Frequency MHz

Frequency Modulation FM

FM (frequency modulation) is a method in which the frequency of a carrier signal is changed according to the information of the modulating signal. In this case, the amplitude of the signal is always constant as the frequency changes with time, which also shows changes in the modulating signal. For example, frequency modulation is used in radio broadcasting. Its main purpose is this. That high-quality audio transmission can be achieved with low noise. A special feature of frequency modulation FM is that such modulation techniques are more robust against noise and interference. Due to this, it is mostly used in music and voice communication.

How does FM (Frequency Modulation) work?

The modulation process in FM (Frequency Modulation) basically works like this.

In contrast to linear modulation, exponential modulation is nonlinear process and therefore the modulated spectrum is not related to the message spectrum in a simple fashion.

The Modulated waveform after exponential modulation can be expressed by the equation.

Xc (t) = Ac Cos (wct+¢t)+Ac Re[ej (wc.t+¢(t)]

Where

¢(t) represent the varying phase or the frequency containing the message,

Ac is the constant amplitude wc.

= 2π fc is the angular frequency of the carrier wave,

And

Re means that we take the real pan of the exponential function in brackets.

As we see, the message in inserted into the angle of the carrier wave or in the expdonent of the fashion describing a cosine wave. This is why these modulated methods are called angle or exponential modulation.

In FM, the instantaneous frequency of the carrier is varied according to the message and its amplitude is kept constant.

The Frequency Modulation, FM, is the process of combining a modulating signal with a carrier wave in such a way that the amplitude of the modulating signal causes the carrier wave to vary in frequency.

The instantaneous amplitude change of the modulating signal varies the frequency of the carrier wave. The frequency of the modulating signal controls the rate at which the frequency of the carrier changes. There are only frequency variations.

The modulating signal is shown in figure below.

Carrier Signal

First a carrier signal is generated. The frequency of which is constant.

Modulating signal

These signals contain information or data. Audio signal etc.

Change in frequency

The frequency Modulation of the carrier signal is also changed based on the amplitude of the modulating signal. So that if the amplitude of the modulating signal increases, then frequency of the carrier signal also increases.

Same as, if the amplitude of the modulating signal decreases, the frequency of the carrier signal also decreases.

Modulated signal:

As a result, the information from this modulating signal can be transmitted by changes in the carrier frequency. A practical explanation of FM modulation is as follows:

Radio Broadcasting:

Radio broadcasting consists of a carrier signal at a specific frequency (e.g., 100.1 MHz). Whenever you listen to music, So in this case, the audio signal (modulating signal) changes the frequency of the carrier signal. Such changes are demodulated from the carrier signal in the receiver, and the audio signals are then recovered.

Uses of FM (frequency modulation)

The frequency modulation has various uses in various fields due to its high quality and robustness against noise. Some of these important uses are as follows:

Radio broadcasting

FM radio broadcasting is most commonly used for the transmission of audio signals.

FM radios operate in the 88–108 MHz band. They provide the highest-quality and clear sound. They are used mainly for music.

The modulated signal is shown in Figure A. The unmodulated carrier wave is shown in Figure B. Waveform in Figure C. Show the resultant FM signal caused by the AF signal Figure 1.1. Modulating the RF carrier signal Figure 1.1a. Notice that as the modulating signal increases the amplitude to its maximum positive value (Point Q), the RF carrier signal increases in frequency (more cycles appear for a given period of time). When the modulating signal amplitude drops to zero (Point R), the RF carrier signal decreases to the center frequency. As the modulated signal swings to its most negative value (Point S), the carrier frequency decreases below the center frequency as the modulating signal reaches zero (Point T) the carrier is again at the center frequency.

When the AF modulating signal goes positive, arid negative, the carrier deviates below the center frequency.

The amount of the RF carrier frequency change is referred to as frequency deviation. This frequency deviation is the result of the amplitude change of the modulating AF signal. Therefore, a strong audio signal loud than will cause the RF carrier to shift above the center frequency to the greater extent than will a weak soft audio signal.

RF Carrier Center Frequency

When an RF carrier is mounted, additional signals called sidebands are proceeded. These sidebands are grouped above and below the carrier center frequency in multiplies of the modulating signal. The sidebands contain intelligence that is transmitted and recovered at the receiver. So long as the frequency deviation is held to a maximum, only two sidebands are developed.

