In the last post we discussed different types of pulse modulation techniques, like Pulse Amplitude Modulation (PAM), Pulse Width Modulation (PWM), Pulse Position Modulation (PPM) and Pulse Code Modulation (PCM). In this post we will deal with the comparative study of PAM, PWM and PPM. Here we will compare these 3 different pulse modulation techniques, on the basis of various parameters.
Comparison of PAM, PWM and PPM Modulation Techniques Video [HD]
1.Definition of PAM, PWM and PPM
PAM- In pulse amplitude modulation, the amplitude of the pulsed carrier wave is varied according to the modulating signal value. PWM- In pulse width modulation, the width of pulsed carrier is varied as per the value of the modulating signal. PPM- In pulse position modulation, the position of the pulsed carrier is varied as per value of modulating signal.
2.Waveform of PAM, PWM and PPM
Waveform of PAM, PWM and PPM is given in the image below. Click on the image to enlarge it.
Comparison of PAM, PWM, PPM Waveforms
3.Variable Parameter of the Carrier in PAM, PWM and PPM
PAM- Amplitude PWM- Width PPM- Position
4.Where the Information is Present in PAM, PWM and PPM
PAM- In amplitude variations of the pulses PWM- In width variations of the pulses PPM- In position variations of the pulses
5.Type of Carrier Wave Used in PAM,PWM and PPM
PAM- Train of pulses PWM- Train of pulses PPM- Train of pulses
PAM- Bandwidth depends on the width of the pulse Bandwidth requirement is low PWM- Bandwidth of the channel depends on rise time of the pulse Bandwidth requirement is high PPM- Bandwidth depends on rise time of the pulse Bandwidth requirement is high
7.Instantaneous Power in PAM, PWM and PPM
PAM- Power varies due to change in amplitude of the pulses PWM- Instantaneous power varies due to change in width of the pulses PPM- Instantaneous power is constant due to constant amplitude and width of the pulses
8.Noise Interference in PAM, PWM and PPM
PAM- Noise interference is maximum and it is difficult to remove noise PWM- Noise interference is less due to constant amplitude PPM- Noise interference is less due to constant amplitude
9.Similarity of PAM, PWM and PPM With Continuous Wave Modulation
PAM- Similar to Amplitude Modulation (AM) PWM- Similar to Frequency Modulation (FM) PPM- Similar to Phase Modulation (PM)
10.Synchronization Between Transmitter And Receiver in case of PAM, PWM and PPM
In this post, I will discuss different kinds of pulse modulation techniques like Pulse Amplitude Modulation (PAM), Pulse Width Modulation (PWM) or Pulse Duration Modulation (PDM), Pulse Position Modulation (PPM) and Pulse Code Modulation (PCM). Here we will also see Pulse Modulation waveforms. But Before starting the topic, it is important to understand the basic difference between Continuous Wave Modulation and Pulse Modulation. 🌓READ THIS ALSO:- #NEED AND BENEFITS OF MODULATION
Pulse Modulation Techniques Video [HD]
Difference between Continuous Wave Modulation and Pulse Modulation
In Continuous Wave Modulation (Amplitude Modulation, Frequency Modulation, Phase Modulation) the carrier wave used is continuous in nature, while in case of pulse modulation, the carrier wave is in the form of pulses.
So now let's discuss different types of Pulse Modulation Techniques in detail.
Classification of Pulse Modulation Techniques
Pulse modulation can be categorized broadly into two types- #Analog Pulse Modulation and #Digital Pulse Modulation
Pulse Code Modulation (PCM) is a digital pulse modulation technique.
