This is Dr. Rupali Sherke from Vulture Institute of Technology, Sulapur working as an associate professor in electronic department. In this video lecture, we are going to discuss on a continuous wave Doppler radar. At the end of this session students are able to illustrate the Doppler effect, derive the Doppler frequency in radar and also discuss the continuous radar system. As previously we have discussed with the classification of the radar system, basically a radar system is classified into two types that is continuous pulse radar system and continuous radar system. In a pulse radar system, the transmitted signals are in a form of the pulses while in a continuous radar system, a signals which has to be transmitted are in a continuous form. In both the cases, they are used to determine the range of the target as well as the velocity of the target. In this case, when the moving target is there, that time we are using the Doppler effect. Let us discuss what is exactly a Doppler effect. When a target is in a moving relative to the radar, it will results in an apparent shift in the carrier frequency of the received signal. This effect is known as a Doppler effect. Analogy to this statement, let us illustrate by using this diagram. This diagram is showing the ambulance which is approaching towards one of the user, let us call that user as a two and it is going away from the user one or we can call it as a observer's, observer one and observer two. As this ambulance is approaching towards observer two, the frequency of the ambulance is sound is increasing to the observer two compared to the observer one. That is because to the observer one, this ambulance is moving away. This is known as a change in the frequency which is given by the Doppler effect. Using this principle, we are going to derive the Doppler frequency. What exactly? How much is the change in the frequency? For that, let us consider a radar setup in which we are considering the radar antenna which is transmitting the frequency and we are assuming the target whose parameters has to be determined as a moving target. The signals transmitted from the radar are the continuous signals. You can see in this figure which are which are denoted by the frequency f0. Now let us assume it is a free space, free space means there is a no interruptions between the target and the transmitting station and let us assume r is a distance for them. And the transmitted signals are receiving reaching towards the target and then intercepted by them and re-radiated, the re-radiated signals are in a form of eco-signals are received by the receiver antenna at the radar station. The received signals have an apparent change in the frequency by some amount of the frequency called fd that is this apparent change in the frequency is observed in a transmitted frequency. Now let us see the equations. The total number of the wavelength in the two-way path is given by 2r by lambda. Why 2r? Because it is traveling in a two-way path as one wavelength corresponds to the phase shift of 2 pi therefore the total phase shift is given by 2 pi into 2r by lambda radians. If the target is in the motion then r is also changing and phi is also changing. The change in phi with respect to time is equal to the frequency. The angular frequency omega d is given by 2 pi fd which is nothing but d phi by dt. By substituting the value of the phi 2 pi by r by lambda we get the equation 4 pi by lambda 2r by dr by dt. The dr by dt is nothing but a velocity where fd is the Doppler change in the frequency and vr is the relative velocity of the target with respect to the rada. And by arranging we get the Doppler frequency which is equal to 2 vr by lambda. The relative velocity from this equation from this figure 2 is given by v cos theta.  If we substitute theta is equal to 0 in this equation then the received Doppler frequency is a maximum which is given by 2 v by lambda because theta 0 is equal to 1. And if you substitute theta is equal to 90 degree then fd will be minimum which is equal to 0. Let us illustrate the same thing with this diagram. If we substitute theta is equal to 0 then this is towards the antenna that is why it is maximum. If we substitute theta is equal to 90 then it is going away from the antenna therefore it is equal to 0. Using this principle let us see the block diagram of continuous wave radar Doppler radar. In a continuous wave Doppler radar a target is moving towards the radar system and the radar system consists of a continuous wave transmitter which is transmitting the frequency f 0 towards the target to determine the parameters related to the target. The received eco signals which is a change in a carrier relative change in the carrier frequency by the Doppler frequency is given by the f 0 plus or minus fd. The fd that is a Doppler frequency is added is plus when it is towards the radar system transmitter or it is minus when it is moving away from the radar system. The received eco signals which has a relative change with the with respect to the radar system are fed through the antenna to the detector receiver detector. The one more input to this receiver detector is from the continuous wave transmitter. Two frequencies are mixed in a detector mixer the output of the detector mixer is a fd. This frequency is nothing but a Doppler frequency. From this equation the carrier frequency or a transmitted frequencies are cancelled and only the change in the frequency is given at the output of the detector. The output of the detector that is fd is fed to the bit frequency amplifier. The function of the bit frequency amplifier is to eliminate or to remove the signals which are received from the stationary target and only pass the eco signals which are due to the moving target. And also the function of the bit amplifier is to amplify the signals received from the stationary moving target so such that they can be displayed on the indicator. The indicator is usually a frequency meter. Now what could be the applications of the continuous wave Doppler radar system? Just pause the video and think for a while a continuous radar can be used to determine the speed because of automobiles, shells, tidied missiles etc. In this case all these elements are in a moving condition that is why we can determine the speed of this using the continuous wave Doppler radar. They are also used to detect the movements of the troops vehicles even in the dark or in a bad weather condition. Also a continuous Doppler radar systems can be used to detect the aircraft in spite of the fixed objects. These are the advantage in a continuous radar system has a low transmitting power, low power consumption, small circuitry and small in size. These are the few references through which this contents have been covered. Thank you.