PopularFX
Home Help Search Login Register
Welcome,Guest. Please login or register.
2022-11-29, 08:00:00
News: If you have a suggestion or need for a new board title, please PM the Admins.
Please remember to keep topics and posts of the FE or casual nature. :)

Pages: 1 ... 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 [30] 31 32 33 34 35
Author Topic: Smudge proposed NMR experiment replication.  (Read 74357 times)
Group: Tech Wizard
Hero Member
*****

Posts: 986
Yes, that is ok,  always adjust the trimmer (or trimmers) for max voltage measured across the pancake coils.   

When the FG gives 4.25 MHz and the pancake tank is also tuned to max amplitude which is say 106 Vpp,   the 3 dB bandwidth of the tank could be estimated by detuning the FG to a lower and an upper frequency at which the thank voltage reduces to 0.707 x 106 = 74.9 Vpp for both frequencies.  The difference of these two frequencies will give the loaded bandwidth of the tank circuit, a useful detail.  If the lower frequency is say at 3.9 MHz and the upper frequency is say at 4.6 MHz, then their difference is 0.7 MHz so the 3 dB bandwidth would be B = 0.7 MHz.  This then would give the loaded Q of the pancake tank in this example as QL = f0/B = 4.25/0.7 = 6 

Gyula
   

Group: Moderator
Hero Member
*****

Posts: 3527
Gyula,

i tried to measure the 3 dB bandwidth, but it seems there is a second peak near by in frequency, so the results are not really clear.

So i first made a 2Mhz to 6Mhz sweep of the pancake coils response while tuned for max. voltage on 4.25Mhz using the trimmer caps, see screenshot.

It shows a broad resonance area with some dips and peaks from which i mentioned the 2 strongest.

In the below video i did another long (2min) sweep of the output coil (690K resistor) from 4 to 4.5Mhz while again tuned for 4.25Mhz and i noticed a jitter around the 4.25Mhz.

I think this jitter might be from the these dips and peaks around resonance.

Not sure where this ragged resonance area comes from, probably the 2 pancake coils are not exactly tuned for resonance and kind of interfere with their resonance points.

Video here:   https://youtu.be/K9SoBdijA7o             

Itsu     
   

Group: Moderator
Hero Member
*****

Posts: 3527

When i tune the trimmers for max signal while using the 2 to 6Mhz sweeping, i get a strong (170Vpp) resonance peak lower in frequency around 3.5Mhz (trimmers all 3 on max. capacitance), see screenshot.

Also here we see these bites out of the trace, pointing to me to the both coils interfering with their individual resonance points.

Why we have such a much stronger resonance peak lower in frequency i can't explain as i would have thought that using the trimmers we could get this strong resonance peak anywhere in the range of the caps.

Perhaps the output coil at the same frequency inhibits the resonance to build up (Q).

 
Itsu
   
Group: Tech Wizard
Hero Member
*****

Posts: 986
Hi Itsu,

Thanks for doing all these tests.  My thoughts: Chances are that the pancake coils have their own resonant frequencies by themselves and they are coupled and both are included in forming a parallel LC circuit too. 
 There may be a certain influence on them from the output coil too but it may not be significant. 

If the self resonance of the pancake coils happens to be in the 3 to 4 MHz range accidentally, this may give such behaviour you found by the frequency sweep.  Whether this oddity is a bad or neutral thing from the searched effect point of view I cannot judge it.

Gyula
   

Group: Professor
Hero Member
*****

Posts: 1716
Hi Itsu,

The anomaly you are looking for is a strong resonance peak, so that 3.5MHz peak may be important.  Is it still there when the water is removed?  If it can be identified with the presence of the water it could be what you are looking for.  After all your hard work you deserve a breakthrough.  My previous estimate of the expected proton resonance frequency was based on an estimate of the static field from the magnets, and that may have been wrong.  It would have been nice if you had a miniature proton spin magnetometer that enabled you to measure that static field....Oh but of course that is what you are building, silly me!  The early work on NMR searching for the predicted nuclear resonance back in 1948 nearly failed because the frequency was just outside the range being searched.  Here's hoping for that breakthrough.

