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Author Topic: Smudge proposed NMR experiment replication.  (Read 3152 times)

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I went in and removed some turns of the both pancake coils so i am left with 11turns each.
What's the O.D. and I.D. ?
   

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O.D. = 112mm,    I.D. = 89mm.


Itsu
   

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Toroidal coil inserted inbetween the pancake coils and loaded with a 100pF trimmer cap.

Adjusted the pancake coils to resonate at 4.3Mhz, see yellow peak.
Then adjusted the toroidal coil trimmer cap to max output on 4.3Mhz, see red trace (differential probing).

Input trimmer cap about half way (50pF?), output trimmer cap barely active (20pF?).


The yellow trace seems to split, not sure what that 2th yellow and red peak / artifact is.

Itsu
   
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I have just done some FEMM simulations for the pair of ring magnets.  FEMM has data for ceramic 5 that closely matches the Y30 data, ie Br = 0.39T.  I did two runs, (a) with the separation between the magnet faces of 14mm the field in the water region is 0.124T, and (b) with a separation of 28mm the field drops to 0.083T.  From Itsu's videos it looks like the 28mm separation is about right.  That 0.083T should give a NMR frequency of 3.53MHz.  However this is only a guide because FEMM is not a 3D simulation, it gives the field between infinitely long strips, see image below.

Smudge
   

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Thanks Smudge,

i measured the separation between the magnet faces to be 25mm in my present setup.

So what for an effect should i be looking for when sweeping the frequency and adjusting the trimmer pots for any NMR?
What would the range of the output load be, 100 Ohm?

Itsu
   

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What would the range of the output load be, 100 Ohm?
Please take out your SWR bridge, connect it to the SA and measure the magnitude of the complex impedance* of the toroidal coil at the frequencies of interest.
This will give you the empirical impedance of the toroidal coil (...and its cap) providing better data for choosing a good load for this coil.


* √(R2+X2)
   
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Thanks Smudge,

i measured the separation between the magnet faces to be 25mm in my present setup.

OK, my FEMM gives a field of 0.0895T at that separation which equates to a NMR frequency of 3.8Mhz.

Quote
So what for an effect should i be looking for when sweeping the frequency and adjusting the trimmer pots for any NMR?
What would the range of the output load be, 100 Ohm?
First off I would want to minimise any unwanted coupling to the toroidal output coil, so what you posted earlier today with red and yellow traces needs looking at.  Ideally with no water present and both coils tuned to the NMR frequency we want zero coupling.  That may not be achievable.  That split or dip in the yellow trace is to be expected when there is coupling since the other coil is sucking energy from the pancake coil.  It does show you that the other coil's resonance is at or about the same frequency.  With the toroidal coil's field at right angles to the pancake coil's field there should be zero magnetic coupling, so it is probably down to capacitive coupling.

Although you correctly have the pancake coils in series opposing there are two ways this can be set up, either the outer connection of one pancake is connected to the inner connection of the other, or the connection is at both outers or both inners (the opposing field determined by which way round the pancakes are placed).  With an unbalanced drive from the signal source I can't see any way to eliminate capacitive coupling for either configuration.  Ideally it requires a balanced drive, then one configuration is better than the other.  If the drive is at both outers while the series connection is at both inners (or vice versa), then the capacitive coupling is minimised.  So I am sorry to say that you need a wide bandwidth balun, something you have already played with on another thread.

One you achieve minimum unwanted coupling, then you can add water and look for the NMR signal which by its gyratory action couples the 90 degree input and output fields together.  While searching for this I would not necessarily deliberately load the output coil, I would go for a high load resistance if any.  This keeps the output coil Q high which give you a better chance of finding a small NMR signal , but at the expense of having to tune both coils at each search frequency.  Not sure using frequency sweep is the right way to do this, but I could be wrong there.  I had envisaged a manual "sweep" in small steps.

