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Author Topic: Smudge proposed NMR experiment replication.  (Read 107397 times)

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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

I verified with manual sweeping that the above used setup where we see negative power, is real and repeatable.

Below (and above) resonance, we have around 80-90° phase shift between voltage and current (ELI) and positive power going into the pancake coils.

Around resonance we see a quick shift to around 107° phase shift and a negative going power.

Above resonance things change back to "normal" again.

Itsu   
   

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So the pancake coils are feeding energy into the "tuner"  :o ?
   

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Does the presence or absence of water have any effect on this negative power?

Smudge
   

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Perhaps more importantly, at this resonant point can power be extracted via a lower value load resistor on the toroidal coil and does the negative power on the pancake coils then reduce?  If so is it possible to get zero power at the pancake coils while extracting power from the toroidal coil?  You may then have achieved the dream.

Smudge
   

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Not sure what is going on, yesterday (and like in post #743) i had this repeatable negative power behaviour: https://youtu.be/kI8QkjPicqo

This morning with the same setup its different (normal?): https://youtu.be/jD7Gh5ObRV0

I think i can have either a defective current probe or the magnetic field of the strong magnets is off-setting it.

The 359 means the amplifier is not recognizing the attached probe.
Degausing the probe away from the magnets is OK, but upon approaching the magnets the 359 returns off and on.

Not sure if this defect / off-set can affect the phase being measured only. 

Perhaps i should try using a csr.

Itsu
   
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Hi Itsu,

Perhaps first you could arrange repositioning the current probe as I show in this picture with the piece of (green) wire, so the probe could get further away from the ring magnet. The length of the new wire would not disturb tuning noticeably.

Gyula
   
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Note:

The negative power can also come from the fact that the cosine of angles above 90° is negative mathematically and the oscilloscope follows the math rules.  cos(103°)= -0.2249

So supposing the current probe is not influenced by the ring magnet as much as say causing higher than 90° phase angle appear on the scope, just by the unwanted magnetic bias of the probe, the negative power may also come from the negative cosine value.   

Partzman wrote yesterday: "However, input current to input voltage phase >90 degrees is possible with the right circuit configuration."    I also mean the 103° phase angle could also be reality, this latter will turn out with the further tests.

Gyula
   

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Perhaps first you could arrange repositioning the current probe as I show in this picture with the piece of (green) wire, so the probe could get further away from the ring magnet.
...or get a hacksaw and cut a piece of a steel water pipe large enough to contain the entire current probe inside it. The steel should effectively screen out any stray external magnetic fields (especially the HF ones).
This might come handy in future projects, too.

With the pandemic, lockdowns and all, we all can use the exercise  ;)

https://i.imgur.com/ksrdbTm.gif
Smudge proposed NMR experiment replication.
« Last Edit: 2021-02-08, 12:38:20 by verpies »
   

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The negative power can also come from the fact that the cosine of angles above 90° is negative mathematically and the oscilloscope follows the math rules.  cos(103°)= -0.2249
Yes, that was my initial take on that, too, because cos(77°) = -cos(103°).
However, Itsu is insisting that sometimes the current really is lagging behind the voltage by more than 90°.

Perhaps his current probe is getting saturated by the external magnetic field or his scope makes a phase measurement error when the signal level is too low or a common-mode signal is confusing the scope.
Repositioning, shielding the contactless current probe should eliminate such modes of error. Substituting a CSR for the current probe should do it, too. Finally, common-mode signals can be attenuated by a CMC as you've described.

Also, in theory, at the LC resonance frequency, the amplitude should be the highest and the the current should lag the voltage by 90°.  However, Itsu's system does not manifest the highest amplitude at  90°.
   

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Thanks guys,

I have arranged the current probe like Gyula suggested.

I think also there is the matter with how the scope is calculating the phase, like both Gyula and verpies mentions, >90° then power gets negative.
Only, it should not get >90°.

But using a 1 Ohm 1% induction free csr does not make more sense to me.

Using that csr, the power into the pancake coils is 9W (stays positive), but is hard to believe as the difference with the current probe (0.5W) is hugh.

Something is not right here.

Video here: https://youtu.be/prx00V2WI5I

Itsu
   
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Hi Itsu,

IF you agree, perhaps check the current probe right at the output of the function generator if you still have the dummy load with the short wire loop as you used at the output of the power amplifier with the current probe.

Perhaps you could insert the same 1 Ohm csr in series with the 51 Ohm dummy load and also check the phase angle that way too.   Adjust the FG to its highest output possible, it would be terminated still correctly by the dummy load (and the 1 Ohm in series with it).   

You could do this same test at the output of the PA but the output of the PA may bring in a certain reactance while the FG output cannot.  You did this dummy load test last week at the PA output and the current probe gave correct current phase vs the voltage across the dummy load.

Gyula
   
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Thanks guys,

I have arranged the current probe like Gyula suggested.

I think also there is the matter with how the scope is calculating the phase, like both Gyula and verpies mentions, >90° then power gets negative.
Only, it should not get >90°.

But using a 1 Ohm 1% induction free csr does not make more sense to me.

Using that csr, the power into the pancake coils is 9W (stays positive), but is hard to believe as the difference with the current probe (0.5W) is hugh.

Something is not right here.

Video here: https://youtu.be/prx00V2WI5I

Itsu

Itsu,

What is your calibration for CH2(blu) that is used to measure the CSR.  Your scope shows 500ma/div with a peak current of ~800ma.  Is this correct?