Smudge
   

Group: Moderator
Hero Member
*****

Posts: 3527

Gyula,  Smudge,

the 3.5Mhz strong peak is still there without the water.

So i will expand the sweeping range somewhat (3 to 5Mhz f.i.) to see if another strong peak appears with the water added only.

Itsu
   

Group: Moderator
Hero Member
*****

Posts: 3527

The last few days i was busy with characterizing the below shown setup by sweeping frequency between 2 and 6Mhz and adjusting the input and output trimmer caps.

Each found peak and/or dip in amplitude was checked both with and without the distilled water present.

The screenshot below shows one such double peak just below 4Mhz (center) with water (white) and without water (yellow).

Up till now no difference between with and without water in peak/dips was noticed.

I tried both with the 690K Ohm resistor (179Vpp) as with a 100 Ohm 1% resistor (29Vpp).

Regards Itsu
   

Group: Professor
Hero Member
*****

Posts: 1716
Hi Itsu,
Not sure at what sweep rate you did this.  The whole object of this unusual water geometry is to minimise the frequency dispersion of the individual proton precessions, and that would result in a very small bandwith of the NMR resonance.  It could be so narrow that it will only be found with a very slow sweep across the frequency range.  I can't offer a feel for how slow this should be, but I see it as a laborious exercise like looking for a needle in a haystack.  I think you are looking for a line width that is much smaller than the typical resonances that you are seeing, so you must be on the lookout for a sudden jump then fall over a very small change in applied frequency.

If this CW approach fails to find the proton resonance it may be necessary to try the pulsed approach looking for a free induction decay or a spin echo.  If successful this will tell you the proton frequency and then you can narrow down the CW approach.  The pulsed approach applies pulsed modulated RF, a pulse of certain width or certain number of cycles.  There should then be a free induction exponential decay of RF following that pulse.  The spin echo approach requires a second RF pulse some time after the first one, again of a certain width, and that causes the RF to build up then decay again as a spin echo.  I would need to look into this further to find the pulse widths required.  Does your FG have the ability to pulse modulate the output RF?   

I am most grateful for your time spent on this, thank you.
 
Smudge
   

Group: Moderator
Hero Member
*****

Posts: 3527

Hi Smudge,

i did a 200ms sweep when tuning the input / output trimmer caps to see how they influence each other, then switched over to 10s sweeps to do the step by step movement of the trimmer caps.

These first rough sweeps did not show much, so i will now move to some more narrower (3 to 5Mhz) sweeps which could be longer in time like 1 minute or more.


I have a fairly advanced FG, so i can modulate the output RF, yes.

Thanks,   regards Itsu
   

Group: Professor
Hero Member
*****

Posts: 2742
So untill i find another solution this has to go on the backburner for now.
Oh!, I can see it is off the back burner now.  I didn't know about it since I was not checking the forum for a long time.
   

Group: Professor
Hero Member
*****

Posts: 2742
Kudos to Gyula for all that practical RF advice and to Itsu for not giving up.

I'd like to notice several elephants in the room:

1) The NMR is so narrow that you need to resolve down to 100Hz or even lower.  The MHz wide sweeps are too wide to notice it.
2) If the RF circuits are tuned precisely to the NMR frequency, how will you distinguish between them ringing and the NMR ?
3) The goal is to maximize the amplitude of the current flowing in the pancake coil's windings, because it is the current that generates magnetic fields (not voltage!).  Since the current amplitude increases together with the voltage amplitude, tuning for maximum voltage at the coils accomplishes the same result, although maximizing the voltage is not the ultimate goal in itself.
4) The SWR between the PA and "tuner*" should be minimized and the SWR between the "tuner*" and the pancake coils should be maximized.  This also means that the current/voltage phase offset between the "tuner*" and the pancake coils should be maximized.
5) The RF power between the PA and the "tuner*" should be very similar to the RF power between the "tuner*" and the pancake coils **, despite the SWRs being wildly different.
6) The grounding point of the Litz shield should make sense in the RF circuit's topology. It should ground away the capacitively coupled energy between the pancake coils and the toroidal sensing coil.  This means RF ground, not necessarily Earth ground.
7) The Litz shield grounding wire should be of very low inductance so it offers very low impedance path to the unwanted capacitively coupled energy of the electric field. This energy should not be coupled into the toroidal coil's circuit !

