possible explanation for the raster problem

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User name johna

Log entry time 14:47:21 on November 14, 2004

Entry number 91630

This entry is a followup to: 91627

keyword=possible explanation for the raster problem

It is possible, though rare, to get the wrong pedestal value for the raster (and BPMs) from the pedestal events. So if it is still nice an flat, just offset, and the raw ADCs look normal, and it goes away on the next run, then this is probably the reason.

p.s. The reason this can happen is because we get the 'pedestal' for the ADC chanel by taking 1000 'random' events. So for things like the raster which always have a signal, the calculated pedestal is really the central value. However, the 'random' trigger is just a pulsar, and if it happens to occur at the same frequency (or some multiple) of the raster, it measures raster at the same point for every event. Since the raster frequency is ~100 times the pedestal event frequency, it is usually close enough to random for these purposes, but once in a while, this can happen.