high res, decimation test
the 20 bandpasses normalized to 1 sec (.ps) (.pdf):
the 20 bandpasses normalized to 1 fft accumulation (.ps) (.pdf)
power, voltage, and spectral jumps vs decimation (.ps) (.pdf)
pass shape (falloff) vs decimation (.ps) (.pdf):
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
lpf. A test of the hrres mode was done on 20may10. The setup was:
The plots show the results of the test:
- The if2 noise source used as input.
- The spectrometer was configured for 8192 channels, 300 1 second
- 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
- The mock setup software was using 12 - sqrt(dec) for the
number of up shifts in the low pass filter.
- Overplot the 20 bandpasses
normalized to 1 sec (.ps) (.pdf):
- The decimations and bandwidths are listed at the bottom.
- The vertical scale is log10(spectrometer counts)*10. These are
for a 1 second integration.
- The number of accumulations in 1 second differs as
(1/bw)*fftlen since the time duration of each fft is different.
- Overplot the 20 bandpasses
normalized to 1 fft accumulation (.ps) (.pdf):
- The spectra are now normalized to a single fft
- The power differences result from the lpf accumulations and the
lpf down shifting.
- Mean power, voltage, and
spectral jumps vs decimation (.ps) (.pdf):
- Page 1: mean power,voltage
- 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
- This was back computed from the output spectruam
- 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.
- Band pass shape (falloff) vs
decimation (.ps) (.pdf):
- The 20 bandpasses are overplotted using a db scale.
- The horizontal scale is +/- .5 *nyquist frequency.
- The spectra have been normalized to unity Near DC.
- No decimation
- This shows a large fall off with little aliasing
- Decimation 2 to 20.
- The edge of the bandpass is only 3db down. There must
be a lot of aliased power folding back in.
- The lpf power levels changes by up to a factor of 3 relative to
- This change follows from the hrshift down shift and the
sqrt(dec) increase caused by the lpf accumulations.
- The bandpass shape for decimation is 3db down at the edge
of the band.