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Author Topic: Smudge proposed NMR experiment replication.  (Read 7605 times)

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Both old pancake coils in parallel bucking (6.4uH), ringmagnets in place, double distilled water in tube.

Fed via 1:1 balun and 10nF:320pF caps (had to add 220pF parallel to the 100pF var. cap to get into 4Mhz resonance range).

1W input (on the power meter) from the FG fed PA.

Sweeping both manual as via FG between 3.5 and 4.5Mhz.

Output toriodal coil tuned with 100pF parallel trimmer cap for resonance loaded with 687K (differential probing).

Output voltage in the 70 to 120Vpp range (without water more).

No dips, peaks or other anomilies seen.................



Itsu
   

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No dips, peaks or other anomilies seen.................
Did you account for the NMR being very narrow in frequency ? 

Sweeping both manual as via FG between 3.5 and 4.5Mhz.
The width of any anomaly would be less than one pixel on the 1MHz wide sweep.
« Last Edit: 2020-08-01, 23:26:42 by verpies »
   

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Yes, thats what i understand also from Smudge.
So starting with a 1Mhz wide sweep, then gradually narrowing it down.
   
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@Smudge

Let's consider two 50mm average radius pancake coils (connected in parallel) and a 10mm 1-turn sensor loop placed between these coils, as depicted on the diagram below in orange color?
When the plane of the small orange loop is perpendicular to the blue line and at a distance of 50mm from the red axis, this loop is the most sensitive to the changes of the flux generated by the pancakes in the radial direction.

What phase of voltage do you expect between the input to the pancakes and the voltage induced across the ends of this small orange loop ?
From the pure magnetic point of view (no displacement current through the air near the pick up coil in a direction to cause induction) I would expect a 90 degree phase between the induced open circuit voltage in the test coil and the current in the pancake coils (the propagation delay at 4Mhz is about 5 thousandths of a degree per mm, so it's effect would be below the phase measurement accuracy).  I would expect a small phase between the voltage across the pancake coils and the induced voltage in the test coil because the resistance of the pancake coils creates a less than 90 degree phase shift between applied voltage and current.

Smudge 
   

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I would expect a small phase between the voltage across the pancake coils and the induced voltage in the test coil because the resistance of the pancake coils creates a less than 90 degree phase shift between applied voltage and current.
What about the voltage phase vs. frequency relationship of the sensor loop and pancake coils connected in parallel ?

The pancake coils are coaxial and parallel and the distance between them is 16mm and their Interwinding Capacitaence (CIW) is 5.8pF at that distance.


The parameters of one pancake coil are:

OD: 112mm
ID: 91mm
Winding: 1 layer, 10 turns of 1mm Litz wire (68 strand), 3.2m long, antiradial weave1.

When the middle break is opened:
Intrawinding/Interturn Capacitance (CIT): 174.75pF (@100kHz)
ESR: 218Ω (@100kHz)

When the middle break is closed:
Inductance: 19.65µH (@100kHz)
DC resistance: 0.22Ω

My sensor coil is 10mm in diameter, 60nH, it is shielded against capacitive coupling and is loaded with 50Ω + 0j.

P.S.
In my system, there is a 5cm mismatch in the transmission line length after the Tee that feeds the pancake coils in parallel.
   
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What about the voltage phase vs. frequency relationship of the sensor loop and pancake coils connected in parallel ?

The pancake coils are coaxial and parallel and the distance between them is 16mm and their Interwinding Capacitaence (CIW) is 5.8pF at that distance.


The parameters of one pancake coil are:

OD: 112mm
ID: 91mm
Winding: 1 layer, 10 turns of 1mm Litz wire (68 strand), 3.2m long, antiradial weave1.

When the middle break is opened:
Intrawinding/Interturn Capacitance (CIT): 174.75pF (@100kHz)
ESR: 218Ω (@100kHz)

When the middle break is closed:
Inductance: 19.65µH (@100kHz)
DC resistance: 0.22Ω

My sensor coil is 10mm in diameter, 60nH, it is shielded against capacitive coupling and is loaded with 50Ω + 0j.

