About a week ago I posted to this group concerning some baseline
problems with our AMX 500 that we traced to early receiver unblanking
following a pulse. On our system, the SE451 unblanking line (EP) was
going high 1.6 usec after the "go" statement, irrespective of the "DE" setting.
The UXNMR manual states that the receiver unblanking should occur at 1/2 of
the DE period, which for the parameters we were using would have been
18.5 usec. What happens with this early unblanking is that tailing edge of
the probe ring-down overloads the receiver causing a 15 V 5 usec pulse at the
output of the receiver. This pulse sets the analogue filters ringing, which
results in severe baseline problems in the resultant spectrum. This effect is
much worse with "X" nuclei than proton because of the narrower bandwidth
of the lower frequency probes and the higher transmitter power employed.
At the time of my original posting I suspected that this might be a software
related problem, and a posting from another user seemed to confirm this
hypothesis.
In order to prove this, I dug-out and re-installed UXNMR 920801 (Thanks
to Rudi N. for pointing out how to re-install an old UXNMR release without
the bother of re-installing a boot tape and re-configuring the system.)
With the older release installed, the receiver gate was back at 1/2 of DE
where it should be, and our baseline problems were cured.
A temporary cure for this problem with 930601.x is to do something like the
following wherever an observe pulse immediately precedes a "go" statement:
.
.
.
p1 ph1
5u
go=2 ph31
.
.
.
The DE value can then be shortened by 5 usec if desired. Even with the
old software, at wide SW the receiver gate is very close to the pulse
(owing to the ca 2 usec delay in the pulse gating) and the above procedure
can also be useful in these cases as well.
I hope that this problem will be be cured shortly. Perhaps a patch to
the appropriate UXNMR module can be put onto the ftp server so that we
will not have to wait for another UXNMR release?
Any comments from Bruker?
-- Kirk Marat NMR Facility Manager Dept of Chemistry / Prairie Regional NMR Centre University of Manitoba, Winnipeg, MB, CANADA, R3T 2N2... Criminal Lawyer (n): redundant term, see WEASEL