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R ILS Receiver
R Figure 007A
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ILS Receiver - Block Diagram R Figure 008
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a front panel display module,
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a memory card module.
(a) RF module This module converts the RF signals received by the glide slope and localizer antennas into analog signals for processing by the Digital Signal Processor (DSP). The module consists of two RF sections, one for the UHF glide slope signals and one for the VHF localizer signals. The primary difference between the two sections is in the front end circuitry due to the frequencies involved. BITE circuitry is included to both test and continuously monitor various stages of each receiver.
1_ Localizer receiver unit
The receiver frequency and gain are controlled by the DSP
section of the main processor board. The antenna input is
coupled to the mixer through the preselector, which is tuned
by the Local Oscillator (LO) input supplied by the frequency
synthesizer. The frequency synthesizer is tuned by the DSP.
The preselector, in conjunction with a balanced mixer,
provides rejection of undesired signals and immunity to
cross-modulation and inter-modulation effects. The localizer
synthesizer controls the localizer VCO frequency ranging from
89.9 to 99.85 MHz. The IF frequency for the localizer is 18.1
MHz.
The output of the mixer is filtered by a highly selective 21.4
MHz crystal filter.
The amplified signal is then buffered and passed through the
detector to provide the composite audio output levels for
processing. A carrier-proportional DC voltage is also provided
for Automatic Gain Control (AGC).
Functional test is accomplished by injecting a test oscillator
signal into the receiver and modulating the signal by the
BITE. During normal operation, signals from the LOs, the
synthesizer and AGC are monitored by the DSP.
2_ Glide slope receiver unit
The glide slope receiver unit receives and detects the
modulated glide slope signals, in the frequency range from
329.15 to 335 MHz. Its architecture is similar to the one of
the localizer receiver, except that the frequency values are
different and that there is no audio identification decoded on
the G/S signal.
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(b) Main processor board The main processor board controls the ILS mode of operation, generates the audio and deviation outputs, controls the aircraft interfaces and performs the control and display functions for the front panel. In addition, it also performs the in-line monitoring (BIT) tasks. The board is divided into three major sections:
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