PopularFX
Home Help Search Login Register
Welcome,Guest. Please login or register.
2021-10-26, 18:12:13
News: Forum TIP:
The SHOUT BOX deletes messages after 3 hours. It is NOT meant to have lengthy conversations in. Use the Chat feature instead.

Pages: 1 ... 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 [18] 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 ... 35
Author Topic: Smudge proposed NMR experiment replication.  (Read 41335 times)
Group: Experimentalist
Hero Member
*****

Posts: 1365
OK, let's take one issue at a time to make sure I understand correctly. 

Is my simplified depiction of H1 in my previous drawings correct for the cross section of a set of bucking coils spaced as shown?

Pm
   

Group: Professor
Hero Member
*****

Posts: 2418
Is my simplified depiction of H1 in my previous drawings correct for the cross section of a set of bucking coils spaced as shown?
IMO the direction of the H1 on the last drawing is correct.
   
Group: Experimentalist
Hero Member
*****

Posts: 1365
IMO the direction of the H1 on the last drawing is correct.

OK, then I think I have the H field sources reversed and incorrect.  As Smudge pointed out, it is the H field from the inter-winding capacitance Ciw that creates that induces the solenoid coil and that field should be coming out of or going into the page and with that I agree.  IOW, each flat coil is a capacitor plate and the H field would be in the same plane as the flat coils. 

Then, the H field generated by the flat coils induces the single turn sense coil and that field is like I had depicted in the drawing.  So, the cross H2 field vector arrows are wrong along with the logic.

However, as I remember some of the 1T loop tests, something doesn't yet add up but I don't want to take anymore time away from the NMR project so I'll do some more testing and when I think I have conclusive evidence one way or the other, I'll post the results.

Pm 
   
Group: Experimentalist
Hero Member
*****

Posts: 1365
This is the schematic of the bucking coils cross section that I believe is now correct.  I will not post any more on this unless prompted.

Regards,
Pm
   

Group: Professor
Hero Member
*****

Posts: 2418
This is the schematic of the bucking coils cross section that I believe is now correct.  I will not post any more on this unless prompted.
Yes, the direction of H1 & H2 appears correct now.
What about H3 field generated by the Cit ?
   

Group: Professor
Hero Member
*****

Posts: 2418
I just realized that I never posted any plots of my naked bucking coils connected in series (coaxial & parallel). These plots are attached below with different spacings between the coils indicated in the filename. The │S21│ plots have linear vertical scales, starting from 1 at the top. The phase is in degrees (depicted linearly on the vertical axis).

Please remember that just because the coil strongly attenuates the signal being transmitted THRU it ( manifested as low │S21│ ), does not mean that the reactive current circulating IN it* is small.
In fact quite the opposite can be true because of "Current Magnification"* determined by the Q of the coil.

* as in a parallel LC network.


All (animated).gif
« Last Edit: 2020-07-27, 10:02:50 by verpies »
   

Group: Moderator
Hero Member
*****

Posts: 3214
It isn't. This resultant single turn forms a loop that will pick up the signal from the pancake coils - not from the innards of the toroidal coil.

I suggest a simple experiment with a circular loop of wire* of the same diameter as the mean diameter of the water tube (100mm) and placed between the pancake coils in place of the water tube and toroidal coil.
Scope how much voltage that loop picks up when the pancake cols are driven as they are supposed to be.  That induced voltage signal will compete with the NMR signal :(

* If you are very ambitious, use a piece of coaxial cable to make this loop.  This way you can experiment how much difference the shield of the coax makes, when it is used as the E-field shield - just like in that video ...but bigger.

Yes, the weave1 scheme can be flattened to a single layer solenoidal coil or single layer toroidal coil. The wire crossings should be at the ID and OD as not to force the increase of the distance between pancakes and magnets.
In other applications where the symmetry and thickness of the winding is not critical (e.g. in transformers, chokes...) a two layer winding coming back on itself would be an option.

I used a single loop and the special prepared coax loop (see above video) for some measurements, but it seems that there is hardly any difference in signal pickup between those 2 loops (both 10Vpp at resonance).

