) M$ z& P$ N( N$ ICross-talk between FM Broadcast Radio Transmitters (88-108 MHz)" M4 O5 M4 W1 x" g0 y
and NMR Spectroscopy: A recent experience
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Recently I had to install a 400 MHz (9.4 T) NMR Spectrometer. The
' `, r r) E+ C2 V/ N7 b/ Esystem worked fine and, using an Indirect Detection Probe , met quickly
; e, G7 K0 E. `* `9 {( O5 n1 Sand effortlessly the specifications. But later on, when the customer8 [* `4 B j1 D- \1 C# p
installed a 13C direct detection Probe, the S/N ratio turned out to be
2 V: Z* i+ y. s& ?8 l8 i0 C4 Fquite low and, on top of it, the sensitivity was subject to erratic and2 D) P' |+ S, P; A. z
very large variations from 50:1 to 130:1 (manufacturer's specs give3 v& H9 r0 y$ y( I
155:1), without any apparent reason.4 b. s" h" Z/ o- v2 t, t) T
Since the nominal 13C observe frequency at 9.4 T is 100,568
. k( U/ y& X: JMHz, right in the middle of the range of commercial FM broadcasts, I5 S8 q! B5 z* i/ a
have immediately suspected that the spectrometer was picking up one of
# Y4 y7 _5 `; d+ ` P. o2 Xthose radio stations. In fact, using a cheap FM radio receiver, a
2 `: w; w+ n0 o% a4 Z9 i4 Y! Kstrong station was quickly found at 100,60 MHz. At this point, I have
. i( K2 p/ G8 v( Hconnected a simple audio amplifier ending with a loudspeaker to the
+ r+ }, g/ t$ n: b: ]+ Woutput BNC of the observe receiver which was there apparently just for
1 k% R' Z4 K5 C3 H4 X; hthis purpose, and all of us were listening to the radio using a 200.000
) Y7 v" _) C1 i4 o$ y+ q2 N, Y, ~Euro NMR spectrometer, except that the audio quality was really poor,
' \& w, b8 z% `9 r zmuch worse than from the above-mentioned gadget radio (a shopping mall6 S8 ? m3 P$ i
gift).
& x# _1 o) i! E( g6 K6 y The problem is well known from the old times, when the highest4 X) W# h$ i, |! v* \4 }
field was 2.45 T and the nominal H1 frequency was close to 100 MHz. One
0 v3 d; G9 l9 s# v% N* u. z# ^; c" V+ Dof the first Italian NMR spectroscopists to experience it, back in% Z: O! w8 [6 }6 R- b2 q" R
1974, was Prof. L.Lunazzi at University of Bologna, on his brand new" }$ c: A2 x s- o8 q; m' g6 O, s
Varian XL100 spectrometer, and the radio station was Radio San Luchino,
, Y( d, R9 F3 a9 t% `0 ]. fwell known to anybody living in Bologna, which broadcasts from the top
' A) _ U- Y$ @6 Pof the nearby Saint Luca hill.
- P* c( V, \; x! }# U The obvious solution is to change the magnetic field, and thus+ Q! s0 V- o/ _8 ]8 _6 [/ K
all resonance frequencies, in order to get out of the modulation
C/ \3 k+ T% R" t) tenvelope of the interfering transmitter. But this is not always easy,( Z! q' K& k m+ S" m/ Q6 M
since the range by which one can move the magnetic field changing just
1 G8 Y7 h. D& @) t! q% S6 ~( Isome software parameters is usually limited to a few tens of kHz in the$ ?6 _5 y( C: B$ O7 Y! U
frequency domain. If larger variations are required the poor engineer
: ?0 ^. Z$ O# Q$ whas to work on the superconducting coils of the magnet, which is a9 W& A5 M* o0 M F
no-trivial job entailing the risk of a total or partial quench.
