(5)
The ground return signal from the receive antenna is coupled through the receive isolator to a mixer. A second input to the receive mixer is derived from the transmit directional coupler. This sample of the transmitter signal plus the ground return signal are heterodyned in the mixer to produce a difference or beat frequency which is proportional to airplane altitude above the terrain. The mixer output is applied to the preamplifier and tracking filter circuits in the modulator.
505
Oct 20/84 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 34-48-03 Page 3
K73393
Low Range Radio Altimeter System Simplified Schematic 595
34-48-03 Figure 2 (Sheet 1) Sep 20/82
Page 4
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
5A7 Low Range Radio Altimeter System Simplified Schematic
Sep 20/82 Figure 2 (Sheet 2) 34-48-03
Page 5
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
(6)
Preamplifier and tracking filter operation is controlled by the altitude processor to improve signal-tracking capabilities. (The processor selects the-correct receiver pass band shape for the altitude being tracked.) The output from the tracking filter is routed to a limiter, which produces a relatively constant amplitude altitude beat frequency signal, and also to a signal presence detector. The detector provides a discrete signal to both the altitude and monitor processors when a signal is present.
(7)
The mixer signal from the receiver is also routed through a high pass filter to a mutual interference detector that contains a threat comparator-circuit. When activated by the presence of a second LRRA system, the threat detector causes a binary addition to a counter in the modulator to induce phase misalignment and thus prevent mutual interference.
(8)
Altitude processing is accomplished by a general-purpose microcomputer, which consists of a central processing unit, a programmable read-only-memory, a random-access-memory, and input/output interface circuits. In the altitude processor, the beat frequency signal from the receiver and the calibration 1 signal from the transmitter are used as data to produce both analog and digital signals corresponding to airplane altitude.
(9)
The monitor processor is basically the same as the altitude processor but it does not produce analog and digital outputs. Instead, it uses the calibration 2 signal from the transmitter to compute airplane altitude and then compares its results with those derived by the altitude processor. If the two answers are within a predetermined value, the monitor processor sends a valid signal to the monitor. Routine tests axe also undertaken by the processor to ensure that signal circuits in the receiver-transmitter unit are operating correctly.
(10)
The monitor accepts the monitor processor validity signal, checks the availability of the units dc power supplies, and monitors circuits in the altitude processor and the receiver. If all functions are satisfactory, an oscillator and driver circuit produces a flag signal to hold the height indicator flags out of view. The flag signal is also supplied to other systems in the airplane.
(11)
The altitude trip subassembly provides six adjustable trip circuits, each of which operates in an identical manner. The altitude signal from the altitude processor is compared to a reference voltage (altitude trip setting). If the input voltage exceeds the reference voltage, the trip relay remains de-energized. When the input voltage is less than the reference voltage, the trip relay is energized. The output from each altitude trip circuit is a ground signal to using equipment.
(12)
In the height indicator, the positive dc altitude voltage from the receiver-transmitter and a dc feedback voltage from the isolation amplifier are applied to the differential preamplifier. The dc feedback voltage represents the position of the altitude pointer, and the dc altitude voltage represents airplane attitudes
(13)
When the two voltages applied to the differential preamplifier are not equal in value, a difference voltage is produced and applied to the servo-amplifier. The servo-amplifier drives the motor which repositions the feedback potentiometer (and altitude pointer) until the voltages are equal. When the two voltages are equal, the altitude pointer indicates the correct airplane altitude.
(14)
The DH control knob is mechanically coupled to the DH cursor and to two contacts of the DH switch. Adjustment of the knob positions the cursor to the selected altitude and positions the contacts of the switch. The switch contacts are connected to an ON coil and an OFF coil in the DH relay. The opposite sides of both coils are connected to the power supply positive flag supply. When the cursor is set to an altitude greater than the airplane altitude, the power supply negative flag supply is applied through the switch to the ON coil closing the relay contacts to turn on the DH light(s). When the cursor is set to an altitude less than the airplane altitude, the switch closes in the opposite direction and the power supply negative flag supply is applied to the OFF coil. The DH relay is a latching relay and remains in one position until the opposite coil is energized.
中国航空网 www.aero.cn
航空翻译 www.aviation.cn
本文链接地址：737 AMM 飞机维护手册 导航 NAVIGATION 2(65)
