DIY low-field continuous-wave NMR spectrometer - Page 1
Forum section: electronics / projects

author: ChristofferB
author_rank: Frequent Contributor
timestamp: June 11, 2019, 11:37:48 pm
content: Finally got my large ferrite magnets! Thought I'd share my progress so far!I'm attempting to build an NMR spectrometer (it's like an MRI but for single molecules). Pole pieces are in the mail.I'm going to make a central block of 30mm thick acrylic with a bore to accept the probe, then have pole pieces and magnets stacked on the outside of that, and outermost acrylic blocks with a pattern of tapped M6 holes to add steel bolts to to shim the field for homogeneity.I've attached the schematics for the magnet.NMR can be done 2 ways: continuous wave, where you have a continuous unchanging frequency and then a ramp generator driving a coil around the magnet or pole piece, sweeping the magnetic field, or pulsed, where you give a short RF burst and then do FFT on the resulting echo (modern approach).If I ever want to sweep the magnetic field, I will add a coil to one of the pole pieces, if not I won't bother.I measured the field strenght, |B| of the magnet in the center of the probe holder and it measures about 1200G or 0.12T. This gives me a hydrogen Larmor frequency of about 5 MHz.I quicly made two crudely orthogonal coils and wrapped around a sample tube of water and tried it out with an RF gen on one coil, oscilloscope on the other and lo and behold! no signal on the scope until i reached around 4.5 MHz, then it grew and tapered off again. Unfortunately I think I've just found the frequency where the coils are in resonance because it makes no difference if i remove my sample tube. aw.Either way, I've found a truly great review of all the electronic oscillators and detectors used in CW NMR, and it's freely accessible! give it a read!My magnet is still very incomplete, I haven't ordered the acryllic yet, and i probably need to do a lot of grinding and hand lapping of the pole pieces to make them polished and parallel enough for this sort of work.They have a small 5mm long 1mm wide cut in the edge towards the centre, I really hope that won't mess with my homogeneity too bad.Either way hope you found this interesting! I'll share more when I've actually gotten something working!Cool review:Morris, Breed, Newton:Advances in Electronics Prompt a Fresh Look atContinuous Wave (CW) Nuclear MagneticResonance (NMR)Electronics 2017, 6, 89; doi:10.3390/electronics6040089--Chris

author: pwlps
author_rank: Frequent Contributor
timestamp: June 12, 2019, 11:05:57 am
content: Quote from: ChristofferB on June 11, 2019, 11:37:48 pmI don't know if it is of interest but I saw the Robinson oscillator detector circuit exactly as the one shown in Figure 10 (Faulkner and Holman, ref. 27) in a commercial NMR magnetometer from the 80's. The probe was using a frequency sweep (rather than field sweep).

author: RoGeorge
author_rank: Super Contributor
timestamp: June 12, 2019, 12:17:53 pm
content: Very interesting!Isn't it supposed to be a closed magnetic circuit, in order to improve the uniformity of the magnetic field uniformity inside the gap?My wild guess is that without coercing the magnetic lines through an external U shape magnetic circuit, the field lines will close through air for each magnet and thus the magnetic field inside the gap won't be very uniform.If the magnetic field is not uniform over the entire probed volume, then different regions of the sample will resonate at different frequencies, thus making any resonance hard to see.Again, I have zero hands on experience with NMR, but looking at the setup I'll say the field between magnets will vary a lot without a confined magnetic path.I'm very curious to see how it will go, please keep posting the progress even when it doesn't work as expected, like you just did. Thanks for sharing.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: June 12, 2019, 04:05:01 pm
content: Thanks for the interest! I found a modernized version of the marginal oscillator using a Jfet instead, and having some more fine control. The original author claims it can be swept easily by adding a varactor in paralel with the tank capacitor. I'm either gonna do that, or make a power amplifier to sweep the magnetic field, either way I'm going to rely on the ramp output on my oscilloscope, as it's already synced to the scope and I don't think I can do better anyway.And you're right, having a flux return yoke would be good, but it's also a lot of additional metalworking, most of which I will have to pay others to do, so I'm gonna start without and see if i can shim it into homogeneity.My probe so far consists of 90 windings 7 mm dia. coil 3 cm long. I have a bit of a cyclic problem, since I can't find the larmor frequency without the probe but I can't really design the probe without knowing which freq. it should be resonant at.I'll keep making updates for sure.--Chris

