[time-nuts] Magnetic Resonance Spectrometer - redux
Tom Van Baak
tvb at LeapSecond.com
Tue Jul 8 16:36:29 EDT 2014
It's always a good sign (being a time nut) to wonder if anything around you can make a frequency standard.
I myself always wonder how close to UTC stop lights are synchronized in time, or what the frequency or ADEV of a car's turn signal is, or how to measure the beat note of two or more turn signals in front of me. In just one minute you face a fine example of most of the art of time & frequency measurement.
Anyway, here's an answer to your Magnetic Resonance question from someone I know:
> Hello Tom--
> Sure enough, this can be done, but it requires the magnet, first and foremost.
> From a frequency-standards point of view, it's a fail, though, since the frequency
> you get is directly proportional to the field strength, and will therefore show
> all the tempco of the permanent magnet's field -- perhaps 0.1% = 1000 ppm per Kelvin.
> With a modern NdFeB magnet, it's even worse, perhaps 0.4%/K.
> The second issue is the requirement for the field to be uniform in magnitude,
> perhaps to 10 ppm, over the volume of the sample. That is hard to assure in a
> random surplus magnet, though once you *find* a signal, you can search around for
> a 'sweet spot' to improve it.
> If you look at the TeachSpin Pulsed NMR system,
> you'll see a modern teaching model of this sort of instrument, based on a
> temperature-controlled NdFeB magnet and running at about 21 > MHz. Naturally the
> circuitry is different, and in fact modern NMR units typically use pulsed excitation
> and Fourier-methods detection, rather than the continuous-wave methods in vogue
> during the vacuum-tube era.
> For people who want to do homebrew NMR, I myself would recommend earth's-field NMR,
> which was also the subject of a Sci Am 'Amateur Scientist' column -- there, you
> give up the concept of a frequency standard, and you use NMR to detect the small
> changes in the earth's magnetic field -- that is to say, your hobby becomes
> magnetometry instead.
> Most questions welcome!
> Best, DVB
----- Original Message -----
From: "Bob Stewart" <bob at evoria.net>
To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Sent: Saturday, July 05, 2014 8:47 PM
Subject: [time-nuts] Magnetic Resonance Spectrometer - redux
First, an apology. When I changed the topic on my original post, I thought that would be OK. Apparently that's still a thread-jacking. Sorry.
I'm still interested in this Magnetic Resonance Spectrometer thing, though. On page 335 of the pdf linked below by Dave, there's an experiment with an MRS using water and the magnet from a magnetron available back then. Apparently the resonant frequency of hydrogen nuclei in water is 6.131325 MHz in that magnetic field. Did anyone ever pursue this with the idea of creating a frequency standard, or was the technology just too primitive at the time? Perhaps it's a repeatability problem from the magnetic flux standpoint? I can guess that temperature changes would cause enough of a flux strength change to cause a problem, but that's just a guess.
Bob - AE6RV
From: DaveH <info at blackmountainforge.com>
To: 'Discussion of precise time and frequency measurement' <time-nuts at febo.com>
Sent: Saturday, June 28, 2014 1:09 PM
Subject: Re: [time-nuts] Loran, GPS, Lightning, Timing
A PDF of the 1960 book can be found here:
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