grounds connected to the toroid coil so the two scope probes would then be taking a differential measurement of any output voltage across the toroidal coil.

I see no output in my setup when there is no ground reference for the solenoid coil no matter what the position of the coil in relation to the flat coils!

Regards,
Pm

Edit: The only output is an equal voltage on both coil leads but this is from the E Field of the flat coils and is not differential between the coil leads.
 
   

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@Itsu

Keeping the quote above in mind, it would be a good idea to compare these two coils with your VNA.

This can be done with these two measurements from 100kHz-50MHz and 100kHz-10MHz (in total, 4 measurements per coil):
  • │S21│ also known as "S21 Magnitude", but I think your VNA calls it "S21 Gain", with the Reference Value = 1U and set to be displayed at the top of the plot.
  • S21 Phase, with the Reference Value = 0º and set to be displayed in the middle of the plot.

The │S21│and Phase can be on the same plot.

These measurements should be done using a good RF test fixture soldered to the coil, similar to this one:




Of course, do a TOSM or SOLT calibration first with the RF test fixture attached and the coil disconnected.  Later solder the fixture to the coil and keep it away from any conductive stuff, when measuring it.

Made some measurements and included some in 1 screenshot, so i have 4 screenshots total.

100Khz to 50Mhz from each coil (front and rear) including 3 measurements in each screenshot (actually 4 with smith chart).

100Khz to 10Mhz from each coil (front and rear) including 3 measurements in each screenshot (actually 4 with smith chart).


the 3 measurements in each screenshot are:
S21 (linear gain as i understand)
S21 Gain (Log gain)
S21 phase

1st screenshot front coil 100Khz - 50Mhz
2th screenshot rear coil 100Khz - 50Mhz
3th screenshot front coil 100Khz - 10Mhz
4th screenshot rear coil 100Khz - 10Mhz

Phase plot was adjusted to + / - 90°

Itsu
   
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