(a)  Received signals from the antenna are routed through the Diplexer/Low Pass Filter and applied to the Preselector/Mixer. In the Preselector the signal is passed through a bandpass filter tuned to 1030 MHz. The filter output is applied to the mixer diode and mixed with the 970-MHz local oscillator (LO) signal. The LO signal is filtered in the preselector, by a 2-pole bandpass filter prior to mixing. The 60-MHz IF signal from the mixer is applied to the IF amplifier/detector module where it is amplified and detected. The output of the IF amplifier/detector is applied to the video processor where it is amplified, pulse shaped, cleaned up by a noise suppression circuit, and elimination of P2 pulse is performed if the received signal is from the ATC ground station main lobe. The signal is then sent to the decoder/encoder circuitry.

 (2)  
Decoder/Encoder

 (a)  The video processor output is applied to the decoder/encoder along with the 2.7586 MHz 2-phase clock from the decoder/encoder (D/E) oscillator. The decoder/encoder then decodes the received message, determines whether mode A or mode C data is requested and then developes the appropriate serial data output to be applied to the transmitter.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL

F1C1A1C2 B2 B2D4 F2
** SPI TRANSMITTED ONLY WHEN IDENT BUTTON IS PRESSED
TYPICAL TRANSPONDER REPLY PULSES
ATC System Pulse Codes Figure 2
T80903
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
 (3)  
Transmitter

 (a)  The modulator accepts the decoder/encoder serial data outputs, adjusts the amplitude and width of the pulses and supplies the proper signals to the transmitter modulator circuit. The modulation signal modulates the transmitter oscillator which is a 1090-MHz cavity controlled pulsed oscillator. The output of the transmitter oscillator is approximately 1.5 watts peak RF power. The pulse RF signal is applied to the exciter module where the signal is amplified to 150 watts peak RF power. This 150 watt signal is then used to drive the power amplifier module where the RF signal is amplified to 500 watts peak RF power. The power amplifier output is then sent to the Diplexer to be routed to ATC transponder antenna.

 (4)  
Functional Test

 (a)  Upon manual activation of the test switch, either on the unit front panel or control panel, the functional test module will generate an RF pulsed signal. This signal is then applied to the local oscillator module where 60-MHz sidebands are generated on the local oscillator signal. These sidebands are a pulsed pair which are properly spaced to represent a valid mode interrogation. The LO signal with the 60-MHz sidebands is applied to mixer and a 60-MHz If signal is generated. From this point on the signal is processed as a valid interrogation and the monitor/reply indicator as well as the light on the front panel will come on. The front panel light will go out immediately when the front panel switch is released; however, the control panel reply indicator will remain on for the prescribed 15 seconds after the last reply.

 (5)  
Monitor

 (a)  On transponders with fault indicators, the R/T monitor circuits check the transponder for proper output power, frequency, mode decoding, clock frequency, and shift register fill-up. The monitor circuit supplies signals to these to provide an indication of the operational status of the transponder. The ANT monitor circuits check the dc resistance of the antenna and coaxial cable. If the circuits detect a dc resistance greater than 200 ohms, the ANT fault indicator will be activated. The ANT fault indicator is reset by depressing the RESET switch on the transponder front panel. The R/T fault indicator is reset from inside the transponder.
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