Normally, repairing
or
replacing cables solves the prob-
lem without retuning. Check the duplexer manufac-
turer's data sheets to determine proper installation.
1.
Connect the service monitor to the repeater/du-
plexer under test, as shown in figure 5.14. Connect
the
"RF
In/Out"
to the duplexer's antenna port
through
an
RF tap-off isolating device.
CAUTION: To prevent RF overload into the generator,
provide sufficient isolation between the tap-off and the
straight-through feed. For example, a
IOOOW
transmit-
ter requires
50
dB isolation.
2. Connect the
"Vert!SINAD/Dist" input to the re-
ceiver speaker connection. Disconnect the cable
from the
"RF
In/Out"
to prevent the transmitter
from keying up on repeat.
3. Set left LCD to
"Dev", "Function" to "Gen", and
enter the receiver frequency.
4. Set the attenuator to
-60
dB and the
"RF
Vernier"
control to mid-range.
5. Adjust the
"1KHz
Lvl" control to 3.0 kHz
DEVon
the LCD.
5-10
6.
Reconnect the cable from "RF
In/Out"
to the RF
tap-off and enable the transmitter to key-up. For
ex-
ample, it may be necessary to provide
0.
75
KHz
PL!DPL
deviation for a
PL/DPL
accessed repeater.
7.
With this generated signal introduced to the RF tap-
off, adjust the signal generator Step attenuator and
the
"RF
Vernier" to the
12
dB SINAD level as in-
dicated on the LCD. Note the generator RF Level
on the right.
8.
Disable the transmitter from key-up. Readjust the
signal generator to obtain
12
dB SINAD again.
Note the generator RF level.
9. The difference between the RF level for
12
dB
SINAD with the transmitter
off
(step
8)
and with
the transmitter on (step 7),
is
the amount
of
system
desensitization.
10.
More than 2 dB desensitization may indicate:
defective antenna system
loose shields
transmitter spurs
improper
or
defective duplexer cable
Check them all, before attempting to retune the duplexer.
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