With my personal expeirence, for trouble shooting, an oscilloscope is
the most useful and handy tool. We can use scope to trace RF, and to
measure approximate frequency, voltage (therefore power with known
impedance). Multimeter and some simple devices, such as attenuators,
combinder, "T" are also essential.
For research, the oscilloscope is essential in pulse sequence development.
We often use it monitor timing and shape of RF pulses in pulse sequences.
I also used it to monitor TTL control signals (from the logic lines on
the XL-interance board of Varian spectrometer) when I built my own RF
channel addition to our two-channel VXR500S. On the other hand, spectrum
analyzer is very useful to optimize spectrometer performance.
A question is if spectrum analyzer is useful for instrument service. A
positive answer will be helpful in justification for buying it. I think
it could be useful in the following case. I have experienced with a few
spectrometers from which spectra are acquired to be contaminated with
undesired spikes. These artifacts are large enough to interfere
NMR spectra, but often too weak to be detected by an oscilloscope (keep
in mind, NMR signals are often as low as -100dBm). Spectrum analyzer is
very sensitive testing instrument, and therefore useful to identify source
of artifacts in such a case. The artifacts may arise from leakage from
nearby RF devices, or be picked up from the air. It could be even worse
for modulation artifacts which are always associate with strong signals
(such as sideband of a solvent peak). In this case, spectrum analyzer
is useful because of good dynamic range and working in frequency domain.
To understand the problem, think about spinning sideband. In the lineshape
test (single peak), 0.5% spinning sideband can be easily seen in spectrum
(frequency domain, spectrum analyzer). However, it is impossible to
identify the 0.5% spinning modulation in the FID (time domain, oscilloscope).
Happy New Year to all!
Chuan Wang
Director, NMR Facility
Department of Chemistry
Rutgers University