(2)
The threshold sensor determines when the amplitude of the output of the RF receiver is sufficent enough to enable the marker beacon annunciations. This corresponds to being directly above the marker beacon transmitter. The output of the threshold sensor is applied to the status monitor.
(3)
The tone decoders determine which of the modulation frequencies is present. This is done to determine which marker beacon alphabetical annunciation to display. The output of the tone decoders is applied to the status monitor.
(4)
The audio amplifier provides the gain necessary to supply an acceptable audio output to the audio management unit.
(5)
The status monitor checks the output of the threshold sensor and tone decoders. From these inputs, the monitor develops a digital status signal which is applied to the CPU. This signal informs the CPU which signal is being received and if the marker beacon is operating properly.
(6)
The CPU module receives the receiver status from the status monitor and also AGC from the RF receiver. From this information the CPU determines if the module is operating properly and if the marker beacon information should be transmitted by the ARINC 429 transmitter.
(7)
The ARINC 429 transmitter sends VOR and marker beacon data to the EIUs which are the interface units to the IDS.
(8)
For the 400 Hz outer marker, the tone output is continuous dashes (----). For the 1300 Hz middle marker, the tone output is alternating dots and dashes (.-.-.-). For the 3000 Hz inner marker, the tone output is continuous dots (....). For the 3000 Hz back course marker, the tone output is continuous paired dots (.. .. ..). For the 3000 Hz airways marker, the tone output is the station identification letters in Morse Code.
(9)
The 75MHz incoming signal from the antenna is applied to a 75 MHz filter. This filter provides tuning and rejection of unwanted signals. The filter output is applied to a RF amplifier. a detector receives the amplifier output and feeds the demodutated signal to the audio system and the AGC filter. The AGC filter monitors the signal level and maintains a constant output level.
(10)
The marker beacon audio system, which receives the signal to three detector, applies the signal to three audio bandpass filter circuits. The active filters centered at 400 Hz, 1300 Hz, 3000 Hz amplify and filter the audio signal and send it to an audio amplifier.
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747-400 MAINTENANCE MANUAL
(11)
The active filters also send the audio signal to a rectifier circuit which receives the filtered audio signal and converts it to a direct current signal. The dc signal is sent to the I/O card which transmits the signal to the EIU's.
(12)
For the 400 Hz outer marker, the tone output is continuous dashes (----). For the 1300 Hz middle marker, the tone output is alternating dots and dashes (.-.-.-). For the 3000 Hz inner marker, the tone output is continuous dots (....). For the 3000 Hz back course marker, the tone output is continuous paired dots (.. .. ..). For the 3000 Hz airways marker, the tone output is the station identification letters in Morse Code.
B. Bite
(1)
The VOR/MKR receiver built-in test equipment (BITE) continously monitors marker beacon receiver module performance. The LEDs on the receiver front panel will indicate system status. A red MKR LED indicates a marker beacon receiver failure.
(2)
A TEST switch is located on the front panel. When the switch is pressed the receiver BITE performs a system self-test. The self-test runs for ten seconds and will display the system status using the LEDs on the receiver front panel.
(3)
The LCD screen on the front panel of the left VOR receiver provides status of the R/T BITE self-test. The self-test does a self check for existing internal and external faults.
(a) A functional self test of the marker beacon system is initiated by push and release either one of the test switches on the front panel of the LCD screen. When this happens, the TEST IN PROGRESS display is shown on the LCD screen of the DME system goes through a self test. The test continue for approximately 5 seconds. The TEST COMPLETE NO FAILURES display is shown for a test pass condition. The TEST COMPLETE FAILURES display is shown for a test fail condition. Push and release the RETURN switch showns the P/N and S/W display. Push and release the WHY switch shows the failure message if a test fail condition has occurred. 中国航空网 www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：747-400 AMM 飞机维护手册 导航 NAVIGATION 2(55)