author: Kleinstein
author_rank: Super Contributor
timestamp: June 12, 2019, 04:42:18 pm
content: For the spacer, holding the two magnets apart, I would not use acrylic or any other hydrogen containing compound. This would give a background from further out. The thin strips shown so far may be acceptable. I would prefer something like ceramics, glass or aluminum.An outer magnetic return path could replace the spacers and give a stronger field (e.g. something like 4-10 times). In addition it would reduce effects from magnetic material in the surrounding. In the current form iron in some 50 cm around the magnet would effect the homogeneity of the field. I agree that it takes quite some effort and material.p.s.: For finding the resonance it could help to measure the field strength with a separate sensor. With a DSO or digitizing DMM one might get away with turning a small coil in the field. One can tune the probe coil with an external trimmer cap to get a larger frequency range with a single coil.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: June 12, 2019, 06:07:54 pm
content: Thanks for the interest, I agree that the spacer might not be ideal but the reason I'm sticking to this specific magnet setup is, aside from ease of construction, is that I've read a series of articles using one very similar, and actually getting structure resolution data out of it.From the article:source:https://www.sciencedirect.com/science/article/abs/pii/S1090780716302713Maybe some stiff expanded polystyrene would be a good compromise, having a very low density and still being workable.Another idea for the return path: getting a lenght of square structural steel tubing 10 cm wide on the inner diameter, with 5-6 cm inner height and put moth magnets and pole pieces on the inside. Tough to machine on the inside, but it could be filed reasonably flat and then shimmed with aluminum foil.But that's in the future, I'll get to work on building the marginal oscillator, so I can actually see the resonance. The width of the resonant area for a single proton environment divided by frequency is a good measure for homogeneity.See attached schematic. I plan on adding a BNC port somewhere to connect a frequency counter to.Thanks for the interest!

author: moffy
author_rank: Super Contributor
timestamp: June 12, 2019, 10:38:54 pm
content: Please keep us updated with your progress. The project is truly fascinating!

author: ChristofferB
author_rank: Frequent Contributor
timestamp: June 14, 2019, 12:00:38 pm
content: So, I think I have designed a yoke that I can manufacture without machining or welding.It's based on the (hopefully correct) assumption that the geometry and tolerances can be much sloppier than in the pole pieces and in the parallelity of the faces of the magnets.I've cut an I beam in half (with a sawzall, it wasn't pleasant), and found some 60mm wide mild steel flat bar pieces.My plan is to have a magnet on each face of the I beams, now T beams, i guess? and use the 60mm for distance.The whole assembly should hold together just by the magnetic force, but I don't like the narrowness of the flat bar, in terms of it suddenly sandwiching in on itself, so I'll put a 6mm hole in each corner of the I beams and put some m6 threaded rod through for stability, to clamp it all together.The faces of the I beams are definitely not flat, they're slightly concave, but I guess I can shim it with some thin foil of steel or aluminium without ruining the flux path too much.I've modeled it, and the cut parts of the I beam doesn't seem to carry any flux, which is good. It also doubles as a magnet stand so that would be handy.What do you think? Is this a yoke or a joke?See attached.Thanks for the interest!

author: ale500
author_rank: Frequent Contributor
timestamp: June 14, 2019, 08:00:52 pm
content: As a chemist, I applaud. as a fellow DIYer great work !. Old machines used to have an oscilloscope, too, the 60 and 90 MHz ones. The 90 MHz machines had a 1 T magnet, they also weighted 1 Ton, too.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: June 14, 2019, 08:15:12 pm
content: Thank you so much! Yeah, permanent use of oscilloscopes for scientific instruments such as XY displays, timebases and amplifiers totally justifies hoarding all those semi-vintage 20 mhz scopes!So, my gaussmeter (hall effect sensor, SS49E) only really goes to 0.1T, according to the datasheet, but the probe reads 0.15T between the pole pieces.I got the magnet assembled and painted! I'm pretty pleased! 8 kg of yoke should be decent! I'll keep off the shimming until I can get the oscillator done, and then I can tune after the narrowest peak. Parallelity will come with the pole pieces currently in the mail.Either way I'm quite happy with it! The threaded rod isn't really needed, it clamps itself together just fine! It's just a "don't crush my fingers" measure, and also to ensure it doesn't shear sideways and fling out the steel blocks.--Chris

