Question

Hello, been editing a wiki page on the subject and this question occurred:

For a Bruker pulse sequence in the following context:

pl3:f3
(p3:sp3):f3

Is it important to set instead 120 db to f3 power (or rather attenuation - in the Bruker scheme of things) before the sp3 statement and restore to pl3 afterwards? That is:

pl0:f3 !where pl0 is set to 120 db, i.e. full attenuation - zero power
(p3:sp3):f3
pl3:f3

I've seen this pattern before and I wonder if it is actually important for any reason or a particular system. - Is there any danger in not nulling the default power before the pulse?

Thanks!

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Yes, it is necessary to preset the PL of the channel immediately prior to a shaped pulse, at least on systems which have ASU or similar device to set RF amplitudes and generate pulse shapes. That would include ARX/AMX/ASX- and DRX/DPX/DMX/DSX-type systems. In principle, it should not be necessary on systems having SGU.

The reason is that the ASU-type RF modulator has some finite response time on the order of 1us. Therefore, at the beginning of a shaped pulse, you will likely get about 1 microsecond of RF at the power level that was previously in effect on that RF channel. If no power level was explicitly set on that channel prior to the shaped pulse, then the power level previously in effect is the default power setting, pl1 on f1, pl2 on f2, and so on.

If you set the prior power level to 120 as in your example, you avoid the 1us spike. This is a serious matter for soft or selective pulses.

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answered Mar 30 '10 at 19:11

Tony Bielecki
131

great, thanks Tony! - Evgeny Fadeev (Mar 30 '10 at 21:14)

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