[AMPS] NMR experience

[John Lyles]

When I used to work for a company in the east, we got calls from all over
the place with RF questions, namely EMI problems. Our speciality was
industrial RF power systems, but we did measure and characterize RF levels
for people, since we were experienced in certifying installations that had
to comply with FCC part 18 regs for ISM equipment. Once I got a call from a
researcher who had a new 200 to 400 MHz NMR system, used for elemental
spectroscopy. (is that the term?) It had a superconding magnet, and a
little RF amplfier under it. They would get these big spurious spikes in
the plots, showing carbon, potassium lines, etc, for things that shouldn't
have them. And these things would move around and up and down in level. It
was really driving them nuts, as they couldn't trust the analysis.

We couldn't find much being generated in the room, but when we listened on
our EMI receiver via headphones, we did discover that there were UHF radio
comm signals that were right on the frequencies being measured. The levels
went all over the place. It was the tug boats going up and down the
Delaware River, just outside the window of the lab. That NMR machine was
wide open to RFI, and they did not build it in a shielded room. Those
things had very sensitive RX stages from the pickup antenna. Everytime a
boat would tug a tanker by the building, the NMR would make up new elements
in the chemical sample! I assume that the manufacturer did a retrofit to
improve the EMI susceptability of their machine. (Not to mention that the
internal computer generated a lot of internal spurs.) I think we put a lot
of pressure for the improvement, as we had not yet paid the bill for the
million dollar machine.

In another case, we had a digipeater on top of one of the buildings at
work, as part of the company ham club. It would get blocked with a weak
signal. Out came the EMI biconcal antenna and RX, and we pinpoined the
source. It was a new networked laser printer, that had high throughput.
Apparenty there were a number of these being installed, and the
manufacturerhad improved their performance in a new version. When I
checked, the FCC type acceptance info on the label was actually for a
different model of motherboard and clock speed. They had made a major
change, without re-checking the design for part 15 FCC compliance. Again,
since we had bought a number of units, all that was needed was to contact
accounts payable and explain that we had a problem with their units. They
were not happy with our complaint, as office equipment is loosly specified
compared to home computer stuff. It's amazing what stuff  gets put on the
market these days.

I believe that the EU directives for reduced EMI (VDE and all that) are
probably driving a lot of companies to hire EMI consultants, and learn RF
shielding techniques. Hams are probably the best qualified bunch of
electronics types to do such work!

John
K5PRO



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