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I'm trying to build my own low fux density bench NMR set-up, and am well-versed in electronics. Continuous Wave is supposed to be the easiest. I see commercial instruments that stimulate elements in the range 200MHz to 1GHz. Yet this is nowhere near the Lamor frequency for any of the heavier elements unless there is a major-league magnetic field present. Do they superimpose an amplitude/frequency modulated sideband at the Lamor frequency to achieve resonance ?

Also on the receive side I see drawings where say 200MHz is digitized to find the Lamor resonance. So are they looking for sidebands produced by the Lamor resonance. Or will I get better results looking directly with a magnetometer ?

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1GHz is not NMR but ESR (unless you can find a 22T magnet...). I wouldn't try anything else than water for an NMR demonstrator experiment. It's cheap and the proton density is high. I think they are usually adding a little bit of copper sulfate or so as a quencher, but I didn't look into that.

Your main worry is the magnetic field. I would try to generate a nice homogeneous 0.5-1T field with an iron yoke and some permanent magnets. That brings your RF electronics into the 20-40MHz range. You can do excitation and measurement with $20 USB-RF synthesizers and receivers that can be bought on Amazon these days, which leaves mostly the design and tuning of the rf coils. Most of the experiments I have done in the past were using saddle coils, but for a setup with a capillary, a simple cylindrical coil will probably do better (it can also be wound easily by hand, while a saddle coil requires a coil form for support). We were almost always tuning with "capacitors" made from RG-58. That's dirt cheap and your "tuning tool" is a pair of wire cutters.

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    $\begingroup$ s/joke/yoke/. Make sure the field is very homogeneous in the region of your sample (small sample volume makes that easier). You can digitize at a frequency below the resonance frequency if you have phase locked the sample clock to the resonance. "Tuning" with twisted pairs of wires is very effective. $\endgroup$
    – Floris
    Sep 16, 2014 at 22:57
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    $\begingroup$ @Floris: Hah! English 1 : CuriousOne 0 :-) Thanks, I'll fix it! Yep. Homogeneity is a real problem and I would follow your advice and keep the sample volume small, as well... beats the heck out of shimming, which is a pain, even with professional equipment. The cool thing about all of these integrated DDS chips and fast DACs/ADCs is, that NMR has basically moved from RF into the baseband. Back then we had to use tens of thousands of dollars worth of test equipment just to get started, today a few chips can do all of that! I love the future! $\endgroup$
    – CuriousOne
    Sep 16, 2014 at 23:04
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    $\begingroup$ I agree - the idea that you can do a demo with a handful of RS parts (quick before they close!) is mind boggling. $\endgroup$
    – Floris
    Sep 16, 2014 at 23:07
  • $\begingroup$ Water is fine, but I was hoping to be able to do the NMR demo using the earth's magnetic field which is 50mT and as far as I know is very homogenous. I do have all of the very expensive RF equipment so frequency isn't an issue. $\endgroup$
    – Englishman
    Sep 23, 2014 at 3:45
  • $\begingroup$ Also I'm puzzled by the NMR frequency 20-40MHz, according to the MIT tables Oxygen has an NMR frequency of 33MHz at 5.8T so at 0.5T-1T I'd expect this to be 3.0MHz-6MHz, so and at 50uT then 330Hz is this correct? Presumably water would be even lower than these values? Also does anyone know if it is possible to use a higher carrier frequency and look for sidebands or won't this work ? $\endgroup$
    – Englishman
    Sep 23, 2014 at 4:34

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