author: Kleinstein
author_rank: Super Contributor
timestamp: June 14, 2019, 10:06:42 pm
content: With the configuration of strong magnet and coil around the sample one can also detect mechanical resonances in metallic (non magnetic) samples. This may work for a first test as the frequency can be calculated from the dimensions and modulus. The frequencies for a few mm samples tend to be in the 0.2-2 MHz range - with quite a few resonances.I have tested this in the pulsed mode of operation (high power (e.g. 50dBm) pulse + detection on decay), but it should also work CW. AFIAK the resonances should be stronger than from NMR.The yoke looks good - maybe the side parts are a little close to the magnet, causing some inhomogeneity at the edge.0.15 T is already quite a strong field and reducing the gap with the pole pieces will increase the field further.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: June 15, 2019, 01:44:49 am
content: That's interesting! Kinda like what is seen in quartz crystals? I'll have to try that. It's also a good-to-know pitfall that could make me think i was seeing the proton resonance.I would like the sides to be all the way out too, but unfortunately the field sucked them in and they're very tricky to move now. All the way out there's 10mm to the magnet edge, maybe I should make some acryllic spacers on the inside to keep them in check.I hope i can shim it by just adding bolts or nuts to the back side of the yoke, that would be ideal. I'm unsure what effect the threaded rod will have there, maybe it'd have been better to add a few more.Either way, I'm considering the probe design, I think the best would be to have a sheet of copper clad pcb riding on top of the magnet, with some nylon screws to adjust height and position of the sample, and then have a soldered copper box or pipe down the centre, with the coil in, so the sample tube is inserted through the pcb. That would also allow for coax connectors and maybe a preamp close to the coil.-60/40 Sn/Pb solder isn't magnetic is it?--Chris

author: RoGeorge
author_rank: Super Contributor
timestamp: June 19, 2019, 07:07:41 pm
content: I don't think 60/40 solder is magnetic, at least I don't feel it attracted by magnets.A way to test for very faint magnetic interaction is to put a small bit of the tested material to float on water then to keep a strong magnet nearby (almost any small blob of material can float on water because of the superficial tension, or by improvising a tiny boat/raft for bigger lumps).Some apparently non magnetic metals are in fact diamagnetic (Landau diamagnetism), but I didn't tested for solder.Last weekend I revisited an old NMR project that I put on hold many years ago (that is why I am so curios about your CW NMR). I've seen solder blobs on some other DYI NMR machine (e.g.https://youtu.be/Bf3r9Db0TSs?t=269at minute 4:29 is the reading coil for a FT NMR, with lots of solder around, but he is sweeping the magnetic field intensity, not the excitation frequency).The FT NMR project I was trying back then was aiming to get 3D MRI (Magnetic Resonance Imaging) in the Earth's magnetic field. The best found online at that time was the 3D image of a fruit and vague hints about its internal structure, with a complete 3D scan for an object the size of an apple being ready in "just" a day or two. Can not find that DIY project any more, but the results were similar with the 3D demo from this educational machinehttp://www.magritek.com/wp-content/uploads/2013/12/Magritek-MRI-2011-web.pdf(note that that 3D image took hours to scan, and the resolution is very low, so pretty unusable).To read the NMR signal there was a sharp band-pass filter to reduce the out of band noise, and a few amplification stages. All I've seen use to had filter + low noise pre-amplifier, thinking here it will be very unlikely to see the NMR signal with just an oscilloscope.Didn't know about the trick with the marginal oscillator, that was very interesting to learn, thank you.Another thing that I found fascinating this weekend was to learn about how NMR can help to identify organic molecular structure (yep, never dive into that before). Judging by the other related project of yours, the gas chromatographhttps://www.eevblog.com/forum/projects/gas-chromatograph-diy-done!/, I guess this is what you are after, organic molecule identification by NMR, is it?These are the videos that I liked so much (linking them here so I could find them in the future, after I'll forget), from the 'Knowbee' channel:

author: Kleinstein
author_rank: Super Contributor
timestamp: June 19, 2019, 07:45:58 pm
content: The marginal oscillator is kind of a very sensitive amplifier, a little like of old days regenerative receivers. So the signal can be quite large at the output of the oscillator. It also acts as a low BW filter.The mechanical resonance would mainly be a problem (possibly confusion with NMR resonance) with a small (e.g. 3 mm ball) relatively isolated solid part. The Q-factor and thus sharpness of the resonance depends on the material, but can be lowered with coupling. The glass test tube with a coil should have a relatively low Q, more like the LC resonance.For tuning the LC resonance one should keep in mind that variable capacitance diodes have quite some loss and thus should not be used for too much. So the varicap should only be a smaller part of the total capacitance. The coarse part should be better with switches capacitors or an air trimmer.

