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5.15 TV Switch Box (NTSC
Version
Only)
Figure 5-16
shows a schematic of the
antenna switch
box.
The antenna switch box consists
of
a
switch and
a
balun, with connectors
provided
for
attachment
to the
computer,
the TV
antenna, and the
home
TV.
The switch box
is
connected
to the
customer's TV through
the
300-
ohm twin-lead output. The TV antenna
is
attached directly
to the
switch
box. The
computer output
is
connected
through
a
75-ohm coaxial cable
to the
phono plug input on
the
switch box.
From
the computer, the signal
is
connected
to
a
balun
in the
switch
box which matches
the
modulator's
75-
ohm output impedance
to
a
TV's 300-
ohm antenna input impedance. This
signal
is
then
connected
to the
switch. The switch
is
specially
designed
to
provide
the 60 dB of
isolation required between
the
computer
and the
TV antenna.
<S>
r
COMPUTER
INPUT
iMsmf
\ms&J
i
ANTENNA
INPUT
300
ohm
BALUN
"="
60
dB
^
SWITCH
.
0*n
r-*o
o^
^o
TWIN LEAD
OUTPUT
TO THE TV
O—
^
Figure
5-16.
Antenna Switch Box Schematic
-53-
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