P.S.
In my system, there is a 5cm mismatch in the transmission line length after the Tee that feeds the pancake coils in parallel.

IMO the displacement current through the intrawinding capacitance is in the wrong direction (I.e.radial) to create flux in your test coil.  And the 5.8pF interwinding capacitance should not come into play with the pancake coils in parallel as the coils are at the same potential.

Smudge
   

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@Smudge

I agree.
Thank you for noticing the directions of the displacement current but that's not a direct answer to my question.
   
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@Smudge

I agree.
Thank you for noticing the directions of the displacement current but that's not a direct answer to my question.

Your question was "What about the voltage phase vs. frequency relationship of the sensor loop and pancake coils connected in parallel ?"

Do you want me to predict the relationship or to comment on a measured relationship?

Smudge
   

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Do you want me to predict the relationship or to comment on a measured relationship?
I would like you to predict the voltage phase relationship.
In you prediction please consider the reactance of the pancake coils as the VNA's AC voltage source is trying to push current into them, as well as its transmission line like effects, e.g reflections from the coil's midpoint.
Also note, that the sensing coil is loaded by the 50Ω+j0Ω internal impedance of the VNA's receiver ( the signal generator has the 50Ω+j0Ω internal impedance, too )

FYI: The sensing coil is constructed using this method out of a 2mm diameter Litz coaxial cable.  Note, that there is a 11cm rigid coaxial cable extension (in 3mm O.D. plastic stiffener) in front of the sensing loop.
   
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I would like you to predict the voltage phase relationship.
In you prediction please consider the reactance of the pancake coils as the VNA's AC voltage source is trying to push current into them, as well as its transmission line like effects, e.g reflections from the coil's midpoint.
Also note, that the sensing coil is loaded by the 50Ω+j0Ω internal impedance of the VNA's receiver ( the signal generator has the 50Ω+j0Ω internal impedance, too )

FYI: The sensing coil is constructed using this method out of a 2mm diameter Litz coaxial cable.  Note, that there is a 11cm rigid coaxial cable extension (in 3mm O.D. plastic stiffener) in front of the sensing loop.
OK.  Predicting transmission line effects is very difficult as it is a complicated transmission line.  Taking a simplistic view, from your data where you have 3.2m of wire it can be considered as a 1.6m long line terminated in a short circuit at the far end (the mid point).  From your capacitance measurement the distributed capacitance is 109 pF/m and from your inductance measurement the distributed inductance is 12.28 uH/m.  That yields a time delay of 36.6 nS/m along the line, i.e. 58.54 nS total.  Its first resonant frequency where the line is one quarter wavelength is then 4.27 MHz, which just happens to be around the frequency we are interested in.  That is not good news as it indicates difficulty in pumping current into the line, which would tend towards an open circuit at resonance.  Your 218 Ohms CSR at 100 KHz is also worrying, as it is more likely due to the HF resistance of the wire than to dielectric losses, and that resistance could be greater at 4MHz.  It is many years since I got involved with the use of Litz wire, but I do remember that above a certain frequency Litz wire is of no use, it is better to use a single conductor, possibly with a low conductivity coating such as silver.  When I look back at the introduction of ferrite-rod antenna for portable radios in place of the old wire frame antenna, the long-wave coils were multi-layer wave-woven with Litz wire occupying a small length of the rod, while the medium-wave coils used a single-layer single-strand wire wound along the length of the rod.  And I think our 4MHz falls into the medium-wave category.  Do you have any measurements that support the use of Litz wire at our frequency? I don't think I can add much more, other than below that quarter wave resonance the input will appear inductive and above it will appear capacitive.

Smudge 
   

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Do you have any measurements that support the use of Litz wire at our frequency? I don't think I can add much more, other than below that quarter wave resonance the input will appear inductive and above it will appear capacitive.
Of course, I have many measurements for you.

