! {7 B' b4 W4 }: `& L9 V0 FCross-talk between FM Broadcast Radio Transmitters (88-108 MHz)
5 M1 p; q9 K5 c% X+ Z8 B& ^/ Mand NMR Spectroscopy: A recent experience) v" y* d) s) x9 y$ P& V+ a
# g! K3 F5 t, ~8 x# P' HRecently I had to install a 400 MHz (9.4 T) NMR Spectrometer. The
5 v( q4 K6 Y! h3 csystem worked fine and, using an Indirect Detection Probe , met quickly9 A) ^5 W, ]7 W0 Z3 P
and effortlessly the specifications. But later on, when the customer n0 z. p4 ?; O/ H( U
installed a 13C direct detection Probe, the S/N ratio turned out to be
9 J. y2 v4 s- `/ w9 L5 I# B* L+ ^( jquite low and, on top of it, the sensitivity was subject to erratic and
& r5 a$ Y0 _ p* Q: D9 z" Pvery large variations from 50:1 to 130:1 (manufacturer's specs give6 g. `" U" E$ d
155:1), without any apparent reason.
! d5 p9 Q! K/ D, l, o# a. X3 o Since the nominal 13C observe frequency at 9.4 T is 100,5688 w" R' C0 {# o7 n& a8 Y& U2 S
MHz, right in the middle of the range of commercial FM broadcasts, I2 C( N3 n1 t9 S' f+ y g$ }
have immediately suspected that the spectrometer was picking up one of
! V6 K2 ?; r4 M* z; F: xthose radio stations. In fact, using a cheap FM radio receiver, a
+ l# C5 |& R/ F) lstrong station was quickly found at 100,60 MHz. At this point, I have
. v6 G2 {7 Z4 [2 _6 P, U. O! ]connected a simple audio amplifier ending with a loudspeaker to the9 v% \4 o8 L7 w- R* l
output BNC of the observe receiver which was there apparently just for, a5 i# e& l( s+ N" _
this purpose, and all of us were listening to the radio using a 200.000
9 O" P* _/ M% h2 x: H9 AEuro NMR spectrometer, except that the audio quality was really poor,
0 T# k& u& D5 ?6 J* s# K" }5 lmuch worse than from the above-mentioned gadget radio (a shopping mall, F# D& z; o! X* u
gift).
- A4 X G. E) R# K6 n/ J The problem is well known from the old times, when the highest
# c) t" B5 R, A/ ^# Y& K0 m: ?6 efield was 2.45 T and the nominal H1 frequency was close to 100 MHz. One0 W1 S* ?" T. |( r
of the first Italian NMR spectroscopists to experience it, back in
8 g' C% a1 X0 w L1 v+ o% s1 g1974, was Prof. L.Lunazzi at University of Bologna, on his brand new; O9 G6 g5 g* i& `$ j7 A
Varian XL100 spectrometer, and the radio station was Radio San Luchino,. |1 e& h& o4 o& o R5 X( B a1 G
well known to anybody living in Bologna, which broadcasts from the top* j" ^+ k! D8 {5 N5 l: W
of the nearby Saint Luca hill.2 J1 t9 i) v Y$ a$ E' b
The obvious solution is to change the magnetic field, and thus
) R& h; d8 Q/ uall resonance frequencies, in order to get out of the modulation
3 `6 u* v2 k# u- z) y5 Y8 Y; qenvelope of the interfering transmitter. But this is not always easy,
. C# p6 N3 C+ Q3 R1 t% Asince the range by which one can move the magnetic field changing just
1 G3 U+ i( D+ w, B- C" \. |, ^some software parameters is usually limited to a few tens of kHz in the
4 W# T$ R) D' M5 pfrequency domain. If larger variations are required the poor engineer# y' H7 e0 r9 i9 M" d
has to work on the superconducting coils of the magnet, which is a$ t E0 g+ m& u+ K
no-trivial job entailing the risk of a total or partial quench.
; |: i& l' j& J' Q' n" r5 S Bitter experience shows that persuading the involved radio
$ z7 F4 w: w5 }; l5 lstation to change its operating frequency is a time consuming,
. X, E! N( r: n$ }# hfrustrating, and apparently quite impossible task.
