Page 60 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

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FIGURE 4-3 An amplitude-modulated (AM) signal.

If the audio signal is faded to silence, we would get zero (flat line) in Figure 4-2,
which when multiplied by the RF carrier waveform of Figure 4-1 would result in a
flat line or zero as well (zero times anything is zero). But we know that when the
audio signal is faded to zero, we should just get the unmodulated waveform or CW

signal, as seen in Figure 4-1.
So the answer really is that the RF CW waveform of Figure 4-1 is multiplied by (1 +
the audio waveform of Figure 4-2) = the amplitude-modulated (AM) waveform of
Figure 4-3.
The 1 added to the audio waveform is essential in preserving the shape of the
modulating audio waveform on the envelope of the modulated RF CW signal in

Figure 4-3. In this way, if the audio waveform fades to zero, we just get Figure 4-1,
a CW RF signal, which is what we would expect.
First Project: A CW RF Test Oscillator

Before the specific schematic is shown, let's take a look at the big picture. A test
generator consisting of a CW oscillator will be used for testing the tuning range,
535 kHz to 1,605 kHz, of the radios. A modulator will be added with an
audio-frequency oscillator to produce amplitude modulation of the CW signal.