Frequency Modulation

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Frequency modulation is one type of modulation, in which frequency of the carrier signal changes. . .. . .

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Frequency modulation:

Frequency modulation

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Presented By Ferdous Pervej Dept. of Electronics & Telecommunication Engineering Rajshahi University of Engineering & Technology, Bangladesh mail: ferdous_pervej@yahoo.com

Frequency modulation:

Frequency modulation Frequency modulation is the process by which frequency of the carrier signal c(t) changes with respect to the modulating signal m(t). FM modulated signal s(t) is a nonlinear function of the modulating signal m(t), thus it is known as nonlinear modulation process. More difficult than amplitude modulation (AM).

Frequency modulation:

Frequency modulation Carrier Signal Modulating Signal Modulated Signal

Frequency Modulation:

Frequency Modulation If the modulating signal is m(t) and Then the modulated signal s(t) is Where, B is the modulation index. Depending to this modulation index FM can ne classified into two types – Narrowband FM : modulation index is smaller than one radian Wideband FM : modulation index is larger than 1 radian

Narrowband FM:

Narrowband FM Modulation index is less than 1 Maximum modulating frequency is usually 3kHz & maximum frequency deviation is =75 kHz. A narrowband FM wave consist of a carrier, an upper side-frequency component & a lower side component. The modulated narrowband signal differs from the ideal response in two fundamental respects :

Narrowband FM:

Narrowband FM 1.The envelope contains a residual AM, so it varies with time. 2.For sinusoidal modulating wave, the angle θ i(t) contains harmonic distortion in the form of 3 rd & higher order harmonics of modulation frequency, f m. By restricting β ≤0.3 radians, the effects of residual AM & harmonic PM are limited to negligible labels.

Narrowband FM:

Narrowband FM A narrow band signal may be represented by phasor diagram as shown in below – Fig: Phasor diagram of a narrowband FM

Narrowband FM:

Narrowband FM Uses : Narrowband FM is used by mobile communication services such as police wireless, ambulances, taxicabs, short range very high frequency(VHF) ship to shore sources and defence.

Wideband FM:

Wideband FM Modulation index normally exceeds unity. Modulation frequencies extend from 30 Hz to 15 kHz. Maximum permissible deviation is=75 kHz. Wideband FM system need large bandwidth, typically 15 times that of narrowband FM system . Wideband FM systems are used in entertainment broadcasting.

Wideband FM:

Wideband FM For large values of β compared to 1 radian, the FM wave contains a carrier & an infinite number of side- frequency components located symmetrically around the carrier. The amplitude of the carrier component contained in a wideband FM wave varies with the modulation index β in accordance with Bessel function Ј n ( β ).

Wideband FM:

Wideband FM We can plot Bessel function Ј n ( β ) versus modulation index β for different positive integer values of n.

Transmission Bandwidth of FM Waves :

Transmission Bandwidth of FM Waves In practice, FM wave is limited to finite number of significant side-frequencies compatible with a specified amount of distortion. Thus an effective bandwidth is required for the transmission of an FM wave. Approximate rule for transmission bandwidth of an FM wave generated by a single-tone modulating wave of frequency f m is This relation is known as Carson’s rule .

Transmission Bandwidth of FM Waves:

Transmission Bandwidth of FM Waves According to international regulations of FM broadcast – Maximum frequency deviation, f d =±75 kHz Allowable bandwidth per channel=200 kHz Frequency stability of carrier =±2 kHz

Generation of FM Signals:

Generation of FM Signals Two basic methods of generating FM waves Indirect : By integrating message signal a phase-modulated signal generates. This is used to modulate a VCO & the result is passed through a frequency multiplier. Direct : The message is directly feeding into the input of a VCO.

Indirect Method of FM Generation:

Indirect Method of FM Generation Modulating wave first used to produce a narrowband FM wave , then frequency multiplication is used to increase the frequency deviation to the desired level.

Indirect Method of FM Generation:

Indirect Method of FM Generation

Direct Method of FM Generation:

Direct Method of FM Generation Here instantaneous frequency of the carrier wave is varied directly in accordance with the message signal by means of VCO. VCO Frequency Multiplier Mixer Frequency Multiplier Fixed Oscillator Bandpass Filter Baseband signal Wide-band FM Fig: Block diagram of direct method of generating wide-band FM

Advantages of FM:

Advantages of FM Amplitude of the frequency modulated wave remains unaffected. Large decrease in noise, hence increase in S/N ratio. Noise may reduce by increasing deviation Frequency allocation allows for a guard band which reduces adjacent channel interference. FM permits several independent transmitters on the same frequency with negligible interference.

Disadvantages of FM:

Disadvantages of FM FM has too much advantages besides it also has some disadvantages - FM wave can’t cover large area. Transmitting & receiving equipments for FM are complex & costly. Reception using conventional method is limited to line of side. A much wider channel, typically 200 kHz, is needed for FM.

Applications of FM:

Applications of FM FM has so many applications such as – Magnetic tape storage: FM is used at intermediate frequencies by analog VCR systems (including VHS) to record both the luminance(black and white) portions of the video signal. Sound : FM is also used at audio frequencies to synthesize sound, which is known as FM synthesis. Radio : FM is used in many radio stations (FM Radio Stations) all over the world. Normally the frequency range of FM channels are between 88.0 MHz - 108.0 MHz.

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