* By "tuner" I mean the capacitive matching circuit, which is between the PA and the pancake coils.

** Any difference between these two powers will be due to dielectric losses in the "tuner's*" capacitors, resistive loses in the wiring and in the "tuner*" and due to the RF radiation resistance (minimal EM radiation by the pancake coils will be indicated by the maximized SWR between the "tuner*" and the pancake coils and maximized i/v phase at the coils).  We really do not want the pancake coils to work as good antennas that radiate EM radio waves away
« Last Edit: 2021-02-06, 11:44:39 by verpies »
   

Group: Moderator
Hero Member
*****

Posts: 3527

Hi verpies,

thanks for the comments, i will take a deeper look to them later today.

The grounding now is on my house earth system on the attic, so no good RF ground.
I do see a hugh difference in signals on the output coil with or without the ground attached, like 200Vpp with and 450Vpp without.

Also there are other peaks on different frequencies with or without.
But this problem could be due to the grounding of the scope probe which is on the same ground as the house grounding.

Itsu
   

Sr. Member
****

Posts: 446
   Verpies:
   Can you explain what is the difference between an Earth ground, compared to an RF ground.

    NickZ

   PS.  I have given up on OverUnity.com. 
   Seams as only the trolls are left there, without any control or moderation over what has been going on there.
   So, I hope they don't come looking to disrupt this forum, as well.
   Just a warning...
   

Group: Moderator
Hero Member
*****

Posts: 3527

verpies,

Quote
1) The NMR is so narrow that you need to resolve down to 100Hz or even lower.  The MHz wide sweeps are too wide to notice it.

i already went down to 500KHz sweeps because also Smudge mentioned something like that.
Now limiting to 100KHz sweeps.

Quote
2) If the RF circuits are tuned precisely to the NMR frequency, how will you distinguish between them ringing and the NMR ?

good question, i understand/guess a narrow(er) spike/peak ontop of the resonance peak would appear.

   
Quote
3) The goal is to maximize the amplitude of the current flowing in the pancake coil's windings, because it is the current that generates magnetic fields (not voltage!).  Since the current amplitude increases together with the voltage amplitude, tuning for maximum voltage at the coils accomplishes the same result, although maximizing the voltage is not the ultimate goal in itself.

Ok, so  monitoring the pancake coils current together with the output coil voltage would  be needed and possible without groundloop problems.

Quote
4) The SWR between the PA and "tuner*" should be minimized and the SWR between the "tuner*" and the pancake coils should be maximized.  This also means that the current/voltage phase offset between the "tuner*" and the pancake coils should be maximized.

Hmmm, the first part is easy as the SWR meter is inbetween the PA and "tuner".
The second part not, as its hard to measure.

Quote
5) The RF power between the PA and the "tuner*" should be very similar to the RF power between the "tuner*" and the pancake coils **, despite the SWRs being wildly different.

Ok,  thats somewhat easier to measure using the SWR/Power meter and the scope math.

Quote
6) The grounding point of the Litz shield should make sense in the RF circuit's topology. It should ground away the capacitively coupled energy between the pancake coils and the toroidal sensing coil.  This means RF ground, not necessarily Earth ground.

Ok, so i now have used a 10cm long, thick litz wire from grounding shield to PA/SWR meter outer coax shield, which dropped the output coil signal from 200Vpp to about 80Vpp depending on tuning.

