(4)  
Memory mode is characterized by temporary or permanent loss of reply signal. If the signal is lost for greater then 11.4 seconds, the system changes back to search mode. If the signal is reacquired during memory (11.4 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.

 (5)  
Transmit

 (a)  
Manually, DME frequency selection is accomplished by tuning to a corresponding VOR/LOC frequency on the VHF/NAV control panel (Fig. 3). This operation tunes the DME through a 2 out of 5 bit coding system which is converted to BCD in the interrogator.

 (b)  
The DME frequency is selected by the FMC when the AUTO mode is selected on the VHF NAV control panel.

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A 737-300/400/500MAINTENANCE MANUAL
 (c)  The BCD output is applied to the frequency synthesizer, and the control circuit. The control circuit decodes the BCD information and the resultant represents the DME frequency band of operation. The BCD frequency information and the frequency band information, control the output from the frequency synthesizer. The signal is applied to the DME driver where it is multiplied, amplified, and pulse-modulated from signals received from the distance circuits. The DME driver output is applied to the transmitter section. The power from the transmitter is then sent to the circulator to be routed to the DME antenna.
 (6)  
Receive

 (a)  Incoming received energy from the antenna is routed through the circulator to the preselector and mixer where they are filtered and heterodyned to produce a 63-MHz IF signal. The IF signal is then applied to the IF amplifier which provides amplification and selectivity. IF amplifier output is applied to the video processor and the AGC circuit. The processor checks the pulse pair spacing, and if correct, provides an input to the range computer gate, to the AGC circuit, and a 1350-Hz ID signal to the aural circuit.

 (7)  
Distance Computation

 (a)  
The ranging circuits, consist of integrated circuits, utilize digital computer techniques. Basically, it may be considered to be similar in operation to a digital electronic counter operating in the time interval mode, with the start pulse to the counter corresponding to the time of transmission of the interrogating pulse pair and the stop pulse to the counter corresponding to the time of reception of a reply pulse pair from the ground station.

 (b)  
The range computer converts the time interval between transmitted and received pulses into parallel BCD and analog pulse pair distance data.

 (c)  
The DME distance data sent from the interrogator to the distance indicators and the digital/analog adapter is on three lines: Clock, Data and Sync. The clock input is in the form of a square wave with a frequency of 11 |3.5 KHz. This clock runs continuously and the data and sync transmissions are sychronous with it. The distance information is in the form of binary ones and zeros, and the distance is coded in Binary Coded Decimal (BCD) form.

 (d)  
The normal range limit of the range computer is 200 nautical miles; however, as the O'ride control input is wired LO in this system, the range limit is extended to 390 nautical miles.

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737-300/400/500MAINTENANCE MANUAL
 (8)  Agility
 (a)  
A program ground identifies the interrogator as agility capable. If the flight management system detects a malfunction in one DME or when one interrogator is being manually tuned, it provides an agility DME discrete signal to the valid or the auto tuned interrogator. The discrete signal configures the frequency decoder and the range computer to process alternating frequency data. The frequency is alternated at 5-second intervals. The no-computed-data bits of the serial output are set, and the indicator displays dashes.
　
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