I must confess that i had to make 3 of those coax (RG58) loops as the first 2 seem to work very erratically (signal / no signal, depending how to hold the loop), so it can be that there is something wrong, but as far as i can see (and measure) this last one should be OK.

Video here:  https://www.youtube.com/watch?v=7lsNtqDSnFo

2 problems noted:
# self resonance frequency very (to) low 3.3Mhz for NMR
# if coax loop is OK, then very much crosstalk is present


Itsu
   

Group: Professor
Hero Member
*****

Posts: 2418
I used a single loop and the special prepared coax loop (see above video) for some measurements, but it seems that there is hardly any difference in signal pickup between those 2 loops (both 10Vpp at resonance).
The lack of difference means that the capacitive coupling does not play a great role IN THAT FIELD ORIENTATION.

I must confess that i had to make 3 of those coax (RG58) loops as the first 2 seem to work very erratically (signal / no signal, depending how to hold the loop), so it can be that there is something wrong, but as far as i can see (and measure) this last one should be OK.
Yes, the erratic performance could be due to the construction of the loop (the aluminum shield does not solder well and is prone to bad contacts and shorts), however there is another explanation which should be investigated further.

Namely, does the axial* movement of the sensing loop cause the received signal to go through zero amplitude?   
On the video, I saw you making only the radial move with the sensing loop. I love that glass cube, btw !

It is important that the sensing loop is very planar (flat) - its shadow should be a straight line.  The planes of the 3 coils must be parallel all the time. A 2x thinner wire would allow for more selective axial positioning, too (e.g. the same wire that the toroidal coil is made with).

"axial" as in moving along the axis of the pancake coils and the sensing loop, ...in other words: initially close to one pancake, next exactly between pancakes and finally close to the other pancake.

2 problems noted:
# self resonance frequency very (to) low 3.3Mhz for NMR
Yes, and that means that achieving a high amplitude H1 field at a higher frequency will require a lot of driving power.  Paradoxically, the high circulating current IC at resonance is accompanied by low current draw from the generator IG. This is caused by high "Current Magnification"  or the ratio IC / IG also known as Q.  Remember, that it is the high IC which is responsible for generating the high amplitude of the desired H1 field - not the IG. This helpful effect does not occur as much outside of resonance.

Also, above the self-resonance frequency, more current flows in the intrawinding capacitance (inter-turn capacitance or "Cit" in Partzman's parlance) than in the turns of the coil.
Furthermore, the interwinding current increases through the interwinding capacitance ("Ciw" in Partzman's parlance).
Currents flowing through the Cit and Ciw generate magnetic fields H2 and H3, respectively,  which are oriented in a different direction than the desired field H1.  Smudge please comment on this.

# if coax loop is OK, then very much crosstalk is present
It was the entire idea of this experiment to demonstrate this.
This means the the toroidal coil should be wound with a special scheme (analogous to weave1 or weave2), that will cancel its circumferential induction.

P.S.
The Cit for one of my coils is 175pF (measured at 100kHz with an LCR meter).
The Ciw for a pair of my coils is 19.7pF at 2mm, 8.4pF at 9mm, 5.8pF @ 16mm, 4.3pF at 23mm (all measured at 100kHz when coaxial and parallel)

My detailed coil dimensions and other data are here.

   
Group: Experimentalist
Hero Member
*****

Posts: 1365
Yes, the direction of H1 & H2 appears correct now.
What about H3 field generated by the Cit ?

Here is my depiction of the Cit H Field.  This is maybe not easy to visualize because with the flat wound coil, Cit is a spiral and the H Field completion is at each end with the lead outs.  Also, the S flow which is not shown is entering the capacitance area from each side due to EXH .

Regards,
Pm
   

Group: Moderator
Hero Member
*****

Posts: 3214

Quote
Yes, the erratic performance could be due to the construction of the loop (the aluminum shield does not solder well and is prone to bad contacts and shorts), however there is another explanation which should be investigated further.