Pm
   

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PM,   

not sure what you mean by "calibration for CH2(blu) that is used to measure the CSR", but i suspect the mV to mA translation which is 1:1 (1A/V).
The 500ma/div is correct, not sure where you see the peak current of ~800ma.

Gyula,

I will be using a 50 Ohm 1% induction free resistor with a series 1 Ohm 1% induction free csr to do some measurements, and it shows some results i do not expect to see, like the amplitude of the signals and the phase offset on 4MHz of this induction free csr.

Using the above 51 Ohms and 3.46V rms i would expect to see 67.8mA rms, but i do not.

Itsu
   
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Okay Itsu, take your time and next time please tell some measured data.

The 50 or 51 Ohm termination across the FG output should halve the open circuit output voltage of course and the current and voltage of the 51 Ohm should be in phase. You will surely find the problem.
Last week you found almost fully in phase current and voltage for the dummy load at the output of the PA.

Thanks for all the efforts.
Gyula
   

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See above, here the video: https://youtu.be/ggAyTt3Y05U

we see 51 Ohm and 3.46V rms across it so i expect to see 67mA rms of current.
Instead, the current probe (green) shows 63mA rms (close) and the csr current (Blue) shows 84mA rms (way off imo).

Also @ 4Mhz we have on the csr a considerable phase offset.

Going to 1Khz improves concerning the csr phase difference as its now almost in phase.
But the amplitude and/or rms value is still to high in my opinion.

The current probe shows a much to low value and very off phase, so i suspect this probe to be defective.
I just found out that this probe does not measure any DC currents, so the DC part of it might be the problem.  >:(


Itsu
   
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Yes, the current probe got defective.  And the 1 Ohm is not 1 Ohm at 4 Mhz but higher than that inductively. This latter explains the "9W" going into the pancake coils, it is obviously wrong. 
Regarding the current probe, try to degauss it a few times repeatedly, perhaps it helps.  But if it cannot measure DC current as you wrote, then it got defective unfortunately. Perhaps there is a helpline or forum discussion for such failure?
   

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Yes, the current probe got defective.  And the 1 Ohm is not 1 Ohm at 4 MHz but higher than that inductively.
I concur.
Also, I think that the inductance of the interconnects and the capacitance of the probes should not be neglected.
   

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With all the 0.1 Ohm csr tests i have done lately using my nanoVNA in my "workbench" thread i also had analyzed my 1 Ohm csr and i remembered that the inductance in the 10KHz to 10MHz range did not increase that much as the 0.1 bare metal csr's.

Therefor i expected that my 1 Ohm csr would be stable up to the used 4MHz.

To be sure i now checked again the used 1 Ohm csr again using the  "Shunt-Thru" measurement method, and it confirmed only a slight increase in impedance on 4Mhz, see green marker in the below screenshots.

It shows that the impedance increases from 927 mOhm on 10KHz to 1.031 Ohm on 4MHz to 1.223 Ohm on 10MHz.

This means a 9% increase on 4MHz compared to 10KHz, so the seen amplitude increase and phase shift at 4MHz compared to 1KHz on my 1 Ohm csr might be plausible when including the "inductance of the interconnects and the capacitance of the probes".



Looking for a new current probe as i think the hall sensor in my present one is toast.

Itsu
   

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Meanwhile sweeping in 100KHz chunks across a 3.5 to 4.5MHz range.....

Itsu
   

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Meanwhile sweeping in 100KHz chunks across a 3.5 to 4.5MHz range.....
Did you add the CMCs to decrease the common-mode cross-talk and make NMR signal relatively larger ?
   

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No,  not yet,  will look into the CMC usage today, see if i can make some and install them.

Itsu
   

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I have lots of different ferrite toroids, but most are of unknown origin.

I still have these small black 10x6x4 toroids for my nano pulser toroids in the dally setup, and some white TN10/6/4-3E25 (U = 5500)

Guess i will use them to start with and put some 10-15 turns on them.

Itsu



   
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Hi all,

Back in 2011 when we did Lithuania experiment, the frequency of ferrite was measured by coils of 90 degrees arrangement.
We had flat coil made from foil around yoke and the standard coil on top of it.
Then after giving white noise to coil from foil I looked on peaks in spectrum analizer for possible points where material of ferrite did resonate - https://www.youtube.com/watch?v=kGGCG2i8u8U

Hopefully this approach would be useful to you as well when looking for material resonant frequencies.

Cheers!
   

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I used the white TN10/6/4-3E25 with 10 turns each (both measured about 270uH @ 100KHz) as a first CMC on the output coil, see picture.

Screenshot 1 shows the difference in output signal without (purple) and with (white) cmc for a 3.5 to 4.2MHz sweep.  ====>>>>   wrong!!,   should be:
Screenshot 1 shows the difference in output signal with (purple) and without (white) cmc for a 3.5 to 4.2MHz sweep.

Screenshot 2 is at a different vertical/division setting of the purple trace.

Itsu
« Last Edit: 2021-02-13, 09:08:33 by Itsu »
   
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Hi Itsu,

That is 10 x  voltage gain at the output, very nice and it was a good idea from verpies to bring up the use of CMCs.

I wonder whether you had to readjust the trimmer capacitor to peak it around the 3.8 MHz or you did not need to do that?

Thanks for your efforts.
Gyula
   
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