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Author Topic: Smudge proposed NMR experiment replication.  (Read 12368 times)

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Quote
Hmm, mine is 4mm in diameter and floats in oil.


Well,  13mm is more then micro enough for my eyes  :o
Would it matter to go even smaller?


Anyway, here the compass behaviour with 1A running through the pancake coil:
https://www.youtube.com/watch?v=a3RQ2Thx_b8

I also used  my hall sensor to sweep by the coil (middle again) and capture the voltage trace on a slow time base setting to show the magnetic field, see screenshot (1A again).


Itsu
   

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Well,  13mm is more then micro enough for my eyes  :o
Would it matter to go even smaller?
It matters if your two pancake coils are closer to each other than 13mm in your next measurement.
Also, these liquid filled bubble micro compasses rotate in 3D so they visualize all dimensions of the magnetic field.

Anyway, for NMR purposes the optimum placement of the water tube is where the compass has the North pointing left or right ( as per the directions in your video), when measuring between two coils.

I also used  my hall sensor to sweep by the coil (middle again) and capture the voltage trace on a slow time base setting to show the magnetic field, see screenshot (1A again).
That was pretty clever.
If you power two coils with any AC, then the hall sensor will visualize where between the two parallel pancake coils, the RADIAL AMPLITUDE of the magnetic field is the greatest.  The 1D Hall sensor has to be oriented carefully to sense magnetic flux in the radial direction only.
For NMR we are interested in maximizing the radial component of the magnetic field where the water tube is.

On the diagram below, the radial direction is depicted as any line which is perpendicular to the red major symmetry axis and is also passing through the blue point.  Two such blue lines on this diagram illustrate two of the infinitely many possible radial directions.
Imagine your sensor or compass sliding along one of these blue lines.
« Last Edit: 2020-06-26, 16:51:07 by verpies »
   

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Thanks, will do the AC test lateron, first i have the 2 pancake coils positioned 15mm apart from each other both pointing the same way (coils to front).
Both pancake coils are similar wound meaning both CW and both measure 115.6uH @ 100Khz.
I have connected them in series aiding first.

Sweeping the compass shows north on the front, south on the back and flipping north to south inbetween the coils in the middle.

When connecting them series bucking,  i have south on front and back and east/west inbetween on the right half swapping over west/east on the left half.

Guess that last one is the desired field.

Video here:  https://www.youtube.com/watch?v=us5_F7dKsy0
Itsu
« Last Edit: 2020-06-26, 21:31:51 by Itsu »
   

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When connecting them series bucking,  i have south on front and back and east/west inbetween on the right half swapping over west/east on the left half.
Guess that last one is the desired field.
Yes

Both pancake coils are similar wound meaning both CW and both measure 115.6uH @ 100Khz.
And what is their inductance when connected as series bucking ?  What does your SA+TG think of their frequency response when they are 15mm apart ?

These coils take a lot of work to build, don't they?  How many meters of wire did you use for them ?
I was lazy and cannibalized my coils out of an inductive cooker.
They are wound with Litz wire in these cookers, which I thought to be very handy because the skin effect depth in copper at 4MHz is 0.033mm :(
This means that a 1mm diameter solid copper wire has 8x more resistance to 4MHz AC, than to DC.  That's not counting the proximity effect and inductive reactance, which I've already calculated before.
« Last Edit: 2020-06-26, 23:33:39 by verpies »
   

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Itsu and Verpies,

Most excellent video!!  This is a precise example of the
kind of research and documentation which is so very
important to the studies of electromagnetic phenomena!

Thanks to both of you for showing all how it should be
done!  Your dedicated efforts are truly appreciated.   


---------------------------
The animal mind ALWAYS reacts to what it does not understand. This is what sets dogs barking. If you are going to tell the truth, you are going to have to be okay with barking dogs, because they will harry your passage until you pass through town.
Les Visible - 27 February 2020
   

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Yes
And what is their inductance when connected as series bucking ?  What does your SA+TG think of their frequency response when they are 15mm apart ?

These coils take a lot of work to build, don't they?  How many meters of wire did you use for them ?
I was lazy and cannibalized my coils out of an inductive cooker.
They are wound with Litz wire in these cookers, which I thought to be very handy because the skin effect depth in copper at 4MHz is 0.033mm :(
This means that a 1mm diameter solid copper wire has 8x more resistance to 4MHz AC, than to DC.  That's not counting the proximity effect and inductive reactance, which I've already calculated before.

A quick measurement shows in series bucking at 15mm apart they measure 106uH @ 100Khz.

I will do some further tests tonight, when hopefully the workshop has cooled down somewhat.

The coils are more difficult to make then i expected, as the plexiglas molds are to weak when trying to tightly winds the 1mm magnet wire, they flex causing the windings to overlap if not carefull.

I did not measure the wire length, but at an average of 31cm circumference (@ 10cm diameter) it would be around 9.3m.

Good idea on the litz wire, i have some, but its to thick (2.5mm dia) and to short (10m), but i keep it in mind when building some better pancakes.

