DIFFRENCE BETWEEN FM AND AM• The power transmission of FM waves is better than that of the AM signals.• In FM wave’s noise can be controlled by increasing the deviation up to some amount. This is impossible in case of AM waves.• AM is amplitude modulation while FM is frequency modulation. But what is modulation? It is the act of modifying a certain aspect of the carrier frequency in accordance to the information being sent. It is then clear that AM modifies the amplitude of the carrier frequency while FM modifies its frequency.
• FM is a development over AM broadcasting and it provides a lot of substantial benefits thus it is much more complex compared to AM. The first and most substantial benefit is its ability to send out two channels of information at the same time with the use of advanced algorithms. This allows the station to broadcast left and right audio channels for full stereo sound. Since most environmental factors that distort radio waves only affect its amplitude and not the frequency where FM stores the actual voice signal, the data in the FM signal doesn’t degrade as easily as AM. This also means that FM signal quality doesn’t degrade linearly as you get further from the transmitting station.• Range wise, AM takes the cake with its much greater distance travelled. FM signals usually drop of at around 50 miles from the station, but AM waves can be refracted in the atmosphere resulting in greater range. The range is also one of the reasons why talk radio prefers AM even if the sound quality is not very high.
.FM. (FREQUENCY MODULATION) FM broadcasting is a broadcasting technology pioneered by Edwin Howard Armstrong which usesfrequency modulation (FM) to provide high-fidelity sound over broadcast radio. The term "FM band" describes the "frequency band in which FM is used for broadcasting".
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).
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 intotwo types –1. Narrowband FM : modulation index is smaller than one radian2. Wideband FM : modulation index is larger than 1 radian
Modulation index is less than 1 Maximum modulating frequency is usually3kHz & maximum frequency deviation is =75kHz. A narrowband FM wave consist of acarrier, an upper side-frequency component& a lower side component. The modulated narrowband signal differsfrom the ideal response in two fundamentalrespects :
1.The envelope contains a residual AM, so it varies with time.2.For sinusoidal modulating wave, the angle θi(t) contains harmonic distortion in the form of 3rd & higher order harmonics of modulation frequency, fm.By restricting β≤0.3 radians, the effects of residual AM & harmonic PM are limited to negligible labels.
A narrow band signal may be represented by phasor diagram as shown in below – Fig: Phasor diagram of a 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.
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.
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 Јn(β).
We can plot Bessel function Јn(β) versus modulation index β for different positive integer values of n.
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 is m This relation is known as Carson’s rule.
According to international regulations of FM broadcast –Maximum frequency deviation, fd=±75 kHzAllowable bandwidth per channel=200 kHzFrequency stability of carrier =±2 kHz
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.
Modulating wave first used to produce a narrowband FM wave, then frequency multiplication is used to increase the frequency deviation to the desired level.
Here instantaneous frequency of the carrier wave is varied directly in accordance with the message signal by means of VCO.Baseband Wide-band signal Frequenc Frequenc FM Mixer Bandpas VCO y y s Filter Multiplier Multiplier Fixed Oscillator Fig: Block diagram of direct method of generating wide-band 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.
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.
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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