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Radio Altimeter - Transceiver Figure 005
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-SYSTEM OK (green) indicates no fault.
-R/T UNIT (red) indicates a fault in the reception-transmission circuit.
-ANT (red) indicates a fault in the antenna circuit.
-IND (red) indicates a fault in the indicating circuit.
The back is equipped with one ARINC 600 electrical receptacle and a
slot for the test connector.
(2)
Internal description (Ref. Fig. 006) The transceiver includes the following boards:
-microprocessor
-search Track control - Sawtooth generator
-UHF source
-power supplies
-alarm outputs and altitude trips
-amplifier - Discriminators.
(3)
Operation
(a)
Layout The transmitted wave Tx is mixed with the reflected wave Rx to provide several beat frequencies the weakest of which alone is selected after separation in the amplifier filter. The tracking discriminator generates an error signal Fb (Fb = Fb
-Fbo) serving to modify the sawtooth wave so as to obtain a
correct Fb ; the sawtooth generator modifies the slope of the
sawtooth the amplitude of which is constant.
The purpose of the monitoring discriminator is to ensure that the
Fb spectrum is definitely centered on the Fbo.
In the absence of this spectrum, the monitoring discriminator
switches the search track control to search mode. The wave is
then transmitted in sawtooth form. The search cycle duration is
0.4 seconds and repeats until Fb is correct.
The energy detected by the monitoring discriminator is then
sufficient to exceed a preset threshold and the monitoring
discriminator performs switching to track mode.
(b)
Monitoring The radio altimeter is permanently monitored by the monitoring discriminator. In addition, calibration is performed twice per second. During calibration :
-
the mixer is cut off and the amplifier filter output signal is no longer present.
-
the monitoring discriminator triggers the search mode.
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Radio Altimeter - Transceiver
Figure 005A
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Radio Altimeter - Simplified Block Diagram Figure 006
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-a second source known as the test source is set into operation and injects an input signal into the amplifier filter. When Fb = Fbo (25 KHz), search is cut off by the monitoring discriminator. The sawtooth generator, which has the same input as the test source, generates a 1 ms sawtooth and the microprocessor calibrates delta F and Fo. The calibration test is then completed.
(c) Microprocessor The microprocessor consists of a Motorola 6802 MCU with a 6840 programmable timer module (PTM) and a PROM memory. The microprocessor controls the loop management, inputs, outputs, self calibration and the self monitoring circuits. It measures the duration T of the modulation sawtooth, subtracts the aircraft installation delay value read on the main connector, computes the altitude, filters out the Doppler effect and issues the output information. The microprocessor monitors the monitoring discriminator output and determines whether the loop should be in the track mode or switched to the search mode. In the later case, it energizes the search generator.
**ON A/C 001-004,
Post SB 34-1171 For A/C 001-004,
(1) External description (Ref. Fig. 005A) The face of the receiver is fitted with a handle, two attaching parts, two momentary pushbutton switches and a front panel Liquid Crystal Display (LCD). The LCD is used as a fault display and as an operator interface during certain modes. The front panel display also provides user interfaces for test and troubleshooting. The front panel contains two momentary pushbutton switches for interfacing with the LCD pages. The back is equipped with one ARINC 600 size one connector, which includes three plugs:
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