You can see the classification of pulse modulation, in the image given below (Click on the image to enlarge it)-
Classification of Pulse Modulation Techniques
To see the classification of all types of modulations including pulse modulation and continuous wave modulation, look at the image given below (Click to enlarge)- 🌓READ THIS ALSO:- COMPARISON OF PAM, PWM, PPM MODULATION TECHNIQUES
Definition:- The modulation technique in which the instantaneous amplitude of the pulsed carrier is varied according to the modulating signal (message signal) is called as pulse amplitude modulation. 🌓READ THIS ALSO:- #PULSE CODE MODULATION (PCM) [ADVANTAGES AND DISADVANTAGES]
Waveform of pulse amplitude modulation
Look at the image given below carefully to understand the formation of pulse amplitude modulated wave. (Click the image to enlarge)-
Pulse Amplitude Modulation Waveform
Here in this image, you can see, three waveforms. The first waveform is of modulating signal (message signal), the second one is of the pulsed career wave and the third waveform is of the pulse amplitude modulated wave (PAM). The message signal can have multiple frequencies and variable amplitude. Here we have used a sinusoidal message signal. The carrier wave that we have used here, is a train of pulses having high frequency. Since we know that in Pulse modulation, the carrier wave is in the form of pulses.
Now we will understand how can we make pulse amplitude modulated wave (PAM). As we have discussed that in Pulse amplitude modulation, the instantaneous amplitude of the pulsed carrier is varied according to the modulating signal (message signal). 🌓READ THIS ALSO:- #Quadrature Amplitude Modulation (QAM)/ QAM Transmitter and QAM Receiver Block Diagram Now look at the image shown above very carefully, observe that, as the amplitude of the modulating signal (message signal or baseband signal) increases, the amplitude of the pulsed carrier wave increases and becomes maximum when the amplitude of the modulating signal reaches at its maximum. Now the amplitude of the carrier wave starts decreasing with decrease in amplitude of the modulating signal. So it is clear from the image that the amplitude of the pulsed carrier wave varies according to the instantaneous value of the message signal.
Important Observations about Pulse Amplitude Modulation (PAM)
#As it is Pulse Amplitude Modulation (PAM), therefore only the amplitude of the pulsed carrier changes. There is no change in the width or position of the pulsed carrier wave. 🌓READ THIS ALSO:- #SAMPLING THEOREM AND RECONSTRUCTION (SAMPLING AND QUANTIZATION) So the width and position of the carrier wave pulse is constant here. Therefore we can say that, all the information of the modulating signal is contained in the amplitude variations of the pulses. No information is present in the width or position of the pulses.
Pulse Time Modulation (PTM)
In pulse time modulation, the width or position of the pulsed carrier wave is varied (variations on the time axis) as per the changes in the amplitude of the modulating signal. As we have discussed already that Pulse time modulation is of two types- Pulse Width Modulation (PWM)/Pulse Duration Modulation (PDM) and Pulse Position Modulation (PPM) So first let's start with pulse width modulation.
Definition:- In Pulse width modulation, width (duration) of the pulses of the carrier wave is varied according to the modulating signal (message signal). Now we will understand the waveform of pulse width modulation. 🌓READ THIS ALSO:- #Watch the VIDEOS PLAYLIST here (Pulse modulation Techniques) Look at the image given below to see the waveform of pulse width modulated signal. (Click the image to enlarge)-
Pulse Width Modulation Waveform
Here you can see waveforms of modulating signal, pulsed carrier wave and pulse width modulated wave.
As per the definition of pulse width modulation, we know that, in Pulse width modulation, the width of the pulses of carrier wave is varied according to the message signal. So you can see here in the image that as the amplitude of the message signal changes, the width of pulses of the pulsed carrier wave changes accordingly. You can easily observe here, the width of the pulse is maximum when amplitude of the message signal is at maximum and in the same way, width is minimum when amplitude of the modulating signal is minimum.
Important Observations about Pulse Width Modulation (PWM)
#Since it is pulse width modulation (pulse duration modulation), therefore only the width (duration) of the pulses of the carrier wave changes. No change takes place in amplitude or position of the pulses. #As the width of pulses changes, according to the modulating signal; information is present only in the width of the pulses. In Pulse width modulation, amplitude or position of the pulses contain no information.
Definition:- In Pulse position modulation, the position of the pulsed carrier wave is varied according to the message signal (modulating signal). Look at the waveforms shown in the image given below, (Click the image to enlarge)-
Pulse Position Modulation Waveform
This image shows, the waveforms of modulating signal, pulsed carrier wave, pulse width modulated wave (PWM) and pulse position modulated wave (PPM).
