AF 121-122, 152-199; AZ 221-299; IB 406-499; CONFIG 3 SN 351-399; AT, GN, MD, 01 Page 18 AND VM ALL Apr 25/95
34-55-00
(1)
The DME system is used in conjunction with either VORTAC or VOR/DME ground stations. The airborne distance measuring equipment interrogates the ground station using one of the 126 frequencies available between 1025 to 1150 MHz (selected at 1-MHz intervals) and the ground station replies at a frequency 63 MHz above or below the interrogation frequency depending on the channel being utilized. Current VOR/LOC channel usage covers only 200 channels and since DME tuning is slaved to VOR/LOC channel selection, DME channel utilization is also limited. Channels (designated 17X thru 59Y and 70X thru 126Y) are associated with the 200 VOR/LOC frequency allocations (34-31-00).
(2)
DME channel selection is accomplished by tuning to a VOR/LOC frequency on the VHF/NAV control panel (Fig. 3, sheet 1). 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 2, 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)
Replay 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 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.
(6)
The IF receiver output consists of video pulses which are applied to the decoder. The decoder distinguishes between valid and invalid pulse-pairs, accepting only valid pulse pairs. It also generates an ac voltage which is applied back to the receiver rf and IF circuits.
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 19 AND VM ALL Apr 25/95
34-55-00
(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. The 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 the 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)
On airplanes with 860E-3 interrogators installed, the monitor circuits provide fault monitoring of the interrogator unit and the DME antenna. A binary coded fault indicator located on the monitor card provides indication that enable precise isolation of malfunctioning circuits within the interrogator unit. The monitor card also contains amplifier circuits that drive the fault indicator on the interrogator front panel (R/T, IND and ANT). These indicators are magnetic latching indicators, that when initially set, must be reset to remove the indication. These fault indicators enable isolation of a faulty receiver/transmitter interrogator unit or antenna. The IND fault indicator is not used.
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