(2)  
DME channel selection is enabled by tuning the VOR/ILS system to a VOR frequency on the VHF NAV No. 1 or No. 2 control panel (Fig. 3). This operation automatically tunes the DME through the frequency synthesizer. The frequency synthesizer tunes the voltage control oscillator (vco), which is the first stage of the transmitter chain, to the frequency chosen by the VHF NAV panel channel selector. Power from the vco is sampled and applied through divider-mixer to card 1 - frequency control. Card 1 divides the frequency by a factor which is controlled by the VHF NAV panel channel selector.

 (3)  
The resultant frequency is compared with 31.25 kHz and an error signal developed. The error signal is applied to the loop filter where it controls the tuning voltage applied to the vco (and the receiver preselector filter tuning voltage). The vco is then corrected until there is no detected error between the vco frequency and the 31.25-kHz reference oscillator. This operation is considered to be an endless "loop" as there is no definite starting point. When the vco has moved to the desired frequency and has stabilized the loop is considered to be "locked up."

 EFFECTIVITY
 LH 011-099;

 AF 121-122, 152-199;  AZ 221-299; IB 406-499;  CONFIG 3 SN 351-399; AT, GN, MD,  01 Page 9 AND VM ALL  Apr 25/95

34-55-00
 (4)  
The first stage of the transmitter, the voltage control oscillator (vco), develops rf power of at least 100 milliwatts on a frequency between 1025 and 1150 MHz. This frequency is determined by the frequency synthesizer. This rf power is applied to the buffer amplifier where it is amplified to approximately 1 watt. The buffer amplifier rf output is used to drive the first power amplifier. The pulse modulator applies 1200-volt pulses to the first power amplifier and 1800-volt pulses to the intermediate and final power amplifiers when a transmit command is received from the ranging circuits. The first power amplifier provides an output power of at least 20 watts to the intermediate power amplifier which in turn amplifies it to at least 200 watts. The final power amplifier amplifies this 200 watts to at least 800 watts. The power from the transmitter is then sent to the duplexer to be routed to the DME antenna.

 (5)  
Incoming receive energy from the antenna is routed through the duplexer and applied to the preselector filter. The frequency synthesizer applies a dc tuning voltage to the preselector filter to tune it to the desired frequency within the selected frequency range. This output of the preselector filter is then applied to the mixer where it is mixed with a sampling of the transmitter power provided by a tap on the buffer amplifier output. The output from the mixer, is then applied to the IF amplifier which provides amplification and selectivity. The IF amplifier has a narrow band (high selectivity) and a wide band (normal selectivity) output. Both outputs are applied to the video processor which determines if the received signal is on frequency, is of sufficient amplitude, measures the 50% level of received pulses for ranging purposes, and supplies agc voltage to the IF amplifier.

 (6)  
The ranging circuits, consisting of integrated circuit cards 2 thru 6, utilizes 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. Ranging always starts at zero miles, searching out in distance for a reply from the ground station.

 (7)  
The point of inspection, or range gate, moves out at a high rate, equal to the round trip propagation time of an anticipated ground station reply. If a pulse pair from the ground station is encountered on one of these fast outward sweeps, the point of inspection is held at that range until it is determined whether the pulse pair is a valid range reply or a squitter pulse pair. The ranging circuits contain a 40,000 fit bounter, which serves as a memory or storage unit.

 EFFECTIVITY
 LH 011-099;

 AF 121-122, 152-199;  AZ 221-299; IB 406-499;  CONFIG 3 SN 351-399; AT, GN, MD,  01 Page 10 AND VM ALL  Apr 25/95

34-55-00

DME INTERROGATOR
DME No. 1 Block Diagram
Figure 3 (Sheet 2)

EFFECTIVITY LH 014-030, 057-070,
CONFIG 3083-099; AZ 221-299; 799 Page 11/12VM ALL
34-55-00
Dec 25/86
607608

115V AC DME 1 ESS RADIO BUS

ELEC CIRCUIT BREAKER PANEL, P7

DME TEST SWITCH
VOR/DME CONTROL PANEL
DME NO. 1 INTERROGATOR
DME No. 1 Block Diagram
Figure 3 (Sheet 3)
　
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