Analog modulation methods
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A low-frequency message signal (top) may be carried by an AM or FM radio wave.
Waterfall plot of a 146.52 MHz radio carrier, with amplitude modulation by a 1,000 Hz sinusoid. Two strong sidebands at + and - 1 kHz from the carrier frequency are shown.
A carrier, frequency modulated by a 1,000 Hz sinusoid. The modulation index has been adjusted to around 2.4, so the carrier frequency has small amplitude. Several strong sidebands are apparent; in principle, an infinite number are produced in FM, but the higher-order sidebands are of negligible magnitude.
In analog modulation, the modulation is applied continuously in response to the analog information signal. Common analog modulation techniques include:
- Amplitude modulation (AM) (here the amplitude of the carrier signal is varied in accordance with the instantaneous amplitude of the modulating signal)
- Double-sideband modulation (DSB)
- Double-sideband modulation with carrier (DSB-WC) (used on the AM radio broadcasting band)
- Double-sideband suppressed-carrier transmission (DSB-SC)
- Double-sideband reduced-carrier transmission (DSB-RC)
- Single-sideband modulation (SSB, or SSB-AM)
- Single-sideband modulation with carrier (SSB-WC)
- Single-sideband modulation suppressed carrier modulation (SSB-SC)
- Vestigial-sideband modulation (VSB, or VSB-AM)
- Quadrature amplitude modulation (QAM)
- Double-sideband modulation (DSB)
- Angle modulation, which is approximately constant envelope
- Frequency modulation (FM) (here the frequency of the carrier signal is varied in accordance with the instantaneous amplitude of the modulating signal)
- Phase modulation (PM) (here the phase shift of the carrier signal is varied in accordance with the instantaneous amplitude of the modulating signal)
- Transpositional Modulation (TM), in which the waveform inflection is modified, resulting in a signal where each quarter cycle is transposed in the modulation process. TM is a pseudo-analog modulation (AM). Where an AM carrier also carries a phase variable phase f(ǿ). TM is f(AM,ǿ)
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