SN 301-350  CONFIG 2  03 Page 5  Apr 25/8634-55-00

TUNING
SERVO
CIRCUITS

fo/12

SOLENOID BAND SWITCHING SIGNAL

TUNING MOTOR CONTROL

TUNING SERVO ERROR

115V AC
ESSENTIAL
RADIO BUS

DME 1 AC
28V DC
ESSENTIAL
RADIO BUS
DME 1 DC

P7 ELECTRONIC CIRCUIT BREAKER PANEL MAIN FRAME POWER SUPPLIES, PROTECTION & CONTROL CIRCUITS
VOR/OFF

ILS
DME RANGE SERVO CIRCUITS

1A14
VOR/ILS - DME
TEST SWITCH

SELF-TEST
VHF NAV CONTROL PANEL
DME 1 Block Diagram Figure 3 (Sheet 1)

RANGE MOTOR CONTROL
RANGE VOLTAGE
CLUTCH SIGNAL
4046

RANGE

EFFECTIVITY

CONFIG 2
SN 301-350
727 Page 6
34-55-00
Apr 25/86
XMIT/REC
ANTENNA

CAPT & F/O RMI
DME 1 Block Diagram
Figure 3 (Sheet 2)

EFFECTIVITY CONFIG 2 SN 301-350

727 Page 7
34-55-00
Apr 25/86
 (8)  
The detected, amplitude-modulated video pulse output, representing the beacon distance reply signal, is sent to decoder module lA5 which produces a single-pulse output for those reply pulse pairs which are properly spaced. Subsequent AGC and identity circuits produce a decoded, amplitude-modulated, single-pulse output to the range circuits and an audio tone beacon identification signal to the flight interphone.

 (9)  
The single, decoded, distance reply pulse from the decoder is coupled to the range circuits which determines the distance measurement. To accomplish this measurement, the range circuits must establish a basic time reference, initiate distance interrogation pulses, detect the reply pulses, and determine the elapsed time between the transmission and reception. The elapsed time interval is then converted to distance readout in nautical miles and displayed on the DME indicators.

 (10)
 Warning flag circuits hold the DME indicator warning flags out-of-view only when accurate distance information is being tracked and displayed. The readouts are covered during standby mode, search mode (when the DME completes the prescribed memory cycle without achieving lock-on), channeling mode, warmup or equipment failure.

 (11)
 When the test button is depressed on the VHF NAV control panel, the self-test circuits function to break lock-on, cancel memory and slew the search drive to lock on at 94.4 miles. When ground potential is removed, the DME resumes normal operation. The self-test circuits provide an accuracy of | 0.2 nautical mile.

 (12)
 115- and 28-volt power is routed directly to the DME R/T unit from the circuit breakers. The DME power supply provides a variety of voltage sources for the interrogator circuits. A built-in time delay maintains the DME in a warmup mode for up to 90 seconds after power is applied.

 (13)
 Mode Description

 (a)  
When the receiver circuits of the interrogator detect no signals above a usable level, the signal controlled search circuits place the interrogator in the standby mode. When received signals increase to a usable level, full power is applied to the interrogator for normal operation.

 (b)  
When the interrogator loses lock-on with a ground station or is channeled to a new station, signal controlled search circuits begin searching outward from 0 miles to the range limit. If this occurs while the indicator is reading less than 50 miles, the search cycle returns to slightly below zero before search begins. This assures that the correct reply is encountered first (for echo protection).

 (c)  
If the interrogator is channeled to a station with no DME, the search cycle stops (if indicator reads less than 50 miles) or returns to zero and stops (if indicator reads more than 50 miles).

 EFFECTIVITY

 SN 301-350  CONFIG 2  03 Page 8  Apr 25/8634-55-00
 (d)  
If the interrogator detects that it is locked on an echo reply pulse, a protection circuit causes the interrogator to break lock-on, turn off the transmitter and returns the search cycle to zero miles where normal outbound search is begun. Echo protection is disabled when the interrogator is in search mode or standby. The echo protection circuit is self-monitoring and will activate the indicator flag in case of most failures.

 (e)  
Should the interrogator receive two ground stations on the same channel, the stronger one is presented. If both stations have equal strength, the aural identification is garbled to give a positive indication of this condition.

 B. Control
 (1)  
The DME system is turned on by closing DME 1 - AC, DME 1 - DC, DME 2 - AC and DME 2 - DC circuit breakers on electronic circuit breaker panel P7 and placing the mode switch on the VHF NAV control panel to the DME position. On the VHF NAV control panel operate the frequency selector to tune the desired VOR/DME channel.
　
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