> RE: AMX receiver gating
>
> The reiver handling in the current UXNMR versions for AMX
> (930601) is as follows:
>
> The DE Delay is calculated acoording to DE=1.427*DW in quad mode.
> Upon execution, DE is splitted in 2 parts:
> 4usec + (DE - 4usec).
I'm not sure of this, but I don't think these changes were mentioned in
the release notes that were supplied with 930601.x
>
> After 4usec, the receiver is opened and the phase is reset.
> After DE, the digitizer is started. The receiver is opened
> earlier in order to get the first point right.
Presumably, this was done in order to reduce the transient effects
of receiver turn-on on the filters when there is a significant
FID amplitude (eg proton in H2O)?
> Due to the fact that the pulse is delayed by 2 usec, the 4 usec
> are actually squeezed to 2usec.
The problem is, with a lower frequency probe excited with considerable
power, the combination of high Q and high power leads to a large
RF amplitude after these 2 usec (it actually measures closer to 1.6
usec on our AMX500). To first order, the probe ringdown time constant
is 1/(probe_bandwidth). If a probe has a 1 MHz (3 dB) bandwidth, then
then there is still ca 14% amplitude at 2 usec. (Non-linear devices,
such as diodes across the pre-amp input, will effectively spoil the
probe Q until the signal is below the diode conduction threshold,
thus making things a little better than I've outlined above)
The receiver is decidedly UNHAPPY about this abuse,
and responds by giving a 15 Volt 5 usec pulse into the filters.
Et voila, baselines that are orders of magnitude worse than before.
It doesn't even have to be a very low frequency either. 13-C
at 125 MHz has a serious problem on our system, and 29-Si is
worse.
I am surprised that the consequences of this change were not more
thoroughly investigated before releasing the software to the
customers.
The ultimate solution would be to make this period
an adjustable parameter.
-- Kirk Marat NMR Facility Manager Dept of Chemistry / Prairie Regional NMR Centre University of Manitoba, Winnipeg, MB, CANADA, R3T 2N2Basic, n.: A programming language. Related to certain social diseases in that those who have it will not admit it in polite company.