author: langwadt
author_rank: Super Contributor
timestamp: June 19, 2019, 08:17:58 pm
content: Quote from: ale500 on June 14, 2019, 08:00:52 pmwhy would it be called a 90MHz machine if it is only 1T? Hydrogen is ~42.6MHz at 1T

author: ChristofferB
author_rank: Frequent Contributor
timestamp: June 19, 2019, 09:34:46 pm
content: Thanks for the continued interest!I'm still waiting for my pole pieces, as well as parts for the marginal osc.I found this design for a permanent magnet sweep coil driver, and honestly it seems much easier than trying to make a stable RF VCO, so I think I'll start there.I'm currently trying to design the mechanical layout of the probe, since I need a coil form that's not conductive, but also doesn't show up on NMR.I'm thinking, making a shielded 1 cm thick copper foil box that can be lowered down in the field, and then have a hole in the top for the sample tube.Tube guide and coil form will be a - don't laugh - drinking straw epoxied in place.It's very thin walled extruded polystyrene, and very low density. and has the right diameter. I think that's a winner. I think I will have both ends open, so I can make a variable height stop to position the tube with.What do you think?

author: ChristofferB
author_rank: Frequent Contributor
timestamp: June 19, 2019, 09:59:19 pm
content: Just did some more design of the probe. I think this is what I'll try.Probe box will be 2 layers of double sided copper clad pcb, sandwiched together with brass standoffs. On the periferie of the brass standoffs a copper foil band will be soldered all the way around. Perhaps one of the PCB layers could be foil too. Either way, straw glued into the center, and a coil (or multiple) wound onto that. I'll probably start with around 50 windings, depending on the straw diameter.You can see how I'll shield the coil signal all the way on the attached schematic.Only problem is getting it straight in the center of the field. I think I'll make some small clamps with nylon screws that can be adjusted and clamp onto my yoke.Maybe I should get some small thick O-rings to slide over the NMR tubes so they'll just hang at a set height.Oh, and I just butchered a transformer for some laminated electrical steel shims. When I get the oscillator running I'll try using small (3-5x10mm) shims on the magnet poles to homogenize the field.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 20, 2019, 01:39:11 pm
content: Adding an electromagnetic field to the magnet could work. However it could make it a little more difficult to make sure the field is homogen.The circuit shown is good for a small current up to about 1 A. This may be enough, if the coil has enough turns. A high electric power is problematic anyway, as the coil would than heat up the yoke or magnet and this way may change the field. It may need some protection (e.g. trans-sorbs parallel to the coil to prevent a possible damage from voltage spikes (e.g. during turn of or the trace return phase). I would expect the ramps to be rather slow anyway, so that the coil inductance in more like small.At some 0.25 T the frequency would be in the 10 MHz range - 50 turns may very well already be too much than. The oscillators usually have an impedance range where they work best. The coil is essentially like an air coil an there are programs to calculate the inductance. The shielding / case would reduce the inductance a little.I would consider have the oscillator really close to the probe, possible even inside the magnet., inside the box. This would reduce losses and possible other effects of cables. At least the main tank circuit should be there.If the field is good, the exact position of the sample should not matter.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: June 20, 2019, 07:42:01 pm
content: I honestly can't imagine I need that much current. I have to sweep the field something like 60 miligauss to get a 50 ppm frequency shift.I am unsure how to design the sweep coils, so after finding the larmor frequency I may just try out two identical coils in series of an arbitrary number of turns, and then have a lab supply to drive them, and then sweep over current ranges to see which range would be useful. maybe 5-10 windings of some thin enameled wire.I've added the pole pieces, after filing and grinding them as flat as I can. If they need to be better I'll get a machine shop to do a gentle mill pass on them.The field strenght is constant within the accuracy of my hall effect probe over almost the entire area at 0.115 T, or 1150 gauss.I think I've found a cheaper alternate to the probe: making it from perspex/plexiglass coated in adhesive copper foil. There's apparantly a company who sells them made like that:Thanks for the hints!

author: ChristofferB
author_rank: Frequent Contributor
timestamp: June 23, 2019, 10:58:49 pm
content: So I've been productive! Almost ready to see magnetic resonance!I've completed the marginal oscillator. It took me forever to troubleshoot the simple AF amp (see schematic in prev.post). It turns out the V- pin of the metal can LF356 is connected to the can, and of course I'd just dead-bug soldered the can to my ground plane... It doesn't even mention it in the datasheet!Now the oscillator works as expected, any change in RF amplitude gives a quick AF spike.either way, I've designed a probe. Haven't settled completely on the coil yet, as it's slightly too high (5-7 MHz).Anyway, I feel the mechanical design turned out great! Hope you agree!--Chris

author: Kleinstein
author_rank: Super Contributor
timestamp: June 24, 2019, 06:34:16 am
content: The mechanical setup look good. However I would prefer to have much of the capacitance for the probe resonance down at the coil to reduce the current going through the BNCs and possible connector effects.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: June 24, 2019, 08:10:23 am
content: That sounds like a good idea. I'm just worried about the component leads being magnetic and skewing the field but I guess it's a very small mass of magnetic material. I'll add 100pf across the coil. Maybe I'll even get it ton desired freq rangeThanks!