8 y! y0 O7 O7 ? Being well aware of the problem, my preliminary spectrometer) c1 a6 K; |& l" H8 M7 H6 x5 c0 N/ H
checks always include some blank acquisitions taken before running up
9 {3 e: M( L1 tthe magnet so that there is no chance to observe an NMR signal. The
2 j2 i8 K) @5 R6 s7 Q$ O- Nresulting dataset should be pure white noise, without significant2 k' l1 X ~. ?$ K" q
spikes. This was done also in this particular installation but, as2 `& `5 x& c. ?8 X" n6 o
usual, in the days following the energization the magnet drifted a bit,) f I- x) q! Q# d& w/ \
getting closer to the radio station carrier. Furthermore, the usual 13C3 J1 ?8 D) R& b
spectral widths are quite wide which makes things even worse. Murphy's
$ S& X; p" H7 {Law has no exceptions!2 `% A" H. J1 D6 a0 s) R8 M1 K
But we are just at the beginning of my real troubles. Before0 V8 S, I+ s% N& {
putting one's hands on the magnet, one should better know how much, in: r; m5 \; G" l8 _0 g
which direction, should the field be moved. I have therefore used a' d: V( |0 A- ?9 A+ p$ q2 Z5 k
good Spectrum Analyzer (Tektronix model 2710) to check the frequency
( r$ R+ S) W6 t( [/ R; x l4 dspectrum around 100 MHz, ready for the worst. And the worst was what I$ `- A* K! S5 \
got! The band was filled with FM signals, evenly spaced by 250 kHz and! ~7 `7 W9 I. P! E# \' ]
with modulation envelopes as wide as 100 kHz, so that when I got far+ ~( d8 q8 ]1 A( m0 I
from one station I started receiving the next one; accounting for
# \" S* o/ H' T9 ~0 ^folding and aliasing effects, there was no chance! The only somewhat
) |* b, I, @4 L0 W- v l0 K* C/ }free region was at 100,120 MHz, but this implied proton frequency of }% X4 u P, L! H* m8 J
398.100 MHz. So now the spectrometer is no longer a "400"!, g* T% c( p% p u- M" |
Before installing a spectrometer, you better get a Spectrum
- m( o* _; {; A; H: |% oAnalyzer and check for the presence of RF fields in the instrument
. ^- }# f3 n' wroom, taking care to explore the areas close to the observe frequencies( F! o3 j; R: g1 G4 ^9 R
of all the most important nuclei. Don't forget the lock: at 14 T
# N+ Y+ p4 }- M; j+ a/ B4 z6 X: o(nominal 1H frequency of 600 MHz) 2H resonates at 92,095 MHz, once3 b' }; J( W$ L) z$ e, |% D
again in the FM broadcast band. The lock channel receiver has quite
$ J; k) R4 z* M9 z! t+ xnarrow bandpass filters, so hitting a radio is a really bad luck, but
: A. M8 y. ]6 b2 t, _- yit had already happened, resulting in fast lock level variations and/ O* e- K$ a% j4 u5 g; I
totally malfunctioning Gradient Shimming which uses deuterium as
4 X3 [5 O6 Q3 @/ i# b" ^2 U+ ~observe nucleus!3 b0 X# r/ y, z
Needles to say, the extremely high sensitivity of an NMR
+ F6 I0 i6 _- N" u( y/ A" OSpectrometer shows up. The signal from the guilty radio, as observed on
! z. N- J5 z) ]3 m1 C* S$ x% q. bthe spectrum analyzer inside the spectrometer room, had very low
( y9 k ?0 h. N! x3 @! y/ fintensity level of about -70 dBm, some microvolt/meter, but that was, V1 t; [" e$ r- G, F2 y
enough to almost completely hide the quite strong 13C signal from the1 m1 r* m/ I( d# r
ASTM sample!1 {* c( J q6 P+ a$ d, U
The radio was clearly picked up by the Probe (closing the% [3 n+ R& Z% I3 o5 E
Preamplifier input with a shielded 50 ohm RF load, all signals5 j) O% r5 q/ q. K2 K4 \. p0 s
disappear) but, quite surprisingly, there is almost no shielding effect5 S T( p. k1 y0 U; E$ L3 A3 p
attributable to the metal body of the magnet, which is after all an
. Z. Q1 [" |% R, Jalmost completely closed cylinder all around the Probe. Most probably a9 k8 f1 J1 s; i( _ @) [/ y
good deal of the signal leaks in through the Shim Coils which are( |$ ]9 i9 Y F
mounted very close to the Probe and, together with their connection
, R+ D- A% a0 `; z& `2 n1 xcables to the Console, constitute a quite good antenna.: z9 D3 I' E$ |
Too bad the Shim Coils are essential, and effective shielding
# A k5 x: Q. b' {% Jof the instrument with a Faraday's cage is always difficult and
1 ^/ M% n' I% P) [0 s. g# Fexpensive (it may be almost impossible once the spectrometer is
2 o* [0 H1 |+ f) w* g" O; W1 rinstalled).
+ }( Z1 b9 s0 L 1 I5 [" D4 M+ S7 U4 _9 X
Before concluding, let me venture some additional advice based on my experience:
% B' e: q0 b: t% l- K1 \
5 d: S- l7 y$ l8 s( v; k0 O= Install the spectrometer in the best shielded room
1 M5 s" C! ?& q+ ~you can get; the best choice is once again in the basement, where you4 [" w* @: G% ?
have the whole building above the ceiling and its [grounded]6 v) B$ Q7 _. q3 c% a( V
foundations all around the rest, done in iron-reinforced concrete,% I G1 K, |3 b* b/ F
amounting to a good Faraday's cage at no extra cost./ F! e0 ^6 B/ l v
" |) T) |, d C% I7 `8 Z
= If possible, avoid top floors. If you can't avoid; S( |2 h$ o3 K: P% p
going upstairs, take a good look out of the window: if you see nearby* S6 a, A: k* J$ C8 p3 @+ ~3 D
transmission antennas, get ready for troubles proportional to their
; T8 @2 ^0 b% f* a2 Jdimensions and closeness (to my knowledge, however, mobile telephony) ] h3 N: V$ K5 C7 ~! K+ o
antennas cause so far no harm).
& {8 [5 @$ ]* X* g5 b
# u- E" _7 ~8 ^) c1 }! N# q= I'm sure that an exchange of experiences and/or2 |( y/ O& E3 |; H4 O& S( {# Q
suggestions regarding this matter would help a lot to solve many+ g' `( M8 L/ C% A' W
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.+ u' v" C3 t8 G J3 O, k
2 h4 f3 x/ A6 n- Y0 d- j5 [Vanni Piccinotti, Firenze, 11 April 2008
摘自stan' NMR Blog.
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