4 C7 U3 m F3 z; N" o0 ?' ~ Bitter experience shows that persuading the involved radio
+ N4 n) `% t9 `station to change its operating frequency is a time consuming,
8 L" o' L) j- c3 `frustrating, and apparently quite impossible task.1 t' U: I, @" o# p
Being well aware of the problem, my preliminary spectrometer- A! s. u0 M- _
checks always include some blank acquisitions taken before running up2 U, R: F* S N8 b
the magnet so that there is no chance to observe an NMR signal. The
1 H8 ]. E' Y& @$ P' Z8 zresulting dataset should be pure white noise, without significant
) u8 D4 w0 C3 t/ E! F! P8 w0 Cspikes. This was done also in this particular installation but, as0 }3 D9 D# f) p* Z/ ~" d- ]
usual, in the days following the energization the magnet drifted a bit,
+ K' s1 S. a4 ]getting closer to the radio station carrier. Furthermore, the usual 13C
4 L6 M% ]+ J3 e! V6 y6 E( Cspectral widths are quite wide which makes things even worse. Murphy's1 ^! E1 l0 D" B- t
Law has no exceptions!9 X* Q I7 n d
But we are just at the beginning of my real troubles. Before( M0 b4 a7 D, B& X7 C
putting one's hands on the magnet, one should better know how much, in
5 t& D* @ P1 U0 k. B' s |2 Iwhich direction, should the field be moved. I have therefore used a0 H8 L# h! n! F" p
good Spectrum Analyzer (Tektronix model 2710) to check the frequency( k5 l7 P: R! B/ E$ s" ~* ?8 R7 a$ P5 N
spectrum around 100 MHz, ready for the worst. And the worst was what I
H7 t+ l, K. _6 y( v5 F* Xgot! The band was filled with FM signals, evenly spaced by 250 kHz and
6 ]( y [& T% h# m* C4 M6 g' ?with modulation envelopes as wide as 100 kHz, so that when I got far. a! p- S( J5 m3 M- g$ y
from one station I started receiving the next one; accounting for* X+ z4 K' U9 R" ?( A
folding and aliasing effects, there was no chance! The only somewhat
0 c5 g0 S+ V4 R4 T- o7 xfree region was at 100,120 MHz, but this implied proton frequency of
( T. U0 ?% o. E* Z# E7 d: K2 {398.100 MHz. So now the spectrometer is no longer a "400"!- u: y# q4 ~% F) N0 l
Before installing a spectrometer, you better get a Spectrum
; y8 k: Q! k/ p! K5 \; C$ r: z* MAnalyzer and check for the presence of RF fields in the instrument% Q3 d3 Q* |9 |3 \) }
room, taking care to explore the areas close to the observe frequencies: ^' I! K& v5 `, i
of all the most important nuclei. Don't forget the lock: at 14 T
7 p! O8 y+ ?/ S. p6 Z$ l(nominal 1H frequency of 600 MHz) 2H resonates at 92,095 MHz, once
7 h b: P: \. P( Vagain in the FM broadcast band. The lock channel receiver has quite
) n f/ t* y- l# e, @ \narrow bandpass filters, so hitting a radio is a really bad luck, but6 @6 Y" w; @6 j* {# p. \
it had already happened, resulting in fast lock level variations and
6 @; k. o1 \- L: b( k6 Atotally malfunctioning Gradient Shimming which uses deuterium as
* r6 q1 c. C) Q6 t. E( {# y+ nobserve nucleus!$ w7 C. H9 k5 a/ I9 B' b9 q
Needles to say, the extremely high sensitivity of an NMR
1 c% s! C9 V F, s( QSpectrometer shows up. The signal from the guilty radio, as observed on8 U' T8 F- f) ~3 l( Q) O
the spectrum analyzer inside the spectrometer room, had very low0 N1 R/ ~1 @$ Y4 o+ R0 N
intensity level of about -70 dBm, some microvolt/meter, but that was
) f8 o0 _9 o* Wenough to almost completely hide the quite strong 13C signal from the
4 {" k( Z& u7 I8 fASTM sample!0 \ c6 H+ N+ I5 g5 |* u& x
The radio was clearly picked up by the Probe (closing the. w' {2 a/ Q5 ^: E5 {4 C R( p
Preamplifier input with a shielded 50 ohm RF load, all signals- m" I9 G9 K6 R i
disappear) but, quite surprisingly, there is almost no shielding effect
) L- b2 f' ]4 U& g7 U& m" sattributable to the metal body of the magnet, which is after all an& ^& s$ _/ l; T4 O
almost completely closed cylinder all around the Probe. Most probably a
4 O; G# E) w' G! Fgood deal of the signal leaks in through the Shim Coils which are, d0 z$ c( p3 I5 c
mounted very close to the Probe and, together with their connection
; F1 Q( O- j8 w- t5 Hcables to the Console, constitute a quite good antenna.! @% H) w4 P$ R9 {
Too bad the Shim Coils are essential, and effective shielding
; B3 G6 @, z9 J4 @$ U/ o5 F3 q) Q* d/ n; Zof the instrument with a Faraday's cage is always difficult and
$ k& ]& E% ^% x! U) q: Oexpensive (it may be almost impossible once the spectrometer is
$ h7 W' U* S1 P$ p0 u' i" C0 Oinstalled).4 D) s$ p; S( ~# L7 A
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Before concluding, let me venture some additional advice based on my experience:: y5 p3 O: I0 [& ~2 n
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= Install the spectrometer in the best shielded room
% `* a& k" t: s. N. h6 hyou can get; the best choice is once again in the basement, where you
3 A) U+ @# s' Y7 A" I3 @8 p) mhave the whole building above the ceiling and its [grounded]
' M4 W5 r, }1 q! [0 d7 A* Sfoundations all around the rest, done in iron-reinforced concrete,# E3 v0 E1 o8 R9 H# p. }( x1 ~
amounting to a good Faraday's cage at no extra cost.
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= If possible, avoid top floors. If you can't avoid2 D, g/ ~5 F$ W* [
going upstairs, take a good look out of the window: if you see nearby# w& f' k( j8 y' @3 L
transmission antennas, get ready for troubles proportional to their
1 I' z9 X/ M2 Q& odimensions and closeness (to my knowledge, however, mobile telephony$ V- e6 [# d: Y: A( a
antennas cause so far no harm)., n( E- c. b) s- b2 A' \
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= I'm sure that an exchange of experiences and/or
- W7 d- d, S: J! R$ O* Fsuggestions regarding this matter would help a lot to solve many: X. J- i1 f/ _ x
existing installation problems and prevent ones yet to come. Stan's Blog is an ideal location and, needless to say, I will be absolutely glad to cooperate.7 B4 K" L1 K" L* a: x" P
- {2 Z# X8 H6 }. ]9 B
Vanni Piccinotti, Firenze, 11 April 2008
摘自stan' NMR Blog.
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