Frequency Modulation
Introduction
Frequency modulation (FM) is a method of impressing data onto an alternating-current (AC) carrier wave by varying the instantaneous frequency of the wave. This technique contrasts with amplitude modulation, in which the amplitude of the carrier wave varies, while the frequency remains constant.
Concept and Theory
In frequency modulation, the frequency of the carrier wave is altered in proportion to the instantaneous value of the input signal, resulting in a change in the carrier frequency. The maximum frequency deviation is often defined by the information signal. The instantaneous frequency can be represented mathematically as a function of time, frequency deviation, and the information signal.
Applications
Frequency modulation is used in a variety of systems and devices that transmit information or process signals. Some of the most common applications include:
Radio Broadcasting
FM is widely used for broadcasting music and speech, in the very high frequency (VHF) radio bands, in a band known as the FM broadcast band. The wide frequency response of FM is used for high fidelity broadcasts of music and speech. Normal (analog) TV sound is also broadcast using FM.
Sound Synthesis
Sound synthesis can be done using frequency modulation. By using a modulating sinusoidal signal, FM synthesis can create both harmonic and inharmonic sounds. For synthesizing harmonic sounds, the modulating signal must have a harmonic relationship to the original carrier signal. As the amount of frequency deviation increases, the sound grows progressively complex.
Radar and Seismology
Frequency modulation is used in radar and seismology applications. In frequency-modulated continuous-wave radar (FMCW radar), radar signals are frequency modulated. In some systems the "distance" information, formed by measuring the frequency shift over time, is used to create images.
Data Transmission
Frequency modulation is also used in telemetry, radar, seismology, and monitoring newborns for seizures via EEG.
Advantages and Disadvantages
Like any modulation technique, FM has both advantages and disadvantages.
Advantages
FM offers several significant advantages, including better signal-to-noise ratio. Due to the increased bandwidth, noise and interference are spread out over a wider frequency range, reducing the impact on the signal. In addition, FM is less susceptible to signal distortion and produces a clearer, more high fidelity sound.
Disadvantages
The main disadvantage of FM is the large bandwidth required. Compared to AM, FM requires approximately 10 times as much bandwidth and is thus not as spectrum-efficient. Additionally, the complexity of the associated circuitry is much higher, potentially leading to increased costs.