For example, the first plot (3rd and 4th, too) is of two unconnected but identical 10mm H-field sensing loops placed 8mm apart (coaxial & parallel). See Fig.1
Notice that together with the VNA's internal impedance of 50Ω they form a high-pass filter. Also note, that their voltage phase shift is close to 90º.

The second plot is of two pancake coils connected in parallel and placed 16mm apart (coaxial & parallel). See Fig.2
From it, you can deduce how much total current flows into these coils (note that this current also includes any displacement currents!)

I don't know how proficient you are in reading VNA plots but the vertical scale for the green │S21│measurement of the pancake coils is 100mU per division (linear scale) and the 1 (or 1000mU) is at the very top of the plot and 0 is at the very bottom of the plot. The labels of the vertical axis do not show this because the │S21│ trace does not have focus.  The blue S21 Phase plot has the focus (this is indicated by the blue S21 rectangle in the upper-left corner of the plot area).
It is important to keep in mind that the │S21│measurement is a dimensionless ratio of two voltage amplitudes: the incident signal and the signal received after being transmitted through the coils.  1 means that the full voltage amplitude is transmitted through and 0 means that nothing is transmitted through.  The 100mU means that 10% is transmitted through, 200mU means 20% ...etc.   Of course, anything that is not transmitted through is either reflected back to the source or dissipated as heat/ultrasonics/far-field EM radiation.
« Last Edit: 2020-08-06, 12:18:17 by verpies »
   

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@Smudge

Considering all of the above, what S21 Phase plot do you expect when the VNA's transmitter is connected directly to the two paralleled pancake coils separated by 16mm and the VNA's receiver is connected directly to the 10mm sensor loop (orange) perpendicular and centered along the blue radial line in the diagram below ?

Also, do you expect the orange sensor loop's distance from the red line (when sliding along the blue line) to influence the plot of S21 Phase vs. f ?



« Last Edit: 2020-08-06, 16:28:24 by verpies »
   
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Of course, I have many measurements for you.

For example, the first one is of two identical 10mm H-field sensing loops placed 8mm apart (coaxial & parallel).
Notice that together with the VNA's internal impedance of 50Ω they form a high-pass filter. Also note, that their voltage phase shift is close to 90º.

OK, you would expect a linear rise with frequency since the induced voltage in the output loop is proportional to the frequency.  As for the phase plot it is near 90 degrees near zero frequency as it should be, and propagation delay will cause that to become less than 90 degrees at higher frequencies.  That 33 degree approx reduction from 90 degrees at 50MHz represents a line length of about 0.5m.  What lengths of cables connected the two loops to the transmitter and receiver?  If that is about 0.5m then all is as it should be.
Quote
The second measurement is of two pancake coils connected in parallel and placed 16mm apart (coaxial & parallel).
From it, you can deduce how much total current flows into these coils (note that this current also includes any displacement currents!)

The coils are connected in parallel so this must be transmission to another coil, presumably the 10mm H-field sensor.  That resonance at 5.695MHz could be the transmission line effect being a quarter wavelength shorted line, but then you would expect the next dip at 3/4 wavelength to be at 3 times this frequency whereas it is around 5 times.  And at around 3 times frequency it is a peak, not a dip.  But you do get this 1-3-5 relationship showing there is a transmission line affect.

Quote
I don't know how proficient you are in reading VNA plots but the vertical scale for the green │S21│ measurement of the pancake coils is 100mU per division (linear scale) and the 1 (or 1000mU) is at the very top of the plot and 0 is at the very bottom of the plot. The labels of the vertical axis do not show this because the │S21│ trace does not have focus.  The blue S21 Phase plot has the focus (this is indicated by the blue S21 rectangle in the upper-left corner of the plot area).
It is important to keep in mind that the │S21│measurement is a dimensionless ratio of two voltage amplitudes: the incident signal and the signal received after being transmitted through the coils.  1 means that the full voltage amplitude is transmitted through and 0 means that nothing is transmitted through.  The 100mU means that 10% is transmitted through, 200mU means 20% ...etc.   Of course, anything that is not transmitted through is either reflected back to the source or dissipated as heat/ultrasonics/far-field EM radiation.

