(3) Not Applicable
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A. TCAS Computer
(Ref. Fig. 005B, 006B)
(1)
Introduction The TCAS computer is divided into four major sections:
-Radio Frequency section (RF Assembly A6)
-Signal Processor (A4)
-Central Processing Unit (CPU A3)
-Input/Output Processor (I/O A2)
(2)
Radio frequency section
(a)
General The RF section consists of receiver, transmitter and RF switch portions. The receiver processes the raw ATCRBS or Mode S intruder reply data received from either the top or bottom antenna. The transmitter portion of the RF section interrogates intruders once per second, also using either antenna. The RF switch portion selects the antenna (top or bottom) for the transmission and reception of the TCAS signals. For ATCRBS intruders, interrrogations are accomplished by controlling a "whisper shout" attenuation system which provides successive responses, starting with the nearest aircraft and continuing progressively to those farther away so as to avoid saturating the radio-electric space and resulting interference.
(b)
Description
1 Transmitter
_ The transmitter is a solid-state circuit that develops 1030 MHz outputs. The TCAS transmitter/receiver has a transmit/listen cycle of approximately 1 s intervals. A series of increasing power mode C interrogations is first transmitted. This is the whisper-shout sequence (4-pulse group repeated with increasing power to 20 dB at 1 ms intervals). Immediately following is a mode S interrogation modulated in DPSK (Data Phase Shift Keyed) format. The remainder of the 1s-cycle is spent in listen mode.
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TCAS Computer - Block Diagram
Figure 006B
R 1EFF : 106-149, 201-209, 211-299, 301-399, 1 34-43-00Page 30 1401-499, 1Aug 01/05 1 1 1CES 1 After the whisper shout attenuator, the signal is fed through diplexers, beam-steering networks and top/bottom antenna switch. The beam-steering network generates and controls the phase relationships of the four RF outputs from the transmitter. These outputs are connected to the four elements of the TCAS antenna.
2_ Receiver
The receiver has four individual RF and IF sections, one for
each directional antenna segment. The receive frequency is
1090 MHz. The signal received from each antenna element is
passed through one of the four parallel top/bottom antenna
switches, beam-steering networks, transmit/receive switches,
then to four identical receiver RF circuits. These four
signals are further applied to mixer circuits to develop IF
signals (the local oscillator frequency used by the mixers is
1030 MHz from the transmitter sub-assembly). These IF signals
are amplified, filtered and applied to splitters for output to
the video processor.
3_ Video processor
One output from each of the four signal splitters is
recombined and output as a video signal. This signal is used
to detect mode C and mode S data pulses. These items of
information are checked and invalid pulses eliminated so the
outputs of the video processor are mode C and mode S data. The
output from each signal splitter is applied to the bearing
detector.
4 Bearing detector
_ One of the IF signals is applied to one limiter as a reference signal. Outputs of two other limiters are mixed with the reference signal for phase detection. The resulting phase detector outputs are dc signals whose levels are directly proportional to the angle of arrival of the RF signal at the antenna.
(3) Not Applicable
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Post SB 34-1179 For A/C 201-207, Post SB 34-1238 For A/C 201-207,
A. TCAS Computer
(Ref. Fig. 005D, 006C)
(1)
Introduction
The TCAS computer is divided into four major sections:
-Beam Steering/Receiver Module (A2)
-Transmitter Module Assembly (A3)
-Digital Signal Processor (DSP) Assembly (A4)
-Input/Output (I/O) CPU Assembly (A5)
(2)
Beam Steering/Receiver Module (A2)
(a)
Beam steering The purpose of beam steering is to vary phasing of the transmit signal to the four directional elements in each (top and bottom) antenna. The antenna elements are numbered 1 through 4 with number 1 element of the top antenna pointing to the nose of the aircraft, number 2 to the right, number 3 to the rear, and number 4 to the left.
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