author: Kleinstein
author_rank: Super Contributor
timestamp: June 24, 2019, 08:16:13 am
content: Magnetic material in the cap is a good point. Some ceramic caps may contain a nickle layer. I would expect film caps (e.g. PS) to be non magnetic, with only aluminum, copper and maybe zinc used as metals.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: June 24, 2019, 11:52:32 am
content: That may be right. I'll see what I have on hand.I'm also considering implementing the gradient coils (at first manually operated with a lab psu) as just being smaller (3 cm dia maybe) coils inserted onto each side of the probe. I havent yet solved how best to put coils around the pole pieces, as there arent much space, and thin wire coils that large would be very floppy. Perhaps The best would be to get some coil forms that fit over the pole pieces 3D printed, and then just slipped over.or maybe I should just wind them on a mandrel slightly larger, using thin wire and maybe not care about neat windings, and when they're in place, epoxy them into a bundle and slip them over, adding little plastic wedges to keep them centered.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 24, 2019, 12:18:40 pm
content: It should not matter much where the windings for the field sweep are. They should not be too close to the sample though, to to couple too much AC wise - though at a different angle, but nobody is perfect. So the extra turn could even be around the spacers of the yoke.

author: pwlps
author_rank: Frequent Contributor
timestamp: June 24, 2019, 07:08:01 pm
content: As I see your project advances fast, I'm eager to see your first NMR signalsWhat kind of sample are you going to try ?Maybe this is too much anticipating your future results but remember in CW NMR knowing the sample T1 is important. In standard CW NMR you need to avoid saturation to see the signal. In fact you have two regimes.: 1)standard CW NMR with slow sweep through resonance where the condition to avoid saturation is: gamma*H1<<1/T1 ; 2) "fast adiabatic passage" (Abragam chapter III), where you deliberately saturate during the sweep through the NMR line, then the condition to avoid T1 effects during the sweep is H1/(dH0/dt)<<T1; but in this case the measured signal vs field does not give you the true NMR lineshape. For (1) if you want to use water it might be good to add some paramagnetic ions to shorten T1.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: June 24, 2019, 07:21:08 pm
content: I am too! I'm honestly uncertain on that part of the theory, but I've read that a standard CW nmr test solution is concentrated copper sulphate (~1M) in water to shorten T1. I plan on attempting both fast and slow scans, as I use my oscillocope ramp gen. as timebase trying different things on an empiric basis should be doable.The marginal oscillator as described earlier (not designed by me) is useless in sweeping the RF freq, since the amplitude isn't constant, so I'm building the sweep coils (first I'm trying 2 x 9 windings 3 cm diameter) connected directly to a lab psu.I did however tune the detector to a point where inserting the CuSO4 sample would quench oscillations, at about 4.3 MHz, so perhaps I did hit magnetic resonance!I'm not counting success before I see a peak, though.The problem with using a ramp generator is that on the vertical down slope it'll rush back through the entire field, giving a graph on the screen that has a sorta good line, and then a return bad trace looping back in the background. Maybe it would be worth using the blanking input with a window comparator to blank the display on the return.Thanks for the interest!

author: Kleinstein
author_rank: Super Contributor
timestamp: June 24, 2019, 07:39:08 pm
content: For the sweep one could in theory use a triangle sweep, going up and down at the same rate. This would give two superimposed curves with possible hysteresis effects from the magnets and also possible difference due to a very high Q resonance and fast sweeping.In this case averaging should be time based, not in XY mode as to get both parts separate.I don't think blanking in run back is really needed. The up / down rates can be quite different if really needed. However the speed is limited by eddy currents - even if the current in the coil changes fast, the field will not change that fast.The sample has two effects that can quench the oscillation: one is the NMR, the other is just eddy current due to the electrical conductivity of the liquid. So the liquid by itself would have an effect just by macroscopic conductivity. To reduce the effect would be by using a non conductive sample, maybe some organic liquid / paraffin.

