Spectral output vs Ashift parameter


        The pdev spectrometer ashift parameter  (see layout of spectrometer scaling)  upshifts the 40 bit accumulator before packing the data for output (it is not used for time domain data).  The output packing can be 32,16, or 8 bits. With 32 bit data the ashift parameter can be left at 0 (since you can't dump very fast anyway). For 16 and 8 bit data the value of ashift is more critical. You want to make sure that the spectra ends up in the output bits and that you have a few bits on the noise. The spectral bandpass will use some of the available bits for the spectral shape of the filters.
    On 01mar08 data was taken to see how the output varies with ashift. The data taking setup was:
    The Ashift was then done off line by  shifting and masking the 32bit output data.
The plots show the results for various values of ashift (.ps) (.pdf): The top plot shows the spectrum (solid) and spectrum plus channel rms (dotted) line. The bottom plot is the channel rms vs frequency. Each color is a different Ashift value.

processing: x101/080301/chklevels.pro

<- page up