(2)
DME channel selection is enabled by tuning the VOR/ILS system to a VOR frequency on the VOR No. 1 or No. 2 control panel (sheet 3, Fig. 3). This operation automatically tunes the DME through a 2 out of 5 bit coding system which is converted to BCD in the interrogator.
(3)
The BCD output is applied to the frequency synthesizer, the digital to analog converter and the control circuit. The control circuit decodes the BCD information and the resultant represents the DME frequency band of operation. The BCD frequency information and the frequency band information control the output from the frequency synthesizer. The signal is applied to the DME driver where it is multiplied, amplified, and pulse-modulated from signals received from the distance circuits. The DME driver output is applied to the transmitter section (Sheet 4, Fig. 3).
(4)
The modulator receives low-level pulse-pair trigger signals from the distance circuits and synchronously generates high-level shaped pulses for application to the transmitter. When the modulation pulses are present, the rf is applied to the diplexer where appropriate isolation of the transmitted and received signals are enabled. The signals are finally applied to the transmit-receive antenna and radiated to the ground station as interrogation signals.
(5)
Reply signals from the ground station are received at the antenna and applied to the diplexer where the signal is then directed to the receiver section of the interrogator. The receiver rf circuits also receive an input from the digital-to-analog (D/A) converter. This input is an analog tuning voltage which the digital-to-analog converter develops from the BCD frequency information at the BCD frequency converter output, and the frequency band information at the control circuit output. The tuning voltage tunes the receiver rf circuits to the DME receive frequency and also tunes the frequency and the frequency synthesizer output.
EFFECTIVITY
LH 011-099;
AF 121-122, 152-199; AZ 221-299; IB 406-499; CONFIG 3 SN 351-399; AT, GN, MD, 01 Page 17 AND VM ALL Apr 25/95
34-55-00
(6)
The IF receiver output consists of video pulses which are applied to the decoder. The decoder distinquishes between valid and invalid pulse-pairs, accepting only valid pulse pairs. It also generates an agc voltage which is applied back to the receiver rf and IF circuits.
(7)
The decoded video pulses are applied from the decoder to the distance circuits. The distance circuits measure the time between the leading edge of the first pulse of the interrogation pulse-pair and the leading edge of the first pulse in the reply pulse-pair. The time measurement is converted to an output consisting of BCD distance word bits. This output is combined with the video signal from the receiver circuits and applied to the ident/blanking circuit where the station identification signal is developed. This signal is then applied to the airplane flight interphone system for reproduction in the captain's and first officer's headphones.
(8)
The BCD distance word bits are clock-pulse synchronized and amplified into a 32-bit serial BCD word that is applied to the DME indicator. The data clock signal, and word sync signal, are also applied to the DME indicator. The 32-bit digital word is loaded into the indicator, and decoded to provide the decimal-number display.
(9)
The logic circuits also provide a flag output signal. The flag signal is a stimulus for the flag alarm circuit which results in the DME indicator showing blank (in lieu of numerals) when a malfunction occurs in the system.
(10)
The system has an automatic standby feature which places it in the standby mode when a ground station is not available or when the ground station signal is too weak to be usable. When this occurs, search operation stops, the transmitter is turned off, and dashed lines appear across the DME indicators. When a usable ground station signal reappears, the system automatically locks on and displays distance on the DME indicators.
(11)
With the DME test switch (on the NAV system test panel) depressed the distance circuits develop a synthetic reply pulse spaced to represent the test distance of zero nautical miles. The distance circuits measure the time between the interrogation and self-test reply pulse and apply appropriate distance information to the control circuits. The control circuits then develop the serial BCD word which is applied to the DME indicator. When the test is activated, the DME indicator reads blank for 1 second, followed by dashed lines for 1 second, then followed by 000.0 nautical mile readout.
C. AF 121-122, 157, 161, 183-184; Functional Description (Fig. 3)
EFFECTIVITY
LH 011-099;
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本文链接地址：747飞机维护手册AMM CHAPTER 34 - NAVIGATION 第34章导航4(96)