author: pwlps
author_rank: Frequent Contributor
timestamp: June 24, 2019, 07:58:05 pm
content: Quote from: Kleinstein on June 24, 2019, 07:39:08 pmEddy currents are independent of the field so it might not be a problem. On the other hand a non conducting liquid sample like parrafin will have a quite ridiculous spin relaxation rate (1/T1) making the detection much more difficult. I wouldn't care too much about eddy currents, after all there is for sure a lot of eddy currents in our body during an MRI scan

author: ChristofferB
author_rank: Frequent Contributor
timestamp: June 24, 2019, 08:00:41 pm
content: Many lab manuals for undergrad physics courses uses a solid plug of natural rubber as sample. That may be the reason. I just find that discouraging because that similar aliphatic chained molecules can't even be resolved on a 500 MHz 10T instrument, so I'd rather have a nice peak.

author: pwlps
author_rank: Frequent Contributor
timestamp: June 24, 2019, 08:15:47 pm
content: Quote from: ChristofferB on June 24, 2019, 08:00:41 pmYes, rubber or another polymer might be a good sample because it is well insulating and at the same time has short relaxation because the motion timescale is much longer than in simple liquids making the dipolar relaxation mechanism very efficient.

author: Mr polarbear
author_rank: Newbie
timestamp: June 29, 2019, 11:32:48 pm
content: First of all wow, your dedication and work is amazing. Before this thread I had never heard of NMR, after reading all the posts I started searching for any information I could find on NMRs, turns out there isn't a lot especially on the sweep/hermholtz coils.In terms of things that actually are helpful and note worthy this is what I found;(1)http://www.cco.caltech.edu/~derose/labs/exp5.html(2)http://courses.washington.edu/phys431/cwnmr/cwnmr.pdf(3)http://experimentationlab.berkeley.edu/sites/default/files/writeups/NMR.pdf(4)http://www.conspiracyoflight.com/NMR/NMR.html(5)http://www.jabolatorium.com/Link 1 is a dumbified sweep coil circuit, might help with designing a circuit to drive the coils.Link 2 uses 80hz triangle waveform instead of a ramp waveform thought that might solve your ramp waveform dilemma.Link 3 has an interesting NMR-head at fig15 don't really understand the use for the paddle, I mean i get that it is used for changing the magnetic field shape, but why is beyond me. The rest of the article is not very helpful the equipment used is outdated.Link 4 is the best and and can be a real building block in designing circuits for your NMRLink 5 I know is an ESR, but it has some interesting circuitry.Anyways best of luck in comleteing your NMR (I really hope you do)

author: langwadt
author_rank: Super Contributor
timestamp: June 30, 2019, 12:39:12 am
content: why continuous wave?

author: ChristofferB
author_rank: Frequent Contributor
timestamp: June 30, 2019, 10:55:22 pm
content: Man, that you guys dig up this many sources and references is making this endeavor a whole lot more achievable - and enjoyable.Thank you so much! I'll sift through it.quick thoughts: 5) The ESR meter looks cool. I did attempt to collect enough waveguide hardware, couplers and gunn oscilators to build an x-band ESR/EPR spectrometer, but honestly, the amount of samples that are EPR active compared to NMR is very limited. Also I don't have experience with the real instruments, but I do with NMR.4) I've wondered too, and I'm unsure what the copper paddle is. It might be flux steering - see fig. 4 in this article:https://www.mdpi.com/2079-9292/6/4/89/pdfLooking further it looks as if the illustration is borrowed from that article.--CW nmr was chosen because it's circuitwise simpler. This entire project stands and falls on whether I can construct a magnet homogenously enough.I won't know until I see the resonance, so until then, simplest, safest, most time-proven is the way to go.Reusing the system, along with a pulse generator, capturing the FID (maybe with a soundcard?) and doing FFT and such on multiple scans requires much more work. Maybe some day but it's definitely an option.

author: pwlps
author_rank: Frequent Contributor
timestamp: July 01, 2019, 01:24:39 pm
content: Quote from: ChristofferB on June 30, 2019, 10:55:22 pmI'm tempted to think a pulsed NMR electronics could actually be simpler to assemble (tweaking a marginal oscillator to get a good signal isn't that simple either) if you can afford buying critical components like a good LNA (short deadtime) and a power amp with a gain quenching capability (to limit noise injection from the power amp). On the other hand it might be difficult to achieve a very short deadtime at such low freqs so that good field homogeneity would still be required.

author: langwadt
author_rank: Super Contributor
timestamp: July 01, 2019, 03:51:02 pm
content: Quote from: pwlps on July 01, 2019, 01:24:39 pmI'd tryhttps://en.wikipedia.org/wiki/Spin_echo

author: RoGeorge
author_rank: Super Contributor
timestamp: July 01, 2019, 11:34:34 pm
content: I just realized I already have magnets big enough to try NMR.Bought them for approx. $10 about a year ago, from LIDL, as helping-hands to keep in place ferrous materials while welding or assembling. If I didn't messed up the calculations, the field measured between the 2 middle spacers is about 66.5 mT.Quote from: ChristofferB on June 11, 2019, 11:37:48 pmHow did you measured \$B\$, and did the value changed much after mounting the magnets inside the metal yoke?