Please note that, here we will obtain the pulse position modulated wave from the pulse width modulated wave. 🌓READ THIS ALSO:- #AMPLITUDE MODULATION (TIME DOMAIN EQUATIONS AND WAVEFORMS) Therefore to draw the waveform of PPM we need to draw the waveform of PWM too. Now observe the waveforms shown in the image. Now we know how to obtain the waveform of pulse width modulated wave. So we will get the waveform of pulse position modulation from PWM. As you can see in the image, that the starting point of each pulse in PPM waveform is obtained from the trailing edge of the PWM pulse. It means, each pulse in PPM starts where the PWM pulse ends (trailing edge of PWM pulse).
Important Observations about Pulse Position Modulation
#It is important to note that in pulse position modulation, only the position of of the pulses changes as per the variations in amplitude of the modulating signal. There is no change in the amplitude or width of the pulses of the carrier wave. So the information about the modulating signal is present in the position variations of the pulses. #Also note here that in pulse position modulation, synchronizing pulses need to be sent by the transmitter to keep the transmitter and receiver in synchronism. 🌓READ THIS ALSO:- #FREQUENCY SPECTRUM OF AMPLITUDE MODULATION (WAVEFORMS AND EQUATIONS DERIVATION) This kind of synchronization is not required in case of pulse amplitude modulation or in Pulse width modulation.
Pulse Code Modulation (PCM)
Now we will discuss the digital form of pulse modulation technique. This form of pulse modulation technique is known as Pulse Code Modulation (PCM). Pulse code modulation is a technique to convert analog signals into digital signals. After converting the signal into digital form, it becomes possible to transmit the digital signal through digital communication network and at the receiving end, it is converted back into it's original analog form. Pulse code modulation process involves the following three stages - #Sampling #Quantization and #Coding
Now observe the image given below (Click the image to enlarge)-
Sampling and Quantization of analog signal
This image shows the following- #Signal in its original analog form #The sampled signal obtained after sampling of this analog signal and #Quantized signal obtained after performing the quantization process.
Now we are going to discuss, the process of sampling and quantization. During the discussion, please see the image carefully - The first waveform given in the image is of the analog signal, that we want to transmit over the digital communication network. But since it is in analog form, therefore first it needs to be converted into digital form. So to do this job, we take the help of Pulse Code Modulation (PCM).
Sampling of signal
The second part of this image, shows the sampled signal. In the process of sampling of the signal, we convert the continuous time signal into discrete time signal. 🌓READ THIS ALSO:- #ADVANTAGES AND DISADVANTAGES OF DIGITAL COMMUNICATION SYSTEM You can see this conversion in the sampled signal shown in the image. The analog signal was continuous in time, since it had some value at every instant of time. But in the sampled signal, which is discrete in time, the value of the signal is present only at certain instants of time. Accuracy of the sampling increases with increase in frequency of the sampling. But some sampling error is introduced because of this sampling process, since it is not possible to have infinite sampling frequency practically. Sampling of the analog signal is performed with the help of sampling theorem. So let me define here, what is sampling theorem.
Sampling Theorem
A continuous-time signal can be completely represented in its samples and recovered back into its original form if the sampling frequency is greater than or equal to twice the highest frequency present in the modulating signal (message signal).
It can be represented mathematically as- fs >= 2fm Here 'fs' is the sampling frequency and 'fm' is the highest frequency present in the modulating signal.
Quantization of Signal
To understand the process of quantization look at the image given above. In the process of quantization, amplitude of the signal is cut horizontally into a number of fixed levels. 🌓READ THIS ALSO:- #SAMPLING THEOREM AND RECONSTRUCTION (SAMPLING AND QUANTIZATION)
Now the value of the signal is rounded-off (approximated) to the nearest level of amplitude. These certain levels of amplitude are shown in the image on the vertical axis. Here we have divided the amplitude range (peak to peak amplitude) into 'l' number of levels. The magnitude of each level is equal to the peak to peak amplitude range divided by the number of levels. It is important to note here that, quantized signal is just an approximation of the original signal. As the number of levels increases, the accuracy of quantization increases. Pulse Code Modulation (PCM) technique uses the process of quantization to convert analog signals into digital signals.
