DIY low-field continuous-wave NMR spectrometer

[Kleinstein]

For a higher Q if the probe LC circuit most of the load capacitance should be at the probe side of the coax. So more like more capacitance at the probe and less at the oscillator side.

The 20 K resistor at the "varactor" diodes could be a little larger, just in case as it loads down the resonator.
Another point may be reducing the 10 Ohms resistor in the feedback.
The 5 nF cap at the drain side is also quite small - lower frequency may want more filter capacitance there.

[iMo]

This is an interesting project!
In past I was thinking on AD9851 for precision measurements
You may get nanoHertz resolution from 0-2MHz crystal stable..

[ChristofferB]

Ok, this is strange..

When putting the inductor directly over the main tuning capacitor, the amplitude is significantly increased - but so is the frequency! Before, I could tune from around 2.4 MHz -3.3 MHz. After it tuned 3.5-7 MHz!

Either way I tried adding a total of 300 pf directly to the probe. I wound a new inductor with 13x6 windings (see the coil form i used late in page 2). No resonance.
I then removed windings, until I could barely get oscillations - at 2.4 MHz. I simply can't get lower.

Can the form of the coil (coil form/wire thickness) influence this? Should i make a coil without the form? Either way i guess the acrylic coil form would be very visible on NMR.

And i've switched from 50 cm RG58 to a slightly better Suhner 40 cm RG223/U cable. this looked to help performance a bit.

When I get it working, I think the permanent probe will be mounted properly and have a feed of a good quality RG213-thickness coax cable to minimize losses.

Thanks for the interest.

[T3sl4co1l]

Is that a typo, the "varactors" are supposed to be antiseries right?

What do you mean "over" the capacitor?

Tim

[duak]

On page 11 of this paper, the varicap diodes cathodes are shown connected together: https://res.mdpi.com/d_attachment/electronics/electronics-06-00089/article_deploy/electronics-06-00089.pdf   I expect the frequency control to be a positive voltage so the varicaps are reverse biased.

Christoffer, The drain circuit LC has an fc of 2.24 MHz.  I don't know if this is supposed to match the fc of the probe LC but if it should, try increasing the value of the drain capacitance.

Cheers,

[ChristofferB]

You are absolutely right, the 'varactor' diodes are installed wrong 

And the drain circuit having an Fc around the freq I observe as the lower limit is interesting. It's just psu filtering as far as I know, it really shouldn't oscillate.

Nice catch. I'll see if I can throw some more capacitance at it.

--Chris

[T3sl4co1l]

Also, what is the 330R + 5n doing?  Across a 100n?  The 100n probably has a Q lower than that to begin with!  The other 5n + 1uH will oscillate easily, no doubt about it.

Try 100n on both sides, with 220n + sqrt(1u / 100n) in parallel with one.

Tim

[RoGeorge]

As it was already pointed out:
- the main culprit was that one of the diodes was reversed
- another must do is to increase the inductance of the 1uH coil near the 12V power filter (a few hundreds uH, maybe more), and increase the 5 nF (between the LRC node to GND) to 100nF or so, too.
- AM detector as it is drawn in your schematic is heavily dependent of the load circuit that it is not shown, a parallel RC from the anode detector diode to GND is essential for the detector to work properly.
- about that 50 cm coax cable - this will also act as a transmission line, but at 1.8 MHz my guess is it that the transmission line effects (like reflections and losses) are negligible.  However, the parasitic capacitance and inductance of those 50 cm cable might lower the resonance frequency.  The cable will be seen as an extra capacitor in parallel with the main oscillating LC tank.

The main idea, as I understand it (but I have no practical experience, so I might be wrong here) is like that:
- we have a very small amount of energy pumped into the main LC tank, so small that the oscillation can barely maintain the oscillator running
- as soon as some of this faint amount of energy is spent to excite the Hydrogen atoms in the probe, we expect to see some disturbance in our oscillator.  This excitation energy will act like a parallel resistor (losses) to the main LC tank.  Such a losses resistor will lower the quality factor Q of our LC tank, and we hope this effect will be detected as a lowering in the output of the oscillator.
- the oscillator should also be shielded, in my experience an unshielded oscillator is very sensitive to surroundings materials and bodies.

From here, the main thing would be to calculate how much energy would be needed to excite the H atoms in the probed liquid, just to know the magnitude order of that energy and thus the expected parallel losses resistor.  This would be good to know what are we expecting to detect.

