Explain Frequency Modulation (FM) and Frequency Deviation, Modulation Index, Frequency spectrum

Frequency Modulation (FM) 

Frequency Modulation (FM) is a form of modulation in which changes in the carrier wave frequency correspond directly to changes in the baseband signal. FM is considered an analog form of modulation because the baseband signal is typically an analog waveform without discrete, digital values.

Frequency Modulation (FM) is the encoding of information in a carrier wave by changing the instantaneous frequency of the wave. FM technology is widely used in the fields of computing, telecommunications, and signal processing.

The process of varying the FREQUENCY of a signal, often PERIODICally. An example of FM is the violinist's VIBRATO, where the length of a string (and therefore the resulting pitch) is rapidly altered by a fast oscillating movement of the finger and wrist.

Applications of Frequency Modulation

If we talk about the applications of frequency modulation, it is mostly used in radio broadcasting. It offers a great advantage in radio transmission as it has a larger signal-to-noise ratio. Meaning, it results in low radio frequency interference. This is why many radio stations use FM to broadcast music over the radio.

Additionally, some of its uses are also found in radar, telemetry, seismic prospecting and in EEG, different radio systems, music synthesis as well as in video-transmission instruments. In radio transmission, frequency modulation has a good advantage over other modulation. It has a larger signal-to-noise ratio meaning it will reject radio frequency interferences much better than an equal power amplitude modulation (AM) signal. For this major reason, most music is broadcasted over FM radio.

FM Modulators

There are several methods that can be used to generate either direct or indirect frequency-modulated signals.

A voltage-controlled oscillator or Varactor diode oscillator:  A voltage-controlled oscillator can be used to form Direct FM modulation by directly feeding the message into the input of the oscillator. In the case of the varactor diode, we place this device within the tuned circuit of an oscillator circuit.

Crystal oscillator circuit: Varactor diode can also be used within a crystal oscillator circuit wherein the signal needs to be multiplied in frequency, and only narrowband FM is attained.

Phase-locked loop: This is an excellent method to generate frequency modulation signals. However, the constraints within the loop should be checked carefully and once everything is stable it offers an excellent solution.

Amplitude Modulation vs Frequency Modulation

While FM and AM function similarly, the main difference lies in how their carrier waves are modulated. In the case of AM, the signal strength varies in order to incorporate sound information whereas for FM, the frequency at which the current changes direction per second for the carrier signal is varied. We will look at some of the differences between FM and AM below.

Amplitude Modulation (AM)Frequency Modulation (FM)
 Frequency and phase remain the same Amplitude and phase remain the same
Can be transmitted over a long distance but has poor sound quality. Better sound quality with higher bandwidth.
The frequency range varies between 535 to 1705 kHzFor FM it is from88 to 108 MHz mainly in the higher spectrum
Signal distortion can occur in AMMore minor instances of signal distortion
Consists of two sidebandsAn infinite number of sidebands
Circuit design is simple and less expensiveCircuit design is intricate and more expensive
Easily susceptible to noiseLess susceptible to noise

How does frequency modulation work?

The encoding of information in a carrier wave by modifying the wave’s instantaneous frequency is known as frequency modulation (FM). In the fields of computing, telecommunications, and signal processing, FM technology is frequently used.

What is the advantage of frequency modulation over amplitude modulation?

FM was first introduced to reduce noise and increase radio reception quality because the frequency of a radio wave is less susceptible to noise than the amplitude. FM radio broadcasts have several times the bandwidth of AM signals to achieve this.

How is the process of frequency modulation done?

The frequency of the radio carrier is modified in accordance with the amplitude of the incoming audio stream to form a frequency-modulated signal.

What are the different types of analog modulation systems?

Frequency Modulation, Amplitude Modulation, and Phase Modulation are the different types of continuous wave analog modulation systems whereas PAM, PWM, and PPM are the pulse analog modulation systems.

What is frequency modulation?

Frequency modulation is the method of changing the carrier signal’s instantaneous frequency in accordance with the message signal’s instantaneous amplitude.

  • List the uses of modulation.
  • Modulation has the following uses.
  • Reduces antenna height.
  • Avoids signal mixing.
  • Increases communication range.
  • Multiplexing is possible.
  • Improves reception quality.

What is frequency deviation?

The FM signal’s instantaneous frequency changes with time around the carrier frequency ωc. This indicates that the FM signal’s immediate frequency varies depending on the modulating signal. Frequency deviation is the maximum change in instantaneous frequency from the average frequency ωc.

What are the drawbacks of frequency modulation over amplitude modulation?

In FM broadcasting, a significantly broader channel is required, approximately 200 kHz, compared to only 10 kHz in AM broadcasting. This is a significant restriction of FM. FM transmitting and receiving equipment, especially that utilized for modulation and demodulation, is more complicated and thus more expensive.

Frequency Deviation

Frequency deviation in FM is defined as the way to describe the difference between the minimum and maximum extent of a frequency modulated signal and the carrier frequency.

Modulation Index

Modulation index is defined as the ratio of the fundamental component amplitude of the line-to-neutral inverter output voltage to one-half of the available DC bus voltage.

Modulation index is a measure of  extent of modulation done on a carrier signal. In Amplitude modulation, it is defined as the ratio of the amplitude of modulating signal to that of the carrier signal.
Its value is kept less than 1 to avoid overmodulation which leads to distortions in the modulated signal and makes it very hard to demodulate and extract the modulating signal.

From the given data,

AM wave is given by,
Hence, amplitude is given by:
Maximum value of AM wave is 
Frequency of lower sideband,  
Frequency of upper sideband,  

Frequency spectrum

The frequency spectrum of an electrical signal is the distribution of the amplitudes and phases of each frequency component against frequency. The use of higher frequencies is desirable because of the smaller antenna size, the improved directional effect of the antennae, and the broader available frequency spectrum.

The radio frequency spectrum is divided into small chunks for a huge number of applications, from AM and FM radio, television, and cellular networks to walkie-talkies, satellite communications, military applications, and even to send and receive signals into outer space, hoping for a reply.

Bandwidth is the portion of the spectrum that a given telecommunications system can use. For example, a system that operates on frequencies between 150 and 200 MHz has a bandwidth of 50 megahertz. An important distinction in spectrum technology is the difference between narrowband and broadband.

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