Dear Fellow Spinners:
I have a question related to using oscilloscope to trouble-shoot NMR
spectrometers ( a Varian INOVA 400 MHz spectrometer in my case, not that it
matters). Your inputs will be greatly appreciated.
As you all might have experienced, from times to times we need to locate a
bad electronic component (e.g. bad cable, filter, preamp, probe...) in the
system. One way to do it is to walk down, using an oscilloscope, the signal
path and try to narrow it down. The following is what I have experienced.
(A) When tracking (with a well locked/shimmed and very concentrated sample
in the magnet), using an 500 MHz oscilloscope, signals along the receiving
path, i.e. between the preamp (in the box by the magnet leg) and the
receiver (in the spectrometer), I found no problem to find the FID signal
(strong and beautiful).
(B) I found no signal (I basically followed the procedure posted by Varian)
on the oscilloscope, when tracking the pulses along the transmitting path in
the following two cases: (a) between the transmitter and the amplifier (in
the spectrommeter, no attenuator was used in this case), (b) after the
amplifier - of course with an attenuator (I tried 30 dB, 20, and 10 dB) on
the oscilloscope input port. The pulses (proton and C-13 pulses) I tried
were from 10 to 50 us (all should be long enough to show up on the 500 MHz
oscilloscope) with a short between-scan delay, depending on the power ( I
tried from 10 to 50 dB - Varian's terminology). The 90 degree pulse for
proton and carbon were both about 9 us at about 55 dB power level.
Any one ever successfully detected a signal on the transmitter path? If
yes, could you point out what I missed?
By the way, any one ever sucessfully detected FID right out of the probe
(i.e. after the probe and before the preamp)?
Again, thank you in advance!!!
Logan Smiths
lslogansmiths_at_GMail.com
Received on Sat Jun 27 2009 - 15:08:15 MST