(b)
Antenna selection and BITE circuit The antenna selection circuit receives a control signal from the Input/Output (I/O) processor circuit. This circuit selects the transmit or receive mode, the top or bottom antenna and then transmits the signal to the antenna switching circuit. It also contains the self-test circuit (BITE).
(c)
Receiver circuit The signal from the top or bottom antenna is demodulated at the input of the receiver circuit. The video processing circuit determines whether the received signal is in a main or a side lobe by comparing amplitude of pulses P1 and P2 in modes A and C and amplitude of pulses P3 and P4 in mode S. The video processing circuit identifies the received signal and validates the interrogations. It then demodulates and transmits the signal to the I/O processor circuit.
(d)
Modulator circuit This circuit receives triggering signals (MOD TRIG) from the I/O processor circuit and produces modulated pulses. These pulses are transmitted to the emitter on the radio transmitter circuit. The modulator contains a self-test circuit (BITE) which receives a sample of RF pulses (TX SENSE) from the radio transmitter circuit. It checks the signal validity and sends it to the I/O processor and self-test circuits (BITE).
(e)
Radio transmitter circuit This circuit comprises a transmitter board, a filter and an RF interconnect board. The output oscillator transmits a 1090 MHz frequency.
(f)
Test oscillator modulator This modulator generates the proper amplitude and phase-modulated signals used by the test oscillator. Two inputs TEST OSC MODE 1 and TEST OSC MODE 2 select the type of desired interrogation. A third input TEST OSCILLATOR START initializes the interrogation.
(g)
Test oscillator circuit This circuit generates the signals required during the transponder self-test as well as mode S or all call type interrogation signals.
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(h) Power supply module The power supply module converts the aircraft 115VAC into +5, +12, -12, +65 and -200VDC for the various internal circuits. It receives a suppression signal from each DME interrogator, the other ATC transponder and the TCAS computer. This signal prevents simultaneous transmissions and inhibits receptions from other systems.
(i) I/O processor circuit This circuit ensures the interface between the transponder and the aircraft. It receives and transmits information via ARINC 429
data bases. It also connects the various internal circuits of the
transponder. It is built around a 16-bit 80186 microprocessor. The transponder is connected to the TCAS through two high-speed 429 buses, one for transmission, the other for reception. The discrete output from transponder is :
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NAME ELECTRICAL LEVEL TO SIGNAL STATUS
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LAMP B FAULT OC/5VDC CTL UNIT - ATC 5VDC = FAULT -------------------------------------------------------------------------------
R (5) Not Applicable
R **ON A/C 051-099,
R R R R R R R R R R (3) Principle of operation (Ref. Fig. 003) Each ground interrogator transmits its interrogations at the frequency of 1030 MHz in the form of a series of two pulses. Depending on the pulse intervals and numbers, they define three different interrogation modes. After receiving these pulses, the transponder identifies and decodes the interrogations. Depending on the detected interrogation mode, the transponder transmits either the identification of the aircraft or its barometric altitude or flight data.
R R R R R R R R R To perform these functions, the transponder is associated with a control unit defining the aircraft identification code, the Air Data/Inertial Reference Units (ADIRUs) the Flight Management and Guidance Computers (FMGCs) and the Traffic Collision Avoidance System (TCAS)(if installed). The transmission of the replies to the ground takes place on a carrier frequency of 1090 MHz. If the interrogation is sent by the side lobe of the radar a characteristic signal is sent allowing the transponder to disregard the interrogation.
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R In addition to its specific transponder functions, it enables
R communication between the TCAS and a detected aircraft if equipped
R with the TCAS.
R (a) Interrogations
R (Ref. Fig. 002)
R Three classes of interrogations are transmitted by the ground
R interrogator, they are:
R - Mode A or C interrogations
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