Page 227 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

FIGURE 14-3 Squaring a sinusoidal signal produces a twice-frequency
component. The X axis shows time, whereas the Yaxis shows the
amplitude.
So what happens if the input signal is a combination of two signals of different

frequencies? Figure 14-4 shows two "high" frequency signals with frequencies F1
and F2 in the lower portion that are just slightly different in frequency. The
smaller-amplitude high-frequency signal (with a frequency of Fl) is lower in
frequency than the higher-amplitude high-frequency signal (with a frequency of
F2). When these two signals are added (summed) and squared, the resulting

waveform is seen at the top of the figure and resembles an amplitude-modulated
(AM) waveform. The envelope of the signal at the top of the figure has a frequency
that is the difference of the two high-frequency signals with a frequency of F2 - F1.