GENERAL AEROSPACE
Boeing 737-300
MAINTENANCE MANUAL
EFFECTIVITY
PW245
34-53-00GA
DRAWING # GA-BC17-ATC-MM
Page 6
Use and disclosure shall be in accordance with the proprietary legend located on the cover of this document
AUG 15/10
B.
Functional Description
(1)
Receiver
(a)
Received signals from the antenna are routed through the Diplexer/Low Pass Filter and applied to
the Pre-selector/Mixer. In the Pre-selector 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 pre-selector, 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 develops the
appropriate serial data output to be applied to the transmitter.
(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 is 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.
(b)
On all transponders when eight consecutive replies are completed, the reply indicator (transponder
and Control Panel) are energized.
C.
Mode-A and Mode-C Operation
(1)
There is no power switch on the ATC Control Panel. Power is applied to the system when the ATC-1
and ATC-2 circuit breakers on the P6 and P18 panels are closed.
(2)
Altitude Reporting.
(a)
The desired altitude reporting condition is set with the altitude reporting switch. The '1' position
causes the altitude reporting function to come on and sets the altitude data source to ADC 1. The
'2' position is not installed. The OFF position disables the altitude reporting function of the ATC
system.