(2)
Each subsystem has a VOR receiver that accepts rf and digital data, processes the data and provides audio and digital output to various interfacing systems.
(3)
VOR receiver inputs
(a)
RF signals from the tuned VOR ground station is provided by the dual VOR/LOC antenna through the VOR/LOC rf power dividers. The rf power dividers allow the VOR/MKR receivers and the ILS receivers to share the VOR/LOC antenna.
(b)
Receiver frequency tuning input is normally provided automatically by the FMC when in autotune mode or manually by the CDU through the FMC. If neither FMC is operational, the CDU provides a source select discrete that allows its onside receiver to accept tuning input from that CDU directly.
(c)
An external self-test request is provided by the CMC when initiated using a CDU.
(d)
A discrete signal is provided by the air/ground relays to allow the VOR/MKR receiver to record the flight segment for flight fault memory.
(4)
VOR receiver outputs
(a)
The VOR digital data outputs from each VOR receiver are provided to the EIU's to generate video signals for VOR displays on the ND.
(b)
Digital output from each VOR receiver is provided to both left and right FMCs for position update.
B. Functional Description - VOR Receiver (Fig. 3)
(1)
A marker beacon receiver module is incorporated in each VOR/MKR receiver. This module is inhibited in the right VOR and active in the left VOR.
(2)
The internal power supply receives 115 volts ac from the NAV RADIO MKR VOR L circuit breaker for the left VOR and from the NAV RADIO VOR R circuit breaker for the right VOR and provides various dc voltages needed for receiver operations.
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BY THE RIGHT CMC AND SWITCHEDTHROUGH THE LEFT CMC
VOR Navigation System Block DiagramFigure 2
A
747-400
MAINTENANCE MANUAL
(3)
The VOR signal received at the antenna is combined in the receiver with the VCO injection signal to produce an 18.1 MHz IF. The IF signal is fed to the detector circuit where the VOR modulation signals are recovered from the IF signal.
(4)
The output of the detector circuit is applied to the audio filter and limiter. The filter passes audio frequencies within the range of 350 Hz to 2500 Hz and refects the other outputs of the receiver. The limiter filters out all noise spikes. The voice and keyed identity tones are fed to the audio amplifier and out to the audio management unit.
(5)
The output of the detector circuit is also applied to the switching select circuit. During normal operation, this output is fed through the selector to the low pass and 9960 Hz band-pass filters.
(6)
The CPU processes the bearing data to provide digital bearing output signals through the ARINC 429 transmitter and output buffer. In addition, the VOR receiver receives a VOR selected course from the FMC or CDU and echoes this out along with the bearing data to the EIUs for display on the NDs. The NDs use the bearing and course select data to compute VOR deviation data and determine TO/FROM annunciation data.
(7)
The VOR receiver contains self-test and monitoring features that check for proper operation of the unit. This BITE program is activated as part of the CPU program. During normal rf signal processing, the BITE network continously checks functions that affect the accuracy of the VOR.
(8)
All receiver faults detected during the BITE program are stored in a nonvolatile fault memory. In addition, the receiver will output failure warning (FW) codes in the sign status matrix of the bearing word output on the ARINC 429 data bus. For receiver failures, this code indicates invalid data. When computed data is not avialable for reasons other than equipment failure, a no computed data (NCD) code is output. Fault displays on the NDs is described in the following IDS VOR display functional discription.
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