Itsu
   

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Itsu and Verpies,

Most excellent video!!  This is a precise example of the
kind of research and documentation which is so very
important to the studies of electromagnetic phenomena!

Thanks to both of you for showing all how it should be
done!  Your dedicated efforts are truly appreciated.

Thanks MuDped,

you are welcome, and your comments are truly appreciated.

Itsu
   

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Good idea on the litz wire, i have some, but its to thick (2.5mm dia) and to short (10m), but i keep it in mind when building some better pancakes.
How much does the Litz wire cost in your area?
In mine, the 1mm diameter Litz wire (66 wires 0.08mm ea.) costs 0.72 EUR for 1m.

   

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On ebay one can find quite good deals for speaker wires which have two cores but can be ripped apart into a single core wire:

100m 2x0.5mm Speaker Cable Transparent Twin Loudspeaker Wire Car Home Audio Hifi
https://www.ebay.nl/itm/100m-2x0-5mm-Speaker-Cable-Transparent-Twin-Loudspeaker-Wire-Car-Home-Audio-Hifi/302652032514

13, 42, 79 Strand Speaker Cable Twin Stranded CCA Figure 8
https://www.ebay.nl/itm/13-42-79-Strand-Speaker-Cable-Twin-Stranded-CCA-Figure-8/333606535701

Greetings, Conrad
   

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On ebay one can find quite good deals for speaker wires which have two cores but can be ripped apart into a single core wire:
Yes, some speaker cables can be easily separated into two separate stranded wires.
Are the strands of one of these wires, isolated from each other?
   

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The 2 pancake coils, 15mm distance parallel, in series bucking on the SA / TG, see screenshot.
Seems the selfresonance has shifted down somewhat to 4Mhz due to the lower combined bucking inductance.

Itsu
   

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It matters if your two pancake coils are closer to each other than 13mm in your next measurement.
Also, these liquid filled bubble micro compasses rotate in 3D so they visualize all dimensions of the magnetic field.

Anyway, for NMR purposes the optimum placement of the water tube is where the compass has the North pointing left or right ( as per the directions in your video), when measuring between two coils.


That was pretty clever.
If you power two coils with any AC, then the hall sensor will visualize where between the two parallel pancake coils, the RADIAL AMPLITUDE of the magnetic field is the greatest.  The 1D Hall sensor has to be oriented carefully to sense magnetic flux in the radial direction only.
For NMR we are interested in maximizing the radial component of the magnetic field where the water tube is.

On the diagram below, the radial direction is depicted as any line which is perpendicular to the red major symmetry axis and is also passing through the blue point.  Two such blue lines on this diagram illustrate two of the infinitely many possible radial directions.
Imagine your sensor or compass sliding along one of these blue lines.


I tried to do that AC test mentioned in post #51, using a 220V:12V transformer feeding a 12V automotive bulb in series with the both pancake coils.
So there was 1.5A AC rms @ 50Hz running through the coils, but the hall sensor picked up only some small signals from the coils with probably a lot of 50Hz noise interference from my workshop

No steady signal was seen on the scope which made sense to me.


Itsu
   

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So there was 1.5A AC rms @ 50Hz running through the coils, but the hall sensor picked up only some small signals from the coils with probably a lot of 50Hz noise interference from my workshop
I was afraid of that.
Do you have a powerful source of higher frequency AC ...e.g. from one of these inductive metal heaters, or cookers, or an old project like this one ?
   

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Seems the selfresonance has shifted down somewhat to 4Mhz due to the lower combined bucking inductance.
How has the resonance frequency shifted down, when lower inductance shifts up this frequency, because it is in the denominator of f=1/2π√(LC) ???  The peak #1 does not seem like the LC resonance (even with the distributed capacitance).  How do you have this connected up for measurement ?

Do you have a suitable diode to try how much you can "pull" the frequency as in this simple experiment ?
   

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I was afraid of that.
Do you have a powerful source of higher frequency AC ...e.g. from one of these inductive metal heaters, or cookers, or an old project like this one ?


Yes i still have that, so will see if i can use it.


Quote
How has the resonance frequency shifted down, when lower inductance shifts up this frequency, because it is in the denominator of f=1/2π√(LC) ??? 

Yeah right, my brain was still in bucking mode i guess.   
So why is with the lower inductance (106uH instead of the 115uH the LITTLE peak at #1 lower in frequency?    Or is the new resonance peak the one at #2  (around 7Mhz), so indeed higher thus at 4.8pF distributed /selfcapacitance?



Quote
The peak #1 does not seem like the LC resonance (even with the distributed capacitance). 
How do you have this connected up for measurement ?

See diagram below,  same as with the single coil measurement as shown in post #25


Quote
Do you have a suitable diode to try how much you can "pull" the frequency as in this simple experiment ?

I don't think i have a varactor diode, but any diode would work a little like that i understand;  i will take a look / test.


I will try to wind the plastic tube coil as the severall layers boat varnish are dried enough by now to make it watertight.

