The mock spectrometer has a high
resolution mode that will digitally filter the input bandwidth from 2
to 1024 in steps of 1. The low pass digital filter includes an
up shift (hrshift) register that adjusts the voltage level output of
the
lpf. A test of the hrres mode was done on 20may10. The setup was:
The if2 noise source used as input.
The spectrometer was configured for 8192 channels, 300 1 second
integrations, polA,B.
Decimation (filtering of ) 1..20 was done. For each decimation,
the input rms levels was to so that 30 counts=1 sigma.
The entire 300 second integration was then averaged for each
decimation.
The mock setup software was using 12 - sqrt(dec) for the
number of up shifts in the low pass filter.
Top mean power vs decimation normalized to the mean power
with no decimation (dec=1).
The power decreases because of the hrshift in the
lpf. The slow rise is the sqrt(dec) increase of the noise power
from the lpf accumulations.
Bottom: mean rmsV vs decimation (normalized to dec=1).
This is the sqrt of the previous plot.
The black line shows the rmsvolts vs decimation
the green line shows where the lpf performed a down shift
(in voltage) ..divide by 2.
The red line show the product of the lpf down shift and the
rms voltage increase from the lpf additions.
The change in the rms volts follows the expected
value (red line).
Page 2: counts at input to the fft butterfly stages. adjpwr=30
counts at a/d.
Top: the rms voltage counts at the input to the fft butterfly
(12bits).
This was back computed from the output spectruam
(pdevlevels.pro).
Bottom: actual rms counts after 1st pshift down shift.
The down shift is done before the butterfly multipley, add
stage. This shows the upper 12 bits of the 16 bit register.
Since the down shift is done in a 16 bit register,
this should notmake any difference since no data is lost.
Page 3: size of spectral jumps vs decimation.
The spectrometer shows jumps in the spectra at 2^n spacing.
This plot shows the jump at channel 2048 vs decimation.
The black trace shows the difference of 20 channels averaged
on the left and right of channel 2048. It is normalized to the average
counts in these 40 channels.
The red line (from page 1 top) shows the relative variation
of the power level over the entire band vs decimation.
The increases in the jump size correlate with the decrease in
the power level.