Honeywell International Inc. DMA-37B2 Manuale Utente

Page 17

10 Oct 2007

DMA-37B DME System

© Honeywell International Inc. Do not copy without express permission of Honeywell.

(1) The DMA-37B DME Interrogator transmits a pair of pulses through the antenna on one 

frequency and waits for a ground station reply on a frequency offset by 63 MHz. The 

ground station replies to this interrogation 50 microseconds later, somewhat like a 

delayed radar reflection.

(2) The 50-microsecond delay allows completion of the interrogation. If not delayed at close 

ranges, the returned signal would come back to the receiver before the transmitted 

pulses have left the aircraft antenna. When a DME unit measures 50 microseconds from 

transmit time until the beginning of the received pulse, the aircraft is at 0.0 mile range.

(3) Like radar, the unit calculates the distance from the station based on speed of the 

transmitted pulses through the air, divided by time. Speed of signal propagation equals 

one nautical mile per 12.369 microseconds.

(4) Each DME unit must distinguish its own pulse return from other DME returns or squitters 

(unsolicited replies emitted by the ground station). To do this, reply/squitter distances 

from multiple interrogations are saved and compared to determine actual reply distance 

to the ground station. The DME is capable of simultaneously maintaining track of up to 

five different ground stations that are tuned by a Flight Management System.

(1) RF Module

(a) This module consists of a single PC board that is mounted in a closed aluminum 

chassis. It contains the transmitter, receiver, circulator and a low-pass filter.

(b) The solid-state transmitter contains a six-stage amplifier followed by a 700 W 

(typical) final amplifier. The pulse modulation is applied to the first six stages of 

amplification. It is tuned to frequencies in the range 1025 to 1150 MHz.

(c) The single-conversion superheterodyne receiver consists of a varactor tuned 

preselector, 1-GHz preamplifier, mixer, SAW band-pass filters, and a 63-MHz 

logarithmic IF amplifier. The preselector is tuned to frequencies in the 962 to 1213 

MHz range.

(d) The circulator routes the transmitter output to the antenna and the antenna received 

signal to the receiver. It provides isolation to protect the transmitter output amplifier 

devices from reflected signals due to high VSWR conditions.

(e) The low-pass filter prevents internal transmitter harmonics from being passed to the 

antenna.

(2) Synthesizer/Modulator Assembly

NOTE: On the -1101 and -1212 units, the synthesizer/modulator assembly has 

been combined into the RF assembly.

(a) This module contains the digital frequency synthesizer and transmitter modulator 

circuits.

(b) The digital frequency synthesizer supplies an RF signal used both as the local 

oscillator input to the receiver mixer and as the low-level CW signal into the 

transmitter. This circuit consists of a phase-locked loop and VCO which is tuned to 

the desired channel frequency by a serial tuning word supplied by the video 

processor module.