Bruker BioSpin Solid State NMR - RF-Filters in the RF Pathway

RF-Filters in the RF Pathway
User Manual Version 002 BRUKER BIOSPIN 21 (327)
can be repaired by a skilled RF engineer. BNC connectors should be avoided;
they are usually off 50 Ω.
As pulses in solids NMR can be rather long, and rather high power, it is also nec-
essary to consider the preamplifier’s power limitations.
Proton high resolution preamps are unsuitable for high power pulses, especially
for durations required for decoupling. High resolution X-BB preamplifiers are limit-
ed to 10 msec pulses at 300 (500) watts. If the back label does not say 500W, it is
300W max.
1
H/
19
F high power preamps need not necessarily be bypassed, but may gradually
deteriorate under many decoupling pulses. It is therefore recommended to bypass
these for decoupling unless the experiment requires that the preamp remain in
line.
Warning: These preamps are not optimized for
19
F, so
19
F decoupling
should never be done through this preamp.
1
H HPLNA preamplifiers need not be bypassed. HPLNA preamplifiers are
strictly frequency selective; a
19
F pulse through a
1
H HPLNA will destroy it!
RF-Filters in the RF Pathway 3.3
RF filters are frequently required if more than one frequency is transmitted to the
probe.
Without filtering, the noise and spurious outputs from the transmitter of one chan-
nel would severely interact with signal detection on another channel. One has to
keep in mind that pulse voltages are in the order of hundreds of volts, but NMR
signals are in the order of microvolt. In High Resolution, where the selection of nu-
clei to run is rather limited, it is possible to apply the necessary filtering inside the
preamplifier. For solids, this is not easily possible, due to the wide range of possi-
ble detection frequencies, but also due to the additional dead time that filters may
cause. So all filtering is done with external filters. If a single channel NMR experi-
ment is run, no filters are required.
Usually, one filter per RF channel is required. Both filters should mutually exclude
the frequency of the other channel(s). Usual attenuations of the frequency to be
suppressed should be around > 80 dB, in special cases, when both frequencies