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Author Topic: Smudge proposed NMR experiment replication.  (Read 3165 times)
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...
 
I tried your TG / SA setup with severall resistors between 1K and 120K Ohm, but it does not want to reveal its resonance point  :(

...

Hi Itsu and Verpies,

Sorry to chime in,
Itsu, you may wish to test your TG / SA setup  using a "normal" series LC circuit instead of the pancake coil? 
To check the setup, use your toroid type T520-2 powdered iron core on which 6 turns gave you 24uH some time ago (this is from a last year post of yours) and make any coil on it in the range  110 -116 uH and resonate it with a series capacitor (18-22 pF) to get resonance around 3 - 3.5 MHZ range.   
I mean using Verpies's drawing with the series resistor but replace the series L1 and L2 in it by the 'normal' series LC circuit to see how it behaves.  This series LC circuit should give a well visible dip (several dB attenuation) at its resonant frequency because it should have higher overall Q factor than the pancake coil(s) have.   

However,  the question may arise, what self resonance a single pancake coil in itself may have: a series or a parallel resonance or both?  In case it has a parallel resonance, then the pancake coil may be connected directly between the TG  - SA 'center' pins  where the optional resistance is shown in Verpies drawing.  And a dip should appear at the parallel resonant frequency while a peak should manifest at a series resonant frequency.  Do you agree?

Gyula
   

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I wonder what its selfresonance frequency will be.
That is hard to measure by SA+TG (actually I use VNA) because my coil rings for a very long time and if I do not put a long delay between sweeps, then when the next sweep comes, the coil is still ringing from the previous one ...and I get garbage results (grass).
Actually, the coil needs a pause to settle not only between sweeps but also between acquisition of adjacent frequency points! ...this is an example where custom software gives greater control of the acquisition process.
Watch out for this gotcha, with DUTs that settle for a long time !

I've seen peaks around 3MHz, 6MHz and 35MHz. The grass is around the second one.
« Last Edit: 2020-06-29, 09:44:47 by verpies »
   

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In case it has a parallel resonance, then the pancake coil may be connected directly between the TG  - SA 'center' pins  where the optional resistance is shown in Verpies drawing.  And a dip should appear at the parallel resonant frequency while a peak should manifest at a series resonant frequency.  Do you agree?
Itsu has already tried connecting the pancake coil between the TG  - SA 'center' pins and he did get at dip, see here. That dip was noisy because in such setup, the pancake coil, which behaves as a parallel LC circuit, had the highest impedance and kept the signal amplitude low at the SA's input. This is to be expected with such connection.
The alternate connection is designed to yield the maximum amplitude at resonance.  To get a peak (not dip) with a parallel LC circuit one can also measure the magnitude of its impedance at various frequencies (having an impedance bridge or VNA helps). Impedance will be highest at the resonance.

This is illustrated on the two plots of my coil below.  The first one illustrates the magnitude of the impedance of the coil vs.frequency.  Notice that near the peak the impedance reaches ~7kΩ !
The second one illustrates only the reactance of the coil vs frequency.  Reactance is the imaginary component of impedance and is supposed to be ZERO at resonance (because at resonance, the inductive reactance and capacitive reactance cancel each other).  ...and they do that at 6.3MHz.
The second plot also proves that the pancake coil behaves as an LC circuit up to 8MHz (anything above the 0 represents inductive reactance and below - capacitive reactance)
« Last Edit: 2020-06-29, 09:38:07 by verpies »
   

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I am also planing a novel way of measuring the self resonance of the pancake coil.
I will short the pancake coil (cats like to be petted and coils like to be shorted  C.C ) and I will put my non-contact current probe around this short.

Then I will discharge my HV stun gun into a 1 turn loop of Litz wire positioned somewhat far from the pancake coil.
This will allow me to observe the decaying self-oscillations of the pancake's current on the scope and independently from any disturbances, that the 1 turn loop may cause, because once the spark terminates, the 1 turn loop will become effectively open-circuited (effectively non-existent).

