| Although
Sound
Card
Interfaces
are
already
there with
good
amount of
free Sound
Card
software,
there are
always
reasons to use a
dedicated
modem.
This
modem is a
true 1200
Baud Bell
202 using
a MX614 from
Maxcom,
making it
ideal
modem for
VHF &
Satellite
digital
work.
This
project is
based on
articles
&
projects
made by
many other
radio
amateurs
using this
particular
IC and
associated
application
notes of
the chip
manufacturer.
MX614
is a low
voltage low
current
modem
chip. This
modem may
be powered
from
RS232
(Just like
good old Hamcom
!! )
However, I
decided to
add some
more
circuit to
improve
its
performance
&
compatibility
hence, it
requires external
power
source of
DC 12V.
Modem
input
design
uses a Microchip
MCP6023
as
an input
filter/amp
to improve
noise
level
&
performance.
You may
change
settings
of this
stage by
changing
values of
presets
and
coupling
capacitors.
An Input
Filter
(Op. Amp
based) Designing
software
is
provided
below for
download
if you
like to
experiment
with
bandwidth
&
other characteristic
of the
input
stage.
Similarly,
a buffered
output
stage using a
TL071
gives an
option to
control
output
level to
match
various rig
Microphone
levels.
A
MX232 (or
equivalent)
is
used to
get true
RS232
levels.
PTT is
available
either
positive
or
negative
by way of
selecting
the H4
Header.
Modem
has front
panel DIN5
connector
to connect
rig
Microphone
and has a
push
button
switch to
change
over from
Mike to
Modem.
Transceiver
may be
connected
at
back-panel
DIN5 or by
directly
solder the
shielded
cable on
DIN5
connections
on
PCB.
Rig
audio may
be
connected
to back
panel DIN5
or to
standard
Phono
jack.
Project
is
designed
on Double
Sided PTH
board and
results
were very
encouraging.
With this
modem board,
perhaps,
you will
not
require
another
modem in
future.
Schematic,
Parts List
and
detailed
documents
are
provided
here for
download. |
