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Lectures on digital modulation techniques [HD]
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First, let's look: What is modulation?
Modulation is a process in which a characteristic of the carrier (amplitude, frequency or phase) is varied in accordance with the instantaneous value of the modulating signal (message or baseband).
So if the amplitude of the carrier is varied it is called amplitude modulation but if the frequency or phase of the carrier is varied depending on the instantaneous value of the modulating signal then it is known as frequency modulation or phase modulation. respectively.
Now let us understand the basic difference between continuous wave modulation and digital modulation-
Difference Between Continuous Wave Modulation and Digital Modulation
Amplitude Modulation (AM), Frequency Modulation (FM) and Phase Modulation (PM) are the examples of Continuous Wave (CW) modulation, while Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK) and Phase Shift Keying (PSK). examples of digital modulation techniques. The fundamental difference between continuous wave modulation and digital modulation techniques is based on the nature of the message signal (modulating signal).
In continuous wave modulation the message signal is of analog nature, but in digital modulation the message signal is of digital nature (binary or M-ary coded version).
In both modulation techniques, the carrier wave has an analog form.
Here it is interesting to note that the three forms of digital modulation techniques namely ASK, FSK and PSK are analogous to AM, FM and PM of continuous wave modulation respectively.
Amplitude shift keying (ASK)
In ASK, the amplitude of the carrier wave is changed (switched) according to the digital input signal (modulating signal). Therefore, amplitude shift keying is analogous to amplitude modulation (analog modulation).
ASK is analogous to AM in that Amplitude Modulation (AM) changes the amplitude of the carrier based on the instantaneous value of the modulating (message) signal, in the same way that ASK switches the amplitude of the carrier. (varied) according to the instantaneous value of the modulating signal (digital input signal). The difference is only in the nature of the modulating signal. In amplitude modulation the modulating signal is analog in nature, but in digital modulation it is a stream of digital bits.
the amplitude of the sinusoidal carrier wave is switched depending on the digital input signal. The carrier is not transmitted or transmitted for digital input signal '0' or '1' respectively. Therefore, amplitude shift keying (ASK) is also called /"ON – OFF Keying (OOK)/".
Frequency Shift Keys (FSK)
If the frequency of the sinusoidal carrier is varied (switched) according to the digital input signal, this is known as the frequency shift key (FSK). It is analogous to frequency modulation (analog modulation).
The reason why FSK is analogous to FM is…
In frequency modulation (FM) the frequency of the carrier is varied depending on the instantaneous value of the modulating signal, similarly in frequency shift keying the frequency of the sinusoidal carrier is also varied (switched) according to the digital input signal. The difference is only in the nature of the modulating signal. With FM the modulating signal is of an analogue nature, while with FSK the modulating signal is digital.
So the basic purpose of Frequency Shift Keying (FSK) here is to modulate (change/switch) the frequency of the carrier wave, depending on the digital input signal.
So in Frequency Shift Keying (FSK) we have two types of carrier frequencies: low frequency for the transmission of '0' and high frequency for the transmission of '1'.
In this way, with Frequency Shift Keying (FSK), the information of the digital input signal is present in the frequency variations of the carrier wave. Therefore, it is known as frequency shift keying.
Now let's discuss Phase Shift Keying (PSK).
Phase Shift Keying (PSK)
In phase shift keying, the phase of the carrier (analog) is varied depending on the digital input signal. Phase shift keying is analogous to phase modulation (analog phase modulation).
The phase shift keying is very similar to phase modulation (PM).
Electronics and Communications (ECE) Lecture Videos (GATE and IES)
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