Spectral output vs Dshift parameter


        The pdev spectrometer dshift parameter  (see layout of spectrometer scaling)  down shifts the output of the power  computation before accumulating the spectral values in the 40 bit  accumulators. It allows longer integrations before overflowing the 40 bit accumulator. As an example:  a 1 second integrations at 170 MHz with: (A/D=48,pshift=0x1555, dshift=0,ashift=0)  has LO feedthrough birdies close to 1.5e9. The maximum allowed value is 4e9. Setting dshift would allow 1 second integrations  without rfi overflowing. The maximum allowable downshift value is set by the requirement that you leave enough bits on the noise.      For a single spectral estimate, the amplitude of this noise is comparable to  the mean value of the power spectrum.

    The allowable range for dshift had been computed from the spectral mean with dshift = 0 (see page 2 of the plots). It showed that you would have about 4% of the channels with an rms < 40 counts with dshift=15, pshift=0x1555 and aToD Rms=48 counts.
    On 08oct07 data was taken with different dshifts to verify that the actual settings matched the computed values.

    The datataking setup was:
The plots show the results for various values of dshift (.ps) (.pdf):


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