I have been watching the discussion about Filters
on here with interest. With DCF39 you have two modes to cope with, the carrier
and the data bursts. A notch filter does not handle this condition very well,
because of the sudden bandwidth extension when the data burst occurs. If you
cater for this then you also desense/attenuate adjacent frequencies of interest
around 137.7 khz a popular frequency for QRS.
The preferred approach would be a
tight Bandpass filter at the front end of the RX and good IF selectivity as
narrow as you can manage. This way you will be able to hear/see even the
weakest of signals. Some of the weaker signals on the band at my qth
are perfectly readable but only in a very narrow bandwidth, giving a good
signal over noise. These same signals are barely detectable with a wide
bandwidth, either aural or visual. To ensure that nothing of interest
is missed on 73 or 136 khz it is preferable to tune the band
slowly with very narrow selectivity.
To try and monitor the whole band visually with the
appropriate bandwidth selected does not work for weak signal reception, these
weak signals would be buried in the noise and not detectable.
Most LF operators suffer
from poor reception on LF because of environmental noise pickup and
the last thing needed is to attenuate signals even more with
unnecessary notch filters. The best approach is to use a good
sensitive RX with good IF selectivity plus a DSP audio filter (or
IF) and if necessary a well designed tight bandpass filter (tuned would be
an advantage) at the front end of the RX.