Spectral output vs Ashift parameter
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.
- 170 MHz sampling, 512 channels, dump time= 64 usecs ??
- pshift =0x1ff ??
- packing=32 bits
- I need to verify that these were the actual settings...
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.
- Page 1, 2 are 16 bit packing polA and polB.
- Page 2,3: 8 bit packing polA and polB.
- These results are a function of the fftlen, integration time, and
the settings for pshift.