Don Wilhelm wrote:
Properly done, a 500 Hz roofing filter would be centered above the audio
zero frequency entirely, so the roofing filter response would extend
from (say) 150 Hz up to 650 Hz - nothing on the opposite sideband, and
even if it does spill over a bit on the filter slope, the DSP can clean
it up.
Then the DSP takes over, and your 200 Hz Hz bandwidth at a 250 Hz pitch
would extend from 150 Hz to 350 Hz - still nothing on the opposite
sideband, and if the roofing filter had allowed a bit of opposite
sideband through (up to 150 Hz), the DSP would not allow it through.
I can tell you that when set the crystal ('roofing') filter in my K3 to
2.8 KHz and listen to a CW signal at a 550 Hz pitch, then whatever I set
the DSP bandwidth to -- from 2.8 KHz down to 50 Hz -- I do not hear an
audio image (the signal on the 'other' side of zerobeat).
Having said that, if there were an S9+30dB signal above or below the
desired signal by 1.4 KHz, then it would activate the hardware AGC and
cause a gain reduction on the desired signal. In this case I would want
a narrower crystal filter.
--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
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