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(2) System Operation (Fig 5)
(a)
The DME systems are shown on the schematic. The DME system receives 115 vac, 400 Hz power through DME L and DME R circuit breakers located on the overhead circuit breaker panel P7. A power supply circuit in the interrogator develops regulated DC and AC voltages.
(b)
Control of the DME is determined by the operational status of the FMC. When the FMC bus is active, the DME ARINC 429 receiver is switched to the FMC input. Otherwise, the receiver will switch to the online CDU for manual operation.
(c)
The ARINC 429 bus receiver contains two ports for frequency and mode control. Frequency tuning commands are received over the ARINC 429 data bus and stored in the range processor. The range processor, hereafter called the CPU, is the central processing unit of the DME interrogator. It controls DME operations by means of a stored program. The CPU is represented by the software portion on the diagram.
(d)
The CPU initiates ground station interrogations. It also computes range from the ground station reply pulse pairs after they are received and decoded by the video processor. The CPU also controls the pulse pair decoding, IF AGC, and ARINC 429 interface functions of the video processor. It also provides the monitor and self-test functions.
(e)
The CPU sends tuning commands to the receiver and to the frequency synthesizer. The synthesizer generates an IF signal at the desired transmit frequency in the 1025-1150 MHz band. The synthesizer uses a crystal oscillator as a frequency source. It uses a phase locked loop to select frequency. The L-band output of the synthesizer is sent to both the receiver and to the power driver as a local oscillator signal.
(f)
The power driver amplifies and modulates the L-band signal to generate a square waveform output signal. The waveform of the output signal is controlled by the modulator. Modulation is controlled by a square trigger pulse applied to the modulator from the range count network in the CPU. This produces an RF pulse pair output from the power driver with a 700-watt peak level.
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L33622
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115V AC
CMC DATA
FREQUENCY
TUNNING
AUDIO PAIRINGDISCRETE TUNNING
AIR/GNDRELAY
NC DME DATA OUTPUT
B81 DME INTEROGATOR (E2-3)
DME Interrogator Schematic
Figure 6
L33625
(g)
(h)
(i)
(j)
(k)
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The rf pulse pair output is connected to the antenna through a circulator. It is then transmitted as an interrogation to the ground station. The multiport circulator provides VSWR protection for the transmitter.The circulator directs received rf signals from the antenna to the input of the receiver. The receiver is tunable over the frequency range of 962 to 1213 MHz. Tuning is controlled by the VCO signal from the synthesizer. The receiver uses the local oscillator signal from the synthesizer to produce video pulse IF output signals. The IF output signals are routed to the video processor.The video processor decodes the IF video pulses and sends the data to the CPU when a valid pulse pair is received. It also sends the IF video signals to the IDENT filter. The filter detects the 1350 Hz tone and sends it to the audio management unit for flight deck audio presentation. This audio Morse code signal is detected and decoded into two digital words containing up to four characters that represent the ground station ident. This information is sent to the EIU over a ARINC 429 output bus for visual presentation on the ND.The received video data pulse pairs are applied to the range counter network in the CPU. This network latches and deposits the range count of the first pulse into the CPU memory. The range count of each additional received pulse pair is sequentially deposited into the memory until the end of the interrogation cycle. The memory now contains the distance (or time) of all random (squitter) and reply pulse pairs received during the interrogation cycle.The CPU then initiates a second interrogation, and the range counts of all decoded receiver pulse are again entered into memory. This process continues for as many interrogations as required. The CPU compares the range count of successive interrogations to locate the reply pulses that occur at the same distance. It then computes range from the selected reply and formats the distance and send the information to the ARINC 429 transmitter. The transmitter splits the information and sends separate outputs to the FMCs and the EIUs. The control word output format is shown in figure 6. 中国航空网 www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：747-400 AMM 飞机维护手册 导航 NAVIGATION 3(79)
