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�� : Wed Oct 30 20:33:52 2013
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lowpass digital fitlers

low pass digital filters

30oct13

idl routines
03oct13:plot filter shapes and aliased power. dec=2..15
02may08: simulate lpdf, look at rms vs decimation.
0

The mock spectrometer has a low pass digital filter after the a/d, digital mixer section. The filters are 8x overlap filters that are symmetric about DC. The decimation can be 2 through 1024. The coef's for a particular decimation:

are 16 bit fixed point numbers
there are 8xdecNum coefs in total
There are 4 files, each containing DecNum coef. These are mirrored about DC to give the 8*decNum coef.

The filter types available are:

rectangle
hamming
hanning (this is the default that we use)
blackman
bartlett

The filters are generated with jeffs program (pnet_mkdlpf_coeff).

IDL routines:

Some idl routines used with the dlpf processing:

pdevklpf.pro - make coef for a decimation and optionally store it on disc
pdevinplpf.pro - input coef for a given decimation from disc.

03oct13:plot filter shapes and aliased power. dec=2..15

Plots were made of the filter shapes (in the time and frequency domain) for decimations 2 through 15 using the filter coefficients.
The plots show:

Page 1: filter shape in the time (top) and frequency domain bottom)

Each of the filters is over plotted in a a different color.
top: time domain

This plots the filter coefficients.

Bottom: frequency domain db plots.

Each filter was normalized to its maximum.
The x axis scale is the unfiltered bandwidth [-.5, -.5] * samplingFrequency
The dashed vertical line shows the band edge after filtering.
The picture plots 3 times the filtered bandwidth to show the filter rolloff.
Example: Decimation = 10

x scale -.5 to .5 of sampling frequency
filter bandwidth 1/10 -.05 to .05
display 3*filtered bandwidth (-.15 to .15).

Page 2. The filter shape with the aliased power over plotted.

The horizontal axis shows the filtered bandwidth ( in units of the sampling frequency).
The black plot is the spectral density
The red trace includes the aliased power.
Each frame shows a different filter type

LPDF filter shapes and aliased power
decimation	decimation	decimation
2 (.ps) (.pdf)	7 (.ps) (.pdf)	12(.ps) (.pdf)
3 (.ps) (.pdf)	8 (.ps) (.pdf)	13(.ps) (.pdf)
4 (.ps) (.pdf)	9 (.ps) (.pdf)	14(.ps) (.pdf)
5 (.ps) (.pdf)	10(.ps) (.pdf)	15(.ps) (.pdf)
6 (.ps) (.pdf)	11(.ps) (.pdf)

processing: x101/pdev/dlpf/simDlpf.pro

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