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 B.  Warning flags (shutters) are held out of view by a warning flag voltage and cover the DME indicators when the system is inoperative, in the search mode or in the standby mode (Ref Fig. 1).
 6.  Operation
_________
 A.  Functional Description (See figure 3.)
 (1)  The DME system is used in conjunction with either TACAN, VORTAC, or VOR/DME ground stations. Each station operates on one of 252 alloted frequency channels. The airborne distance measuring equipment interrogates the ground station using one of the 126 frequencies available between 1025 to 1150 MHz (selected at 1-MHz intervals) and the ground station replies at a frequency 63 MHz above or below the interrogation frequency depending on the channel being utilized. These channels (designated lX and lY through 126X and 126Y) are associated with 252 VOR/LOC frequency allocations (34-31-00).
 EFFECTIVITY
 LH 001-010,031-050,071-081;

 AF 101-120, 123-151;  AZ 201-219;  CONFIG 1 IB 405  01 Page 3
   Apr 25/95

34-55-00

28V DC RADIO BUS NO. 2
115V AC
RADIO BUS NO. 2
115V AC ESSENTIAL
RADIO BUS 28V DC
ESSENTIAL RADIO BUS
BREAKER PANEL
CAPTAIN'S DME DISPLAY FIRST OFFICER'S DME DISPLAY
DME System Interface Diagram
Figure 2

EFFECTIVITY CONFIG 1
LH 001-010,031-050,071-081; 799 Page 4
AF 101-120,123-151; AZ 201-219;

34-55-00
 Apr 25/95IB 405
 (2)  
DME channel selection is accomplished by tuning a VOR/LOC frequency on the VHF NAV control panel which automatically tunes the DME interrogator to the DME frequency asociated with that channel. Tuning is controlled by two resistance bridges (one coarse and one fine), each with half in the control panel and half in the interrogator. Switching to various resistances in the control panel causes bridge imbalance which activates a motor-driven switch, rebalancing the bridge and performing all necessary tuning in the interrogator within 10 seconds.

 (3)  
Current VOR/LOC channel usage covers only 200 channels and, since DME tuning is slaved to VOR/LOC channel selection, DME channel utilization is also limited. VOR/LOC tuning from 108.00 MHz to

 111.95 MHz covers 80 channels where the DME search range is limited to 50 nautical miles (short range search). The remaining 120 channels (VOR/LOC tuning from 112.00 MHz to 117.95 MHz) have a search range of 192 nautical miles.

 (4)  
Transmit and Receive Functions

 (a)  
In the frequency synthesizer (figure 3), the tuning process controls the outputs of a high- and low-frequency oscillator which are mixed to produce an output at 1/24 of the selected DME channel transmit frequency. This output is applied to the frequency multiplier.

 (b)  
The signal from the mixer in the frequency synthesizer is applied to a tuned filter that selects the sum frequency of the oscillator signals. The filtered signal is then amplified and applied to a X24 multiplier to increase the frequency of the signal to the transmit frequency of the selected DME channel. The output of the multiplier is applied to the power amplifier and to the mixer in the receiver circuits as the first injection signal.

 (c)  
The plate-pulsed power amplifier increases the power of the signal from the frequency multiplier to the required transmit level. The pulse-modulated output of the power amplifier is applied to the antenna through a diplexer, which allows one antenna to be used for both transmitting and receiving.

 

 EFFECTIVITY
 LH 001-010,031-050,071-081;

 AF 101-120, 123-151;  AZ 201-219;  CONFIG 1 IB 405  03 Page 5
   Apr 25/95

34-55-00
 (d)  
The ground station reply signal is applied to the receiver preselector through the diplexer. The preselector provides image rejection and filters the received signal. The output of the preselector is heterodyned in the mixer with the injection signal from the frequency multiplier and applied to the first IF amplifier. Since the frequency of the DME transmit (injection) signal is always 63 MHz away from the ground station reply signal, the mixer has an output at 63 MHz which is selected as the first IF. The output of the first IF amplifier is converted to 10.7 MHz and applied through the second IF amplifier to the signal detector. The signal detector rectifies and filters the IF signal to provide an output of video pulses which are applied directly to the deblocker amplifier in the decoder, and through the video amplifier to the decoder.

 (e)  
The detected pulses from the receiver are applied to a comparator in the decoder. The output of the comparator consists of pulses of fixed amplitude (for input pulses large enough to switch a tunnel diode) which are amplified and applied to the pulse-shaping multivibrator. The output of the amplifier is shorted by the suppressor amplifier when a pulse is received from either the suppressor multivibrator in the interrogation generator, the air traffic control system, or the other DME system. This suppression disables the decoding circuits when the interrogator or other pulse equipment is transmitting.
　
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