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737-300/400/500MAINTENANCE MANUAL
An oscillator in the R/T modulator generates a 155-Hz modulation signal. This signal is wobbulated by a 31-Hz input signal from the wobbulation generator and then applied to a generator which produces the triangular wave modulating signal for the R/T transmitter.The signal from the modulator frequency modulates an oscillator signal in the R/T transmitter. The resultant signal is amplified and frequency multiplied to produce the nominal 4.3-GHz transmitter output signal.A portion of the transmit signal is coupled to the AFC circuits. The AFC loop acts to hold the center frequency of the transmitted signal to 4.3 GHz. Another sample of the transmitted signal is coupled to the calibration loop mixer and delay line. The resultant difference frequency output from the calibration loop mixer is applied to the calibration circuits where it is converted to a dc voltage (altitude). This voltage is compared to a standard reference voltage and any difference is sensed and applied to the modulator for altitude self-calibration. The calibration is accomplished by varying the slope of the triangular wave to reduce the calibration circuit difference (error) signal to zero.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.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).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.
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A 737-300/400/500MAINTENANCE MANUAL

LRRA-2 R/T EFIS SYMBOL GEN-2 F/O EADI
Low Range Radio Altimeter System Signal Block DiagramFigure 2

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A 737-300/400/500MAINTENANCE MANUAL

TRANSMIT RECEIVE
(SHT 2)

ANTENNA ANTENNA
Low Range Radio Altimeter System Schematic Figure 3 (Sheet 1)
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A 737-300/400/500MAINTENANCE MANUAL
EFIS 2(E2-3)

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