Namely, does the axial* movement of the sensing loop cause the received signal to go through zero amplitude?   
On the video, I saw you making only the radial move with the sensing loop. I love that glass cube, btw !

It is important that the sensing loop is very planar (flat) - its shadow should be a straight line.  The planes of the 3 coils must be parallel all the time. A 2x thinner wire would allow for more selective axial positioning, too (e.g. the same wire that the toroidal coil is made with).

*  "axial" as in moving along the axis of the pancake coils and the sensing loop, ...in other words: initially close to one pancake, next exactly between pancakes and finally close to the other pancake.

I used a 1mm magnet wire as loop now and made some axial movements which showed NO "received signal to go through zero amplitude".
There is a dip in the middle from 10Vpp with loop against either (rear / front) coil and 8Vpp in the middle.

Another observation is the sweep from 1Khz to 10Mhz with this loop, which shows a high amplitude signal BEFORE resonance when the is loop against the front pancake coil and a high amplitude signal AFTER resonance when the loop is against the rear pancake coil, see screenshots.


Quote
# self resonance frequency very (to) low 3.3Mhz for NMR

This means that i have to reduce the number of turns on the pancake coils to get to NMR resonance (~4.2Mhz) which causes that it does not overlap on the toroidal coil completly.

Itsu
   

Group: Professor
Hero Member
*****

Posts: 2418
This means that i have to reduce the number of turns on the pancake coils to get to NMR resonance (~4.2Mhz) which causes that it does not overlap on the toroidal coil completly.
...or make  pancake coils with a double parallel windings.
...or increase the spacings between the turns to reduce Cit.
...or both.

Notice that my coils have a higher SRF. This is due to their slightly different diameter or larger spacing between turns provided by the silk braid,...or both.

I used a 1mm magnet wire as loop now and made some axial movements which showed NO "received signal to go through zero amplitude".
There is a dip in the middle from 10Vpp with loop against either (rear / front) coil and 8Vpp in the middle.
I expected that dip. Perhaps it is only -2V deep because the sensing loop is not ideally parallel to the pancake coils....or the capacitive coupling makes up for the remaining 8V of amplitude, after all.

Another observation is the sweep from 1Khz to 10Mhz with this loop, which shows a high amplitude signal BEFORE resonance when the is loop against the front pancake coil and a high amplitude signal AFTER resonance when the loop is against the rear pancake coil, see screenshots.
This would suggest a capacitive coupling to pancake coils which are not driven symmetrically with respect to ground. Does the loop made out of coaxial wire also manifest this variation with the same movement ?

What if you connect only one end* of that 1mm wire sensing loop to the tip of the scope probe and short the probe's tip to ground with a 10kΩ resistor (that can be done on the probe itself) ? 
This will treat the entire sensing loop as a plate of a capacitor. 
How much voltage do you see when getting closer and further away from the pancake coil which is driven asymmetrically wrt ground ?

* hold the other end in the air with tape to keep its circular shape but without shorting the loop
« Last Edit: 2020-07-27, 16:03:30 by verpies »
   

Group: Professor
Hero Member
*****

Posts: 1502
Here is my take on the H2 and H3 fields, see images below.  Note that I show the Ciw displacement current going across the center of the assembly, and really it depends on where the Ciw capacitance resides, going from the "hot" end of one coil to the "cold" end of the other and that could be a current that is not horizontal, and that would alter the H3 field pattern.

Smudge
   

Group: Professor
Hero Member
*****

Posts: 2418
@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.
   

Group: Moderator
Hero Member
*****

Posts: 3214
I just realized that I never posted any plots of my naked bucking coils connected in series (coaxial & parallel). These plots are attached below with different spacings between the coils indicated in the filename. The │S21│ plots have linear vertical scales, starting from 1 at the top. The phase is in degrees (depicted linearly on the vertical axis).

Please remember that just because the coil strongly attenuates the signal being transmitted THRU it ( manifested as low │S21│ ), does not mean that the reactive current circulating IN it* is small.
In fact quite the opposite can be true because of "Current Magnification"* determined by the Q of the coil.

* as in a parallel LC network.


