David Woolley (E.L) wrote:
>
> Ed Muns wrote:
>
>> What DSP bandwidths were you using and where did you have your crystal
>> filters engaging? There is only a 65 Hz bandwidth difference between the
>> "400" and "250" crystal filters and both are wider than the what is
>> needed
>> for 170 Hz shift RTTY. The crystal filter function is to protect the DSP
>
> For clarity, 170Hz is less than the minimum required bandwidth for RTTY.
> That needs to be extended by some multiple (> 1.0) of the baud rate,
> to avoid excessive truncation of significant sidebands. I would suggest
> that a filter that was significantly down at 250 Hz would be introducing
> significant distortion.
>
David you may be suggesting that "in theory" but I would pay *close
attention* to what W0YK says. Ed wins many RTTY contests including several
current world records from P49X, and results sometimes speak louder to me
than theory (and I'm an engineer if that means anything).
http://www.radio-sport.net/wpxrtty08_post1.htm
FYI, the DSP filter in the K3 follows the following formula for shape factor
according to Lyle KK7P:
Shape Factor = (6 dB BW) / (6 dB BW + 300 Hz)
At 200 Hz BW, the DSP shape factor computes to 2.5. The 200 Hz crystal
filter was measured by Elecraft to be 224 Hz wide with a shape factor of
4.0, so whether you use a 400, 250, or 200 Hz roofing filter, that has
little bearing on the final BW when WIDTH is set to 200 as Ed suggested
(since all XFILs are wider than the DSP's BW).
Many folks seem to have problems confusing the role of roofing filters for
determining the final bandwidth in the K3. That is simply not the case.
The purpose of the roofing filter is mainly to reduce strong (approximately
S9+30) signals from over-driving the ADC in the DSP. If they do that, then
the DSP determines the final selectivity over its ~100 dB dynamic range.
73, Bill
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