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.
What is Frequency Modulation?
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.
Image will be uploaded soon
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.
Frequency Modulation Equations
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.
What is Amplitude Modulation?
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.
Image will be uploaded soon
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.
Amplitude Modulation Equations
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
Types of Amplitude Modulation
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.
Disadvantages of Amplitude Modulation
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.