G3NYK wrote:
I must admit to being a little confused now. I am told a 500Hz
filter is not good enough for 136, but the theoretical advantage of a
300Hz filter is only a couple of dB better, I believe. A 100Hz filter
would have a theoretical advantage of 7dB (I believe) this should be the
same as going from 2.5kHz to 500Hz (where I find the practical advantage
more like 3-4dB). Yes there is a lot of QRM on HF cw and you need to
isolate a signal to listen to. My experience (with 'inexperienced cw ears'
!) is that I don't get the theoretical advantage by going to narrow
filters. Am I missing something? or am I maybe not sufficiently practiced
to take full advantage of the difference? I guess as Toni says that 1 to
2dB can make the difference between working a station and not working it.
I can hear, and copy, most of the signals on 136 with a 2.5KHz SSB filter,
and I don't get trouble from DCF39. I use lower sideband and put the
carrier at 138.05 or 138.10. With a cheaper rx like the Lowe HF150 the SSB
filter does not have steep enough skirts (and there is no cw option) and
the carrier of DCF39 is only 60dB down, and I get a light trace of the
signal on the waterfall display. Now listening like this does mean you
have to tune the 'grey matter filter' to morse at frequencies up to about
2kHz. My tally of calls was about 20 or so listening like that, and it
gave me the incentive to get better gear. On a receiver with a decent SSB
filter shape factor the signal from DCF39 is 90dB down and no trouble
(provided the front end has enough dynamic range)
I'm keen to get a few tens of dBs advantage on weak signals over the band
noise but I'm afraid I dont see it as a reality yet. The problem seems to
be that those last few dBs are beginning to get expensive now.
Ah, the key words here are "grey matter". The DSP in the brain is
far superior to anything you can get from MFJ, but only for certain
things. Certainly, the theoretical advantage in signal/noise of a
narrower filter is never achieved in practice with reception by ear
because the brain is able to do its own filtering. The main advantage
of a narrow filter is in reducing QRM which the brain has difficulty
filtering out. And perhaps Alan would find the need for a narrower
filter if he had to copy all of a QSO.
I use a 500Hz filter - the shape factor of the SSB filter in the IC-706
is too poor for 136 - but have not felt the need to spend the money
on a 250Hz filter. My score of 70 stations heard on 136
demonstrates how good the Rx is.
I do agree, though, that an absolute beginner may well have
problems with a wider filter because he/she may not be experienced
in digging signals out of the noise (the brain's DSP needs some
programming) and often there will be spurious mixing products
audible in-band until the beginner realises just how good the front
end needs to be. I am sure there must have been many people who
gave up when they could not get anywhere near the noise floor.
Of course, narrower filters (including DSP) can be useful and will
give the brain a rest, so I am not suggesting that they should not be
used. I have a DSP filter on my bench but do not often use it as I
rarely find it improves on what the ear/brain combination can do.
One last point, someone (Vaino, perhaps) commented that some
contest operators do not use narrow filters. Yes, this is common
because in a contest most stations are both strong and off
frequency, and overs are too short to do much fiddling with the Rx.
So using too narrow a filter will result in lost contacts. The beauty of
a DSP filter is that, once tuned in, a weak signal or one very close
to QRM can be quickly selected by progressively reducing the
bandwidth without retuning - something that is difficult to do with
switched analogue filters.
Mike, G3XDV (IO91VT)
http://www.dennison.demon.co.uk/activity.htm
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