Amplitude and frequency modulation Essay

Modulation – In general, the information signal can rarely be transmitted in its original form. It is first converted into an electric signal, and then is used to modulate a carrier signal. A carrier signal is used – (a) To reduce wavelength of the signal for efficient transmission, and (b) To allow multiplexing i.e. using same channel to transfer multiple signals. The term modulation, is defined as the addition of the information signal to some parameter of the signal carrier. For most of radio and telecommunication today, the carrier is alternating current in a given range of frequencies i.e. a single-frequency sinusoid.

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A basic sine wave can be represented as V (t) = Vo sin (2 p f t + f), where V (t) is the voltage of the signal, Vo is the amplitude of the signal, f is the frequency of oscillation and f is the phase of the signal (“http://www.fas.org/man/dod-101/navy/docs/es310/AM.htm”) Any one of these three parameters, amplitude, frequency, phase, could be modified to achieve modulation of the signal. Modulation index, m, is the fraction of carrier parameter that is modulated. For e.g. – If m = 0.5, it means that the carrier is varied by up to 50 % of its original value. Two most commonly used modulation techniques are amplitude modulation and frequency modulation.

Amplitude Modulation

Amplitude modulation is considered to be the simplest form of modulation. Here, the amplitude of the carrier wave is varied in order to transmit analog or digital data. For e.g. – the information signal, Vm(t)  is used to vary the amplitude of the carrier, Vco to produce a modulated signal, VAM(t). In mathematical form, this can be represented as below:

Information: Vm(t)
Carrier: Vc(t) = Vco sin (2 p fc t + f )
AM: VAM(t) = { Vco + Vm(t) }sin (2 p fc t + f) (“http://www.fas.org/man/dod101/navy/docs/es310/AM.htm”)
The figure below explains the concept of amplitude modulation.

Fig -1 http://techweb.com/encyclopedia/defineterm.jhtml?term=amplitudemodulation

Fig- 1 shows the modulation of an analog wave, using amplitude modulation. In amplitude modulation, the carrier frequency is constant. The total power of the transmitted signal varies with the modulating signal, whereas the power of the carrier wave remains constant. The bandwidth of AM signals depends only on the maximum modulation frequency. Also the frequency of AM is limited to 33%, beyond this distortion would be introduced in the receiver. Modulation index is limited to a maximum value of 1.0.

Amplitude modulation is the oldest method of transmitting audio signals electronically. In the beginning, radio waves were only used for sending signals, like Morse code. Reginald Aubrey Fessenden, in the year 1900 developed Amplitude Modulation, and in the year 1906 used it to make the world’s first radio broadcast by transmit his voice and music signals over a distance of several hundred miles, to ships on the Atlantic Ocean. His technique is used even today in AM radio.

Main advantages of AM are small bandwidth and simple transmitter and receiver designs. Amplitude modulation is widely used on the long, medium and short wave bands for broadcast stations. AM is also used for aircraft communications in the VHF band. However, apart from this AM is not widely used these days. Amplitude modulation has its limitations. First interference caused by the earth’s ionosphere is a big problem. Changes in the ionosphere at night alter its reflecting characteristics; some AM signals fade while others travel greater distances to interfere with other AM signals. Again, lightning, which is a strong source of electromagnetic radiation, breaks up the modulated radio signal, causing a great deal of static. AM signals also lack high-fidelity quality. These problems were solved with Edwin Howard Armstrong’s invention of frequency modulation (FM) broadcasting in 1924.

Frequency modulation

It is a type of modulation in which the frequency of the carrier wave is varied in order to transmit analog or digital data. Frequency modulation uses the information signal, Vm(t) to vary the carrier frequency f within some small range about its original value, to produce the modulated signal VFM (t). This can be represented mathematically as below:

Information: Vm(t)        http://www.fas.org/man/dod-101/navy/docs/es310/FM.htm
Carrier: Vc(t) = Vco sin ( 2 p fc t + f )
FM: VFM (t) = Vco sin (2 p [fc + (Df/Vmo) Vm (t) ] t + f)
Df, represents the the peak frequency deviation. The carrier frequency term: fc + (Df/Vmo) Vm (t), varies between the fc – Df and fc + Df. The figure below explains this concept:

Fig 2

http://techweb.com/encyclopedia/defineterm.jhtml?term=frequency+modulation

In case of frequency modulation, the amplitude of the carrier wave is constant. Because of this reason, FM modulation is a low-noise process and provides a high quality modulation technique. Today FM is one of the most important types of modulation schemes in radio transmission. It is used in radio and television broadcasts, and also mobile communications.

In 1920s and 1930s, AM was used all over the world in radio broadcasting using the superheterodyne circuits, made by Major Edwin Howard Armstrong.  However, AM was very susceptible to static, electric noises and also lacked high-fidelity. Major Armstrong saw need for an improvement and invented frequency modulation radio. Initially, he was met with strong opposition from the people who were using amplitude modulation broadcasting techniques and did not want to use new methods. But, soon after when World War II started, the FM system was used in U.S. Army radio sets with great success. Much later FM gained a strong hold in broadcasting field, after a series of legal battles.

Main advantages of FM are improved signal to noise ratio and less radiated power. The bandwidth of an FM signal depends on both the modulation index and the modulating frequency. The efficiency is of FM is high, and the modulation index varies from 1.0 – 5.0 depending on the application.  FM systems are inherently immune to random noise, as the noise is distributed uniformly and varies mostly in amplitude.

References

Books

Bailey, D., “Practical Radio Engineering and Telemetry for Industry”, Jun 2003, IDC Tech.

Poole, I., “Basic Radio”, Mar. 1998, Adrio Communications Ltd.

Websites

“Amplitude Modulation”

            http://techweb.com/encyclopedia/defineterm.jhtml?term=amplitudemodulation

“Amplitude modulation – Introduction to Naval Weapons Engineering”

            http://www.fas.org/man/dod-101/navy/docs/es310/AM.htm

“Frequency Modulation”

            http://techweb.com/encyclopedia/defineterm.jhtml?term=frequency+modulation

“Frequency modulation – Introduction to Naval Weapons Engineering”

            http://www.fas.org/man/dod-101/navy/docs/es310/FM.htm

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