DESENSITIZATION TEST
IN-SERVICE
DUPLEXER
Perform this test after retuning a duplexer or after detecting inadequate isolation between transmitter and receiver. The
following conditions may make a duplexer appear mistuned:
• High VSWR antenna
•
1M
between the repeater's transmitter and an external signal
• Faulty cables or connectors
• Improper cable length or configuration
Normally, repairing or replacing cables solves the problem without retuning. Check the duplexer manufacturer's
data
sheets to determine proper installation.
1.
Connect the Analyzer to the repeater/duplexer under test,
as
shown in figure 4-24. 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 generate, provide sufficient isolation between the tap-off and the straight-through
feed. For example a 1000 W transmitter requires
50
dB isolation.
2. Connect the DVM input to the receiver speaker connection. Disconnect the cable from the RF
In/Out
to prevent the
transmitter from keying up on repeat.
3. Set DISPLAY to
Oen/Mon
Mtr, FUNCTION to Generate FM, and enter the receiver frequency.
4.
Set the attenuator to
-60
dB and the Variable control to mid-range.
5.
Adjust the I KHz Level control for 3.0 KHz
DEVon
the screen.
6.
Reconnect the cable from RF
In/Out
to the RF tap-off and enable the transmitter to key-up.
For
example, 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 level Step attenuator and the
Variable control to the
12
dB SINAD level
as
indicated on the display. Note the generator RF LEVEL.
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.
4-44
'-......--'
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