(2) Internal description (Ref. Fig. 006A) The transceiver includes the following boards:
-an RF module
-
a main processor module
-
a monitor processor module
-
a power supply module
-
a rear interconnect module
-
a display data module
-
a memory card module.
(a)
RF module The RF module, controlled by the main processor, transmits and receives the altimeter signal. BITE circuitry is included to both test and continuously monitor the RF module functions. The process of generating a transmission begins with the Voltage Controlled Oscillator (VCO) output which is split between the transmitter and receiver Local Oscillator (LO). The received signal is mixed with the VCO signal, producing a difference frequency signal that is amplified and fed to an Analog-to-Digital (A/D) converter on the main processor module.
(b)
Main processor module The main processor module controls the radio altimeter operation, performs signal processing of the difference frequency and test signals, and controls the aircraft interfaces and the data displayed on the front panel. The main processor module is divided into three major sections: the Digital Signal Processor (DSP) section, the 486 CPU microprocessor section, and the Input/Output (I/O) section.
1_ DSP section
The DSP section is used to process the analog outputs from the
RF module and to generate some of the control signals to the
RF module for transmit modulation, automatic gain control, and
test signals.
The DSP processes the difference frequency and calibration
frequency into altitude information.
Data is exchanged with the CPU section through a dual-port
Random Access Memory (RAM), providing maximum throughput of
both processors.
2_ 486 CPU microprocessor section
The CPU section does frame-to-frame processing of the altitude
data from the DSP section providing the resulting altitude to
the I/O section.
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The microprocessor in the CPU section controls all major functions of the radio altimeter. Programmable logic devices serve as the microprocessor controller and provide the interfaces to the memory devices (boot routine, program, fault, and data), the data recorder/data loader flash card, and the front panel display driver.
3 I/O section
_ The I/O section provides the two ARINC 429 altitude outputs as well as ARINC 429 interfaces with other aircraft systems including the Centralized Fault Display Interface Unit (CFDIU). All discrete inputs external to the radio altimeter are processed by the I/O section. The I/O section also generates the external discrete outputs which are buffered to prevent damage to the processor circuitry.
(c)
Monitor processor module The monitor processor module provides a second signal processing path using a Digital Signal Processor (DSP). The DSP processes the received signal and calibration signal supplied in digital form from the main processor A/D converter. Also present are a clock, an ARINC 429 receiver/selector and static memory.
(d)
Power supply module The power supply module is a self-contained, high-efficiency, switching power supply that converts the 115VAC, 400 Hz A/C power into the required +5VDC, +12VDC, -12VDC, and +24VDC operating voltages. A power-down interrupt provides advanced notice of a power loss allowing the processors to temporarily retain their status.
(e)
Rear interconnect module To prevent High Intensity Radiated Fields (HIRF) from entering via rear connector cables, an HIRF compartment is formed in the rear of the RA. The signal and power cables are filtered by using discrete and distributed filter elements and limiting devices on the rear interconnect module located inside this HIRF compartment. The filtered lines are then fed to the appropriate points in the RA.
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Radio Altimeter - Simplified Block Diagram
Figure 006A
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(f)
Display date module The display data module is mounted behind the front panel and provides an interface to an operator via a low-power Liquid Crystal Display (LCD) which is visible from the front of the RA. In addition to the LCD, the module contains LED "Light Box" backlighting, temperature compensation circuitry, and a PC board containing an associated LDC driver, two pushbutton switches, and a 9-pin D connector.
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