Smudge
   

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Please take out your SWR bridge, connect it to the SA and measure the magnitude of the complex impedance* of the toroidal coil at the frequencies of interest.
This will give you the empirical impedance of the toroidal coil (...and its cap) providing better data for choosing a good load for this coil.


* √(R2+X2)

Thanks,  i will play with that tonight.

Itsu
   

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OK, my FEMM gives a field of 0.0895T at that separation which equates to a NMR frequency of 3.8Mhz.
First off I would want to minimise any unwanted coupling to the toroidal output coil, so what you posted earlier today with red and yellow traces needs looking at.  Ideally with no water present and both coils tuned to the NMR frequency we want zero coupling.  That may not be achievable.  That split or dip in the yellow trace is to be expected when there is coupling since the other coil is sucking energy from the pancake coil.  It does show you that the other coil's resonance is at or about the same frequency.  With the toroidal coil's field at right angles to the pancake coil's field there should be zero magnetic coupling, so it is probably down to capacitive coupling.

Although you correctly have the pancake coils in series opposing there are two ways this can be set up, either the outer connection of one pancake is connected to the inner connection of the other, or the connection is at both outers or both inners (the opposing field determined by which way round the pancakes are placed).  With an unbalanced drive from the signal source I can't see any way to eliminate capacitive coupling for either configuration.  Ideally it requires a balanced drive, then one configuration is better than the other.  If the drive is at both outers while the series connection is at both inners (or vice versa), then the capacitive coupling is minimised.  So I am sorry to say that you need a wide bandwidth balun, something you have already played with on another thread.

One you achieve minimum unwanted coupling, then you can add water and look for the NMR signal which by its gyratory action couples the 90 degree input and output fields together.  While searching for this I would not necessarily deliberately load the output coil, I would go for a high load resistance if any.  This keeps the output coil Q high which give you a better chance of finding a small NMR signal , but at the expense of having to tune both coils at each search frequency.  Not sure using frequency sweep is the right way to do this, but I could be wrong there.  I had envisaged a manual "sweep" in small steps.

Smudge

Smudge,  thanks.

I will readjust to around 3.8Mhz.

My pancake coils are connected this way: the inner connection of one pancake is connected to the outer connection of the other, but set up in parallel mirror mode (both coils facing each other).
So i drive the both: one inner, other outer.

I to had in mind the manual "sweep", not the FG sweep i used up till now.

Itsu
   
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Smudge,  thanks.

I will readjust to around 3.8Mhz.

My pancake coils are connected this way: the inner connection of one pancake is connected to the outer connection of the other, but set up in parallel mirror mode (both coils facing each other).
So i drive the both: one inner, other outer.

I to had in mind the manual "sweep", not the FG sweep i used up till now.

Itsu

Then you will not be able to minimise the capacitive coupling between the pancakes and the toroid.  I think you need to not have both coils facing each other, feed at both inner or outer and series connect at the other end.  When fed with a balanced input this puts the radial centre line between the pancake coils at zero electric potential.

Smudge
   

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Ok, but when i flip over one pancake coil, the 4mm thick plexiglas will be inbetween the toroidal coil
and this flipped pancake coil causing some unbalance, see light brown substrate in below picture.

Guess i need to rewind 1 pancake coil opposite direction as the other one.

Itsu
   

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First off I would want to minimise any unwanted coupling to the toroidal output coil, so what you posted earlier today with red and yellow traces needs looking at.
Yes, we should determine what the ratio of this coupling is. e.g. the ratio of the current amplitude fed into the pancake coils vs. received by the toroidal coil.
For the calculation of this ratio the I/O impedance should be known.

A plot of the insertion loss vs. frequency (via two-port S21 measurement), would also give us information whether the crosstalk is capacitive or inductive in nature, because the impedance of acoustic and capacitive crosstalk decreases with frequency, and the impedance of inductive crosstalk increases with frequency.

Ideally with no water present and both coils tuned to the NMR frequency we want zero coupling.
That would be ideal, indeed.

