K73512
DME System Component Location 5C8
34-55-02 Figure 1 Feb 20/86
Page 2
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
2. Control Panel
A. Each ground station installation is assigned a specific VHF/NAV frequency and associated DME frequency. Since the DME system operates in conjunction with the VOR/ILS navigation system, controls for both are located on the same control panel and consist of two VHF/NAV frequency selectors and a DME function selector. DME system No. 1 is controlled from the captain's control panel and DME system No. 2 from the first officer's control panel. The position of the transfer (TFR) switch determines which displayed frequency is to be used. The DME function selector is a three position switch with STBY, NORM and O'RIDE positions. A test button in the DME control switch provides a system self-test.
3. Antennas
A. The DME antennas are conventional omni-directional blade of flush-mounted units that are connected to the interrogators by coaxial cable. The No. 1 system antenna is on the bottom centerline of the fuselage at body station 468. The No. 2 system antenna is on the bottom centerline of the fuselage at body station 580.
4. Interrogator
A. The interrogator units are standard 1/2 ATR short assemblies which mount (side-by-side) on a shelf in the electrical/electronic compartment (Fig. 1).
B. Each interrogator unit provides the capability to compute the time interval between transmitted and received pulse-pair signals, convert this time difference to distance, and apply this information to the HSI MILES indicator and RDDMIs.
C. The DME interrogator sends a suppression pulse to the ATC radar transponders during the transmission and reception of signals. Similarly, the ATC transponders, when transmitting, suppress the DME interrogators.
D. On airplanes with performance data computer installed, the DME interrogator also provides distance information to the performance data computer by an analog pulse pair and DME valid signal output (Ref 34-18-0).
E. The interrogator operates in the frequency range of 962 to 1213 MHz. There are 252 transmitting channels between 1025 and 1150 MHz and two 126 channel receiving bands from 962 to 1024 and 1151 to 1213 MHz.
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Oct 20/84 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 34-55-02 Page 3
5. Indicators
A. Dual DME indicators are installed in each RDDMI as shown in Fig. 1.
B. The DME indicator is a magnetic wheel type counter which receives a BCD pulse signal from the related DME system. Logic in the indicator decodes this signal and converts it into a digital display miles-to-go to the interrogated ground station.
C. Warning flags (shutters) cover the distance display when the system is inoperative.
6. Operation (Fig. 2)
A. Functional Description
(1)
The DME interrogation signal from the airborne equipment consists of pulses of rf energy transmitted in pairs. The pulses in a pair are spaced 12 microseconds apart for the assigned 126 DME channels. A reply signal is transmitted from the ground station for each interrogation pulse-pair from the airplane equipment. The reply signal consists of pulse pairs with the same characteristics as the interrogation pulse pairs, except for rf frequency. The reply pulse rf frequency is 63 MHz above or below the interrogation pulse frequency. The exact relationship of the frequencies depends upon the particular DME channel selected. The DME system searches for synchronous pulses and provides valid distance data when locked on and tracking.
(2)
DME channel selection is enabled by tuning to a VOR/ILS frequency on the VHF NAV No. 1 or 2 control panel (Fig. 2), this operation tunes the 2x5 tuning matrix, which tunes the frequency synthesizer in the interrogator. 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. Power from the vco is sampled and applied through divider-mixer to the tuning command converter. The tuning command converter divides the frequency by a factor which is controlled by the VHF NAV channel selector. The resultant frequency is compared with 31.25 kHz, and an error signal developed. The error signal is applied to the loop filter where it controls the tuning voltage applied to the vco. The vco is then corrected until there is no detected error between the vco frequency and the 31.25-kHz reference oscillator. This operation is considered to be an endless "loop" as there is no definite starting point. When the vco has moved to the desired frequency and has stabilized the loop is considered to be "locked up".
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