Hi STeve
I would think a 40dB stopband would be sufficient.
bear in mind the modulator distortion in the rig will probably be about
no better than 3% at best.
IE put a 600Hz tone in your sideband radio and at various drive levels,
have a look at the harmonics...
there will be harmonics from ahead of the sideband filter, (whatever the
implementation) .
then have a look at IMD products with say 1000Hz and 500Hz. probably at
best -25 down.
On 18/12/2018 8:14 AM, Steve wrote:
I have played around by using a smaller number of coefficients.
Currently the filter uses 160 and I created a filter that uses 63
instead.
The current filter is very sophisticated (designed by James Ahlstrom),
as it uses complex coefficients (essentially doubling the numbers I
listed above, one for the I and one for the Q sides. This filter can
be tuned to other center frequencies.
It's kind of neat how it works, but you need about 2 GHz CPU. It
provides 100 dB attenuation for a @1100 Hz BW and looks great on the
SDR spectrum view :-)
My change to 63 filters cut the time in half, but I only used a 40 dB
attenuation. This left some higher amplitudes on the ends, but
probably acceptable. I didn't transmit the signal, just looked at it
from a saved file in Audacity and using FreeDV spectrum view.
I also tried an 81 coefficient filter with 55 dB attenuation, and to
tell the truth I didn't see much change.
Be a good area to experiment on the air. I believe the filter was
implemented early on because there was kind of a step in the spectrum,
so it cleaned that up, but I don't think the step would be a killer,
and leaving the filter off if you are out of CPU ought to be fair game.
73, Steve
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