 Hello everyone, welcome to this video lecture, myself Dipali Vadkar, working as assistant professor at WIT, Swollapur. In this session, we will study pulse time modulation. At the end of this video lecture, student will be able to describe working of pulse width modulator using IC-TRIPLE-5 circuit. Also student will be able to explain the working of PPM modulator using IC-TRIPLE-5 circuit. Purl's time modulation that is the PTM, it is the class of signaling techniques that encodes the sample values of analog signal onto time axis of digital signal. There are two types of PTM, first is a pulse width modulation that is the PWM and second is pulse position modulation that is the PPM. Let us see what is the PWM or pulse width modulation, it is pulse width modulation it is defined as the process of wearing the width of signal pulse in accordance with the modulating signal variations. So, in the pulse width modulation the width of train of pulses changes according to the modulating signal. So, here the amplitude and position of this train of pulses remains constant only there is a change in width. Now, first we will see the PWM modulator its block diagram. In this modulator basically the comparator is used, the one input of the comparator it is nothing but modulating signal X of t or continuous time signal which is to be modulated and another input to this comparator that is the sawtooth generator comparator compares both this input and it gives the output that is the variable width of train of pulses that is the PWM pulse width modulated signal. This is the circuit diagram for PWM modulator using IC555 here the clock signal is applied at the pin number 2 of this IC555. Another input to this circuit that is nothing but X of t continuous time signal or modulating signal and we get the output that is the PWM at the pin number 3 of this IC. So, let us see in detail the circuit this circuit basically consists two parts one is a differentiator and another is a IC555. So, here the this circuit is for differentiator circuit here this register R1 is connected to the VCC that is the positive supply. This differentiator differentiates the input clock signal and this differentiator generate positive and negative spikes at its output but here one diode is connected so this diode bypass this positive spikes means it pass the positive spike and it avoids the negative spike. So, we get the negative spikes at the output which is here across this these two points. So, the output of this differentiator that is nothing but negative spikes and its frequency is same as that of the frequency of clock signal. What is the frequency of clock signal? So, here the clock signal is adjusted exactly at the sampling frequency. So, distance between these all clocks it is nothing but sampling period that is nothing but TS. Now, these negative H trigger pulses are given to the IC555. Let us see the IC555 circuit here the IC555 circuit this negative H trigger pulse from differentiator introduced to pin number 2 and the modulating signal to pin number 5 and we get the output we get the output at the pin number 3 of this IC. In this circuit the first clock signal is applied input to the differentiator then this differentiator circuit generate a positive and negative spikes out of that positive spikes bypass through this diode D1 and the output here the negative spike. This out negative spikes that is the negative H trigger is applied to the pin number 2 of IC555. When this negative H trigger is applied to the pin number 2 then at that time here the output of this IC is becomes high. Now, due to this the capacitor which is present here this C2 capacitor this capacitor start charging. Now, when this capacitor charging here another input it is nothing but modulating signal this input is applied to the control pin of IC555. Now, here two inputs one is a modulating signal which is this X of t and another is the charging of this charging voltage of this capacitor charging and discharging voltage of the capacitor. Now, when the voltage across this capacitor C2 becomes equal to the input voltage or control voltage at that time the output of this IC changes means it states changes to low voltage. So, the starting H is decided by the trigger pulse and the ending H or lagging H of this pulse is decided by the modulating signal or input signal. So, we can say that the width of this pulse is depend upon modulating signal. So, this process is continuous and we get the variable width pulses train of pulses at the output of this IC that is the at the pin number 3 and this pulses are nothing but pulse width modulated output. Now, next is a pulse position modulation it is the analog modulating scheme in which the amplitude and width of pulses are constant while the position of each pulse varies according to the instantaneous value of message signal. So, in the pulse position modulation the position of all these pulses varies according to the amplitude of modulating signal. Let us see the block diagram for PPM modulator. In this PPM modulator again comparator is used to input to the comparator modulating signal and sort of signal. So, here at the comparator we get the PWM signal that is the pulse width modulated signal. So, this pulse width modulated signal is applied to the monostable multivibrator again this multivibrator is using the IC555 here and the at the output of monostable multivibrator we get the PPM output that is the pulse position modulated signal. Let us see the circuit diagram for PPM modulator. In the PPM modulator here again two parts one is a differentiator circuit another is the IC555 circuit, but here the control voltage is connected to the wind capacitor and which is connected to the ground. So, here the input for this circuit this input is only PWM signal and this pulse width modulated signal is given to the differentiator. Then here the differentiator consist R1-C1 capacitor this R1 is connected to the VCC. When we give the train of pulses to the differentiator then differentiator generate the negative spikes that is the negative edge trigger. Now this negative edge trigger applied at the P number 2 the output of this IC is at the P number 3. Now when this negative edge trigger is given to the IC we have seen it is working in the last slide. Now the question what will be the output of IC555 when trigger pulse is given pause the video for a while and think. So when we give the negative edge trigger at that time the output of this IC555 that output becomes high. So here first the input that is the PWM signal that is that wave is here and at the ending edge of this pulse there is a negative edge trigger and when the negative edge trigger is applied to the IC this is the output across this IC555. So the output becomes high. Output remains high for a particular time period. So this time period is a constant here and that time period is depend upon the value of this C2 and R2. So in pulse position modulation this pulse remains high for a particular time period. That time period is a constant and its amplitude this amplitude is equal to the amplitude of input signal the amplitude of PWM signal. Now this process is continuous and at the output we get a train of pulses, width is constant amplitude is constant but its position changes. So in this way we give the PWM signal input to the differentiator. The differentiator generate a negative edge trigger pulses when the negative edge trigger pulses given to the IC555 then the IC555 its output becomes high and this output remains constant for a constant time period and this time period is depend upon R2 C2 and after that time period the output of this IC becomes low. So in this way at the P number 3 of this IC555 we get a pulse position modulation output. These are the references. Thank you.