Thank Robert Santini and L.S. Mombaswala for their helpful replies. See
below for the original messages.
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Robert Santini <rsantini_at_purdue.edu> wrote
Hi Jerry,
I have been building and acquiring commercial NMR systems for about 40
years. I have had to deal with this interference on more than one occasion!
The short answer to your query is -- locate as far as possible from a power
transformer! The residual AC field will produce field modulation side
bands on all peaks in the spectrum. As a practical test, take a CRT
oscilloscope or a CRT color monitor near a power transformer. You should
see artifacts in the display to a distance of about 20 meters depending on
transformer orientation. If you are forced to consider a location within
about 50 meters, try to have the power transformer oriented to minimize the
coupling with (I assume) a vertical bore cryomagnet. Probably the worst
case is to be directly above or below the power transformer. BTW, some of
the modern "line regulating" power transformers operate in a core
saturation mode. This can give a net DC field component that varies with
the load current at the transformer secondary winding(s).
I have tried various kinds of magnetic shielding material and theoretical
models over the years. There are usually too many variable to use these
tools effectively. I think that the basic problem is that NMR is the best
tool to characterize a low level AC field. Unfortunately, NMR
spectrometers are not portable.
The ultimate test is the level of 60 Hz plus harmonics that appear as
modulation side-bands on a strong 1H signal under high resolution
conditions. BTW, this test is characterizing the effective vertical
component of the AC field. The horizontal component has indirect effects
on a high resolution spectrum. This might not be the case for a
spectrometer with a magic angle probe
An NMR gauss meter could be used at relatively low 1H frequencies with a
portable air-core permanent magnet to characterize a lab environment before
installation of a spectrometer. Modifications will be necessary as most
simple NMR gauss meters use 60 Hz as a modulation frequency (from the AC
line). If a 50 Hz modulation frequency is used, the 60 Hz field may show
up as side band information. It would be necessary to have a sample with a
fairly sharp line and excellent stability of the magnet and frequency
source. I would try to probably "zero beat" the resonance and implement a
phase detection scheme to detect the 60 Hz AC signal. It may be better to
detect a higher harmonic (say, 180 or 240 Hz.
Regards,
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"L.S. Mombaswala" <laiqsm_at_cc.iitb.ac.in> wrote
Hello Jerry,
The transformer is designed to have maximum magnetic flux
linkages within itself and very small percentage of it is leaking out may
be 0.01 % and as distance increases the flux density reduces. Secondly, we
have to see,what kind of magnet you are using. If it is shielded magnet,
you need not worry at all. Merely keep distance of about 15 feet from the
transformer to be interference free. In case of normal unshielded magnet,
you should take precaution to install it away from the transformer. It
would be good idea to check the magnetic field with a gauss meter and keep
the NMR magnet where the magnetic field is around 0.36 gauss. Probably
this figure would be between 20 to 25 feet away from transformer. It is
better to take precaution or else you may have artifacts in
2D-experiments.
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cheers,
Jerry Hu
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Jerry Hu, Ph.D. email: jghu_at_mrl.ucsb.edu
RD Engineer Tel: (805)893-7914
Materials Research Lab, UCSB Fax: (805)893-8502
Santa Barbara, CA 93106
Received on Thu Feb 28 2002 - 16:49:33 MST