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R circuitry required to accept the Radio Frequency (RF) signal and
R convert it into a voltage signal to be used by the signal
R processing elements.
R (b) Transmitter
R The transmitter, which consists of a 1090 MHz local oscillator,
R RF transmitter gain stages, transmit/receive pin diode switches,
R forward power and antenna monitors and a low-pass harmonic
R filter, is unique in that it is an all solid-state unit with the
R capability for the Mode S message requirements, which impose
R increased operating requirements on the transmitter.
R (c) Interface/CPU assembly
R The interface/CPU assembly contains a microprocessor and
R supporting memory and logic circuitry. The support circuitry
R includes Flash EPROM memory devices for the operational program,
R boot program and system data, system RAM memory for program
R execution, and an EEPROM memory for calibration and self-test
R error logging. The assembly contains Application Specific
R Integrated Circuits (ASICs) for the RF processing functions
R (interrogations and replies), and communication over ARINC 429
R data buses. In addition to the RF ASIC the assembly contains
R circuitry for processing receiver video signals. The assembly
R supports all external interface functions, including processing
R discrete inputs/outputs and analog synchro interfaces.
R (d) Power supply/modulator assembly
R The transponder has its own internal self-contained power supply.
R The 115V, 400 Hz power supply input is applied through the rear
R connector of the unit.
R The power supply modulator assembly has an EMI filter, 115VAC to
R DC converter, power interrupt monitor, +60VDC switching supply
R for transmitter energy storage, a fly-back supply for generating
R internal unit voltages, and a +35VDC and +28VDC regulator for
R supplying transmitter voltages. The assembly contains PIN diode
R driver circuitry for driving the RF transmit/receive switches,
R pulse and envelope modulator circuitry for modualting the RF
R transistors. The assembly also contains a self-test modulator and
R power supply monitors.
R (5) Theory of operation
R Interrogations which are received by the transponder are filtered and
R amplified by the 1030MHz receiver, converted to a 60MHz IF by the
R mixer and IF filter on the receiver. The interrogations (pulses) are
R then converted to a base-band analog video signals by the log amps.
R The DPSK portion of a Mode S interrogation is turned into a digital
R video signal by the DPSK demodulator. The analog video signal is then
R 1EFF : 051-099, 1 34-52-00Page 52 1 1 May 01/05 1 1 1CES 1 processed by the video processor circuit on the interface/CPU assembly, where it is converted to a digital video signal. The digital video signals and DPSK video signals are then processed by the RF ASIC, which determines the validity and type of interrogation. If a valid interrogation is received, the RF ASIC in conjunction with the microprocessor and software generates a reply. The RF ASIC generates digital pulses which are used to modulate the transmitter. The digital signals are converted to analog modulator signals by the pulse and envelope modulator circuitry on the power supply/modulator. The analog modulator signals are then used to turn the transmitter stages on or off as is appropriate to generate the pulsed replies.
R **ON A/C 106-149, 221-299, 301-399, 401-499,
(3) Principle of operation (Ref. Fig. 003A) 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. The transmission of the replies 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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