signals on the carrier.
Notice that the modulating signal with the
greater amplitude causes a greater amount of variation to the carrier
frequency.
Notice also that the amplitude of the carrier remains
constant. Only the frequency is varied.
(3) A study of figures 2-12 and 2-13 shows that both the frequency and
amplitude of the modulating signal have an effect on the FM carrier.
(4) FM communication channels are located in the VHF range or higher. This
is because the bandwidth requirement is 50 kHz per channel as compared
with the 6 kHz for DSB and the 3 kHz for SSB. There is more room in
the VHF and higher ranges to accommodate FM.
(5) One characteristic of VHF and higher ranges is line-of-sight
transmission. Consequently, it is possible to use large numbers of FM
radio sets operating on the same frequency, provided they are
physically separated a sufficient distance to prevent interference.
(6) Another feature of FM radios is that lightning storms, diathermy
machines, automobile ignition systems, and interfering radio stations
have little effect on FM reception.
These noises cause an amplitude
variation of a radio wave.
An FM receiver can eliminate most of the
amplitude variations, so most of the noise is eliminated.
e. FM Transmitters.
There are two types of modulation that are used most
often in Army FM transmitters. In direct modulation, the RF wave is modulated at
its source (the oscillator). In indirect modulation, the RF wave is modulated in a
stage following the oscillator.
The FM wave created by either system can be
received by the same receiver.
The block diagram of a typical direct-modulation
transmitter is shown in A of figure 2-14, In B of figure 2-14, a typical indirect-
modulation transmitter is shown.
2-5.
RECEIVING SYSTEMS
a. Principles of Radio Reception.
The receiving antenna intercepts a small
portion of all radio waves passing by.
These waves are of many different
frequencies, each carrying its own information. The ideal receiver will select the
desired signal from among the many intercepted by the antenna, and will reproduce a
signal identical with the one originally modulated and transmitted.
b. Basic Radio Receiver. A radio receiver must be able to select the signals
from the various radio stations, extract the information, and produce the sound for
the listener.
Block diagrams of basic radio receivers are shown in figure 2-15.
In A, the antenna, detector, and headset provide a very weak signal and only one or
two stations can be heard. In B, there is an improvement in volume because an AF
amplifier has been added, but the ability to select stations is not much better.
In the block diagram in C, the signals from the antenna are amplified before
detection.
Some of the weaker stations can now be heard, and overall volume has
been improved.
309 L2
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