TABLE OF CONTENTS Ever since the early days of radio, modulation techniques have played an important part in electronic communications. A low-frequency voice or data signal is used to modulate some characteristic (usually the amplitude, phase, or frequency) of a carrier signal. This type of system represents an intentional use of modulation. In addition, unintentional modulation may be present, such as power line sidebands on an oscillator output or residual frequency modulation on an amplitude-modulated signal. Whether the modulation is intentional or not, a spectrum analyzer can be used to characterize and measure it.
Analog modulation techniques start with a carrier which is a pure sinusoid.
| (6-1) |  |
where
| A | = | carrier amplitude (zero-to-peak) |
| f c | = | carrier frequency (hertz) |
| ? | = | The carrier phase |
The carrier may be modulated in a variety of ways, but the various techniques fall into two main categories: amplitude modulation and angle modulation. Amplitude modulation implies that the amplitude is no longer simply a constant but is a function of time. Similarly, angle modulation occurs when the angle of the cosine term is varied. Angle modulation may take the form of frequency modulation or of phase modulation, depending on the particular modulation technique.
All the modulation techniques result in increasing the occupied bandwidth of the carrier by spreading out sidebands in the frequency domain. Originally, the carrier is presumed to be a single spectral line, infinitely thin, occupying only one exact frequency. When modulated, the signal bandwidth increases depending on the...
