(3) Not Applicable
R **ON A/C 051-099, 106-149, 201-299, 301-399, 401-499,
(2) Operation
(Ref. Fig. 006A)
(a) Interrogation When the frequency for a DME ground station is selected on the RMP or by the FMGC, a serial digital tuning word is transmitted along an ARINC 429 data bus to the range processor in the interrogator. The range processor uses the frequency data to provide a tuning word for the synthesizer. In the synthesizer the Voltage Controlled Oscillator (VCO), which operates in the L-band range of 1025 to 1150 MHz, generates the transmitter signal which after amplification is applied to the driver. The transmitter part of the interrogator consists of a VCO with a quartz-stabilized, phase-locked-loop frequency synthesizer and a power amplifier. The synthesizer is controlled by the 8OC/86 CPU. The power amplifier is energized only during the pulse transmission and the transmitted energy goes to the antenna through a circulator. The receiver part consists of a preselecting filter, an RF amplifier, a mixer and an IF amplifier centered on 63 MHz. The IF amplifier output pulses are then detected.
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DME Interrogator - Simplifed Block Diagram
Figure 006A
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(b)
Reply signal processing Reply pulses from a DME ground station are received by the antenna and applied through the circulator to the receiver. In the receiver the reply pulses are filtered, amplified and mixed with the local oscillation from the driver to produce a 63 MHz IF signal. After further amplification and filtering, a second mixer combines the 63 MHz signal with a local 52.3 MHz oscillation to produce a 10.7 MHz IF signal which is applied to the video processor through a video detector. In the video processor the signal is applied to a threshold detector and a pulse pair decoder. The threshold detector determines the range measurement timing. The video processor verifies that a valid pulse is received and applies the valid video pulse to the range processor module. An audio identification algorithm executed by the processor board decodes and synthesizes a 1350 Hz audio signal which reproduces the source identification signal transmitted by the DME ground station and provides an output to the audio integrating system in the aircraft.
(c)
Distance measurement A range counter in the range processor module, which generates the driver and power amplifier trigger pulses, measures the time interval between the first interrogation pulse and the reception of the first valid video pulse. The time delay of the reply pulse is converted to a distance value and stored in memory. During an interrogation cycle, additional reply pulse delay measurements are made and processed to produce an accurate range distance. The range distance data is transmitted through ARINC 429 data ports to the DMC, the RMI and the FMGC.
(d)
Suppression A suppression coaxial connects all these units: the DME interrogators and ATC/ TCAS system. This prevents transmission from one system while the other is in reception mode. This is necessary because all these systems operate on the same frequency range.
(e)
Modes The DME system has three basic modes : standby, directed and free scan.
-Standby mode: in this mode no distance data is computed, the NCD indication appears in the RMI, and the NCD signal is also transmitted to the FMGC. The DME reverts to the standby mode when it is not communicating with a ground station.
-Directed mode: in this mode up to five foreground stations are designated by the FMGC to the DME interrogator.
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-Free scanning mode: in this mode any or all of five stations can be designated by the FMGC. If less than five stations are designated, the DME fills the empty slots with the closest stations. The DME scans the background loop, consisting of the other 247 stations, searching for station pulses.
(3) Manual self-test and automatic monitoring
(a)
Purpose The fault monitor of the DME interrogator periodically checks the circuits and operation of the interrogator while the unit is installed in the aircraft. The fault monitor uses Built-In Test Equipment (BITE) to gather operating parameters relating to the fault and stores historical fault data relating to the failure in the maintenance fault memory. If a failure is detected, the crew and/or the FMCS is alerted that the unit does not operate correctly. The stored fault data can be used by the shop technician as an aid in trouble shooting the faulty LRU.
(b)
Operation The fault monitor self-test periodically interrupts normal DME operation and uses BITE to monitor a number of parameters selected to determine the status of functionally replaceable modules. Under normal conditions, the self-test is performed every 60 s. If a failure is detected, the self-test is performed every 5 s. Safety related failures detected while the aircraft is in the air cause DME service to be denied to the crew and/or the FMCS. A detected failure causes the DME to output ARINC data words containing failure and No Computed Data indications. Safety related failures include wrong distance, wrong channel, etc. If the failure is not safety related (low RF power, poor receiver sensitivity, etc.), the DME continues to attempt to operate normally until the aircraft is on the ground. If any failure is detected while the aircraft is on the ground, DME service is denied to the crew and/or the FMCS. The DME outputs ARINC data words containing failure and No Computed Data indications. Data relating to any failure detected when the aircraft is in the air is stored in Non-Volatile Memory on processor card A7. The Non-Volatile Memory can be accessed by the shop technician as an aid to fault isolation. The DME self-test procedure can be manually initiated by either the ARINC 429 input control word or the DME functional test rear connector discrete. When the self-test procedure is initiated in one of these ways, the ARINC output data words indicate a failure
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