[Lowfer] The benefits of audio filtering - #2

[Andy Bell]

The benefits of audio filtering:: For those who may
not be able to find - or - afford the narrow " CW "
filter for your rig; this may be of interest to you.
As promised in an earier post, I want to share with
those interested, what I have determined from my
recent experiments in audio filtering. I started with
passive homebrewed circuits, lots of experimenting,
but quickly determined that audio filtering adds
benefits to the reciever setup. The " Cons " to
passive filtering is the tremendous insertion loss.
After a few components you start to run out of
reciever volume. Using junkbox parts I put together a
Highpass and a Lowpass filter and connected them
together and ended up with about 1.8KHz of usable
bandwidth using USB mode. Signals were about 35 to
40dB down at each end of the BW.
Seeing the benefit; I purchased a Vectronics Active
audio filter ( type left out to avoid advertising )
It has an 80Hz narrow setting, and never using narrow
filters before, I was astonished at what it did to
recieved signals in the passband. about a 15dB
increase in S/N ratio. Signals were coming through
that before were in the noise. In just a few hours of
use; I am addicted to narrow BW now. No more ear
busting static crashes. The only thing I modified in
the filter, was the addition of a Highpass filter on
the input. without it, the ARGO display showed heavy
60 ( -35dB ), 120, 240Hz AC hum from the ground
commons. With it only the 60Hz is barely visible and
-85dB down. 
Thanks for your time reading this; and I hope someone
will find it useful information.
73 to all, Andy Bell - KU4XR



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