(3)
The loop antennas receive two input signals (sine and cosine) from the directional loops and provide bearing signals to the ADF receiver. One winding is mounted parallel to and the other is perpendicular to the airplane fore-to-aft centerline.
(4)
The sense antenna provides a nondirectional reception pattern and a high strength reference phase signal to the receiver. This signal is used to eliminate phase ambiguity of the loop antennas and obtain a true bearing to the station.
C. Control Display Unit (CDU) (Ref 34-61-00)
(1)
The CDU tunes the ADF receiver directly or through the FMC. The CDU provides mode and frequency selections for the ADF receiver. Frequency and mode data are entered on the left or right CDU. The CDU sends frequency and mode data on an ARINC 429 digital data bus to both FMCs. The master FMC sends tuning to the left and right ADF receivers. The master FMC is determined by the position of the FMC master switch.
(2)
The operation of CDU on ADF system is as follows:
(a) Pushing the NAV RAD key on the left or right CDU causes the NAV RADIO page to be displayed on the associated CDU display CRT when FMC is the tuning source. (The ALTN NAV RADIO page will be displayed when both FMCs fail). The frequency and mode of operation for the ADF receivers can be selected on this page. This information is sent to both FMCs. The master FMC then sends the tuning information to both ADF receivers.
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747-400 MAINTENANCE MANUAL
(b)
To tune the left ADF receiver, enter a valid ADF frequency by pressing the appropriate numeric keys on the CDU keyboard. Valid ADF frequencies consist of a three or four digit number optionally followed by a decimal point and a tenth digit and must be in the ADF frequency range. This frequency will appear in the CDU scratchpad.
(c)
To transfer this frequency to the line below ADF L, press the line select key 3L. The left ADF receiver is now tuned to this frequency and operating in the ADF mode.
(d)
If the ANT mode is desired, enter the letter A after the frequency. If the BFO mode is desired, enter the letter B after the frequency. If only the frequency is entered without specifying which mode is desired, the ADF receiver will default to the ADF mode.
(e)
To tune the right ADF receiver, use the same procedures described above except line select key 3R is used.
D. Navigation Display (ND) (Ref 31-61-00)
(1)
The Navigation Display (ND) is a multifunction display for navigation and weather radar information. On ND, ADF bearing information is displayed as the ADF pointer, ADF identifier and ADF frequency on all EFIS modes as long as a valid signal is being received.
(2)
For an invalid signal or no computed data received from the tuning source, the ADF-L and ADF-R fail flags appear on ND, and ADF bearing information is not displayed.
3. Operation (Fig. 3)_________
A. Functional Description
(1)
The left and right ADF systems receive 115-volt ac, 400-Hz single phase power from the left and right main power buses, respectively. The receiver's internal power supplies provide dc voltage for the system operation. They also provide | 12 volts dc to their respective antenna preamplifiers.
(2)
The ADF frequency and mode selections are done manually from CDU through FMC or directly from the CDU if both FMCs fail. When CDU finds that both FMCs have failed, it sends a discrete signal on SSD line. This discrete signal causes the ADF receiver to accept frequency and mode data directly from the CDU.
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MAINTENANCE MANUAL
(3)
The CDU sends frequency/function selection to the signal processor in the ADF receiver. The selected frequency is encoded in 32-bit word for VCO control. The data is examined and if it is valid, the processor supplies a serial word freq/function control to the synthesizer module. The synthesizer module contains an ADF LSI that produces modulation control signals (31 Hz four phases). The modulation control signals are provided to convert the antenna sense and loop signals to a composite phase modulated signal that provides bearing information. The synthesizer also has a function to tune the receiver to the desired frequency.
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