I think the same could be accomplished with a little less extreme HV source and an avalanching BJT transistor in place of the spark gap.
   

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Hi Itsu and Verpies,

Sorry to chime in,
Itsu, you may wish to test your TG / SA setup  using a "normal" series LC circuit instead of the pancake coil? 
To check the setup, use your toroid type T520-2 powdered iron core on which 6 turns gave you 24uH some time ago (this is from a last year post of yours) and make any coil on it in the range  110 -116 uH and resonate it with a series capacitor (18-22 pF) to get resonance around 3 - 3.5 MHZ range.   
I mean using Verpies's drawing with the series resistor but replace the series L1 and L2 in it by the 'normal' series LC circuit to see how it behaves.  This series LC circuit should give a well visible dip (several dB attenuation) at its resonant frequency because it should have higher overall Q factor than the pancake coil(s) have.   

However,  the question may arise, what self resonance a single pancake coil in itself may have: a series or a parallel resonance or both?  In case it has a parallel resonance, then the pancake coil may be connected directly between the TG  - SA 'center' pins  where the optional resistance is shown in Verpies drawing.  And a dip should appear at the parallel resonant frequency while a peak should manifest at a series resonant frequency.  Do you agree?

Gyula

Hi Gyula,

thanks to chime in, yes as verpies mentioned, i tried "the TG  - SA 'center' pins" setup which i normally do as the other setup is kind of unnatural to me (shorting out the TG / SA), but of course works OK on different circuits as explained by verpies.

But this shows with the pancake coil some, for me, strange outcome (see post #61) which probably is caused by the specific series bucking mode setup of the both pancake coils.

Changing over to the verpies setup (TG / SA parallel) shows virtually nothing.

Regards Itsu
   

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verpies,

thanks for the plots, nice stuff.

As max. impedance in a parallel LC is at resonance, so that would be at 7.8Mhz looking at plot #1.
But looking at plot #2, Xc and Xl cancel each other out at 6.3Mhz which also points to resonance.

I would have expected to have both resonances to be shown at the same frequency.

Again there is far more there then meets the eye  :D


I will try to experiment with your novel way of measuring the self resonance of the pancake coil  O0

Itsu
   

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As max. impedance in a parallel LC is at resonance, so that would be at 7.8Mhz looking at plot #1.
But looking at plot #2, Xc and Xl cancel each other out at 6.3Mhz which also points to resonance.

I would have expected to have both resonances to be shown at the same frequency.

Again there is far more there then meets the eye  :D
Yes, apparently something increases the REAL part of the impedance after the IMAGINARY part (reactance) gets cancelled. There is more to the coil's impedance then mere imaginary reactance. For example, the real resistance of the coil matters, too (in mine it is 0.5Ω).
The skin effect works in the proper direction to explain this since as the frequency increases, it artificially increases the real resistance of the wire (8x in your case) but I don't think it can make such a large difference because 8x the real resistance of my coil is only 4Ω and not the 741Ω difference we see.  So maybe some standing waves or the proximity effect are responsible for this increase.

But most likely I still have problems with my measurement technique, i.e  the coil keeps ringing and does not settle between the acquisition of adjacent frequency points.

Ringing coils are also a big problem for the reception of the AC magnetic field form the Larmor precession in NMR.  It is a problem without RF switching because they both occur at the same frequency and are only separated in time domain (yes, yes in spatial direction, too, but that does not matter practically).
« Last Edit: 2020-06-29, 12:54:32 by verpies »
   
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All,

It may be that the SRF measurement is getting somewhat overly complicated.  Here is an example of measuring the pancake coil shown below using a signal generator with a 50 ohm source impedance, a 100k ohm series resistor and the coil under test connected between the output of the 100k resistor and ground.  A 50kHz pulse is fed to this network and the SRF is easily seen in the ring down on the scope trace.

Regards,
Pm
   

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We've already tried it, see here.  We've gotten different results with a very similar coil.

