RADIOBEACON TRANSMITTER
ND500II (125 WATTS) DOUBLE SIDEBAND - NO VOICE
Page 4-2
15 September 2003
PRE-START-UP CHECKS
4.4
Prior to applying AC or DC power to the
transmitter, observe the following:
(a)
Verify all assemblies are installed and their
mating connectors are fully engaged.
(b)
Verify the external input/output wiring is
connected as detailed in paragraph 3.9.1.
(c)
Visually inspect the internal electrical wiring
for defects such as damaged insulation, broken
wires, wrong connections and/or loose
connections.
(d)
Verify all attaching hardware is installed and
securely tightened.
(e)
Verify the requirements of section 3 have been
completed.
(f)
Verify the transmitter RF output is terminated
into a 50 ohm load - an antenna that is
interfaced by an appropriate matching system
for normal operation or a 50 ohm resistive
dummy that is rated at 250 watts (twice the
rated carrier power of the transmitter) for
calibration and testing procedures.
(g)
Verify the appropriate secondary winding taps
of the power transformer have been selected to
match the voltage of the input power source
(refer to paragraph 3.8.3.2).
(h)
Verify the AC power source has a minimum
rating of 350 volt-amperes and the DC power
source (if used) is capable of delivering 12.8
amperes (+24V DC source) or 5.6 amperes
(+48V DC source).
TURNING ON TRANSMITTER
4.5
Turn on transmitter as described in
paragraph 3.11 for initial startup and after repairs
that may have affected the calibration. At other
times, set the switches to position tabulated for
'Calibration Setting' in table 4-1 initially and then to
settings tabulated for 'Operating Setting'.
RESETTING TRANSMITTERS
4.6
Transmitters that have transferred to
'shutdown' may be reset by momentarily switching
the transmitter 'off' and back to 'on'. This action may
be taken locally by using
POWER
switch S3 or
remotely by using external 'on/off' control.
MODULATION DEPTH WHEN USING A HIGH
'Q' ANTENNA
4.7
When the transmitter's output is connected
to a high 'Q' antenna system, the modulation envelope
observed on the RF current waveform may differ
from that on the RF voltage waveform. This
difference is caused by antenna impedance mismatch
at the sideband frequencies, which results in reflected
power standing waves on the feed cable. Depending
upon feed cable length, the sideband impedance may
be more or less than 50 ohms. When the sideband
impedance is less than 50 ohms, the sideband current
will increase and may place undesirable stress on the
solid state devices in modulator/power amplifier A6.
Stress current limits are being exceeded if the
over-modulation detector circuit is properly
calibrated (see paragraph 5.5) and
OVERMOD
ALARM
lamp DS5 is turned on. When this occurs,
the modulation depth is greater than that displayed on
TEST
meter M1. The modulation depth should be
reduced by adjusting
MOD %
potentiometer A4R2
counter clockwise until
OVERMOD ALARM
lamp DS5
just turns off or stops flashing. The resultant
modulation depth is the maximum that is obtainable
with that particular antenna system.
4.7.1
The following explanation is provided to
assist the reader in understanding this phenomenon.
4.7.1.1
The subject transmitter contains two
circuits that monitor the modulation envelope of the
RF output. An over-modulation detector circuit
monitors a sample of the RF current, which is
provided by a current probe at the input to harmonic
filter A7.
TEST
meter M1 displays the modulation
percentage, which is extracted from a sample of the
RF output voltage provided by a voltage probe at the
output of harmonic filter A7. The over-modulation
detector circuit is calibrated with the RF output
applied to a precision, 50-ohm, resistive dummy load,
therefore, the load impedance will be precisely 50
ohms at the carrier and sideband frequencies.
