(4)
The rf inputs are provided by the ADF antenna. These inputs include an input from the non-directional sense antenna and an input from each of the two directional loop antennas.
(5)
The loop signals are summed with the sense signal. This eliminates the 180-degree phase ambiguity. The 180-degree phase ambiguity results from the inability of the loop antenna to detect the correct bearing from two relative bearings which are 180-degrees apart. The combined signal is used to compute bearing information to the transmitting station.
(6)
The loop signals are balance modulated by reference 31 Hz modulation control signals. The 31 Hz control signal is composed of four selected phases in quadrature. The balance modulated loop signals are summed together with the sense signal in a controlled phase relationship to form a composite phase modulated carrier signal. The signal is then coupled through the receiver to the IF circuits and then to the coherent demodulator module.
(7)
The coherent demodulator senses the presence of a signal from the IF circuit. It then phase locks a tracking VCO to the signal. The phase modulation component of the signal is recovered from the VCO phase lock circuit. The recovered signal contains the original bearing and audio information received by the ADF antenna. The bearing signal is filtered and then coupled to the processor module. In the processor module the recovered bearing signal is then compared to the reference modulation control signal by the signal processor. It is converted to a 12-bit bearing data field. The audio signal is filtered and amplified by the audio circuit. The audio output is sent to audio management unit.
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ALL ú ú 15 Page 8 ú Feb 15/98
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL
M50152
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ALL ú ú 21 Page 9 ú Feb 18/02
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL
M50153
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ALL ú ú 20 Page 10 ú Feb 18/02
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A
747-400
MAINTENANCE MANUAL
(8)
Quadrantal Error Corrector (QEC) compensates for signal distortion due to airplane structure. It is unique for each type of airplane. QEC is programmed on the receiver connector. Pins are connected (open or ground) to reflect the QEC required for that type of airplane.
(9)
The bearing information is sent to the EIUs and RMI (if installed) via an ARINC 429 digital data bus. From EIUs, ADF bearing information is displayed on both NDs.
(10)
For audio signals, the output is sent to the audio processor. When a CW station is selected, a BFO tone (1020 Hz) and muting signal are also sent to the processor. The muting signal prevents an audio output when a weak or no signal is being received. The output passes through the filter and transformer to the audio control panels.
B. ADF - System Monitoring
(1)
The ADF receiver continuously monitors system operation. The BITE circuits monitor various circuits in the ADF receiver for faults. All detected faults are stored in the fault memory.
(2)
The BITE module contains a circuit which when enabled, injects an on-channel test signal into the circuits of the receiver. The receiver response is compared to the expected response to determine if the unit is operational. Any active faults will be annunciated on the RMI or on the receiver front panel LCD during a self-test. The fault memory can store faults for up to 63 flights. The maximum number of faults that can be stored for any flight segment is 13. If a fault exists continuously, it is stored only once. If a fault remains intermittent for one second or more, it is stored each time it occurs. Fault memory may be accessed on the ground by an ATE connector on the front of the unit. Also, detected faults can be displayed on the front panel LCD using maintenance screens for troubleshooting.
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