Actually, the crystal filters have two purposes:
1) Eliminate the 15 KHz image that would otherwise be a problem
2) As Don said; however, it's the 2nd mixer that needs more protection than the
DSP.
Wes N7WS
On 2/6/2014 11:38 AM, Don Wilhelm wrote:
Arie,
Sorry about the added "n".
Yes, the DSP algorithms will not allow the passband to extend into the
opposite sideband.
What I was responding to was the initial statement about "centering" the
passband (on the CW sidetone pitch), and I was pointing out the fallacy of
that when the filter is wide.
The analog filters at the 8 MHz have only the task of keeping strong nearby
signals out of the ADC and not overloading it - they also reduce the exposure
of 'pumping' for the hardware AGC by those strong signals near in frequency
(such as may be encountered in a DX pileup or contest conditions).
The DSP does the final filtering, and the DSP filters do not actually need the
roofing filters at all other than for the reasons stated above.
73,
Don W3FPR
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