With the toroidal coil's field at right angles to the pancake coil's field there should be zero magnetic coupling, so it is probably down to capacitive coupling.
No. You forgot that Itsu's toroidal winding has only one layer ...and that means that this coil has non-negligible sensitivity to the circumferential currents present in the pancake coils.
Another way to look at it is noticing that the wires in the pancake coil are not perpendicular to the wires in the toroidal coil. The latter has a winding pitch, which results in 1 turn circumferential component.
Of course, this can be cancelled with properly connected bifilar coil or even-layered coil.

Although you correctly have the pancake coils in series opposing there are two ways this can be set up, either the outer connection of one pancake is connected to the inner connection of the other, or the connection is at both outers or both inners (the opposing field determined by which way round the pancakes are placed). 
Winding pancake coils in opposite directions constitutes another option.  This can also avoid the plexi coil-formers being on different sides of the coils.
Also, an effort should be made to cancel their collective radial current, which arises as the electric charges move from the outer to the inner radius of the pancake coil (or vice versa).

With an unbalanced drive from the signal source I can't see any way to eliminate capacitive coupling for either configuration.  Ideally it requires a balanced drive, then one configuration is better than the other.  If the drive is at both outers while the series connection is at both inners (or vice versa), then the capacitive coupling is minimised.  So I am sorry to say that you need a wide bandwidth balun, something you have already played with on another thread.
I agree that balanced drive and Rx will help a little.

One you achieve minimum unwanted coupling, then you can add water and look for the NMR signal which by its gyratory action couples the 90 degree input and output fields together. 
The unique property of this signal is that it persists after the stimulus from the pancake coils ceases, so temporal separation is another method to minimize the crosstalk.

P.S.
At the risk of sounding nauseating. Litz wire provides clear benefits besides presenting lower AC impedance at RF.
For example, I just noticed that an unlooped piece of 1mm solid copper wire gets too hot to touch in the proximity of a 75W 100kHz inductive heater (w/ solenoidal coil).
Notice, that this is a straight piece of wire, which is not connected in a loop, so circular eddy currents in it, can have the maximum diameter of 1mm.
Yet, significant energy transfer* happens anyway. It happens regardless of this piece of this wire being oriented perpendicularly to the heater's coil or parallel to it.  It also does not matter whether this piece of wire is not inside the heater's solenoid - 5mm outside of heater's coil is enough (with perpendicular and parallel wire orientations, too).

An unlooped piece of a Litz wire (of the same diameter) does not experience any noticeable heating in any orientation and position.


* I'll let Smudge calculate whether eddy currents in this straight unlooped solid wire, which can raise its temperature by +25ºC, can affect the conductivity along this wire or induce electric potentials along this wire ...and how much quantitatively.

« Last Edit: 2020-07-01, 19:51:09 by verpies »
   
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Ok, but when i flip over one pancake coil, the 4mm thick plexiglas will be inbetween the toroidal coil
and this flipped pancake coil causing some unbalance, see light brown substrate in below picture.

Guess i need to rewind 1 pancake coil opposite direction as the other one.

Itsu
Or add another layer of plexiglas.  Do the  same to the other coil, then the magnet field and NMR frequency will be even lower.

Smudge
   

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Or add another layer of plexiglas.  Do the  same to the other coil, then the magnet field and NMR frequency will be even lower.
...and the fringing of the static magnetic field - higher.
Also, doesn't the S/N ratio of the NMR signal decrease with the magnetic flux density of the static field and the frequency?
   
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No. You forgot that Itsu's toroidal winding has only one layer ...and that means that this coil has non-negligible sensitivity to the circumferential currents present in the pancake coils.
Yes I agree, but that can be minimised by returning the end of the toroidal coil via a single "turn" following the water ring.