Pulse Code Modulation Waveform
The image given below shows pulse code modulated waveform.
Here we will discuss, What is an Optical Fiber, the Construction of the Optical Fiber and Working principle of the Optical Fiber. A complete video lecture has also been included to clear the concepts in a better way. So let's start with the definition of Optical Fiber-
What is an Optical Fiber?
An optical fiber is a flexible and transparent fiber, made by drawing glass (silica) or plastic to a diameter slightly more than that of a human hair (including outer coating, its diameter is 0.25 mm-0.5 mm).
Optical Fiber Construction and Working Principle Video [HD]
The light rays propagate inside the optical fiber by the phenomenon of Total Internal Reflection (TIR). Now we will know the basic principle of operation of the optical fiber.
Working Principle of Optical Fiber
Total Internal Reflection in Optical Fiber (TIR)
The optical fiber has a core and a cladding layer. The refractive index of the core is more than the refractive index of the cladding. So when the light rays move from denser medium to rarer medium (core to cladding) with an incidence angle greater than the critical angle (90 degrees), the total internal reflection phenomenon takes place and the light rays return back into the same denser medium (core). So the propagation of light rays take place inside the core of the Optical Fiber with successive Total Internal Reflection (TIR).
To understand, how this phenomenon of total internal reflection takes place inside the optical fiber when the light rays propagate through it, see the image given below
As we know, when the light rays move from denser medium to rarer medium, then it deviates away from the normal, as you can see in the image. This deviation (refraction) can be seen in all the three cases shown in the image.
Therefore the angle of refraction is more than the angle of incidence when the light rays move from denser medium to rarer medium. Now look at the image, here you can see that, in the first case, when the incidence angle is less than the critical angle (the incidence angle, when the angle of refraction is 90° degrees), the light ray moves away from the normal in the rarer medium and goes into the rarer medium. The second case shows the case of critical angle. Critical angle is the angle of incidence, when angle of refraction is 90 degrees (when the light ray moves from denser tor rare medium). Now see the third case, it is the case where total internal reflection takes place. This is the same case that happens inside the optical fibre, when the light rays propagate through it. In this third case, when the light ray moves from denser to rarer medium, with an angle of incidence more than the critical angle, it returns back into the same denser medium. This is called as total internal reflection. 🌓READ THIS ALSO:- #ADVANTAGES OF OPTICAL FIBER COMMUNICATION #OPTICAL FIBER SOURCES (DESIRABLE PROPERTIES) This phenomena is known as total internal reflection, because in this phenomena, the light ray is reflected totally back into the same medium like reflection phenomena.
Construction and Working of the Optical Fiber
Now we will see how this phenomenon of total internal reflection takes place inside the optical fiber. To understand the concept clearly look at the image shown below (Click on the image to enlarge it)-
Propagation of light inside optical fiber with Total Internal Reflection (TIR)
This image shows the structure and working principle of the optical Fibre. You can see in this image the two layers of the Optical Fiber, known as Core and cladding. 🌓READ THIS ALSO:- #Single-Mode Optical Fiber Advantages Now observe the image carefully. When the light ray moves inside the core and reaches to the core- cladding interface, then the phenomenon of total internal reflection takes place. This phenomena is seen here because, the light ray moves from the denser medium to the rarer medium (core to cladding).
But for the total internal reflection to take place, it is necessary that the angle of incidence must be more than the critical angle. Only the incident light rays for which the angle of incidence is more than the critical angle, can propagate through the core of the fiber with total internal reflection. The light ray moves through the core of the fiber, with total internal reflection taking place each time when the light ray reaches at the core-cladding interface. For the light rays that enter into the Optical Fiber with an angle of incidence lesser than the critical angle; the phenomenon of total internal reflection doesn't take place, and these light rays move into the cladding instead of returning back into the same denser medium (core). So, we can see that for the light rays to travel through the Optical Fiber, it is necessary that the light rays must have the angle of incidence more than the critical angle at the core-cladding interface, for the total internal reflection to take place. Read More- Go To HOME Page FREQUENCY SPECTRUM OF AMPLITUDE MODULATION (WAVEFORMS AND EQUATIONS DERIVATION)
