Frequency modulation (FM) is an analog modulation technique used in radio communications to encode information onto a carrier wave by varying its frequency. The frequency of the carrier signal is modulated according to the amplitude of the message or information signal.
How FM Modulation Works
Frequency modulation involves varying the frequency of a carrier wave by the amplitude variations of the message signal containing the information being transmitted.
A high-frequency sinusoidal carrier wave is generated by an oscillator. The frequency of this carrier is constant until modulation begins. The message signal is then superimposed on the carrier by a device called the modulator.
In the modulator, the amplitude variations of the incoming message signal are converted to equivalent variations in the frequency of the carrier wave. This is achieved using a reactance modulator circuit consisting of a reactance tube or varactor diode.
The reactance of this circuit varies in proportion to the amplitude of the message signal. This reactance forms part of the tuned circuit that generates the carrier. So the tuning of the carrier generator fluctuates with the message signal, leading to corresponding changes in the carrier frequency.
If the message signal amplitude increases in a positive direction, the reactance modulation causes the carrier frequency to increase above the center carrier frequency. When the message amplitude decreases negatively, the carrier frequency is pulled down below the unmodulated carrier frequency.
Thus the carrier frequency at any given instant is either higher or lower than the center carrier frequency based on the amplitude of the modulating signal at that instant. The extent of frequency deviation is proportional to the message amplitude. This represents the frequency modulation process.
The FM signal with the carrier frequency modulated carries the message information just like AM. This FM signal is amplified and transmitted through the air using an antenna. The FM receiver captures the signal through its antenna and demodulates it using a discriminator circuit to extract the original message signal.
So by varying the carrier frequency to represent the changing message amplitude, the message information gets encoded onto the FM carrier. This conversion of amplitude variations to frequency variations provides advantages like high fidelity and resistance to static noise.
Advantages & Disadvantages of FM
Here are the main advantages and disadvantages of frequency modulation (FM):
- High fidelity - FM provides much better audio quality than AM, with higher fidelity and little noise. The wider bandwidth allows for stereo transmission and greater dynamic range.
- Resistant to interference - FM is less affected by external signals and natural phenomena like storms. The change in frequency instead of amplitude provides better interference immunity.
- Consistent signal strength - FM reception does not fade out as the receiver moves out of range of the transmitter. The signal remains strong until it drops out completely.
- Harder to intercept - FM signals are more difficult to intercept and decode compared to AM. This provides some level of privacy.
- No impact of noise - Any noise picked up during FM transmission only affects the amplitude, not the frequency. So noise does not distort or degrade the message signal.
- Susceptible to "capture effect" - When two FM signals of similar strength reach the receiver, only one will be demodulated while the other gets rejected.
- Requires more complex circuits - FM transmitters and receivers are more complex requiring specialized components to handle the wide frequencies. This makes FM more expensive.
- Interference between adjacent channels - FM stations spacing needs to be carefully managed to avoid interference between adjacent band frequencies.
- Limited range - The line-of-sight propagation of FM means range is limited compared to AM. Power requirements are also higher.
- Not suitable for long-distance - FM is better suited for local broadcasting than long-distance transmission like international broadcasting done better via AM.
Here are some of the major applications of frequency modulation (FM):
- FM radio broadcasting - FM is used for high-fidelity stereo sound broadcasting by radio stations in the 88-108 MHz band. FM provides static-free, high-quality audio.
- Two-way radio communications - Two-way radios used by emergency services, taxis, delivery fleets, etc. often use FM to enable private two-way voice and data communications.
- Commercial broadcasting - FM broadcasting is used for TV audio, cable TV channels, satellite broadcasts, and other commercial applications requiring high-quality sound transmission.
- Storage media - Analog and digital FM modulation help store audio and video on media like magnetic tape, VHS tapes, optical discs, etc. by changing the frequency of recorded signals.
- Wireless intercoms - Private and secure wireless intercom systems used in offices, factories, homes, etc. employ FM technology.
- RF transmitters - Wireless microphones, guitar systems, radio telemetry, and radio control equipment use FM to transmit audio and data.
- Optical fiber communications - FM can be used to transmit analog signals over fiber optic communication links using laser diodes.
- Test and measurement - FM test signals are generated by signal generators and used for alignment, calibration, and testing of radio receivers.
- Synthesizers - Electronic music synthesizers use FM synthesis to generate a variety of complex instrumental tones digitally.
So in summary, FM is widely used in radio, wireless, and wired communications applications where high-fidelity, noise-free transmission and reception of audio, video, and data signals is required.
The widespread adoption of FM technology for radio and wireless communications is thanks to its noise resistance and audio clarity, which revolutionized broadcasting fidelity. Though digital modulation schemes like QPSK are prevalent today, analog FM continues to be a versatile modulation technique for many radio applications.
If the un-modulated carrier signal is represented by A sin2pf t and the modulating signal is represented by B sin2pfa t.
Then the carrier frequency “f” will be changed around the resulting frequency fc thus
f = fc + Df Sin2pfa t.
The voltage of the frequency-modulated signal is represented by
V = A sin[2p(fc + Df Sin2pfa t)t]
Df = frequency change (deviation)
The total variation from the lowest to the height is called carrier swing.
So carrier swing = 2 x Df
Making use of calculus, the equation for the frequency-modulated signal can be written as
V = A Sin [2pfc t + Df/fa) Cos2pfa t]
Df/fa = mf = modulation index
The carrier signal, modulating signal, and modulated signal are shown in the below fig.