Quote
7) The Litz shield grounding wire should be of very low inductance so it offers very low impedance path to the unwanted capacitively coupled energy of the electric field. This energy should not be coupled into the toroidal coil's circuit !

Right, but also this is hard to measure.



Meanwhile sweeping along..........

Itsu

   

Group: Professor
Hero Member
*****

Posts: 2742
Verpies:
Can you explain what is the difference between an Earth ground, compared to an RF ground.
It is the point to which the Litz shield attaches. The purpose of this atypical shield is to block the capacitive coupling of the electric field between the pancake coils and the toroidal coil (while leaving the magnetic field unaffected) so the RF ground should be capacitively symmetrical with respect to the toroidal coil terminals. 

Also, one should be careful to measure only the toroidal coil's differential signal and not the common mode signal, so the grounding of the scope matters, too.
The common mode interference signal, which can be picked up by an Earth-grounded scope, can be attenuated with multiple ferrite beads (or toroidal RF cores) which encompass BOTH wires connecting the toroidal coil.  These beads significantly increase the inductance of these wires, so they present a high inductive reactance path to the common mode signals. ...and because these beads encompass BOTH wires - they do NOT attenuate the differential signals.

Come to think of it, the common mode ferrite beads (or toroidal RF cores) would also attenuate the undesirable common-mode signals between the "tuner" and the pancake coils.

P.S.
I understand what you mean about the O.U. forum and welcome.  Things are different here.
« Last Edit: 2021-02-06, 17:18:50 by verpies »
   

Group: Professor
Hero Member
*****

Posts: 2742
Hmmm, the first part is easy as the SWR meter is inbetween the PA and "tuner".
The second part not, as its hard to measure.
You can use the i/v phase offset to gauge the SWR between the "tuner" and the pancake coils, because the larger this phase offset becomes (ideally 90º), the higher the reactive current becomes between the "tuner" and pancake coils and the SWR also becomes larger there.
Large Reactive Current (ideally: PowerFactor=0) is desirable at this point in the circuit.  Large Real Current would be bad because it would mean that EM waves are being radiated or resistive heating or dielectric heating is happening.
   

Group: Professor
Hero Member
*****

Posts: 2742
@Gyula

How is your experience with common-mode chokes at these frequencies ?

Also,  what do you think about an addition of a shorted turn, which would employ the Lenz law against common-mode RF signals, too... ?

   

Group: Moderator
Hero Member
*****

Posts: 3527
You can use the i/v phase offset to gauge the SWR between the "tuner" and the pancake coils, because the larger this phase offset becomes (ideally 90º), the higher the reactive current becomes between the "tuner" and pancake coils and the SWR also becomes larger there.
Large Reactive Current (ideally: PowerFactor=0) is desirable at this point in the circuit.  Large Real Current would be bad because it would mean that EM waves are being radiated or resistive heating or dielectric heating is happening.


Sweeping from 3.5 to 4.5Mhz.

At 3.9Mhz:

the SWR meter shows 6W @ SWR 1:1.4 (max. / min.)
The math shows ELI phase (103°) see screenshot 1 (measuring at pancake coils yellow voltage, green current)
The sweep shows a resonance peak around 3.9Mhz see screenshot 2 (measuring at output coil)

Itsu
   

Group: Moderator
Hero Member
*****

Posts: 3527

The strange thing about the above first screesnhot is that when calculating the power from the shown pancakes voltage and current we get a negative 17W.

This probably is due to the fact we are measuring "inside" the parallel input circuit.

Itsu
   

Group: Professor
Hero Member
*****

Posts: 2742
The math shows ELI phase (103°) see screenshot 1 (measuring at pancake coils yellow voltage, green current)
Looks like your phase measurement is reversed 77º → 103º
   

Group: Moderator
Hero Member
*****

Posts: 3527

Hmmm,   i measured phase: yellow -> green = 103°.
When measuring phase: green -> yellow i get -103°.