So, I shorted the pancake coil with a 0.22Ω resistor. Stimulated it with a short magnetic pulse from a 1T coil (20cm away, parallel, on the same axis) powered by HV and spark gap ( Tesla style ! ).
Scoping across this 0.22Ω resistor gives me this result:



As you can see, the magnetic pulse induces approximately 2Apk-pk current pulse in the pancake coil and as soon as the spark terminates, the pancake coil is free to oscillate in isolation ( i.e. without any interference from the 1T coil )
Dead shorting the pancake coil and using a contactless current probe on the short, works too.  I didn't post it because my current probe has 30MHz bandwidth limitation and doesn't look as sharp.
« Last Edit: 2020-06-29, 16:28:46 by verpies »
   

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But the ringing signal looks distorted, so is this the 3MHz, 6MHz and 35MHz signals mentioned earlier all together?

It finally settles down to about 20Mhz (50ns between 2 peaks)?

Itsu
   

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But the ringing signal looks distorted, so is this the 3MHz, 6MHz and 35MHz signals mentioned earlier all together?
It is distorted while the arc is still going but as soon as the arc terminates it calms down. The arc has a 1ns rise time and 600ns fall time. Any ideas how to make it shorter without blowing on it ? Magnets ?
Anyway, when it calms down, counting the peaks over 5 divisions yields 21MHz.  Before you ask: "Do you see it on the VNA impedance measurement ?",  A: Yes, I do.
   

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Ok, yes, blowing air over the arc i have seen Tinsel do that, magnets also suppose to work.
   

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I put together the pancake coils (15mm apart), the tube coil inbetween and the 2 ring magnets on the outside for some resonance testing.

The input circuit was as shown below in the drawing as specified by Smudge.
I used a 100x higher value (10nF) then the 100pF trimmer.

The FG is sweeping again from 1Khz to 10Mhz, so each scope division is 1Mhz.

Resonance moves from 1.4Mhz (peak) to 2.8Mhz, so way to low for the needed 4.3Mhz NMR frequency.

So something has to be adjusted.

Video here:  https://www.youtube.com/watch?v=QunSj9id3ME

Regards itsu
   

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The FG is sweeping again from 1Khz to 10Mhz, so each scope division is 1Mhz.
Resonance moves from 1.4Mhz (peak) to 2.8Mhz, so way to low for the needed 4.3Mhz NMR frequency.
So something has to be adjusted.
Well, the trimmer cap is certainly too big for the coils.  I would be interested, how the removal of the toroidal coil alters that FM sweep. ...and what about the magnets?
The really bad news will be when you discover, that the signal injected into the pancake coils appears on the toroidal coil ...without water.
« Last Edit: 2020-06-30, 23:07:48 by verpies »
   
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Hi Itsu and Verpies,

Thanks for your answers,  and I should have read the earlier posts to collect all the info before my post.   8)

Itsu,  I think in the present setup so far what you could do to increase the resonant frequency for the pancake coils is to reduce their number of turns to bring down the 115 uH to around 60-80 uH or so. 

Gyula




...
Resonance moves from 1.4Mhz (peak) to 2.8Mhz, so way to low for the needed 4.3Mhz NMR frequency.

So something has to be adjusted.
...
   

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Well, the trimmer cap is certainly too big.  I would be interested, how the removal of the toroidal coil alters that FM sweep. ...and what about the magnets?
The really bad news will be when you discover, that the signal injected into the pancake coils appears on the toroidal coil ...without water.

I can check if removing the magnets and the toroidal coil has any influence (probably yes) on the resonance point, but the bottom line is, they have to be there.

I did put a probe on the toroidal coil output and i did saw some signals there, but it depends on if there also is a probe on the input (grounding problem) or not at what frequency and amplitude.
I will do some further tests.

Itsu
   

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Hi Itsu and Verpies,

Thanks for your answers,  and I should have read the earlier posts to collect all the info before my post.   8)

Itsu,  I think in the present setup so far what you could do to increase the resonant frequency for the pancake coils is to reduce their number of turns to bring down the 115 uH to around 60-80 uH or so. 