Quote
Also, an effort should be made to cancel their collective radial current, which arises as the electric charges move from the outer to the inner radius of the pancake coil (or vice versa).
I agree and if the radial currents in both coils are in the same direction then this should be achieved.
Quote
P.S.
At the risk of sounding nauseating. Litz wire provides clear benefits besides presenting lower AC impedance at RF.
For example, I just noticed that an unlooped piece of 1mm solid copper wire gets too hot to touch in the proximity of a 75W 100kHz inductive heater (w/ solenoidal coil).
Notice, that this is a straight piece of wire, which is not connected in a loop, so circular eddy currents in it, can have the maximum diameter of 1mm.
Yet, significant energy transfer* happens anyway. It happens regardless whether the piece of this wire is oriented perpendicularly to the heater's coil or parallel to it.  It also does not matter whether this piece of wire is not inside the heater's solenoid - 5mm outside of heater's coil is enough (with perpendicular and parallel wire orientations, too).

An unlooped piece of a Litz wire (of the same diameter) does not experience any noticeable heating in any orientation and position.


* I'll let Smudge calculate whether eddy currents in this straight unlooped solid wire, which can raise its temperature by +25ºC, can affect the conductivity along this wire or induce electric potentials along this wire ...and how much quantitatively.
I will look into that.
Smudge
   

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For the time being i have:

# flipped over 1 pancake coil, so both coils "face" the same direction, not mirrored.
# added a piece of 4mm thick plexiglass between the not flipped pancake coil and the toroidal coil.
# added a piece of 4mm thick plexiglass in front of the flipped over pancake coil.
# connected the pancake coils so that its driven to both outside connections.

This means that the both pancake coils are 4mm extra away from the toroidal coil and there is an extra 4mm between the pancake coils and the magnets.

The total distance between the magnets is now 33mm.

Tuning input for about 3.8Mhz resonance and matching the output to that shows the following signals:
Yellow is input pancake coils
red is differential probing output signal.

So we still have a considerate amount of crosstalk between input and output.

Itsu
   

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Yellow is input pancake coils
Is that the current flowing through the pancake coils ?
   

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No  yellow is the voltage across the pancake coils connection.

Below i have added in green the current through the pancakes.

Red still the differentiale probed output voltage signal

Itsu
   

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Below i have added in green the current through the pancakes.
Is it really the current flowing through the pancakes only ...or the current flowing to the pancakes and caps from the AC source ?
Sorry for pestering...

Red still the differential probed output voltage signal
Is the voltage output from the toroidal coil tweaked & peaked with the var.cap (trimmer) each time the test frequency changes ?

...I would not necessarily deliberately load the output coil, I would go for a high load resistance if any.
I know that this advice was only for signal search but when trying to determine the I/O crosstalk ratio, I really don't like measuring the output voltage amplitude in the unloaded LC circuit of the toroidal coil, because the resonant rise can unpredictably inflate the signal amplitudes there and make it difficult to compare them at several test frequencies (after "peaking").

I think it is really important to determine the crosstalk vs. frequency relationship well, because such measurement allows us to determine whether this crosstalk is predominantly capacitive in nature ...or inductive. I really hope it is not acoustic via Ampere's forces.

Depending on the results, we can deal with it differently. For example capacitive interaction can be shielded with grounded Litz cloth, which does not affect HF magnetic fields (yes, capacitive coupling also falls off with the k of the dielectric and with distance, but distance has other detrimental effects on the entire system). 
Undesirable inductive coupling can be managed by controlling the field geometries more precisely (e.g. cancelling longitudinal currents and generally "squaring" the fields).
For acoustic coupling, preventing the coils from touching each other or transferring ultrasonic vibrations through solid objects its usually enough (copper to air to copper ultrasound transmission is very inefficient)
   

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Trying to answer, but i get:

Forbidden

You don't have permission to access this resource.

Additionally, a 403 Forbidden error was encountered while trying to use an ErrorDocument to handle the request.


Itsu
   

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Send through the proxy......


Quote
Is it really the current flowing through the pancakes only ...or the current flowing to the pancakes and caps from the AC source ?
Sorry for pestering...

