The information or the data is available in audio, video, written and in other forms. To transmit these data radio signals are used. The transmission takes place with the help of radio carriers. The process of converting and transmitting the data into radio signals by the addition of information to an optical or electronic carrier signal is called Modulation. Usually, a carrier signal has a steady waveform i.e. it has constant frequency and amplitude. The data is added to the carrier wave by varying its frequency, amplitude, polarization, spin and phase. Modulation has wide applications in radio, optics/laser and computer networks.
There are three basic types of modulation,
Frequency Modulation (FM)
Amplitude Modulation (AM) &
Phase Modulation (PM)
As Phase Modulation and Frequency modulation works on the same principle, they are considered as the inter-related.
We shall now discuss FM and AM one by one.
In frequency modulation, the amplitude of the signal is constant but the frequency is modified according to signals being sent. It is developed by Edwin H. Armstrong, an American electrical engineer. To overcome the effect of noise and interference with other signals rising due to thunderstorm and eddy currents generated in transmission devices, a method of Frequency Modulation is used. As these interferences can easily affect the magnitude of the amplitude but cannot diversify frequency, FM is preferred by users.
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There are many techniques used for Frequency Modulation.
Varactor diode Oscillator- In this technique, a varactor diode is placed within the circuit to modulate frequency. This method gives only narrow band transmissions.
Phase Locked Loop- This technique provides excellent FM. In this method, phases are constrained within the loop to modulate frequency.
m(t) - modulating signal
Am - Amplitude of the modulating signal.
ωm - Angular frequency of the modulating signal.
Ɵ - is the phase of the modulating signal.
FM equation can be represented as,
m(t) = Am cos (ωmt + Ɵ)
Deviation of frequency Δfmax is given by the expression,
Δfmax = fmt-fc׀
= K Am Cos (2 π fm t)
Modulation index is given by,
μ=fm Δfmax = fmKAm
Advantages of Frequency Modulation
The main advantage of FM is, it is less susceptible to noise and other interferences during transmission of signals. As the only frequency modulation takes place, transmission remains unaffected.
It can be used for low power transmitters.
For an expected output, less battery power is required and thus can be used for two-way radio applications.
Disadvantages of Frequency Modulation
The equipment cost for FM is higher. Also, it gives larger bandwidth.
The output is small compared to other types of modulations.
For better transmission you need closely placed antennae.
The transmission of data by radio carriers with the help of addition of signals in which frequency is kept constant and amplitude is modulated, is known as Amplitude Modulation. It is widely used in electronic communications. For transmission of long, medium and small waves, AM is widely used.
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There are two methods of Amplitude Modulation
Low- level Generation- In this method of AM, frequency less than the desired frequency is generated and then later amplitude is modulated.
High- level Generation- In this technique, the gain of the transmitter final output is amplified.
The Amplitude Modulation equation is given as,
y(t) = A.sin(ωct)+A M2[sin((ωc+ωm)t+φ)]+A M2[sin((ωc−ωm)t−φ)]
C- the carrier amplitude
φ- phase of the signal at the start of the reference time
M - carrier amplitude
Modulation Index is given by,
μ= Ac/Am=Amplitude of carrier wave/ Amplitude of modulating signal
There are three types of Amplitude Modulation
Single sideband (SSB) modulation- In this type of AM, the modulation wave consists either of the upper sideband or the lower sideband.
Double sideband-suppressed carrier (DSB-SC) modulation- In this type of AM, the transmitted wave consists of only the upper and lower sidebands still bandwidth remains the same.
Vestigial sideband (VSB) modulation-Here, one sideband is transmitted almost completely and just a part of the other sideband is taken. The required bandwidth is in excess amount.
Advantages of Amplitude Modulation
Amplitude Modulation is cheaper.
It is simple to implement and it can be demodulated easily.
The receivers of AM do not need special arrangements and they are inexpensive.
The efficiency is very low as it needs a lot of power.
It uses the amplitude frequency many times to control the signals by radio carrier.
The quality of the original signal on the receiver is poorer.
The Amplitude Modulation systems are prone to disturbances like noise and interferences of the external causes.
The applications are limited to one to one communication only.
1. What are the applications of FM and AM?
Applications of Frequency Modulation-
It is widely used in radio broadcasting.
In radar systems,
In EEG machine
Applications of Amplitude Modulation
To broadcast transmissions- To broadcast small, medium and long signals, Amplitude Modulation is used. As they are easier to demodulate, radio receivers need cheaper apparatus and are simple to manufacture.
In air band radio- In many airborne applications, Amplitude Modulation is used. For better ground to air communication it is used and vice-versa.
2. What are the differences between AM and FM? Which one is better?
First successful Amplitude Modulation transmission was done in mid 1870s
Developed by Edward Armstrong in 1930
The frequency and magnitude of phase remains the same
The magnitude of amplitude and phase remains the same.
Susceptible to noise and interference
Less susceptible to noise and interference
The frequency range is between 535 to 1705 kHz
The frequency range is between 88 to 108 MHz
Can transmit waves to a longer distance but with poorer quality.
Gives higher bandwidth but transmits good quality of signals.
If one needs less disturbances during signal transmission then Frequency Modulation is always better to choose but when one needs transmission from a longer distance the choice is Amplitude Modulation.