 Hello everyone, welcome to this session. I am Dr. Asha Therngi and today we are going to learn about one of the digital modulation techniques amplitude shift key which is popularly known as ASK. At the end of this session you will be able to explain need for digital modulation techniques, differentiate analog and digital modulation techniques, explain different ASK modulation techniques represent binary data into ASK modulated signal. These are the contents we will be covering in this session. We all know what is modulation technique and the need for modulation in communication system. Before moving ahead pause this video for few seconds and recall what is modulation technique. Welcome back. Modulation is a process in which the properties or characteristics such as amplitude, frequency or phase of high frequency sinusoidal carrier signal is modified with respect to the instantaneous amplitude of information signal which is also known as modulating signal. There are two types of modulation techniques analog modulation and digital modulation. In analog modulation the modulating signal is a low frequency analog signal while the carrier is a high frequency sinusoidal signal. Examples of analog modulation are amplitude modulation, frequency modulation and phase modulation techniques. Whereas in digital modulation modulating signal is digital signal and carrier is high frequency sinusoidal carrier. Examples of digital modulation techniques are amplitude shift keying, frequency shift keying, phase shift keying, etc. But what is the need for this digital modulation techniques? Let us see. In digital communication the information or the modulating signal is in the form of 1s and 0s. For wired channels or for short distance communication they can be transmitted directly by using different line coding techniques that is by using baseband data transmission system. But by using this method they cannot be transmitted for longer distance. Thus to transmit this data through wireless medium or for long distance communication passband data transmission system is used that is they are modulated using high frequency carrier signal and then transmitted. During modulation the amplitude, frequency or phase of the carrier is changed with respect to the instantaneous value of the data stream that is 1s or 0s in that bit duration. These modulation techniques are known as amplitude shift keying, frequency shift keying and phase shift keying respectively. As the modulating signal consists of only two levels that is 1s and 0s these modulation techniques are known as digital modulation techniques. Let us see ASK in detail. In ASK amplitude of the carrier is varied with respect to the input logic 1 or 0 of modulating signal and the frequency and phase of the carrier is kept constant. For this modulation technique two carriers with different amplitudes but same phase and frequency are used to represent 1 and 0. It is the simplest of all the digital modulation techniques. Let us now see how ASK signal is represented. Figure shows the digital bit stream and its respective ASK signal. It can be seen that each logic 1 is represented by a higher amplitude carrier and logic 0 is represented by smaller amplitude carrier signal but the frequency and phase of both the carriers are the same. In mathematical form it is represented as shown. For each bit duration the output V ASK of t is equal to either A cos 2 pi fct or A by 2 cos 2 pi fct if the input is logic 1 or 0 respectively. Thus giving two different amplitudes A and A by 2 with same frequency fc and same phase for the two carrier signal. ASK can also be represented in another form where one is represented by high frequency sinusoidal carrier and 0 is represented by no signal that is the presence and absence of the carrier with respect to 1 and 0 of the input data. This type of ASK is known as OOK that is on-off keying. Equation represents the mathematical expression for on-off keying scheme which states that during the bit slot output is carrier signal A cos 2 pi fct if data bit is 1 and 0 if the data bit is logic 0. Let us see first how OOK signal is generated. In this type of ASK modulator a simple product modulator or a mixer or a multiplier is used. To one input of this mixer input binary bitstream is applied in a NRZ coding form with two different voltage levels for logic 1 and 0 as shown in the figure. To the second input of the mixer a high frequency sinusoidal carrier signal is applied. The output is simply the product of these two signals. It can be seen that for the bit duration when input is 1 output is presence of carrier signal whereas for the bit duration when input is 0 output is absence of carrier. Thus the modulated signal is on-off keying or OOK signal. Let us now see ASK signal generation method. In this type of ASK modulator a switch is used as shown in the figure whose switching position is controlled by the value of the input bitstream during the each bit slot. This is similar to a 2 is to 1 multiplexer with a single select pin which connects the respective input to the output depending on which the value of the select pin either 1 or 0. To the one input of the switch a carrier signal with a large amplitude is connected as shown say V1 of t is equals to a cos 2 pi fct. To the second input another carrier signal with the same phase and frequency but with different amplitude with respect to the first carrier is connected as shown. Say V2 of t is equals to a by 2 cos 2 pi fct. The switch is made to move between input 1 and 2 based on the input data bitstream during the bit duration which is used to control the switching position. Thus the rate of switching is same as the bit rate. Thus you can see the resultant output ASK modulated signal as shown in the figure. When input bit is 1 the switch is connected to input 1 and thus the output during that bit slot is the carrier with larger amplitude. While when the input bit is 0 the switch is connected to input 2 and thus the output during that bit slot is the carrier with the smaller amplitude. Thus ASK signal can be represented with the mathematical expression as V ASK of t is equal to a cos 2 pi fct or a by 2 cos 2 pi fct if input is logic 1 or 0 respectively. These are the references you can use for further study. Thank you.