(3)
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 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.
(4)
Incoming receive energy from the antenna is routed through the duplexer and applied to the pre-selector filter. The frequency synthesizer applies a tuning voltage to the pre-selector filter to tune it to the HI or LOW frequency band. The output of the pre-selector 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.
(5)
The video processor determines if the received signal is on frequency, is of sufficient amplitude, if the pulses are properly spaced for the channel selected, provides tone identification, AGC voltage and decoded video signals. The tone identification is applied to the audio access unit of the flight interphone system (Ref 23-52-0). The AGC voltage is applied back to the IF amplifiers and the decoded distance is applied to the ranging circuits.
(6)
The ranging 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.
(7)
When the DME is searching, the range gate is caused to slew out in distance (starting at zero miles) by stepping clock pulses into the memory counter until a reply is encountered. When a pulse pair from the ground station is received, clock pulses are sent to the memory chain counters for the remainder of that counter cycle. A short pulse, called the range gate, is generated at the time the memory counter reaches its full capacity. The occurrence of this gate should coincide with the time or arrival of a replay pulse pair from the ground station.
(8)
Every interrogation scan will result in either the presence or absence of a pulse pair at the time of the range gate. Interrogations that result in the occurrence of replay in the range gate are termed valid, while the absence of a reply is termed invalid.
(9)
Two separate counters perform the search-track decision. The valid counter consists of a BCD divide by ten counter and the invalid counter consists of two flip-flops. The valid counter requires a total of ten valid replay input pulses to produce a decision to track, while the invalid counter will produce a decision to continue searching out if four consecutive interrogations fail to result in a valid reply. Therefore, if a series of interrogations result in ten replies before four consecutive interrogations fail to produce a reply, the decision to lock on and track will occur.
(10)
Once locked on, the range gate and the received reply are constantly compared to detect any difference between them. If a difference is noted, the number of clock pulses sorted in the memory counter is updated so that the range gate and the received reply are once more in coincidence.
(11)
The DME distance data sent from the interrogator to the RDDMIs are 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 synchronous with it. The distance information is in the form of binary ones and zeroes, and the distance is coded in Binary Coded Decimal (BCD) form. A BCD code contains four positions, each one clock cycle long, each of which may either be a high (one) level or a low (zero) level.
5C8
34-55-02 Page 4 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. Feb 20/86