It is many many years since I had dealings with network analyzers, we used Hewlett Packard ones back in the 1960's.  I don't know what to suggest to get that first dip away from 4-ish MHz other than winding (or printing) fewer turns using wide strip, maybe even just a single turn of very wide strip.

Smudge
   
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@Smudge

Considering all of the above, what S21 Phase plot do you expect when the VNA's transmitter is connected directly to the two paralleled pancake coils separated by 16mm and the VNA's receiver is connected directly to the 10mm sensor loop (orange) perpendicular and centered along the blue radial line in the diagram below ?

Also, do you expect the orange sensor loop's distance from the red line (when sliding along the blue line) to influence this S21 Phase plot ?


Haven't you just sent me that measurement or did I presume wrongly?  I would much rather analyze a measurement than predict an outcome.  I would not expect sliding along the blue line to significantly influence the phase plot.

Smudge
   

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Haven't you just sent me that measurement or did I presume wrongly?
No, no. I posted this diagram before but I never posted the signal received by this orange 10mm sensing loop.
This diagram serves mainly to define the directional/orientational nomenclature.
   

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OK, you would expect a linear rise with frequency since the induced voltage in the output loop is proportional to the frequency.
That is only one side of the coin.
The other 10mm loop (the transmitting one) forms a filter with the VNA's internal impedance.
We cannot expect this transmitting loop to generate a constant amplitude flux variations, because it is driven by a constant voltage source and the current amplitude flowing through it is not constant with frequency.

What lengths of cables connected the two loops to the transmitter and receiver? 
2cm and 6cm.  Yes, they are not equal length, because I burned out my SMA cables in an earlier high power run. But since then I have acquired new ones. Do you want me to repeat this measurement with two 15cm cables after the Tee ?

The coils are connected in parallel so this must be transmission to another coil, presumably the 10mm H-field sensor. 
O, o!  Back up....
There was no H-field sensor used in that measurement.  It was just a S21 measurment of the naked pancake coils connected in parallel.
   

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I would much rather analyze a measurement than predict an outcome.
I will post a 4 dimensional measurment, i.e. an │S21│ and S21 Phase vs. frequency plots of the signal received by the 10mm H-field probe as its distance from the red line varies in micrometer steps (robotically positioned).
First, let's just straighten out the confusion about the previous measurement of the naked pancake coils, which did not involve the H-field probe.
   
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That is only one side of the coin.
The other 10mm loop (the transmitting one) forms a filter with the VNA's internal impedance.
We cannot expect this transmitting loop to generate a constant amplitude flux variations, because it is driven by a constant voltage source and the current amplitude flowing through it is not constant with frequency.
OK, a constant voltage source should create a constant rate of change of current, which for sine waves is a current proportional to the reciprocal of the frequency.  That should cancel out the induced voltage being proportional to frequency, so we should see a flat response but we don't.  So is that fall-off with frequency due to increased resistance, radiation or a combination of both?  I would not expect the resistance change (skin effect) to show such a linear response.
Quote
2cm and 6cm.  Yes, they are not equal length, because I burned out my SMA cables in an earlier high power run. But since then I have acquired new ones. Do you want me to repeat this measurement with two 15cm cables after the Tee ?
Well comparison with the existing result would tell you whether cable length is significant.
Quote
O, o!  Back up....
There was no H-field sensor used in that measurement.  It was just a S21 measurment of the naked pancake coils connected in parallel.
OK. tell me if I am wrong in now assuming you have 50 ohm cables connecting transmitter to receiver but somewhere in the middle where they join you have the parallel coils connection shunted across that 50 ohm line.  I think I need to know the cable dimensions, i.e. the physical set-up, in order to comment usefully
Smudge

Edit, I have just realised that I am talking rubbish in my first reply here.  There is not a fall-off, there is a linear rise.  So if the current remains constant with frequency then the coil resistance must dominate and be constant, but again I wouldn't expect that.  I'll look again at the slope of that line and see what I come up with.
   

