RADIOBEACON TRANSMITTER
ND500II (125 WATTS) DOUBLE SIDEBAND - NO VOICE
Page 5-9
15 September 2003
5.3.8
SHUTDOWN CHECK
: Verify carrier
level/modulation depth fault thresholds are set to the
levels that will cause transmitter to shut down when
RF output falls below predetermined minimum
carrier level, when modulation envelope falls below
predetermined percentage or when keying is not
present, as follows:
NOTE
The dummy load's power indicator is specified when
setting carrier levels and the modulation envelope of
an oscilloscope waveform is specified when setting
modulation depths in the following procedures. The
user may monitor the appropriate reading on TEST
meter M1, provided the accuracy of the meter
reading is acceptable.
5.3.8.1
Preliminary Shutdown Procedures
:
Verify all preceding functional tests/calibration
prerequisites have been completed and then pre-set
associated potentiometers and switches to the settings
required to start the shutdown procedures, as follows:
(a)
Verify carrier level checks specified in
paragraph 5.3.5 have been completed.
(b)
Verify modulation depth checks specified in
paragraph 5.3.6 have been completed.
(c)
Set
MOD THRESH
potentiometer A5R8 fully
counterclockwise.
(d)
Set
LOW CARRIER
potentiometer A5R10 fully
counterclockwise.
(e)
Connect an oscilloscope, using a suitable
coaxial cable, to
RF MON
connector J1.
(f)
Set switches to positions tabulated for
Calibration Setting
in table 4-1.
(g)
If necessary, adjust
O/P PWR
potentiometer
A4R31 for the 'intended' carrier level, as
indicated by dummy load's power indicator.
NOTE
The 'intended' carrier level is the RF carrier output
required to provide the desired range for a specific
installation. It must not exceed 125 watts or be less
than 25 watts. If an 'intended' carrier level has not
been established, set the RF output to 125 watts (for
carrier frequencies between 190 kHz and 650 kHz)
or 100 watts (for carrier frequencies between 651
kHz and 1250 kHz or between 1600 kHz and 1800
kHz).
(h)
Set
MOD
switch S2 to
ON
.
(i)
If necessary, adjust
MOD %
potentiometer
A4R2 for the 'intended' modulation depth, as
indicated by modulation envelope on
oscilloscope waveform.
NOTE
The 'intended' modulation depth is the maximum
modulation depth a specific antenna system will
accommodate before excessive reflected sideband
power and/or distortion is produced. It must not
exceed 95 percent. If an 'intended' modulation
depth has not been established, set the modulation
depth to the maximum achievable percentage
without causing clipping and distortion.
(j)
Set
KEYING
switch S1 to
ON
.
(k)
THRESH
lamp DS1, on monitor PWB A5, shall
be flashing on and off, provided keying frame
contains identification code.
5.3.8.2
Low Carrier Level Fault Threshold
Calibration
: Set the carrier level fault threshold to
the level that will shut down the transmitter when the
RF output has decreased 3.0dB or to a user
determined minimum level, as follows:
NOTE
International Civil Aviation Organization (ICAO)
standards dictate the transmitter be turned off or a
warning alarm be generated when the RF output has
decreased by 3.0dB or more from the intended
carrier level. The following procedures use -3.0dB
as the carrier fault threshold level. If the user
decides to select a different threshold level, the user
selected threshold level should be inserted where -
3.0dB is referenced.
