Nautel XL12, Technical Instructions

Page: 57 / 238

12 000 WATT AM BROADCAST TRANSMITTER
XL12
Page 2-33
01 January 2002
2.4.6 MODULATION CHECKS:
Verify the
RF output is appropriately modulated when audio is
applied, as follows:
(a) Verify transmitter's RF output is connected to
a 50 ohm, resistive, 20kW, dummy load.
(b) Verify the RF power stages have been
checked, the preset power levels have been
established and the RF monitor output voltage
has been set as detailed in paragraph 2.4.5.
(c) Connect an appropriate modulation monitor to
the remote interface PWB's
RF MONITOR
connector (J2) in lieu of the 50-ohm resistive
load used to establish the RF monitor voltages.
(d) Verify forward power is selected to be
displayed on the
RF KILOWATTS meter [ FORWARD
POWER (0-14kW) lamp is on].
(e) Select the highest preset RF power level and
verify the RF output, as displayed on the
RF
KILOWATTS
meter, is the desired forward power
level.
Do not change the
AUDIO GAIN
potentiometer
setting on exciter A's interphase PDM driver
PWB if a C-QUAM AM stereo exciter is installed
as exciter A's RF drive source. The setting of
this potentiometer was precisely calibrated for
AM stereo operation during manufacture and a
change in its setting will necessitate a time
consuming recalibration.
2.4.6.1 Modulation Envelope (Monaural
Output):
Check the RF output's modulation
envelope is acceptable when a monaural exciter is
the RF drive source.
NOTE
If exciter A's RF drive source is a C-QUAM AM
stereo exciter, it must be set for monaural
operation as detailed in step (k) of paragraph
2.4.2 if it is to be checked when monaural audio
is being applied.
If an external AM stereo generator is the RF
drive source it must not be modulated and the
audio source must be configured as detailed in
steps (g) and (h) of paragraph 2.4.2.
(a) Verify the audio input is configured as a
common source for exciter 'A' and 'B' (exciter
interface PWB's
AUDIO SOURCE switch is set to
its SINGLE position), an audio signal generator is
connected between TB2-8(+) and TB2-9(-) on
the remote interface PWB and its output is
preset to 1000Hz at a zero output level (turned
off).
(b) Increase the output level of the audio signal
generator until a modulation depth of 100% is
attained, as indicated on the modulation
monitor. The audio signal generator's output
level should be +10dBm.
NOTE
The
AUDIO GAIN
potentiometer on an interphase
PDM driver PWB associated with a monaural RF
drive source was factory set to provide 100%
modulation, when the audio input level is
+10dBm.
If a C-QUAM AM stereo exciter that is set for
monaural operation is exciter A's RF drive
source, the audio signal generator's output level
may not be pre10dBm in step (b) when
exciter A is the active exciter. If the exciter will
be reset to provide AM stereo operation,
disregard steps (c) thru (g).
(c) If the monaural program audio can be set to
+10dBm when 100% modulation is required, it
is recommended the
AUDIO GAIN potentiometer
on the interphase PDM driver PWBs be left at
the factory setting.
(d) If the monaural program audio can not be set
to +10dBm when 100% modulation is required,
the
AUDIO GAIN
potentiometer on interphase
PDM driver PWBs associated with a monaural
exciter must be reset as follows:
- Determine monaural program audio level (in
dBm) that will be applied when 100%
modulation is expected, noting it must be
between 0dBm and +12dBm.
- Set the audio signal generator's output to the
level that will be applied by the program audio
when 100% modulation is expected.
- Adjust the AUDIO GAIN potentiometer on the
active exciter's interphase PDM driver PWB
for 100% modulation, as indicated on the
modulation monitor.