9 Jun 2018

Digital Modulation Techniques (ASK, FSK, PSK, BPSK) - Amplitude, Frequency and Phase Shift Keying

In this post we will discuss three kinds of digital modulation techniques that are- Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK) and Phase Shift Keying (PSK).

So first of all let's understand, what is modulation?

Digital Modulation Techniques (ASK, FSK, PSK, BPSK) Video




What is Modulation- 

Modulation is a process, where some characteristic of the carrier wave (amplitude, frequency or phase) is varied in accordance with the instantaneous value of the modulating (message or baseband) signal.
So if amplitude of the carrier wave is varied, then it is called as amplitude modulation (AM), but if frequency or phase of the carrier wave is varied, according to the instantaneous value of the modulating signal, then it is known as frequency modulation (FM) or phase modulation (PM) respectively.

So now let's 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) are examples of Digital Modulation Techniques. The basic difference between continuous wave Modulation and digital modulation techniques is based on the nature of message signal (modulating signal).

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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 encoded version).
In both of these modulation techniques, the carrier wave is of analog form.
Here it is interesting to note that, the three forms of digital modulation techniques that is ASK, FSK and PSK are analogous to AM, FM and PM of continuous wave modulation respectively.

So now let's discuss each digital modulation technique (ASK, FSK and PSK) in detail-

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, because in Amplitude Modulation (AM), amplitude of the carrier wave is changed according to the instantaneous value of the modulating (message) signal, in the same way in ASK also, the amplitude of the carrier wave is switched (varied) according to the instantaneous value of the modulating signal (digital input signal). The difference is only of the nature of the modulating signal. In amplitude modulation, the modulating signal is of analog kind but in digital modulation, it is a stream of digital bits.

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Now Let's understand the concept of amplitude shift keying (ASK) with the help of an example.
Look carefully the image shown below-


Amplitude Shift Keying, ASK, ASK Waveform, Amplitude Shift Keying (ASK) Waveform
Amplitude Shift Keying (ASK) Waveform

Here in this image observe that we are going to modulate a sinusoidal carrier wave (shown in green colour), with the digital input signal (0 1 1 0 0 1).
This image also shows the ASK waveform (modulated signal).
So now it's time to understand, how this amplitude shift keying takes place.
In amplitude shift keying, we change the amplitude of this sinusoidal carrier wave according to the digital input signal which is acting as modulating signal (message signal) here.
So the basic concept is, we do not transmit the carrier wave when the digital input signal is '0', and transmit the sinusoidal carrier as it is, for digital input signal '1'.

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You can observe this phenomena in the image carefully, that in this example, we have digital input signal '0', at three places, so for these three digital '0', no carrier signal has been transmitted. But for binary '1' at three places, the full carrier wave has been transmitted without any change.
Here the amplitude of the sinusoidal carrier wave is switched, as per the digital input signal. The carrier wave is either not transmitted or transmitted for digital input signal '0' or '1' respectively. That is why amplitude shift keying (ASK) is also called as "ON - OFF Keying (OOK)".
Now we will discuss the Frequency Shift Keying (FSK)

Frequency Shift Keying (FSK)


If the frequency of sinusoidal carrier wave is varied (switched) as per the digital input signal, then it is known as the frequency shift keying (FSK). It is analogous to frequency modulation (analog modulation).
The reason behind why FSK is analogous to FM, is....
In Frequency Modulation (FM), the frequency of the carrier wave is varied according to the instantaneous value of the modulating signal, in the same way in frequency shift keying also, the frequency of the sinusoidal carrier wave is varied (switched) as per the digital input signal. The difference is only of the nature of the modulating signal. In FM the modulating signal is of analogue nature while in FSK modulating signal is digital.

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Now to understand the concept of frequency shift keying, look at the image shown below carefully-


Frequency Shift Keying, FSK, FSK Waveform, Frequency Shift Keying (FSK) Waveform
Frequency Shift Keying (FSK) Waveform

This image shows three parts-
#Digital Input signal (0 1 1 0 0 1)
#Sinusoidal carrier wave and
#FSK waveform

So here the basic purpose of Frequency Shift Keying (FSK), is to modulate (change/switch) the frequency of the carrier wave, according to the digital input signal.
Now observe the image, the places where digital input '0' is to be transmitted; the frequency of the sinusoidal carrier is decreased but when we transmit '1'; the frequency of the carrier wave is increased.
(Frequency is the number of cycles passed per second, or 1/Time period).
So in Frequency Shift Keying (FSK), we have two types of frequencies of the carrier wave, low frequency for the transmission of '0' and high frequency for the transmission of '1'.
In this way, in Frequency Shift Keying (FSK), the information of the digital input signal is present in the frequency variations of the carrier wave. That is why it is known as frequency shift keying.
Now let's discuss the Phase Shift Keying (PSK)

Phase Shift Keying (PSK)


In phase shift keying, phase of the carrier wave (analog) is varied as per the digital input signal. Phase shift keying is analogous to Phase Modulation (analog phase modulation).

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The phase shift keying is very much similar to Phase Modulation (PM), because in both of these modulation techniques, the phase of the carrier wave is changed, according to the instantaneous value of the modulating signal. The difference is only of the nature of the modulating signal. In phase modulation, the modulating signal is analog but in case of phase shift keying, modulating signal is of digital nature.
The carrier wave is of analogue kind in both of these modulation techniques.
Now we will understand the basic concept of Phase Shift Keying (PSK), with the help of an example shown in the image given below-


Phase Shift Keying, PSK, PSK Waveform, Phase Shift Keying (PSK) Waveform
Phase Shift Keying (PSK) Waveform

The Image contains three parts-
#The digital input signal (011001)
#The sinusoidal carrier wave (analog)
#PSK waveform

Now let's understand the basic concept, how the phase shift keying takes place-
In Phase Shift Keying (PSK), the phase of the carrier wave is changed (switched) according to the digital input signal. Therefore the information of this digital input signal is present in the phase shift variations of the carrier wave.

Now observe the image carefully-
Here we will Try to understand the concept of PSK with the help of an example given in this image. 
Notice here that, whenever the digital input changes the bit (either from '0' to '1' or from '1' to '0'), a phase shift of 180 degrees (π) takes place in the carrier wave. But no phase change occurs when there is no change in the digital bit.
In this image, observe; the phase shift of 180 degrees takes place in the carrier wave at three places. At all these three places, the digital input bit has either change from '0' to '1' or from '1' to '0'. No phase shift takes place when two two successive (back to back) 1's or two successive 0's are to be transmitted (as per the image). Hence we get the PSK waveform in this way. 

This was all about three kinds of digital modulation techniques, amplitude shift keying, frequency shift keying and phase shift keying.

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