TEST J1,J210 MHZ K9 MAN/AUTO TUNE21
D2165A
J3-J7 1 MHZ J8-J12 0.1 MHZ G9
DN/RT
FMCA104
DATA IN
DME J13,J14 0.05 MHZA11 H9
D26317G6773
18 M1175 FMC-P (E1-2)
M1163 D/A ADAPTER-1 (E1-2)
1 DISPLAY TEST VALUES SHOWN 2
SECONDS ON, ONE SECOND OFF
DME System Schematic
Figure 3 (Sheet 1)

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M1163 D/A ADAPTER-1 (E1-2) M1175 FMC-P (E1-2) M924 FCC-A (E1-3)
DME System SchematicFigure 3 (Sheet 2)

A 737-300/400/500MAINTENANCE MANUAL
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A
737-300/400/500MAINTENANCE MANUAL
 (e) Transmit 1) Manually, DME frequency selection is accomplished by selecting a VOR/ILS frequency on the VHF NAV control panel. This operation provides a 2 of 5 coding system which is sent to the DME interrogator. 2) A microprocessor converts the 2 of 5 coding input to BCD (binary coded decimal), which is inputed to the range and velocity LSI (large scale integrated circuits). The range and velocity LSI tunes the frequency synthesizer. The frequency synthesizer tunes the voltage control oscillator (VCO), which is the first stage of the transmitter chain, to the frequency chosen by the VHF NAV channel selector. 3) The DME frequency is selected by the FMC when the AUTO mode is selected on the VHF NAV control panel.
 4)  The first stage of the transmitter, the voltage control oscillator (vco), develops rf power on a frequency between 1025 and 1150 MHz. This frequency is determined by the frequency synthesizer. This rf power is applied to the buffer amplifier where it is amplified to approximately 200 milliwatts. The buffer amplifier rf output is used to drive the first power amplifier. The pulse modulator applies pulses to the power amplifiers when a transmit command is received from the range and velocity LSI. The first power amplifier provides an output power of at least
 1.5 watts to the second power amplifier which in turn amplifies it to at least 13 watts. The third power amplifier amplifies it to at least 90 watts. The final power amplifier amplifies this 90 watts to at least 300 watts. The power from the transmitter is then sent to the duplexer to be routed to the DME antenna.
 (f) Receive
 1)  Incoming received energy from the antenna is routed through the duplexer and applied to the preselector filter. The output of the preselector filter is then applied to the mixer where it is mixed with a sampling of the transmitter power provided by a tap on the VCO output. The output from the mixer, is then applied to the IF amplifier which provides amplification and selectivity. The IF amplifier has a narrow band (high selectivity) and a wide band (normal selectivity) output. Both outputs are applied to the video processor. The video processor determines if the received signal is on frequency, is of sufficient amplitude, if the pulses are properly spaced for the frequency selected, provides AGC voltage and decoded video signals. The AGC voltage is applied back to the IF amplifiers and the decoded distance is applied to the range and velocity LSI.
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737-300/400/500MAINTENANCE MANUAL
 (g)  Distance Computation
 1)  Range and velocity computation is performed entirely within two LSI circuits. The Range LSI performs the primary task of computing the DME range; however, it also performs several secondary tasks: generating the transmit pulses, decoding the video, and generating the suppression and detector sample pulses. The velocity LSI is coupled with the range LSI to produce a second-order range computing circuit. This means that if the received signal is temporarily lost (example, during an identification burst) then the computed range continues to move at the same velocity that it had before the signal was lost, so when the signal returns there is no abrupt change in data.
　
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