K73588
DME System Interface Diagram  5C8 
34-55-0  Figure 2  Jun 20/85 
Page 4 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

5.  Indicators
A.  Two dual BCD (binary-coded-decimal), four digit indicators are installed in the airplane as shown in Fig. 1.
B.  Each indicator provides a dual display of distance information from DME system No. 1 and 2, with each half of the dual indicator operating independently of the other (Fig. 2).
C.  Each DME indicator receives a BCD pulse signal from the related DME system. Logic in the indicator decodes this signal and converts it into a digital display showing miles-to-go to the interrogated ground station.
D.  The display numerals are formed by energizing discrete segments of an incandescent, seven-segment, lamp matrix in response to the pulse-coded information. Any numeral (from 0 to 9) can be formed using a maximum of seven segments. For example: the numeral 0 uses six segments (Fig. 1).
6.  Operation
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. 3), 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".

(3)  
The first stage of the transmitter, the voltage control oscillator (vco), develops rf power of at least 100 milliwatts 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 1 watt. The buffer amplifier rf output is used to drive the first power amplifier. The pulse modulator applies 1200-volt pulses to the first power amplifier and 1800-volt pulses to the intermediate and final power amplifiers when a transmit command is received from the ranging circuits. The first power amplifier provides an output power of at least 20 watts to the intermediate power amplifier which in turn amplifies it to at least 200 watts. The final power amplifier amplifies it to at least 200 watts. The final power amplifier amplifies this 200 watts to at least 800 watts. The power from-the transmitter is then sent to the duplexer to be routed to the DME antenna.

(4)  
Incoming receive energy from the antenna is routed through the duplexer and applied to the preselector filter. The frequency synthesizer applies a tuning voltage to the preselector filter to tune it to the HI or LOW frequency band. 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 buffer amplifier 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.

(5)  
The video processor determines if the received signal is on frequency, is of sufficient amplitude, if the pulses are properly spaced for the channel selected, provides tone identification, AGC voltage and decoded video signals. The tone identification is applied to the audio access unit of the flight interphone system (Ref 23-52-0). The AGC voltage is applied back to the IF amplifiers and the decoded distance is applied to the ranging circuits.
　
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