 So welcome to next lecture that is on double sideband and single sideband. So these are the learning outcomes. At end of this session, students will be able to demonstrate double sideband suppressor carrier modulation and single sideband. Before starting to this lecture, just recall what do you mean by modulation and the double sideband modulation. So we will start with the lecture that is DSBSC. So as we have seen in the previous lecture that AM modulated wave consists of three frequencies that is carrier signal and two sidebands that is LSB and USV. The carrier signal does not contain any information therefore two-third of the transmitted power is wasted so to make an AM signal more efficient we can suppress the carrier signal. When the carrier signal is removed, the remaining signal contains only two sidebands that is LSB and USV. Such a signal is referred as DSBSC. So the advantage of suppressing the carrier is what we can save the power which is being transmitted so that we can increase the efficiency of an DSBSC signal. So it is one type of an AM signal only. So we will see now the mathematical expression for DSBSC signal in a time domain. As we have seen this is an LSB signal and this is known as an USB. Similar from an AM signal we have removed the carrier signal that is VC sin omega CT. So this is known as an carrier signal. So these are the amplitude of the LSB and this is an amplitude of the USB. In the similar manner if we find out how much bandwidth is required to transmit an DSBSC signal so it is same like an AM signal that is two FM is required because it consists of an LSB and USB that is FC plus FM minus FC minus FM is equal to two FM. So there is not decrease in the bandwidth but more power is saved. So to generate an DSBSC signal this is the block diagram used for that. So the balanced modulator is used to generate an DSBSC signal. So when two different signal frequencies are being passed through a balanced modulator such as the carrier signal which is of different frequency and modulating signal which is of different frequencies are being passed then the DSBSC signal is produced. So we will see. So these methods are used to suppress the carrier signal. One is ring modulator and one more is the fade modulator. So in this lecture we will see in detail how the ring modulator is used to suppress the carrier signal. So this is the circuit diagram for the ring modulator. So here the diodes are being connected in the ring manner. So that's why it is known as a diode ring modulator. So it consists of T1 transformer and T2 transformer. This is an input transformer. This is an output transformer and diode D1 and D2, D3 and D4. Here the four diodes are used here. So to know how the carrier separation takes place assuming that the modulating signal is absent and only the carrier signal is applied first. Since here we are not going to apply any modulating signal first we are going to see how the carrier is being suppressed when the carrier is applied. So when the carrier is being applied here with respect to this, this is more positive with respect to this signal, this point. So when the positive half cycle of the carrier signal is there, so the current will flow in this manner. For that reason diode D1 and D2 are in the forward bias and D3 and D4 are in the reverse bias here. So see the directions of the current flowing through D1 and D2 which are equal and opposite in direction. So that is known as in the primary currents are equal and opposite. So what happens the magnetic flux generated due to this get cancelled to each other that is known as a zero output. So for this type it is known as a carrier as being suppressed here. Next if you see the same for the negative half cycle, the diode D1 and D2 will become in the reverse bias and diode D3 and D4 will be in the forward bias. Here also the same principle is used here see the direction which is opposite in direction the magnetic flux produced due to this will be get cancelled each other producing zero output here when there is no modulating signal. So now we have understood that the carrier is being suppressed without applying the modulating signal. Let us see now the operation of an RF signal carrier and with the modulating signal both are applied here. When the modulating signal is applied here it is of due to the center type transformer this is positive and this is the negative. So that is why the directions are plus minus plus and minus here. So for the first positive half cycle the diode D1 and D2 will conduct as we have seen in the previous and diode D3 and D4 are in the reverse bias. So this will be the plus minus plus minus here so you will get the positive half cycle. In the same manner when the negative half cycle is there so the polarity will be get changer. Now this is more negative with respect to this positive and diode D1 and D2 will be in the reverse bias and diode D3 and D4 will be in the forward bias but the polarities are going to be changed due to the switching of diode from D1, D2 to D3, D4 here. So here you will get minus and plus at the output side here producing then output. If you see in the graph so this is an modulating signal that is X of t and this is the carrier signal C of t. So D1 and D2 and is on so it will be in on state here producing this output and here is the phase reversal due to the polarity change of an diode D1 and D2. So this is an DSB SC signal where the carrier is being suppressed here so advantage of separation of the carrier signal is what you are going to save more power. So efficiency automatically it will go on increasing here. So let us see now what is doomed by an SSB modulation. So as we have seen in the double sideband transmission so the carrier is being suppressed and only two sidebands are being sent as the information contained in the lower sideband is same as the information contained in the upper sideband. So the transmitting at the both sidebands at the same time it is not essential at all. This means that no information will be lost by suppressing the carrier as one of the two sidebands of an AM signal that says nothing but an SSB signal. As LSB and USB contains same information so for some application where the only the speech is required so you can suppress the one of the sideband with carrier. So that such type of signals are known as an SSB. So if you compare the bandwidth required for an SSB is now only FM because in DSB SC signal and full carrier signal that is an AM signal you require two FM signal. Now the requirement of bandwidth has been reduced to only FM signal. So how much is your FM signal that much bandwidth is required to transmit an SSB SC signal. There are three methods of extracting the desired sidebands out of two sidebands. One is a filter method one more is the phase shift method and the third method. So let us discuss now the basic principle of an the SSB signal. First that is the carrier signal is applied to the modulating signal to the product modulator. The product modulator is nothing but the balance modulator. As we have seen at the time of the carrier suppression when two different frequencies are being passed through and balance modulators you will suppress only carrier and only two sidebands are present at the output here. That is the carrier signal and the modulating signal here you will get LSB and USB. So depending on the filter operation you are going to use. If you want to pass USB then the high pass filter is used. If you want to pass lower frequency then you are going to use the low pass filter here. So one of the sideband will get suppressed here and only producing one sideband at the output here. So whatever the output is produced is same whatever the carrier signal is there means the transmitting frequency and the input signal is of same type of same frequency. But to increase the frequency the balance mixer is used here. The crystal oscillator is used to boost the frequency which is known as an up conversion means the boosting of the frequency here it is done here. Why? Because the transmitting frequency and whatever the input signal are of same type here. So that is why we have to increase the frequency that is known as the done here that is a balance mixer to the linear amplifier. So this is the basic operation of an SSB to generate an SSB signal. It is very simple here because first we have to use a balance modulator to generate two sidebands that is LSB and USB and to suppress one of its sideband we are going to use a filter method here. So there is one more two method that is the phase shift method and the third method here. So these methods can be discussed in the later sessions. So these are my references that is analog communications by AP Godse and UA Bakshi and analog communication by V.H. Chandra. Thank you.