Overview
In previous articles we discussed the properties of electromagnetic waves, but electromagnetic waves alone do not provide communication. Information must be loaded onto an electromagnetic wave so that the wave can act as an information carrier. How is a signal loaded onto an electromagnetic wave? This article reviews several fundamental modulation methods used to map signals onto electromagnetic carriers. Although modern communication standards use more advanced schemes, these basic methods remain foundational for understanding signal transmission.
Amplitude Modulation (AM)
Amplitude modulation (AM) changes the amplitude or strength of the carrier signal. AM was the first type of modulation used for broadcasting sound. While other modulation forms are increasingly used today, AM remains in widespread use in some applications.
Frequency Modulation (FM)
Frequency modulation (FM) varies the frequency of the carrier according to the modulating signal. A key advantage of FM is its ability to limit amplitude noise effects, since only frequency variations carry the required information. This can be achieved by passing the signal through a stage that limits amplitude, eliminating amplitude variations that may result from noise or general signal fluctuations. If the stage is driven into limiting by sufficient signal strength, amplitude changes will not affect the demodulated audio level, assuming audio is being transmitted. Because of these properties, FM has been widely used for higher-fidelity analog audio broadcasting.
Phase Modulation (PM)
Phase modulation (PM) changes the phase of the carrier according to the modulating signal. PM and FM share many similarities and are closely related: frequency is the derivative of phase, i.e., frequency is the rate of change of phase. Phase modulation has become widely used for digital data transmission.
Angle Modulation
Angle modulation refers to modulation methods that change the angle or phase of a sinusoidal carrier. In angle modulation, the carrier amplitude remains constant. The two primary forms within this category are frequency modulation and phase modulation. Because frequency is the derivative of phase, FM and PM are interrelated: a frequency-modulated signal can be generated by integrating the modulating waveform and using the result as the input to a phase modulator; conversely, a phase-modulated signal can be generated by differentiating the modulating waveform and using the result as the input to a frequency modulator.
Modulation Combinations
Modulation schemes can combine amplitude and angle components to achieve improved performance.
Quadrature amplitude modulation (QAM): Information is encoded in both amplitude and phase. Data modulates the in-phase (I) and quadrature (Q) components, forming a constellation with multiple points in two planes.
Amplitude and phase-shift keying (APSK): Compared with QAM, APSK arranges the constellation to optimize peak-to-average power ratio and can use fewer amplitude levels. This enables RF power amplifiers to operate more efficiently.
Signal Bandwidth
Bandwidth is a key property of any signal because it defines the required channel bandwidth and therefore how many channels can fit into a given radio spectrum segment. As spectrum pressure increases, signal bandwidth becomes a critical characteristic for any radio transmission.
Bandwidth is controlled mainly by two factors:
- Modulation type: Some modulation schemes use bandwidth more efficiently than others. This factor alone can determine modulation choice when spectral efficiency is important.
- Bandwidth of the modulating signal: Shannon's theorem sets a lower bound on the bandwidth required to transmit a given signal. In general, the wider the bandwidth of the modulating signal, the wider the required transmitted bandwidth.
Choosing a Modulation Type
When selecting a modulation method, consider the advantages and disadvantages of each. AM and FM are widely used for analog audio transmission, while phase shift keying and QAM are common for digital data transmission. The choice depends on factors such as spectral efficiency, robustness to noise, and hardware constraints.