Itsu
   

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I did a quick FM sweep from 1Khz to 10Mhz with the 2 pancake coils 15mm apart parallel while in series bucking, see screenshot.

The loosely coupled FG loop was placed inbetween the 2 coils, but its position has big influence on
the amplitude of the both peaks and a lesser influence on the resonance frequencies.
So we see resonance peaks on 3.5 and 4.6Mhz.


I did some tests 3 years ago using bifilar pancake coils closely parallel to each other (3:40) here:
https://www.youtube.com/watch?v=AQEJ08TRNkU&t=245s

It showed that putting them to close their resonance points never merged.


Itsu
   

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So why is with the lower inductance (106uH instead of the 115uH the LITTLE peak at #1 lower in frequency?    Or is the new resonance peak the one at #2  (around 7Mhz), so indeed higher thus at 4.8pF distributed /selfcapacitance?
See diagram below,  same as with the single coil measurement as shown in post #25

The key to understanding this behavior is remembering that at the resonance frequency:
1) An ideal series LC circuit acts as a short circuit.
2) An ideal parallel LC circuit acts as an open circuit.

But these are not ideal LC circuits, thus at resonance:
a) The current flowing in a series LCR circuit is at the MAXIMUM.
2) The current flowing in a parallel LCR circuit is at the MINIMUM.

Because of the distributed intrawinding capacitance, these pancake coils act as two imperfect parallel LCR circuits...and when they are connected in series and are far apart, they act as two parallel LC circuits connected in series
When they are close together, their mutual inductance and  and interwinding capacitance comes into play.

It is surprising that the two frequency peaks are so far apart for coils that are virtually identical.

Anyway, the SA essentially measures voltage at its input, but since its input has the 50Ω impedance, this voltage is proportional to the current flowing in this input impedance, too (V= i * 50Ω).
When you are measuring one coil and we treat it as a parallel LC circuit, we can expect it to act as on open circuit at resonance.  Obviously, an open circuit doesn't allow any current through. No current flowing through the 50Ω input impedance, means no voltage appearing at the SA's input, because 0A * 50Ω = 0V.
With an imperfect LCR circuit, the voltage at the SA's input, would not be 0V, but it would be at the minimum, which represents the dip denoted as dip #2. (it is noisy because the amplitude is low at that point - as it should be in this measurement setup).

If you want the parallel LC resonance to manifest itself as a high amplitude peak (not noisy), then connect it as on the diagram below. (calibrate it to a flat line after all the coil's cables and interconnects are in place ...but before the coil itself is connected).
« Last Edit: 2020-06-28, 14:06:30 by verpies »
   

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Thanks for the info / reminders / refreshing.

I put up some measurements as can be seen in the video.

Concerning your TG / SA setup diagram, this shows almost nothing as i had to put everything on its maximum power / sensitivity to see a very slight dip around 4.15Mhz, see screenshot below.


LCR meter measurements:

single coil 1: 116.2uH
single coil 2: 115.2uH
both coils 15mm apart, series aiding: 354uH
both coils 15mm apart, series bucking: 107.3uH


Sweep measurements:

single coil 1: 4.1Mhz resonance
single coil 2: 3.8Mhz resonance
both coils 15mm apart, measured separately: 2 peaks: 1st 2.9Mhz, second 6Mhz
both coils 15mm apart, series aiding: 2.2Mhz resonance
both coils 15mm apart, series bucking: 2 peaks: 3.5Mhz and 4.5Mhz.

TG / SA parallel measurement:

both coils 15mm apart, series bucking: 4.15Mhz

Video here:  https://www.youtube.com/watch?v=gQivPoqBl0A

Itsu
« Last Edit: 2020-06-28, 18:15:08 by Itsu »
   

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Looks like the coil has low Q and is too weak* to short the TG's output to ground . The TG has output impedance of 50Ω.
Did you try to use the optional resistor to weaken the TG's output so the coil has a chance to "win" with the TG ?


* the coils'ss minimum impedance is too high
   

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Yes, i tried 10K in series with the TG lead, but did not see any difference.

Let me try some different value's.
   

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First pancake coil is drying, it measures 115uH @ 100Khz.
Inner diameter is 7cm,  outer diameter 13cm, 30 turns of 1mm wire
I cannibalized the other side of the cooker and wound a 30 turn pancake coil with 1mm Litz wire.
It measures 93.55µH (@100kHz) and 0.51Ω.
OD=127mm, ID= 59mm

Soon, I will measure its frequency params, too.
   

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 O0

Thats closer to the by Smudge calculated 100uH then my coils.
I wonder what its selfresonance frequency will be.

Itsu
   

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I made the tube coil, it is 242 turns of 1mm wire, measuring 31uH @ 100Khz and 0.3 Ohm.

Itsu
   

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Ok, that 4Mhz looks like the real one.

But i wonder how we get to 4.3Mhz resonance when the coils already selfresonante that low.

 
I tried your TG / SA setup with severall resistors between 1K and 120K Ohm, but it does not want to reveal its resonance point  :(

itsu
   
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