Hello,
Other than changing the field, you need to shield the probe and ALL rf signals
(transmitters, receivers and LOs) from the external RF. The best way to do this
would to be to build a screen room around the console and magnet. This can be a
pain and expensive but will give the best results.
If this is not possible, then do some tests to find out where the rf
interference is coming from. Remove the probe and terminate all cables with
shielded 50 ohm rf loads. See if you still have interference. If so start
bypassing the shields for all transmitter, receiver, and LO signals where they
go in and out of the modules. Do this until all interference goes away. It is
a pain but with careful work it can be done.
Once this has been done, you can make the magnet bore an rf shield if you use
GREAT care. Make a good electrical contact to the copper magnet bore tube. If
this is not possible, then remove everything you can from the bore and get a
copper tube just big enough to fit into the remaining hole. Make sure the tube
is continuous throughout the bore. Now modify everything to fit into the
remaining space. Now prepare removable copper end caps for each end of the
bore. Make sure the end caps make good rf contact to the copper bore tube. Now
mount non-magnetic connectors into the end plates for ALL wires which pass into
bore making sure all cables are shielded and the shields are directly connected
to the metal end caps. If there are non rf signals going into the bore then rf
bypass them with small caps to ground at the end caps if possible. Now connect
the probe while inside the magnet bore to the console and test for interference.
Once this configuration is running interference free, then add the other cables
to the bore, one by one, seeing if the interference comes and goes with the new
cable. If it does then work on cleaning up the signals on this cable. Continue
until all cables can be connected without interference.
Now you should be able to be your experiments. Don't forget that liquid
solutions going into the bore can also carry rf signals, particularly if they
contain some salts.
woody_at_acornnmr.com
> -----Original Message-----
> From: Jeff Walton [mailto:jhwalton_at_ucdavis.edu]
> Sent: Tuesday, December 19, 2000 11:55 AM
> To: ammrl_at_wwitch.unl.edu
> Cc: LAWRENCE BYRNES
> Subject: RF Interference
>
>
> Dear All,
>
> Thanks to all who replied before. I FINALLY figured out where this
> interference is coming from. It turns out that it is the campus
> radio station at 90.3 FM. The interference was so bad one day that I
> was able to hear it (the radio station) over the console speaker by
> turning the gain way up.
>
> This system, and I believe all the old GE systems, use an IF at 58.25
> MHz. The difference between 90.3 and 58.25 is very close to the
> difference between 58.25 and 26. Both signals beat against the IF
> and produce THE SAME DIFFERANCE FREQUENCY! For now the problem is
> particularly bad for one particular imaging coil. I can only imagine
> that there something about this coil which interacts with the to
> setup some sort of resonance at 90 MHz. Apparently, between this and
> because the station is relatively close, we are picking it up.
>
> I have never heard of this occurring before. I don't know how we are
> going to get rid of this aside from changing fields. We run this
> magnet at 0.6 Tesla rather than its design field of 2 Tesla due to
> environmental reasons (very tight space), so this is an unusual
> situation. Please let me know if any of you have run across something
> like this. I don't know if this has has anything to do with Lawrence
> Byrnes' problem on his Aspect 3000, but he may want to consider this
> as a possible source.
>
> Thanks Again!
>
> Jeff
>
>
> ******* Original Message *************
>
> Dear All,
>
> We have a 45 cm horizontal bore imaging system operating at 26 MHz (0.6
> Tesla). The spectrometer is an old GE 2T CSI system (we are running on the
> lowband) retrofit with a Tecmag Libra. Recently, radio frequency
> interference has plagued us. The amplitude of the interference depends on
> which coil we use. The interference appears to be frequency modulated. It
> appears whether or not there is sample in the coil and appears over the
> entire tuning range of the coil, 25.5 MHz to 26.1 MHz. I can terminate the
> coax with a 50 ohm resistor instead of the coil and it disappears. Thus,
> it appears we are picking something up through the air.
>
> Question: Does anyone know if there are any commercial, two-way hand held,
> or ham radios that regularly transmit around 26 MHz? Also, if anyone can
> think of another mechanism by which we might get this sort of interference,
> I am open to suggestions.
>
> Thanks!
>
> Jeff
>
> ******* Original Message *************
> Jeffrey H. Walton
> Associate Research Physicist
> One Shields Ave.
> UCD NMR Facility, Bldg. MS1-D
> University of California
> Davis, CA 95616
> (530) 752-7794 (office)
> (530) 752-6480 (MS1-D lab)
> (530) 754-9064 (Cruess Hall lab)
> (530) 752-8109 (FAX)
> jhwalton_at_ucdavis.edu
> NMR Facility URL http://www.nmr.ucdavis.edu
Received on Wed Dec 20 2000 - 15:17:24 MST