(d)
The control panel can be returned to the MANUAL mode by pressing the AUTO/MANUAL switch/indicator.
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737-300/400/500MAINTENANCE MANUAL
(4) Test Functions
(a)
When the airplane master test switch (P2 Panel) is actuated, test relay K3 is energized. The AUTO and MANUAL annunciators are illuminated and a test discrete is applied to both displays. They indicate 188.88, alternating 2 seconds on and 1 second off.
(b)
VHF NAV and DME tests are enabled by grounds from the test switches. When a VOR, UP/LT, or DN/RT test is initiated, the flight management system interlock is interrupted, which will disengage auto tuning of the DME system if auto tuning is in operation.
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A 737-300/400/500MAINTENANCE MANUAL
MANUAL TUNING AUTO TUNING
MANUAL OR AUTO SELECTED ON VHF NAV CONTROL PANEL
RDDMI EHSI (VOR/ILS MODE)
WHEN AUTO IS SELECTED ON THE VHF NAV CONT PNL, DASHES ON THE RDDMI INDICATE NCD OR AGILITY TUNING.
DME Distance Displays Figure 4
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IN ORDER TO SEE TEST ON EHSI, SELECT MAN ON THE
DME SYSTEM SELF-TEST
VHF NAV CONT PNL-1, SELECT VOR/ILS ON THE EFIS
CONT PNL-1, SELECT NORM WITH THE VHF NAV XFER SWITCH.
DME System TestsFigure 5
A 737-300/400/500MAINTENANCE MANUAL
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737-300/400/500MAINTENANCE MANUAL
D. Functional Description, Interrogator
(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 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.
(2)
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 (450 minimum), causing the interrogator to go into the search mode.
(3)
In the search mode pulse pairs are transmitted to the ground station at the rate of 90 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 22.5 pulse pairs per second, and the reply delay time is converted to distance in nautical miles.
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