The frequency spread of digitally modulated radio-frequency (RF) signals has a characteristic shape that depends on the bit rate of the transmitted data. The higher the data rate, the greater the frequency bandwidth required for its transmission. Figure 1 is a photograph of the display of a spectrum analyzer receiving a digitally modulated RF signal. It shows the transmitted RF spectrum of the main lobe of a binary-phase-shift-keyed (BPSK) signal between the two nulls at the edges of the picture as it would be displayed on the analyzer. The carrier frequency is at the center of the display, and the characteristic nulls at the edges of the main lobe are clearly shown.
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