AMMRL: RF noise spikes from MAS probe, summary and solution

From: Kirk Marat <kirk_marat_at_umanitoba.ca>
Date: Fri, 21 Jul 2006 15:06:10 -0500

Many thanks to all who replied to my question regarding RF noise bursts from an
MAS probe. Most people's ideas centred around triboelectric charges, or contact
of the rotor with the stator or coil, or some combination of these. Any charge build-up on
the rotor could discharge in a sparking fashion (thus creating the bursts) if the rotor
contacted stator or coil. A contact could also cause vibrate the coil. Vibrating coil
plus magnetic field equals a microphone, which would generate a voltage that could
have harmonic components well into the RF spectrum. This would be similar to the
magnetoacoustic ringing that is the bane of anyone working with low gamma nuclei.

Now, I should have pointed out that the effect was observed with the probe both in
and out of the magnet. This eliminates any magnetoacoustic effect. We also noted the
effect on two probes, and with multiple rotors. No scuffing or coil contact marks were
observed on the rotors, and spinning was very stable. Although this doesn't eliminate
contacts as the source of the problem, this and the fact that the effect started suddenly
on two probes, suggested that we should look elsewhere for the problem.

It was Szalontai Gábor commented that they had seen the same effect on a Bruker 4
mm probe, but only when the air was damp. It turns out that the drier on our compressed
air (outside of our control) had failed and that we were indeed spinning with damp air.
Not wet enough to send liquid water through the lines, but still damp. We tried spinning with
dry nitrogen and the problem disappeared! My intuition would be that problems with
static discharges would be worse with dry air. However, I can imagine that cooling caused by
expansion of the air in the drive or bearing jets could cause condensation of microscopic
water droplets that could trigger a static discharge (or momentarily unbalance the rotor)
and case the problem.

That's my best guess as to what was causing the problem. Nevertheless, our drier is fixed,
and we have no more lightening storms in our probes!

Thanks again for all the suggestions!
-Kirk

Kirk Marat, Ph. D., NMR Facility Manager
Dept. of Chemistry
University of Manitoba
Winnipeg, MB, R3T 2N2, CANADA
ph. (204) 474-6259 FAX: (204) 474-7608
kirk_marat_at_umanitoba.ca

ALL SPAM forwarded to Spam Cop

Earlier I wrote:

>I have the most bizarre problem that I have encountered in almost 30 years of
>working with NMR systems.
>
>Two of our Chemmagnetics high speed MAS probes have started generating
>random RF noise bursts at speeds exceeding ca 18 kHz. These bursts are strong enough
>to trigger the receiver overload light on the console, and are COMPLETELY independent
>of any timing events in the console. The acquisition can be stopped and the transmitters
>can be disconnected and they are still there. They can easily be seen by putting a scope
>on the output of the pre-amp, pretty much eliminating anything in the console as a source
>of the problem. They ONLY occur when the sample is spinning, they start at a spin rate
>of about 18 kHz, and get worse as the spin rate increases.
>
>These are 3.2 mm Chemmagnetics/Varian probes. I think a similar 5mm probe may be
>exhibiting some of the same problems as well, starting at about 10 kHz , but I'm not sure.
>
>The problem is independent of sample.
>
>The problem is independent of nucleus. We've seen it from 39K all the way up to 13C.
>
>We were initially led astray by a console freeze-up (with receiver overload light stuck on)
>that happened at the same time that the above symptoms started. We (and Varian) thought that
>it might be a receiver problem, but replacing the receiver board made no difference.
>
>It's almost as if there is a little lighting storm inside the probe generating the bursts.
Received on Mon Jul 24 2006 - 12:04:06 MST

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