73 Amateur Radio Today - Free and Open Source Software

73 Review
Number 9 on your Feedbllek card
by Larry Antonuk lYB9RRT
JPS Communications, Inc.
P.O. Box 97757
Raleigh NC 27624-7757
Telephone: (919) 790-1048; Fax (919) 790-1456
Order line (BOO) 533-381 9
Price cess: $249.95
JPS Communications' NRF·7
Clean up your audio with state-of-the-art OSP!
"Real hams com use audio filters." This
was the prevailing wisdom, at least until
a lew years ago. If you were using an "addon-
audio CW filter, lor instance, it simply
meant that you were too cheap to spring tor
the optional narrow crystal IF filter that went
lr uc your rig. Even worse , it mighl have meant
that your rig was so cheap that it didn't even
have an optio n for diffe rent IF bandwidths!
Heavens!
A narrow crystal IF filter was obviously the
way to go-as long as money wasn't a conce
rn . An IF filter would p roduce sleep er
"sxtrts" than an audio filter, and prevent adjacent
signals from overloading the AGe circuits
and wiping out your receiver entirely. Audio filters
were also prone to -ringing.- While they
might enable you to pick signals out of a pileup,
the very act of listening to a cheap audio
CW filter for an entire contest could drive an
operator bananas. (Imagine listening 10 someone
hitting a steel pipe with a wrench-with
your head underwater.)
It's hard to gel rid of preconceived nolions,
but audio filters are not what they once were.
The main reason, of course, Is that it's now a
digital world. The NAF-7 from JPS Communications
makes use of technology th at was
non-existent (or economically unavailable) just
a few years ago. But what makes a digital audio
litter better than a conventional analog audio
filter?
The main function of a filter is just what the
name implies-it filters out unwanted signals.
An IF filler does this at the IF frequency by
providing a low impedance at the main IF frequency,
and higher impedance as the frequency
varies trom the center frequency. Obviously,
the laster the impedance goes up as
the frequency varies from the main tF frequency,
the sharper the response of the filter
(the steeper the MsIo;irts1. In other words, the
sharper the filter response, the thinner the
"snce" of band that the filter will pass.
An audi o fill er works in a similar manner,
except that it operates on th e audio tones
from the spea ker rather than the IF frequencies.
For Instance, an audio filler configured
fo r CW might have a cente r freq uency of
800 Hz. It will provide lillie or no anenuation
at 800 Hz, but the attenuation will increase
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42 73 Amateur Radio Today ' August, 1994
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