A 737-300/400/500MAINTENANCE MANUAL
TRANSMIT RECEIVE ANTENNA ANTENNA
Low Range Radio Altimeter System Schematic Figure 3 (Sheet 1)
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A 737-300/400/500MAINTENANCE MANUAL
INDICATOR
Low Range Radio Altimeter System Schematic Figure 3 (Sheet 2)
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A
737-300/400/500MAINTENANCE MANUAL
(5)
The output signal from the transmitter 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.
(6)
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).
(7)
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.
(8)
Operation of both frequency counter and driver circuits is identical. The amplifier input to a frequency counter is converted to a dc analog voltage which is proportional to the input frequency (altitude). This voltage is amplified by a driver and the driver output is the dc altitude voltage supplied to external using equipment. The output from one drive is supplied to the height indicator and the output from the other driver is available for use by other airplane systems. In addition, the output from the two drivers is applied to the receiver-transmitter high and low altitude trip circuits.
(9)
Operation of all six fixed altitude trip circuits in the R/T is identical. The dc altitude input voltage from the driver 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. Not all available trip circuits are used. Each trip circuit may be adjusted to a fixed setting.
(10)
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.
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本文链接地址：737-300 400 500 AMM 飞机维护手册 导航 NAVIGATION 3(135)
