Setting the pdev levels for 16 bit sampling


Computing the register levels
Plotting the bandpass and rms for 16 bits, standard register settings

Computing the register levels

        The pdev spectrometer has a number of parameters that configure the processing pipeline (see layout of spectrometer scaling). This page discusses how they should be setup. It starts at the A/D  input and works it way back towards the power accumulation and output. 

location in pipeline
A/D input
1 sigma=30 a/d counts
butterfly down shifts(pshift)
9 down shifts then alternate each butterfly stage.
no down shift at output of power Computation before power accumulate.
upshift 40 bit accumulator before taking upper 32,16, or 8 bits for output.

A/D input:

Butterfly down shifts (pshift)

Computing power:

Downshifting power before accumulator.

    You can down shift the output of the power computation prior to adding it into the 40 bit accumulator. This is needed for long integrations (1 sec) with short fftlens (around 512 channels). If not, the 40 bit accumulator may overflow.


    The power accumulation is done in a 40 bit accumulator.

Packing and output.

Computing which bits to output.

Plotting the average spectra and rms.

    The average spectra (over about 1 second) and the rms by channels was computed using data taken on 05dec08. The setup was:
   Plotting mean and rms vs fftlen  and dump time(.ps) (.pdf):
    Plotting the average spectra and rms vs channel
For each fftlen and dumptime the average spectra and rms by channel are plotted (the rms/channel is the lower plot in the frame). Black is polA and red is polB (For stokes I data black is stokes I).

Avg Spectra & rms/by channel
dump times (millisecs)
128 .ps (.pdf)
.064, .1,1.,2.,5.,10
> 5 ms rms  < 10 counts
256 .ps (.pdf) .064, .1,1.,2.,5.,10 >10 ms rms < 10 counts
512 .ps (.pdf) .064, .1,1.,2.,5.,10
1024 .ps (.pdf) .064, .1,1.,2.,5.,10
2048 .ps (.pdf) 1.,2.,5.,10
4096 .ps (.pdf) 1.,2.,5.,10
8192 .ps (.pdf) 1.,2.,5.,10

processing: x101/081205/,

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