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VHF NAV unit-1 and unit-2 are located on the electronic equipment bay on rack
E3-4. The unit is an airborne receiver that processes ground station transmission
signals, the output of the unit are VOR bearing, localizer deviation, and glide
slope deviation.
[3] The VOR ground station transmitted signal is an RF carrier ( frequency
range 108.00 through 117.95MHz), the RF carrier is modulated by a 9960Hz
subcarrier. The subcarrier is frequency modulated (FM) by a 30Hz oscillator,
after modulation, the 9960Hz subcarrier frequency is changing from 10440 Hz
to 9480Hz 30 times per second. The VOR station transmit antenna points at any
instant is very carefully coordinated with the phase of the 30Hz FM signal at
that instant. When the antenna is pointed north, the phase of the 30Hz FM signal
is peaked positive. When the antenna is pointed south, The phase of the 30Hz
FM signal is peaked negative. It is this careful coordination between the 30Hz
FM signal and the direction which the antenna is pointing that makes it possible
for the airplane receiver to establish its position with respect to the ground
station transmitter. Figure 26-3 illustrates the coordination between the phase of
the FM signal and the direction the antenna is pointing. Usually, the 30Hz FM
signal is called reference signal.
Fig. 26-3 subcarrier transmitted
[4] As received, however, the signal has an additional modulation. This
additional modulation is a 30Hz amplitude modulated (AM) signal which result
from directional antenna rotation. For each rotation of the antenna, the airplane
receiver sees a very strong signal when the antenna is pointing at the receiver.
Which diminishes as the antenna moves away from that position. The amplitude
of the signal as seen by the receiver is at a minimum when the antenna has
moved half a turn and is pointing away from the receiver. The amplitude of the
received signal increases to a peak during the next half turn, when the antenna is
once again pointing at the receiver. So although the VOR ground transmitter
does not amplitude modulate its transmitted 9960Hz subcarrier, the airplane
receiver receives the total signal including 30Hz FM signal and 30Hz AM signal.
Usually, the 30Hz AM signal is called variable signal. Figure 26-4 illustrates the
30Hz AM signal and 30Hz FM signal as they are seen by a receiver located
directly north of the VOR station. They are shown in phase with each other.
Fig 26-4 subcarrier received
[5] In figure 26-4, the upper waveform is one-half of the complete 30Hz AM
envelope of 9960Hz subcarrier. The amplitude of the AM signal is maximum
when the directional antenna is pointing north toward the receiver. This is the
same time at which the transmitter is peaking the FM signal positive. The
amplitude of the AM signal falls off as the antenna rotates ,when the antenna is
pointing south, the received signal is minimum in amplitude. At this time, the
FM signal is peaked negtive. As the antenna continues to rotate, is build up its
amplitude toward another peak which is reached when the antenna is pointing
north and the FM signal is also peaked positive.
[6] The detected AM signal (variable signal ) and detected FM signal (reference
signal ) are exactly in phase if the airplane receiver is located exactly on the
north radial. If the airplane receiver should be off to one side of the north radial,
these signals could not be exactly in phase. the fundamental task of a VOR
receiver is to compare the phases of the detected AM and FM signals, in order to
discover on which radial the airplane receiver is located. Processing of the
phase-difference signals results in output signals that are used to position VOR
bearing pointers. In the lower portion of figure 26-5 , we know the FM signal
(reference signal )is peaked positive when the antenna is pointed north. The AM
signal (variable signal )is peaked positive when the antenna is pointed east.
Therefore, the AM signal lags the FM signal by 90 when the airplane receiver
is on the 90 radial. If the airplane receiver is on 45 radial, the AM lags the FM
by 45 . If the receiver is located west of the VOR station, since the AM signal
cannot peak positive until the antenna points at the receiver, the antenna will
have rotated through 270 beyond its north pointing position before the AM is
peaked positive. The AM lags the FM by 270.
Fig 26-5 VOR operation and theory
EXERCISE
A. Find out the explanations of the following words in you "English-Chinese
technical tictionary"
1. deviation [ P1-42 ]
2. localizer [ P1-93 ]
3. glide slope [ P1-69 ]
4. modulation [ P1-102 ]
　
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