author: ChristofferB
author_rank: Frequent Contributor
timestamp: July 02, 2019, 08:31:37 am
content: Cool! The more the merrier!They're a little on the narrow side for homogeneity I think, maybe unscrew them and put them the other way? On the other hand then there's a hole in the middle..I measured my field strenght with an ss49e Hall effect sensor I'd epoxied to a non magnetic handle. Then just reading out the voltage on a multimeter and using datasheet value of 1.4 mv/mG (uncalibrated) to calculate field strenght.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 02, 2019, 11:07:55 am
content: The large read magnet may still be on the small size. The difficulty is usually to get a very homogeneous field. Just to get it running the filed does not even have to be that large - for the extreme case there is "zero" field NMR, using just the earths field.The usual way to get the field homogeneous, it to use only a relatively small part in the center of the magnet. So it's a good idea to have the field over at least some 5 cm across, even if the actual probe volume in more like 5 mm across.It may still be possible to add larger pole pieces to get a larger field volume. Chances are the steel plates on both sides are a little on the thin side.

author: shadow.dark
author_rank: Contributor
timestamp: September 08, 2019, 03:00:34 am
content: I am very interested in NMR. I have a VNA. Can I implement the NMR function by adding simple attachments?

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 10, 2019, 10:04:21 pm
content: Thanks for the interest!I think you'd have more success with a spectrum analyzer. Modern NMR is done by short RF pulses and then the decay is fourier transformed. Problem is the differences in frequency are very small so you'd need a VERY narrow-band high-res spectrum analyzer.If your VNA can do some kind of FFT it might be feasable though.The project is on a bit of a hiatus at the moment, since im developing HPLC detectors at the moment. I'll revisit it at some point though.--Chris

author: mrflibble
author_rank: Super Contributor
timestamp: September 16, 2019, 03:03:08 pm
content: I've been following this with interest. Cool project!Did you have a chance to measure the homogeneity of the magnetic field? I wonder if maybe a Halbach array can create a stronger field with good homogeneity. The main tradeoff being smaller volume. And the other tradeoff being having to take more care to not get your fingers crushed during assembly of the array.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 16, 2019, 08:43:20 pm
content: Thanks for the interest!I haven't gotten further with homogeneity, my plan for testing is to make circles of 5x5 mm graph paper and attach to the pole pieces and then measure the field directly on top in every square and see how things look.I'm still at the problem of making magnetic sweep coils, and it's not looking good. I don't think haphazardly wrapping enamel wire around my yoke is going to give anything usable, so i think ill re-visit using some capacity diodes to sweep the frequency slightly. maybe just slowly manually at first or maybe connected to the scope ramp out. I think that's by far the easiest solution.In reality i'll just parallel up 5-10 1N40XX diodes, since all diodes exhibit some capacity. I can always tune myself away from the uncertainty by attenuating the ramp signal and adjusting the freq.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 16, 2019, 11:13:25 pm
content: [ Specified attachment is not available ]Ok so the pressure of people following the project inspired meI made a varicon control with 2 1N4006 diodes in series, parallel over the tuning cap and coupled the center point out to a BNC through a 28K resistor, and was able to tune 80 KHz from 0 to 12V ! that's good enough for me! quite probably far too big a sweep! Anyway I hooked it up to my scope and adjusted the feedback to obtain this. (attached). This clearly shows a sweep is going on, as the oscillator is quenched in half the sweep! i imagine NMR resonance would look like a very sharp quench/unquench in this setup but I need to play around more with that.Either way diode tuning is a go!