All (animated).gif


Nice animated gif from those plots, shows nicely what the between distance does.

Below a plot of my new pancake coils at 16mm apart with similar settings as you:
Blue S21 LinMag, Purple S21 phase (22.5°/div).

Taken from the nanoVNA device as the PC app (nanoVNA saver) does not allow Linear settings.


EDIT:  just found out that the PC app does show |S21| as linear, see right bottom.     S21 Phase is right top.


Itsu
   

Group: Moderator
Hero Member
*****

Posts: 3214

Quote
This would suggest a capacitive coupling to pancake coils which are not driven symmetrically with respect to ground. Does the loop made out of coaxial wire also manifest this variation with the same movement ?

Yes it does but less.



Quote
What if you connect only one end* of that 1mm wire sensing loop to the tip of the scope probe and short the probe's tip to ground with a 10kΩ resistor (that can be done on the probe itself) ?
This will treat the entire sensing loop as a plate of a capacitor.
How much voltage do you see when getting closer and further away from the pancake coil which is driven asymmetrically wrt ground ?

* hold the other end in the air with tape to keep its circular shape but without shorting the loop

The front pancake coil (driven with red FG lead) gives 17Vpp on the 10K terminated loop when touching, the rear pancake coil (connected to the black FG lead) gives 22Vpp when touching.


 
Itsu
   

Group: Professor
Hero Member
*****

Posts: 2418
The front pancake coil (driven with red FG lead) gives 17Vpp on the 10K terminated loop when touching, the rear pancake coil (connected to the black FG lead) gives 22Vpp when touching.
How much difference to that test does the connecting of  the scope's ground to FG's ground (at coil's terminal) make  ?
   

Group: Moderator
Hero Member
*****

Posts: 3214
So without the 10K but with scope and FG grounds tight together its:  front 20Vpp,  rear 27Vpp.
« Last Edit: 2020-07-27, 20:41:57 by Itsu »
   
Group: Experimentalist
Hero Member
*****

Posts: 1365
Here is my take on the H2 and H3 fields, see images below.  Note that I show the Ciw displacement current going across the center of the assembly, and really it depends on where the Ciw capacitance resides, going from the "hot" end of one coil to the "cold" end of the other and that could be a current that is not horizontal, and that would alter the H3 field pattern.

Smudge

Smudge,

I'm trying to understand what you're explaining so I will focus mainly on the H Field creating the induction in the solenoid coil.  Do I understand correctly that your H3 Field creates the induction in the offset solenoid coil?

I originally thought that the H3 Field you show going across the center of the assembly was the cause of the solenoid induction but it didn't fit the experimental results IIRC.  I now view the Ciw as two circular plates parallel to one another which should exhibit a single direction H Field filling the cross section of those plates.

If I understand your view, I should be able to position two solenoid coils, one on top of the other in the cross sectional area and should see opposite polarities on their output, correct?

Regards,
Pm
   

Group: Professor
Hero Member
*****

Posts: 2418
So without the 10K but with scope and FG grounds tied together its:  front 20Vpp,  rear 27Vpp.
No,no, with the scope and FG grounds tied together AND with the 10kΩ resistor in place (from tip to ground).

Also, since you i-probe is does not require a galvanic connection to the sensing loop, what happens when you short this loop and put the i-probe on it (thus making a giant isolated H-probe) ?
Does the amplitude of the signal sensed this way go through zero as you move the loop axially ?
   

Group: Moderator
Hero Member
*****

Posts: 3214

Quote
No,no, with the scope and FG grounds tied together AND with the 10kΩ resistor in place (from tip to ground).


ok,  i thought so, but as nothing changed then, i went for the removal of the 10K.   
So nothing changes when connecting scope and FG grounds together and 10K between scope tip and ground (front 17Vpp,  rear 23Vpp)



Quote
Also, since you i-probe is does not require a galvanic connection to the sensing loop, what happens when you short this loop and put the i-probe on it (thus making a giant isolated H-probe) ?
Does the amplitude of the signal sensed this way go through zero as you move the loop axially ?


