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Hw narrow of BW can be received? The sliders get you 4 Hz, is that real?
Use USB or similar and tune 1kHz off from a carrier, then reduce the bandwidth about the carrier. Can you still hear the carrier, assuming it is actually within the very narrow passband.
If this works, try it with fast morse or a single RTTY tone instead. Can you hear the modulation changes, or does filter ringing (due to the small bandwidth) prevent you ?
not sure how I saw 4 Hz earlier but with 20 Hz now, I can detect morse
Edit: I get 4 min in AM and 20 min in SSB/CW
Sure, "4 Hz" is not really that. Even digital filters will have all kinds of artifacts, side lobes and whatnot that creep in. But I have definitely observed cases where the S/N of CW signals are improved between the 60 Hz default of CWN and adjusting the passband lower than that. And the ringing is tolerable in most cases.
The passband can be adjusted down to 4 Hz independent of the mode. Use the p key and zoom all the way in. Then mouseover the passband symbol until it shows you the bandwidth.