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OK. tell me if I am wrong in now assuming you have 50 ohm cables connecting transmitter to receiver but somewhere in the middle where they join you have the parallel coils connection shunted across that 50 ohm line.  I think I need to know the cable dimensions, i.e. the physical set-up, in order to comment usefully
The diagram below should make it clear what I was describing in this post.  I am not listing the lengths of the transmission lines before the calibration plane because their influence is calibrated out.

Also, the pigtail transmission lines AFTER the Tee, were 2cm and 6cm in the old setup, but in the new setup, they can be 15cm and 15cm ...or... 5cm and 5cm

« Last Edit: 2020-08-05, 23:37:13 by verpies »
   

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What would be a preferred input matching circuit when using parallel opposing pancake coils?
Not the one shown below which is for series opposing.

Itsu
   

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Why not? To these caps it does not matter whether these coils are connected in series or in parallel. These caps will absorb the coils' internal CIT anyway.

Just don't expect a low SWR because these coils are not designed to radiate their energy into the far field (as radio waves). Nor are they designed to dissipate this energy as heat.
They are designed to maintain a high reactive current (which stores the energy in local near-fields that never leave the coils) and to reflect almost all of this energy, back into these caps. IOW: They are designed to slosh the energy between inductance and capacitance.
The circulating current will be much much higher than the input current, because of the the current magnification ratio iCIRCULATING / iINPUT ...also known as Q.
   
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Edit, I have just realised that I am talking rubbish in my first reply here.  There is not a fall-off, there is a linear rise.  So if the current remains constant with frequency then the coil resistance must dominate and be constant, but again I wouldn't expect that.  I'll look again at the slope of that line and see what I come up with.
That tiny H-field loop has very low inductance, so it will look like a short circuit against the 50 Ohm output impedance of the transmitter, hence the current will remain constant as the voltage is kept constant.  Then induction into the receiving loop explains the linear rising output voltage, except I would have expected the transmitter loop inductance to show up at the higher frequency 50MHz end.

Smudge
   

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Because, as i understood, the input caps act as a impedance step up from the low (50 Ohm) input impedance to the high series impedance, while the parallel circuit will be low impedance already.

An S11 scan at the input shows the low (1 Ohm) input impedance with a shallow (2.5 Ohm) peak at reasonance (4.3Mhz).

But i forgot the purpose of these coils as to not radiate so i am sure you are right in that it behaves differently as i expect.

Itsu   
   

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Because, as i understood, the input caps act as a impedance step up from the low (50 Ohm) input impedance to the high series impedance, while the parallel circuit will be low impedance already.
A parallel LC circuit at resonance acts as an open circuit.  Even one pancake coil acts as a parallel LC circuit. Two pancakes connected in parallel - even more so.
If you look at the 2nd plot, then you can see this response at 5.7MHz ( and the higher dip is from shorted-transmission-line behavior ).



An S11 scan at the input shows the low (1 Ohm) input impedance with a shallow (2.5 Ohm) peak at reasonance (4.3Mhz).
You should print out this diagram and permanently glue it to the back of your VNA.
   

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That tiny H-field loop has very low inductance,
Yes, around 20nH.

The transmitter's internal impedance and receiver's impedance in parallel are 25Ω.
Don't this impedance and inductance form a high pass RL filter ?

... except I would have expected the transmitter loop inductance to show up at the higher frequency 50MHz end.
At higher frequencies it looks like this:


The phase wraps-around from -180º to +180º.

The second plot below was done with one H-Field probe physically flipped 180º.


This phase in the second plot also wraps-around from -180º to +180º.
« Last Edit: 2020-08-06, 13:13:52 by verpies »
   
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