Petr Maly wrote:

Hello all
I am just designing a new RX for 136 kHz band. It will have IF on 1 MHz with
high quality x-tal filter with bandwidth 200 Hz. The main selectivity will
by obtained on AF with passive filter with coils on pot cores. Till now I
used similar filter with bandwidth about 50 Hz and it seems to be still too
wide for LF. I was about to make it new with 15 Hz width. Somebody here
mentioned 1 Hz wide filter which seems to me to be too narrow. At least, the
filter cannot be narrower than obtainable osc stability. Can we make a
little questionnaire here? What bandwidth do you use and what is the
experience?

73, Petr, OK1FIG

My own experience is that your narrow-band filters, if physical filters  (i.e,
coils and capacitors, or crystals), are very liklely to ring. How about using
software filters?
73
Andre' N4ICK