747飞机维护手册AMM CHAPTER 34 - NAVIGATION 第34章导航4(104)

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 B.  The couplers are mounted on the aft wall of the right and left wheel wells near their respective sense antennas.
 7.  Receiver (Fig. 1)
________
 A.  The receiver is a transistorized, superheterodyne radio receiver that processes amplitude-modulated, rf signals within the frequency range of 190 kHz to 1750 kHz, and produces a three-wire synchro output representing relative compass bearing to the rf transmitter site. It also produces an audio output from any modulation on the received rf signal for use in the flight interphone system.
 8.  Control Panel (Fig. 1)
_____________
 A.  The control panel remotely controls receiver band switching, tuning, gain and operational modes by passive, electrical means. It is manually operated and provides the operator with visual indications of selected mode and frequency.
 9.  Operation
_________
 A.  Functional Description (Fig. 3)
 (1)  In each automatic direction finder system, the omnidirectional (sense) and directional (loop) antennas provide rf signals to the receiver. The loop antenna signal lags 90 degrees behind the sense antenna signal. Rf voltage from the sense antenna (Sheet 1, Fig. 3) is impedance matched in the sense antenna coupler and routed through a double-shielded coaxial cable under the passenger deck to the receiver. Rf voltage from each loop of the fixed loop antenna is compensated for field distortion in the quadrantal error corrector and coupled into the receiver through the special loop antenna cable.
 EFFECTIVITY

 AF 101-156; IB 405;  CONFIG 1 LH 001-003; SN 301-302  04 Page 5  Apr 25/86

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 (2)  
The doghouse assembly attached to the receiver's upper front panel contains a goniometer which provides an electrically equivalent means of rotating the fixed loop antenna. Loop antenna inputs induce two separate magnetic fields into a stator winding where they combine vectorially to produce a single resultant magnetic field proportional to the relative bearing of the rf transmitter (Sheet 1, Fig. 3). This magnetic field induces a voltage into the goniometer rotor winding, the phase and amplitude of which depend upon the angular position of the rotor in relation to the stator. In the ADF operating mode, this error signal is processed by the receiver and fed back to the goniometer rotor shaft as mechanical motion which positions it at an electrical null. Rotor shaft rotation is through a full 360 degrees with 0 degrees corresponding to aircraft heading. When the goniometer is nulled, the angle between 0 degrees (aircraft heading) and nulled rotor shaft position is directly equivalent to the relative bearing of the rf transmitter.

 (3)  
The receiver circuit that processes the goniometer error signal is a triple conversion superheterodyne with tuned rf input circuits. The rf stages are tuned through three frequency ranges by varactor diodes (Sheet 2, Fig. 3). The third frequency range is extended by switching in a parallel inductor when required. The first tuned rf stage has a separate varactor diode for each of the three tuning coils. Each of the other tuned rf stages has a single varactor used for all three tuning coils. The required coils and varactors are selected by diodes switched from the control panel.

 (4)  
The three oscillators of the three-stage frequency converters are crystal-controlled. The crystal for the first oscillator is selected from a bank of nine crystals. The crystals for the second and third oscillators are selected from separate banks, each containing two crystals. The required crystals are selected by diodes switches from the control panel. Selection of crystals is related to the required radio frequency so that the final intermediate frequency lies within the band 180 kHz to 130.5 kHz.

 (5)  
The first three IF stages are tuned by varactor diodes across IF transformers arranged in bandpass pairs. Combined top and bottom capacitance coupling is used between the two elements of each bandpass pair to ensure that the bandwidth passed remains constant throughout the IF tuning range of 180 kHz to 130.5 kHz. The fourth IF stage (which is not tuned) is connected to the audio, AGC and ADF detectors.

 (6)  
AGC voltage is routed to the control panel for combination with manual rf gain control. The control voltage is applied to the three tuned IF stages and to one of the rf stages. Audio output is similarly routed to the control panel for audio gain control.

 EFFECTIVITY

 AF 101-156; IB 405;  CONFIG 1 LH 001-003; SN 301-302  04 Page 6  Apr 25/86

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 (7)  
In the ADF mode, the rf error signal from the goniometer coil (rotor) is processed, compared with the rf from the sense antenna, detected and fed back to the goniometer rotor through a servomotor to achieve a null. An oscillator generates a 220 Hz signal (Sheet 1, Fig. 3). The amplified signal provides the power supply to the reference-phase winding of the two-phase gonio servomotor. The amplified signal is also used, after a 90-degree phase-shift, to switch a balanced modulator. Output from the balanced modulator, released at each half cycle of the switching voltage, consists of blocks of the tuned rf loop error signals which alternate in phase. This output is added to the sense antenna input in the following tuned rf stage.
　
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