(c)
The transmit signal is applied through the transmit circulator to the transmit antenna. The signal from the antenna is radiated to the earth and reflected back to the receive antenna. The received signal is applied to a filter to eliminate interference frequencies and the output is routed through the receive circulator to the mixer. The transmit and receive circulators transfer any reverse direction power to high loss loads to provide protection (isolation) for receiver-transmitter circuits.
(d)
In the mixer, the received signal is beat against a sample of the transmit signal and the difference frequency output is applied to the filters. The mixer output is directly proportional to the time required for the transmit signal to travel to the ground and back, and is proportional to the distance from the ground (altitude).
(e)
The AGC circuit determines the relative signal strength in the receiver section by monitoring the amplifier output. An AGC signal is applied to the filters to vary the filter cutoff frequency and maintain the receiver signal level near a preset threshold. Signal strength normally increases with decreasing airplane altitude. The output of the filters is applied to the amplifier where it is amplified to a level suitable to drive the two frequency counters, AGC circuit, and antenna coupling monitor.
EFFECTIVITY
SN 301-350 CONFIG 3 02 Page 4 Apr 25/8834-33-00
(f)
The output from the amplifier is applied to the dual frequency counter and driver circuits. Pulse outputs from the frequency counters are converted to dc altitude signals by operational amplifiers in the driver circuits. The amplifiers provide the dc altitude signals used to drive the height indicator and to provide altitude data to other systems on the airplane. The autopilot/flight director system uses the altitude signal to facilitate gain programming of the glide slope and localizer deviation signals during approach modes. The auto throttle system uses the altitude signal to enable speed control during the low altitude flare maneuver.
(g)
The dc altitude signals are also applied to the receiver-transmitter altitude trip circuits. Each altitude trip circuit consists of an adjustment potentiometer, altitude detector, dc amplifier, and a switch which drives a relay. The dc altitude input voltage 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.
(h)
The dc altitude voltage from the receiver-transmitter is applied to the height indicator pointer drive comparator. The comparator compares this voltage with the voltage from the altitude pointer potentiometer. If there is a voltage difference, the comparator supplies an error signal to the servo-amplifier. The amplifier output signal drives the servomotor which repositions the altitude pointer potentiometer until the comparator output error signal is nulled. The potentiometer and altitude pointer are both geared to the servomotor. This causes the altitude pointer to be positioned to the proper altitude.
(2) Altitude Indication Operation
(a) The receiver-transmitter dc altitude voltage is also supplied to the height indicator adjustable trip comparator. The comparator compares this voltage with the voltage from the adjustable trip potentiometer. As long as the airplane altitude exceeds the altitude of the adjustable trip setting (DH index), the trip relay and DH light(s) will remain de-energized. When the airplane altitude is slightly less than the altitude of the adjustable trip setting, the trip relay will be energized causing the DH light(s) to illuminate.
EFFECTIVITY
SN 301-350 CONFIG 3 03 Page 5 Apr 25/8834-33-00
TRANSMIT RECEIVE RECEIVER TRANSMITTER ANTENNA ANTENNA
LRRA 1 or 2 System Schematic
Figure 3 (Sheet 1)
EFFECTIVITYSN 301-350
CONFIG 3 799 Page 6
34-33-00
Apr 25/88
SB 34-2069
LRRA 1 or 2 System Schematic
Figure 3 (Sheet 2)
EFFECTIVITY
CONFIG 3
SN 301-350
799 Page 7
34-33-00
Apr 25/88
(b)
The adjustable trip comparator also controls the 800-Hz audio warning output supplied by the audio amplifier. The audio warning is initiated by the comparator when the airplane altitude is 50 to 100 feet above the DH altitude setting. The audio initiation point is internally adjustable in the height indicator within this 50- to 100-foot range. As the airplane descends in altitude, the tone increases in level until the DH altitude is reached. When the airplane reaches the DH altitude, the tone is reduced to a low level and remains at the low level as long as the airplane is below the DH altitude. If the airplane now ascends to an altitude which causes the DH light to be turned off, the audio warning is initiated at a maximum level and decreases as the airplane altitude increases. At the 50- to 100-foot altitude range above the DH setting, the audio tone is reduced to a low level.
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本文链接地址：747飞机维护手册AMM CHAPTER 34 - NAVIGATION 第34章导航2(66)
