Page 6-16
Vector-LP Radio Beacon Transmitter Technical Instruction Manual
Issue 1.1
Section 6 Theory of Operation
6.4.4.5 FORWARD POWER MONITOR
A dc voltage, proportional to the transmitter’s
forward power, is applied to the
Forward
Power Sample
input (P1-1). This voltage is
filtered by operational amplifier U9 and
associated components and applied to
U11’s internal ADC. If the forward power
sample is above or below user-defined limits
for a user-defined time period, the
Changeover
output (J4-15) becomes active
(logic low). If a changeover had already
occurred prior to this occurrence, a
Shutdown
output (J4-16) will also be
generated. All user-defined parameters are
established using the front panel GUI and
applied to the exciter monitor/generator
PWB via the internal serial bus.
6.4.4.6 RF MONITOR PARAMETER
SELECTION
Multiplexer U14 and associated components
allows the user to select the parameter (RF
voltage or RF current) to be applied to the
RF MONITOR
output at the rear of the
exciter.
6.5 CONTROL/MONITOR STAGE
See Figures 6-2 and SD-3. The control/
monitor stage contains the control/display
PWB, which contains the transmitter’s
embedded microcontroller. The remote
interface PWB, graphic user interface (GUI),
analog/digital displays and audio speaker are
also contained here. The NDB site interface
PWB (A4) and a modem (U11) are optional
items that can also be part of the control/
monitor stage.
6.5.1 Control/Display PWB
See Figures SD-5 through SD-8. The control/
display PWB (A1) performs the following
functions:
x
Produces
dc
voltage
Carrier Ref (A)
and
(B)
outputs that ultimately determine the
forward power (RF carrier) level of the RF
output. These voltages are the reference
voltages for the variable pulse duration
modulation (PDM) generator in each
exciter (A and B).
x
Monitors critical parameters and turns off
(shuts back) or reduces (cuts back) the
Carrier Ref
voltage when defined fault
threshold limits are exceeded.
x
Identifies out-of-tolerance parameters
when the carrier level is shut back or cut
back by providing a visual indication for
local monitoring or an electrical status
output for remote monitoring.
x
Selects the main exciter (A or B) that will
provide the RF drive and PDM drive to
the RF power stage’s RF amplifiers, if no
changeover has occurred.
x
Determines on/off status of RF power
stage.
x
Interfaces with the diagnostic display.
6.5.1.1 CARRIER REFERENCE VOLTAGE
The
Carrier Ref (A)
(J7-38) and
(B)
(J7-40)
outputs are applied to the interphase PDM
driver PWB of each exciter to generate the
PDM drive pulse that ultimately controls the
RF output of the transmitter. DACs U31 and
U32, in conjunction with microcontroller U22,
establish a dc reference voltage for the
Carrier Ref
outputs. The voltage is buffered
through operational amplifiers U27B, U28B
and associated components.
6.5.1.2 ALARM THRESHOLD CIRCUITS
Certain parameters that are monitored by the
control/display PWB can initiate an alarm
indication and appropriate transmitter control
if an out of tolerance condition occurs.
