[Lowfer] The benefits of audio filtering

[Andy Bell]

Does an audio bandwidth filter help ARGO reception in
any way ??? - YES !!! Of course the most effective
filtering  would be in the " IF " stages of your
receiver, However::: an audio bandpass filter ( active
- or - passive ) has its merits as I proved to myself
in reception testing for the past 3 weeks. I hombrewed
a very crude passive audio bandpass filter from
junkbox parts, the width is around 1.1KHz. My
narrowest filter in the rig is 2.4KHz. I placed the
filter between the rig and soundcard; using alligator
clips for quick removal / comparison. Low rolloff is
150KHz ( no 60 - or - 120Hz anymore ) high rolloff
starts gently around 900Hz and most high audio is gone
around 1.5KHz. This being a passive device; yes there
is noticable insertion loss, but nothing the system
can't overcome. It's much easier on the ears, but in
regards to ARGO copy, it stopped the static crashes
from over limiting ARGO. Before the filter, the ARGO
level indicator would " PEG " and the limit light came
on with heavy crashes. With the filter in line, level
indicator at the half-way point; static crashes just
barely move the indicator now, which is a tremendous
improvement. The crashes would destroy marginal
signals before, where now, they come through. Also
strong, adjacent heterodyne would desense the
receiver, and weak signals were swallowed up. With the
filter in line, if I tune the offending signal about
800Hz away, the filter attenuates it enough that the
weak signal begins to come through.This testing
convinced me to order an active CW filter kit with
bandwidth down to 80Hz and do more testing on
filtering received audio and the benefits of doing so.
I look forward to posting my results in the near
future. Sorry this is so long, I tried to keep it
short but informative: 73 to all - Andy Bell - KU4XR


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