[hpsdr] FIR filtering and HPSDR

[Tom McDermott]

Hi Erik,


Each filter is usually custom-designed for a specific purpose, so it's hard
to just grab some filter - likely you won't
find it interesting. Instead if you want to experiment with FIR filters,
one possibility is to find a filter design program
that will let you experiment with the taps and response. I use gnuradio
filter designer, which is bundled with gnuradio,
so it's the best possible price.

A google search on 'FIR filter design software free' yields a lot of hits.
Use one to design a small filter, then code
it and see how it works.

The large matched filter is a correlator. The taps are simply the
time-reversed value of the expected sequence, so
computing the taps is trivial. A very large number of taps can give a lot
of correlation gain.



Here's a filter: when sample rate = 48 ksps, the passband ends at 3 KHz,
the stopband starts at 8 KHz, and the stopband
attenuation is 35 dB. It has 15 taps, and a gain of 2.

taps = [0.0035640986170619726, 0.012058229185640812, 0.038125719875097275,
0.08916111290454865, 0.16120941936969757, 0.2382189929485321,
0.29765212535858154, 0.3200206458568573, 0.29765212535858154,
0.2382189929485321, 0.16120941936969757, 0.08916111290454865,
0.038125719875097275, 0.012058229185640812, 0.0035640986170619726]


-- Tom, N5EG
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