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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
 (2)  Interrogator
 (a)  
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 or 36 microseconds apart for the assigned 126 DME transmit frequencies. The transmit frequencies are in the band of 1025 to 1150 MHz. Each frequency in this band may have x or y pulse pair spacing. The x pulse pair spacing is 12 microseconds and the y pulse pair spacing is 36 microseconds. This allows the DME to be tuned, using the 252 VHF NAV frequencies. 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 frequency is 63 MHz above or below the interrogation pulse frequency. The exact relationship of the frequencies depends upon the particular DME frequency selected.

 (b)  
The ground station continuously transmits random pulses called squitters, whether or not it is being interrogated. When the interrogator is first turned on, it is in the signal-controlled search (standby) mode, the transmitter is off and the receiver counts the number of squitter pulses received from the ground station. When the airplane is within range of the ground station, sufficient squitter pulses will be counted by the receiver (700 minimum), causing the interrogator to go into the search mode.

 (c)  
In the search mode pulse pairs are transmitted to the ground station at the rate of 100 pulse pairs per second. The ground station receives the interrogation and after a 50-microsecond delay, transmits a reply to the airplane. When synchronous reply pulses are received, the interrogator switches to the track mode, the pulse repetition frequency is reduced to 25 pulse pairs per second, and the reply delay time is converted to distance in nautical miles.

 (d)  
Memory mode is characterized by temporary or permanent loss of reply signal. If the signal is lost for greater then 11 to 15 seconds, the system changes back to search mode. If the signal is reacquired during memory (11 to 15 seconds), the system returns to the track mode. During memory mode, the indicator continues to display distance update at the same rate as when in track mode.

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
D161B115V AC
26V AC
12 POWERELEX PWR-1 DME-1 DC
SUPPLY
13 28V DC
BAT BUS
VOR/LOC-1
5

P18 LOAD CONTROL CENTERVOR/ILS
0-5V AC
P2 CONTROL PANEL FREQ
COMM D263
MANUAL
AUTO
18888 1 18888 1
20
PANELTEST
21
LIGHTS
49
RECORDER/

RECORDER/D303
DISPLAYDISPLAYDISPLAY43
NAV
DRIVER

BACK-
DRIVER + -
+-
44R304 VOR/ILS NAV
GROUND
LIGHTING
XFR RLY-1 (53-4)MAN
33TEST 35
10 MHZ 34
33
D161B DASH
CLOCK
45 ENABLE
3536
46 37
1 MHZ
24SYNC
27 NC 38 48 AUTO2 X 5 TUNING
39 MAN
29
60 A 25NC
28
MICRO 21
20DATA
PROCESSOR 58
MHZ 0.126
22 BRT/DIM
23 59 47 24

LIGHTING VALID
40 RLY
 MHZ 0.01
2325
D263
TEST

W
AUTOCOMMON
CONT 14 26
K1
W

AUTO
D303
TUNE ARM

19
48
K3

46
TEST

32MASTER45
DIM/TEST
16 NC
D263
1,2
ENABLE

D1795A
3-7
K28-1242 MAN/AUTO TUNE
UNSWITCHED OUTPUT
AGILITY 13
53 DME TEST M164 DME-1 INTERROGATOR (E2-2)2-OUT-OF-5
D303 ENCODER1,5D357M924 FCC-A (E1-3)
6-10
B1-14 TUNING
11-1517,1822
SWITCHED
B15
36
VOR

D2165A
M154 VHF NAV D2179B
UP/LTG4 FREQ SEL COMMON
　
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