On Jun 1, 2010, at 6/1 10:04 AM, Jerry Flanders wrote:
> It is likely that your 200 Hz filter is a bit wider than 200, or
> rolls off so slowly so that it is much wider at only a few dB down.
Yep, Don has left enough hints in his white paper to show that what
you said is true.
Notice that Don mentioned the QRM being 40 Hz away from the Mark tone?
After applying the "200 Hz" filter, notice that you can still see the
QRM, albeit somewhat attenuated. So the transition band of the filter
is a little more than 40 Hz beyond the Mark shift.
I.e., the bandpass filter is still passing appreciable signals at a
width of 170+40+40 Hz (the second 40 Hz accounts for the Space side of
the bandpass filter). The 3 dB point could well be 200 Hz (or
not :-), but the filter itself appears to be still passing a lot of
signal at 250 Hz.
BTW, I suspect that in this particular case, it might work better to
offset the rig's filter so the QRM falls completely outside the
bandpass and let the RTTY demodulator's ATC handle a slightly
attenuated Mark signal (it would look like a mark signal that is
partially selectively faded).
In fact, if the demodulator has a reasonable ATC (automatic threshold
correction or adaptive slicer) implementation, you can use the rig's
filter to eliminate the mark tone completely to emulate a Space-Only
demodulator. That is how you implement a mark-only or space-only
demodulator anyway -- a good slicer plus a narrow filter that passes
only the mark or only the space signal. Just that a modem that has a
Mark-only or Space-only demodulator has the narrow filters built in,
and with MO and SO selectable with a single turn of the knob or press
of a button.
73
Chen, W7AY
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