Gyula

Hi Gyula,

i think reducing the number of turns of the pancake coils would be the only solution.

Originally, the design was for 11 turns each, see picture, but after some debate it was increased to the present 30 turns.

Also Smudge mentioned here in post #23  https://www.overunityresearch.com/index.php?topic=3924.msg82567#msg82567 that according to his FEMM calculations, the pancake coils in series opposing would be around 67uH, which match better with your 60 - 80 uH needed for 4.3Mhz.

Itsu.
   

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I can check if removing the magnets and the toroidal coil has any influence (probably yes) on the resonance point, but the bottom line is, they have to be there.
Yes they do, but judging whether their presence increases or decreases this frequency...and by how much, points the way forward.  There are things that can be done, such as spacing, isolating, litzing, antilongitudal windings.  There is even a difference whether the center of the signal coax is connected to the middle of the pancake coil vs. the outer diameter.

I did put a probe on the toroidal coil output and i did saw some signals there, but it depends on if there also is a probe on the input (grounding problem) or not at what frequency and amplitude.
Yes, signal crosstalk through the ground path is a show stopper. 
« Last Edit: 2020-06-30, 11:54:59 by verpies »
   

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i think reducing the number of turns of the pancake coils would be the only solution.
I told you so here.
OK, I still have some 1mm Litz wire to make smaller coils, which will match yours.  I just have to buy more SMA panel connectors and CD spools for coil-formers. Because of this, my O.D. will be 127mm again.

   
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Don't forget that the NMR frequency is determined by the static field within the water from the magnets.  Has this field been measured yet?  Moving the magnets further apart will reduce the field and the NMR frequency, but at the expense of the field not being parallel to the horizontal axis.  So if winding new coils is a pain then perhaps moving the magnets will allow some NMR work to proceed at a lower frequency.

Smudge
   

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Don't forget that the NMR frequency is determined by the static field within the water from the magnets. 
Moving the magnets further apart will reduce the field and the NMR frequency, but at the expense of the field not being parallel to the horizontal axis.
Yes, my dear doctor. This is elementary


Has this field been measured yet?
I don't think Itsu has a calibrated gaussmeter.
   

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Don't forget that the NMR frequency is determined by the static field within the water from the magnets.  Has this field been measured yet?  Moving the magnets further apart will reduce the field and the NMR frequency, but at the expense of the field not being parallel to the horizontal axis.  So if winding new coils is a pain then perhaps moving the magnets will allow some NMR work to proceed at a lower frequency.

Smudge

Technical specifications for the ring magnets  grade Y30

Gauss (G)      Tesla (T)    kOe            kA/m      kOe            kA/m    MGOe    kJ/m 3    °C
≥ 3900          ≥ 0.39       ≥ 2.3           ≥ 184    ≥ 2.3         ≥ 188    ≥ 3.4    ≥ 27.6    ≤  250

from: https://www.magnet-shop.com/ferrite/ringmagnets/ringmagnet-140.0-x-60.0-x-20.0-mm-y30-ferrite

Looking for a decent gaussmeter app for my Iphone, most only handle uTesla's.

Itsu

   

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Yes they do, but judging whether their presence increases or decreases this frequency...and by how much, points the way forward.  There are things that can be done, such as spacing, isolating, litzing, antilongitudal windings.  There is even a difference whether the center of the signal coax is connected to the middle of the pancake coil vs. the outer diameter.
Yes, signal crosstalk through the ground path is a show stopper.

Removing the magnets and/or the toroidal coil has no noticable influence on the pancake coils resonance frequency range.

So i guess i need to start removing some turns.....

Itsu
   

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Removing the magnets and/or the toroidal coil has no noticable influence on the pancake coils resonance frequency range.
That is excellent news !
   

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I went in and removed some turns of the both pancake coils so i am left with 11turns each.

Sweeping with the FG now shows the 4.3Mhz resonance peak is somewhere in the middle of the 100pF trimmer cap.

I will do some further measurements tomorrow.

Regards Itsu
   
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