author: Kleinstein
author_rank: Super Contributor
timestamp: September 17, 2019, 07:15:48 am
content: The diode tuning is definitely an option to tune the frequency. However to be sure to get a magnet field dependent effect, one might still want a small coil to change the field. The sweeps may still use the diodes and the coil only for slow shifts to a few different field setting.The extra coil could have problems with the joke part of the magnet not instantly reacting to changes in the magnetic field. A missive piece of iron kind of slow down field changes and may also cause jumps / nonlinear effects. So a coil around the joke could cause trouble.On the other side a coil close to the sample could dampen the detector coil.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 17, 2019, 04:10:58 pm
content: My main problem right now is I don't know what field strenght i'm working at, so i'm changing the tune range more or less at random.My hall effect sensor don't go up to the strenghts i expect so I don't trust its reading. I think the ideal next step would be to ask my university's NMR responsible guy to borrow a research grade teslameter and get the magnetic strenght absolutely nailed down.Being able to vary the magnetic field slightly might be a really nice idea. especially in the event that there's some strong RF noise source that's right on my resonance freq. Maybe just 10 turns of some 2.5 cm2 stranded installation wire on either side of the yoke would be sufficient at low voltage/high amp.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: November 11, 2019, 11:52:23 pm
content: Ok so sorry for updating this long-dead thread but I thought it better to keep all info here than to make a new one..I did the terrible thing I didn't want: I made my magnet worse to get it into a range where i can measure the field strenght. I made the gap bigger (almost 10 cm now) and now I have a field strenght in the gap at around 43 mT. this gives a Larmor freq of around 1.8 MHz.I also re-designed the probe to have a larger inductance coil, but for whatever reason, I cannot get the frequency below 1.85 MHz. Any larger inductance/capacity and the oscillator collapses. I'm clearly missing something about oscillator design. I have the varactor diodes and main tuning capacitor in the electronics box, then a 50 cm RG58 cable to the probe, which has a 40 pf trimmer and the coil.Can anyone propose an improvement to the oscillator design that would help with this? Do i need to move more of the tank reactance into the probe? Or can i have a smaller coil in the probe as pickup and have the rest of the L in series in the oscillator enclosure, maybe wound on a toroid?I'm at a loss.Thanks in advance!--Chris

author: Kleinstein
author_rank: Super Contributor
timestamp: November 12, 2019, 09:02:24 am
content: I don't think it's a good idea to have another inductor at the oscillator, as this would makea weaker coupling and more power going through the coax - so possibly more loss. It is more like some other changes to the oscillator (e.g. larger caps at the supply, higher or lower current).

author: RoGeorge
author_rank: Super Contributor
timestamp: November 12, 2019, 09:59:57 am
content: Great to hear news about this project!From the history of the thread, I remember you tried a few different methods.- Which one is now, oscillator frequency sweeping while looking for an amplitude peak, or magnetic sweep?- A schematic to the oscillator you are testing now, or a link+page to the exact schematic would help, maybe a draft hand-draw of the setup, too.- Does the oscillator work at low frequencies if you solder the LC tank on it, without using the 50cm transmission line?

author: ChristofferB
author_rank: Frequent Contributor
timestamp: November 12, 2019, 05:33:29 pm
content: You're right! This should come with an updated schematic, see attached!I am counting on a frequency sweep in a static magnetic field for now, When i get a low-impedance magnet driver made I might just use that as a fine adjust.I was assuming with a big tuning cap, I couldjust make a range of coils for different frequencies, as is often seen in dipmeters and old ham receivers.Trying to eliminate the larger R from having a coax between my L and C is a good idea, I'll update you on how that goes shortly.Thanks again for the interest!

author: Kleinstein
author_rank: Super Contributor
timestamp: November 12, 2019, 06:04:21 pm
content: 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.

author: iMo
author_rank: Super Contributor
timestamp: November 12, 2019, 06:14:24 pm
content: This is an interesting project!In past I was thinking on AD9851 for precision measurementsYou may get nanoHertz resolution from 0-2MHz crystal stable..

author: ChristofferB
author_rank: Frequent Contributor
timestamp: November 12, 2019, 10:19:16 pm
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 13, 2019, 12:09:05 am
content: Is that a typo, the "varactors" are supposed to be antiseries right?What do you mean "over" the capacitor?Tim

author: duak
author_rank: Super Contributor
timestamp: November 13, 2019, 12:55:41 am
content: 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.pdfI 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,

author: ChristofferB
author_rank: Frequent Contributor
timestamp: November 13, 2019, 07:30:36 am
content: You are absolutely right, the 'varactor' diodes are installed wrongAnd 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

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 13, 2019, 11:15:26 am
content: 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

author: RoGeorge
author_rank: Super Contributor
timestamp: November 13, 2019, 03:54:19 pm
content: 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!

author: ChristofferB
author_rank: Frequent Contributor
timestamp: November 13, 2019, 04:55:29 pm
content: 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

author: ChristofferB
author_rank: Frequent Contributor
timestamp: November 13, 2019, 06:44:17 pm
content: 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

author: Kleinstein
author_rank: Super Contributor
timestamp: November 13, 2019, 07:19:23 pm
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 13, 2019, 08:00:20 pm
content: 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

author: M.noohi
author_rank: Newbie
timestamp: August 23, 2024, 08:03:15 am
content: helloI 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