-
a receiver (A5)

 -
a transmitter (A6)

 -an Input/Output (I/O) module (A4)
 -two Central Processing Units (CPU) (A2, A3)
 -a power supply unit (A1)
 **ON A/C 051-054,
 (Ref. Fig. 005A)
 **ON A/C 051-099,
 Post SB 34-1159 For A/C 051-054,
 (Ref. Fig. 005C)
 **ON A/C 051-099,
 (Ref. Fig. 006A)
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R  TCAS Computer - Block Diagram 
R  Figure 006A 
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 (2) Radio frequency section
 (a)
 General The RF section consists of a receiver, a transmitter and an I/O module. The receiver processes the raw ATCRBS or Mode S intruder reply data received from either the top or the bottom antenna. The transmitter portion of the RF section controls all interrogation, it interrogates intruders once per second, also using either antenna. The I/O module is used to route signals to and from the antennas. For ATCRBS intruders, interrogations 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)
 I/O module The I/O module controls the transmit and receive RF path selection, generates self-test signals and waveforms. All RF switching is done electronically by pin-diodes. The antenna steering is an electronic switch consisting of electronic components which send receive and transmit signals into different paths according to the antenna direction (0, 90, 180 and 270 degrees). The selected path is routed to the appropriate switch which drives the signal to either a top or bottom antenna. There are 8 ports on the switch: 4 for each direction of the top antenna and 4 for each direction of the bottom antenna. The received signals (all 4 received channels are on at one time) are channeled to the antenna steering where they are electronically connected to the receiver. The I/O module incorporates self-test circuitry for the receiver and the transmitter. There is also the antenna self-test function. The receiver self-test circuitry simulates ATCRBS/mode S replies which are injected at an appropriate level into each receiver RF path. The transmitter self-test verifies proper power/path integrity and the I/O module continuously monitors antenna integrity as well.

 (c)
 Transmitter The transmitter controls all interrogations waveforms and power levels. It consists of both pulse and DPSK modulators, all power amplification stages, high-power precision step attenuator

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 (whisper-shout), as well as harmonic filter and spectrum filter,
 and BITE power detection circuitry.
 The L-band transmitter has a peak power output of 1.2 kW at 1030
 MHz. The transmitter signal is amplified and a DPSK modulator is
 also provided for Mode S interrogations.
 This signal is divided into 4 in the proper phase relationship.
 The modulator/driver provides power and control signals, it sends
 discrete signals to the CPU which provides the correct pulse
 width for modulation.
 The final signal is increased up to 1500 W and it is fed into the
 whisper-shout module. This module is a precision 27 step
 attenuator (1 dB by step) which controls the power level.
 The output is fed through the filters before it is driven to the
 I/O module which applies the power to the appropriate antenna.

 (d) Receiver The receiver consists of 4 matched channels which provide very accurate bearing information. Each receiver channel consists of a single conversion receiver, high-quality logarithmic amplifier and high-resolution A/D converter. The receiver consists of 4 virtually identical receive channels coupled to 2 separate digital video sections. Each receiver uses a single conversion heterodyne circuit. The preselector, included in each receiver, contains an L-band, band pass filter which selects the 1090 MHz frequency band. It provides selection of unwanted signals in excess of 60 dB outside the 30 MHz bandwidth. This signal is converted from 1090 MHz to 60 MHz. The processed signal is applied to the logarithmic amplifier. The logarithmic amplifier video output is fed to a video amplifier where the actual magnitude of the slope and offset level is controlled precisely. This video signal is fed to the digital video circuit. This one samples the analog video signal at an 8 MHz rate for mode S and
　
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