About the exciting coil in the pic:
- I see the coil has a plastic support.  AFAIK, plastic materials are based on "hydrocarbures" (I don't know the correct chemical term, I'm thinking chains of C and H, or maybe that ring of 6 Carbon atoms where many of the corners are filled with Hydrogen atoms), so I guess plastic will have a significant amount of H atoms in it, H atoms that can interfere with the measurements of the probed liquid.  I don't know if this disturbance is relevant or not, but I think it should be analyzed.
- your coil is made out of 6 sections, which means that the electromagnetic field will not be uniform.  There will be EM field gradients between the sections, and any gradient in the field is expected to widen the H resonance peak.  A wide resonance will be harder to detect than a narrow resonance, that is also why a uniform magnetic field is required in NMR.

So far I was talking only what should be, and only according to my limited understanding, but without any hands-on experience with such an oscillator.

At this point, my main concern is if it is possible at all to detect any NMR by simply sweeping the frequency instead of sweeping the magnetic field.  Calculating the amount of energy dissipated into the probe's H atoms in order to excite them (and thus deducing an apparent loss resistance in the main LC tank) would be a great help in approximating the required LC quality factor (Q without any probed liquid), but I need to read more about how to calculate the expected energy absorbed by the probe.

Since NMR is one of my wish projects for a long time, I'll try to experiment myself with a similar oscillator.  Will let you know the results.

Please post any progress or difficulties you might encounter, and best luck! 

[ChristofferB]

I'll get right on tidying up the choke/power filtering side of things, and of course the reversed varactor diodes.

Loading the detector didn't occur to me, the LF part of the circuit had such a circuit, I just removed it as my LF ampifier stages didn't function properly.

The entire circuit is built into a tinned steel RF can, and the probe is constructed out of double-sided PCB. I've bought some 0.2mm copper foil I will use to completely encapsulate the probe once I have settled on coil topology.

And I'm aware of the issues with my coil form. It's just so damn handy to wind on, adding/removing windings from an ordinary coil is a pain. Maybe I will wind it on a lenght of 7mm copper tubing for the final version. Or some quartz tubing.

I haven't gotten to any of the atomic physics problems yet, I think it's best to get the 'ordinary' RF stuff working as one would expect beforehand.

Thanks for all the interest! It really is a motivator!

--Chris

[ChristofferB]

Changes has been implemented, see attached. There is no improvement to be seen, lowest frequency that doesn't kill the oscillator is around 2.3 MHz.

Would it be a solution not to separate the probe from the oscillator? Have everything in a flat long-ish copper or brass enclosure where one end protrudes into the magnet gap and the electronics sits right outside? That would require a complete re-build though.

--Chris

[Kleinstein]

To give the oscillator more feedback and thus a wider working range, one could skip the drain side resistor, use more current for the JFET (e.g. an extra resistor from source to ground) and maybe change the 2 capacitors in the FB path a little (e.g. more capacitance to the GND side).
The 300 Ohms and 5 nF to ground at the supply do not make that much sense - maybe better have a similar part at the other side of the 34 µH inductor. Another small point could be making the 100 K resistor at the gate larger. This should increase the Q a little.

With a lower frequency the extra coax cable should be more like less critical than with a high frequency. So I don't think it would help much having the circuit more compact. Still the coarse trimmer would still need to be outside.

[T3sl4co1l]

If you're expecting a wide tuning range, consider a different topology.  The capacitive feedback forms a smaller and smaller fraction of the total as tuning capacitance goes up; the Q factor and resistance of the resonant tank also fall at the same time.

What's the drain resistance about, anyway?  It doesn't hurt anything if it's small, but it only makes things worse in general, as far as I know.

Tim

[M.noohi]

hello
 I am building a CW-NMR spectrometer.
I have used AD9850 module to generate sinusoidal signal. which I command through STM32f103c8t6 microcontroller.

I have connected the sine signal to the two-stage amplifier model ERA () and then to the power amplifier model HF-MINPA () and then to the birdcage coil.

In the receiver part, I have connected about 50 turns of thin wire around the NMR tube to the INA849 chip.

The strength of my magnetic field in the air gap between two magnets is about 0.928 tesla, and the distance between two magnets is 10 mm.

I just sweep the frequency.

No matter what I do, I did not see any signal on the oscilloscope (DSO1202B) that indicates nuclear radiation.

In the first step, I just want to see a detected NMR signal for 1H, and I don't want high accuracy and sensitivity.

Please can anyone help me to make the device work properly.

mostafa.noohi@yahoo.com