SN 301-350 CONFIG 2 01 Page 4 Apr 25/8634-55-00
(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)
When the distance from a particular ground station is desired, the frequency selector on the VHF NAV control panel is set to the assigned frequency channel. Operation of the frequency selector activates the AVQ-70 tuning servo circuits which tune frequency synthesizer lAl, frequency multiplier lA2, and RF head lA3 to the proper frequencies. Frequency synthesizer lAl generates a signal which is exactly 1/12 of the required DME transmitting frequency.
(5)
The output of frequency synthesizer lAl is coupled to frequency multiplier lA2 where it is quadrupled in frequency, amplified, and passed to rf head lA3. Since the frequency multiplier also receives the modulator pulse output, the resulting output is a pulse-modulated signal at a carrier frequency which is 1/3 of the desired transmitting frequency. The pulsed output from the frequency multiplier is first applied to the RF head tripler cavity where it is tripled in frequency and then fed through the first, second, and final amplifier cavities to the antenna via the duplexer and low-pass filter.
(6)
The selected ground-based beacon station receives the interrogation signals and transmits distance reply signals on the associated transmitting channel which is 63 MHz removed from the receiver frequency.
(7)
The beacon reply signals picked up by the DME antenna are fed through the low-pass filter and the duplexer to a tuned preselector cavity which allows only the selected receiving frequency to reach the crystal mixer. The crystal mixer also receives a CW local oscillator signal from the rf head tripler cavity. This signal, when heterodyned with the receiving frequency, produces a 63-MHz intermediate frequency output to IF amplifier lA4. The IF amplifier first amplifies and then heterodynes the 63-MHz input signal with a 51.7-MHz local oscillator signal to produce an 11.3-MHz signal which after suitable amplification is fed through a video detector.
EFFECTIVITY