However, as we increase the amplitude of the modulating AF signal above the minimum value, additional significant sidebands in multiplies above and below the carrier frequency are produced. If a strong AF signal of 1 kHz is used to modulate an RF carrier centered at 100 MHz, as shown in figure b, sidebands are produced at 100.001 MHz, 100.002 MHz,….. 100.007 MHz, an 99.999 MHz, 99.998 MHz, 99.993 MHz.

The bandwidth for this 1 kHz modulating signal is 14 kHz, 100.007 MHz – 99.993 MHz, which is relatively narrow.

However, should be used a modulating AF signal of 15 kHz, as shown in figure c. A bandwidth of 210 kHz is required. We can see that the bandwidth required depends upon two factors. The amplitude of the AF modulating frequency and the frequency of signal.

100MHz. Unmodulated

The energy contained in each sideband pair two sideband’s spaced equally above and below the carrier center frequency decreases as the sideband pairs are removed from the center frequency. A point is reached where a sideband pair contains so little energy that they can be disregarded.

Modulation Index

This point is determined by using a modulating index. The modulation index is a ratio of RF carrier frequency deviation to the modulating AF signal frequency.

Television Broadcasting Audio Transmission

In TV broadcasting, the audio signal is modulated by FM. So that the audio signal along with the video is sent with high quality and minimum noise.

Wireless communication:

Personal Mobile Radio (PMR):

FM modulation is also used in wireless communication systems. For example, walkie-talkies, handheld radios, etc.

Aviation Communication

FM modulation is also used in the aviation industry. To enable clear and noiseless communication between pilots and control towers.

Data Communication Telemetry:

Telemetry systems also use FM modulation to easily transmit sensor data and other types of information wirelessly. Which includes space missions, etc.

Modem

FM modems are also used for data transmission where digital data can be easily converted to analog signals.

Medical field

FM Herring Aids:

Hearing aids also use FM modulation to help users hear clear and distinct sounds, especially when they are in classrooms or noisy environments.

Science and Research and Radar Systems

FM modulation is also used in radar systems to detect distant objects.

Spectroscopy

FM modulation is used in various scientific experiments and instruments to make more accurate measurements.

These diverse uses of FM modulation make it of central importance in various fields due to its efficiency, quality, and resistance to noise.

Better sound quality:

In FM modulation, the frequency of the signal is changed. Due to this, noise and interference are reduced. As a result, the sound quality has greatly improved. This type of technique is especially useful for music and speech.

Noise Resistance:

FM signals are highly resistant to noise and interference. These signals produce much less noise than AM. Due to this, the definition of these signals has improved.

Bandwidth Performance:

FM signals are constantly changing in frequency. This increases the bandwidth efficiency. Through it, various types of data can be transmitted effectively.

Reduction of Distortion:

FM has less signal distortion. Because only the frequency changes and the amplitude does not change. Because of this, the signal remains more stable and clear. Long distance performance of frequency modulated signals Can be transported over long distances without any major loss. By which clear and stable broadcasting can be possible even in remote areas.

Spectrum Performance:

By frequency modulation itself, different channels can be easily broadcast at close frequencies. Due to which spectrum efficiency is also increased and more channels can be fit in less bandwidth.

Disadvantages of FM (Frequency Modulation) include:

Bandwidth Requirement:

Frequency modulation is requires more bandwidth than Amplitude Modulation.

Why does it resonate over a wide frequency spectrum?

This can limit the effective use of the frequency spectrum.

Complex transmitters and receivers

FM transmitters and receivers are more complex and expensive, to design and manufacture than AM. These require specific types of electronic components and circuitry. Which are quite expensive in price.

Decreased remote access:

FM signals are generally limited to line of sight. These signals are less effective for direct transmission over longer distances. Buildings, mountains and other obstacles greatly affect the quality of these signals.

Effect of intervention:

FM signals are less susceptible to interference. But when ever there is an intervention, then the effect is more significant. This happens especially when the frequencies of two FM stations are close.

Expensive infrastructure:

The infrastructure used for FM broadcasting is very expensive.

These are required high power transmitters, antennas and other types of equipment. It can be a big challenge for small and low-budget stations.

Need for better quality:

To take advantage of FM transmission, both the receiver and the transmitter are of high quality. With non-standard types of equipment, it becomes difficult to reap the full benefits of FM transmission.

FM Frequency Modulation