No problem, it is the current through the pancake coils at position shown in the diagram below.


Quote
Is the voltage output from the toroidal coil tweaked & peaked with the var.cap (trimmer) each time the test frequency changes ?


Yes, but its very touchy and i can only set it up correctly using the FM sweep setup where i trim the red output signal ontop of the input signal with the output trimmer cap.

But during the switch over to normal sine wave signal display i loose that ability to trim that output cap correctly ontop, so am kind of blind.

I also noticed a 10Vpp yellow signal loss between setups (see above 2 last screenshots, 1 = 28Vpp other 18Vpp yellow signal).

Seems that when setting up the FG trigger port (ground only) to the scopes green probe port it gains 10Vpp.
When removing that FG trigger port connection for doing the normal signal / current probe setup it looses that 10Vpp again.


Means to me we have some serious groundloops with severe impact.

Itsu
   

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Send through the proxy......
Gripe to Peterae !  He will need your IP number to investigate this (error 403 is different than the 406 error with the percent character).

No problem, it is the current through the pancake coils at position shown in the diagram below.
OK, this signal is very immune to interference so you can rely on it. If it was not for the current flowing through the intrawinding capacitance, then this signal would reflect the magnetic field generated by the pancake coils precisely.

Yes, but its very touchy and i can only set it up correctly using the FM sweep setup where i trim the red output signal ontop of the input signal with the output trimmer cap.
Touchy means very selective, which is good. Touchy can also mean that the trimmer cap is too big, and its small adjustments cause large its capacitance changes.
Do you have two trimmer caps? ..the big one and a small one, that you can connect in series ?


I also noticed a 10Vpp yellow signal loss between setups (see above 2 last screenshots, 1 = 28Vpp other 18Vpp yellow signal).
Seems that when setting up the FG trigger port (ground only) to the scopes green probe port it gains 10Vpp.
When removing that FG trigger port connection for doing the normal signal / current probe setup it looses that 10Vpp again.
I had a problem like that once. I used an isolation transformer form an old Ethernet card to feed the trigger signal to the scope (I tried optocouplers too, but they require a PS and are low speed).
Also, I have 1:1 isolation transformers on the 240VAC power supplies of all my gear and I do not use the ground lead on them (unless I want to).

Means to me we have some serious groundloops with severe impact.
You're not kidding!
   

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Quote
Gripe to Peterae !  He will need your IP number to investigate this (error 403 is different than the 406 error with the percent character).

Message send to Peterae

Quote
Touchy means very selective, which is good. Touchy can also mean that the trimmer cap is too big, and its small adjustments cause large its capacitance changes.
Do you have two trimmer caps? ..the big one and a small one, that you can connect in series ?

Yes i have som smaller trimmer caps which i can use in series.

Quote
I had a problem like that once. I used an isolation transformer form an old Ethernet card to feed the trigger signal to the scope (I tried optocouplers too, but they require a PS and are low speed).
Also, I have 1:1 isolation transformers on the 240VAC power supplies of all my gear and I do not use the ground lead on them (unless I want to).

Ok, so i could have had this problem earlier when using the FM sweep setup or when just having that FG trigger port connected (video shot here: https://www.youtube.com/watch?v=P0pTPVgrhFs )
I will try to think of that during these setups.

Itsu
   

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OUR still only accessable via the proxy.

I made a wideband 1:1 balun, what is the best place to put it,  between signal source and the caps or between the pancake coils and caps?

Itsu
   

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OUR still only accessable via the proxy.
Apparently the server "thinks" that you are a dangerous hacker.

I made a wideband 1:1 balun, what is the best place to put it,  between signal source and the caps or between the pancake coils and caps?
If it is a galvanically isolated balun with 1:1 ratio then my answer is "between signal source and the caps".
If the balun ratio was something else than 1:1 then you could be tempted to match the impedance of the pancake coils to the impedance of the signal source transformed by such balun.
   
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