Page 59 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits
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sending coded messages or for controlling a device. For example, Morse code is
sent for providing alphanumeric information. Or the condition of a CW signal that is
on can be used to keep a relay switch in the "on" state.
To convey voice or music, however, the CW signal must be modulated in some
form. That is, the amplitude, phase, and/or frequency of the CW signal: must be
related to another signal. In this book, amplitude modulation (AM) is discussed
because the receivers we will be building are AM radios. Figure 4-2 shows a
lower-frequency signal that is a modulating signal representing an audio signal.
x = 2.04545. Y = 2 .41351
FIGURE 4-2 A lower-frequency signal such as a modulating signal.
The Amplitude-ModulatedSi'gnal
An amplitude-modulated (AM) signal is generated by changing the amplitude of the
RF CW signal (e.g., see Figure 4-1) with a modulating signal (e.g., see Figure 4-2).
At first glance, Figure 4-3 looks as if the CW RF signal from Figure 4-1 has been
multiplied by the audio signal, as seen in Figure 4-2. Well, not quite, but almost.
