In this post we will discuss three kinds of
digital modulation techniques, Amplitude Shift Keying (ASK), Frequency Shift
Keying (FSK) and Phase Shift Keying (PSK).
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So first of all let's understand, what is
modulation?
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, 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 or phase modulation
respectively.
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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).
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.
Now let’s understand the concept of amplitude
shift keying (ASK) with the help of an example.
Look carefully the image shown below.
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Amplitude Shift Keying (ASK) Waveform
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Here in this image observe that we are going
to modulate a sinusoidal carrier wave (shown in green color), 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'.
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)
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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.
Now to understand the concept of frequency
shift keying; look at the image shown below carefully
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Frequency Shift Keying (FSK) Waveform
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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 (analogue phase modulation).
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.
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Phase Shift Keying (PSK) Waveform
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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.
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 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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Source：engineeringmadeeasypro
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