So still 103°, but i know what you mean it should be 77° as more then 90° would not be possible for lagging or leading phases in capacitive or inductive circuits.

Itsu
   
Group: Experimentalist
Hero Member
*****

Posts: 1582
Hmmm,   i measured phase: yellow -> green = 103°.
When measuring phase: green -> yellow i get -103°.

So still 103°, but i know what you mean it should be 77° as more then 90° would not be possible for lagging or leading phases in capacitive or inductive circuits.

Itsu

Itsu,

Check your current probe direction in the circuit.  If the current was 180 degrees reversed, then the phase would be ~77 degrees and the input power would be positive.  However, input current to input voltage phase >90 degrees is possible with the right circuit configuration.

Pm
   

Group: Moderator
Hero Member
*****

Posts: 3527
Hi PM,

i did check the probe as i found the negative power strange, but to me it is in the right direction, see diagram below, where in green the current probe position / direction.

Indeed if i reverse the probe direction, the current will be leading (ICE) by 77° and the calculated power will be positive.

So am i thinking wrongly that the current probe in the drawn position is the correct one?

Added a picture of the setup including current probe and the thick litz wire for grounding the shielding litz to the outer coax from the amplifier.

Itsu
« Last Edit: 2021-02-07, 16:44:41 by Itsu »
   
Group: Experimentalist
Hero Member
*****

Posts: 1582
Hi PM,

i did check the probe as i found the negative power strange, but to me it is in the right direction, see diagram below, where in green the current probe position / direction.

Indeed if i reverse the probe direction, the current will be leading (ICE) by 77° and the calculated power will be positive.

So am i thinking wrongly that the current probe in the drawn position is the correct one?

Added a picture of the setup including current probe and the thick litz wire for grounding the shielding litz to the outer coax from the amplifier.

Itsu

I agree with you.  The current probe direction is correct in the schematic so what you see is what you have!  :)  I don't understand it though!

Pm
   
Group: Tech Wizard
Hero Member
*****

Posts: 986
Hi Verpies,

Thanks for your kind words. Regarding your questions, I have good experience with common mode chokes, I used ferrite toroids on the outside of coax cable to reduce current in the outside braid.

At these frequencies involved here the material of ferrite beads or toroidal cores should still have to have low loss and ample cross section area for the differential RF currents, especially when higher power levels of some ten or hundred watts are involved. The use of several individual cores is preferred to a single one core, i.e. stacked one after the other. this reduces core losses when high differential currents are to be attenuated. 
 
I agree with the suggestion in your reply#739 on using two common mode chokes at the input and at the output of the pancake coils as you indicated in the photo. It will involve a small retuning of the trimmers. I cannot exactly recall Itsu's toroidal core arsenal, maybe an OD of 1.5 or 2 cm from 3F4 or similar ferrite material if he has two of them would give some kOhm common mode reactance with max 10-15 turns which are easily windable.
The permeability of such cores may range from some hundred to a few thousands, the 3F4 has about u=900. 

About the idea of the addition of a shorted turn to reduce common mode RF signals: interesting idea, I never tried it. My initial guess would be that one or only a few shorted turns would have small attenuating effect (this also depends on the frequency), but many, shorted single turns, probably isolated electrically from each other would start blocking differential currents. The Triax coax type has a continuously braided and conducting shield around the two inner conductors and I wonder how this would compare to your idea of shorted individual turns.

Gyula


@Gyula

How is your experience with common-mode chokes at these frequencies ?

Also,  what do you think about an addition of a shorted turn, which would employ the Lenz law against common-mode RF signals, too... ?
   
Pages: 1 ... 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 [30] 31 32 33 34 35
« previous next »


 

Home Help Search Login Register
Theme © PopularFX | Based on PFX Ideas! | Scripts from iScript4u 2022-11-29, 08:00:00