Using the H-probe is again kind of cumbersome as it does not fit inbetween the pancake coils, but with some fiddling and increasing the distance, it looks like
the front pancake (red lead FG) gives more signal then the rear pancake (black FG lead) without any dip or peak in the middle (linear).

Itsu

 
   

Group: Professor
Hero Member
*****

Posts: 2418
,...it looks like the front pancake (red lead FG) gives more signal then the rear pancake (black FG lead) without any dip or peak in the middle (linear).
WTF?!  The H-sensing loop is galvanically isolated from the scope's probe completely.  Also, it should not couple preferentially to one of the pancakes and the sensed H-field's amplitude should cross zero as it is moved axially.
Smudge, just consider the flux lines that penetrate the sensing loop's plane.

Using the H-probe is again kind of cumbersome as it does not fit in between the pancake coils
Oh, sorry - I forgot about this silly/serious problem.
Perhaps you can twist an extension in that sensing loop wire like on the attached diagram (I drew the twist with different colors but it can be made out of the same wire).
   

Group: Professor
Hero Member
*****

Posts: 2418
Nice animated gif from those plots, shows nicely what the between distance does.
If you liked it, here is another one.
It depicts the S21 │Z│ and Phase of one naked coil on a linear scale, as the terminating resistance (in the middle break) varies form 0Ω to 200Ω.
Notice, that the impedance magnitudes at Peak#1 & Peak#3 decrease as the coil's terminating resistance at the middle break increasesC.C



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 at the middle break is matched better and better.

* relative to Dip#2
« Last Edit: 2020-07-28, 13:10:42 by verpies »
   

Group: Moderator
Hero Member
*****

Posts: 3214
WTF?!  The H-sensing loop is galvanically isolated from the scope's probe completely.  Also, it should not couple preferentially to one of the pancakes and the sensed H-field's amplitude should cross zero as it is moved axially.
Smudge, just consider the flux lines that penetrate the sensing loop's plane.

Oh, sorry - I forgot about this silly/serious problem.
Perhaps you can twist an extension in that sensing loop wire like on the attached diagram (I drew the twist with different colors but it can be made out of the same wire).

That picture is exactly as i tried it first and it gave the result i mentioned.
Lateron i widened the distance between the pancakes and tried the i-Probe on the main loop with similar results.

Let me focus on the first method again for more accurate measurements.

Itsu
   

Group: Moderator
Hero Member
*****

Posts: 3214
If you liked it, here is another one.
It depicts the S21 │Z│ and Phase of one naked coil on a linear scale, as the terminating resistance (in the middle break) varies form 0Ω to 200Ω.
Notice, that the impedance magnitudes at Peak#1 & Peak#3 decrease as the coil's terminating resistance at the middle break increasesC.C



You can see that the │Z│ Peak#3 at 30MHz is due to reflection since it decreases significantly as the coil's termination at the middle break is matched better and better, while the │Z│ Peak#1 stays relatively constant at 4.7MHz, which makes it look like it is related more to the parallel LC self-resonance of the coil (i.e. L*Cit resonance), ...rather than to the transmission-line like reflections.

Nice, so it goes from a high Q reactive environment to a low Q resistive one.
   

Group: Professor
Hero Member
*****

Posts: 1502
I'm trying to understand what you're explaining so I will focus mainly on the H Field creating the induction in the solenoid coil.  Do I understand correctly that your H3 Field creates the induction in the offset solenoid coil?

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.

Quote
I originally thought that the H3 Field you show going across the center of the assembly was the cause of the solenoid induction but it didn't fit the experimental results IIRC.  I now view the Ciw as two circular plates parallel to one another which should exhibit a single direction H Field filling the cross section of those plates.

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.

Quote
If I understand your view, I should be able to position two solenoid coils, one on top of the other in the cross sectional area and should see opposite polarities on their output, correct?

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

   
Pages: 1 ... 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 [18] 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 ... 35
« previous next »


 

Home Help Search Login Register
Theme © PopularFX | Based on PFX Ideas! | Scripts from iScript4u 2021-10-26, 18:12:13