 Hello friends, myself Ilya Archanveni, assistant professor from Department of Electronics, Valchen Institute of Technology, Solapur. Today we are going to see the topic frequency detectors part 2 from the subject analog communication. So what are the learning outcomes in this? So basically at the end of this topic students are able to first design, discuss the term FM detectors in detail, design the different FM detectors and its types, discuss the advantages of FM detector. So how FM signal is generated? So this is given by generation of FM, this is done by the two different methods. First one is known as direct method which includes reactance modulator, directed diode. The second method includes indirect method in which Armstrong's indirect method is used for generation of FM. Detection of FM can be done by using the four different methods. First one is known as slope detector, the second one is known as balanced detector, the third one is known as foster sleet detector and the fourth one is known as ratio detector. In the frequency detectors part 1 we had seen slope detector and balanced detector circuit and their frequency responses. In this video we are going to see the other two that is foster sleet and ratio detector, the circuits and frequency response. So this is the circuit for foster sleet discriminator or detector, here this circuit is nothing but a full wave rectifier from the secondary, from the secondary windings and the primary windings include a capacitor C1 which is a variable which is connected to the inductor and from the secondary windings of the circuit here a C2 capacitor which is the variable one which is connected at the secondary winding of the circuit, the C acts as a coupling capacitor between the primary and the second, R2 and R3 and C3 act as the RC filter and R4 and C4 acts as the RC filter or the load for this particular circuit. So the voltage across these two are V01 and V02 and the total output is given by the addition of V01 and V02, below is the frequency response of the same, here useful range lies between Fc minus Del F and Fc plus Del F, here the useful range that means here the circuit works efficiently in this range, whereas Fc is nothing but the carrier frequency and Del F is the frequency deviation. So for the working we have to consider three cases, in case one Fn is greater than Fc hence voltage at D1 is equal to voltage at D2 therefore the two voltages across RC circuit are equal hence the output is equal to 0, for case two we are considering Fn to be greater than Fc hence voltage at D1 is greater than D2 therefore V01 is greater than V02 therefore the output is positive, for case three Fn is less than Fc voltage at D1 is less than voltage at D2 V01 is less than V02 therefore the output goes negative, hence by these three cases we can understand that the FM waves are demodulated in these three cases and we are getting the modulating signal. So what are the advantages and disadvantages of this detector, firstly alignment tuning can be done for tuned circuits that is similar frequencies can be tuned in this circuit by using capacitors C1 which is connected at primary and C2 which is connected at secondary, these circuits are tuned to the same frequencies, thirdly the circuit works linearly hence the frequency response is very much linear as we have seen in the response the useful range is a linear one. The disadvantages are the circuit does not provide amplitude limiting here in this circuit amplitude varies as the modulating frequency varies. So this produces error at the output, so next is a ratio detector in this, this is the circuit and this is the frequency response for the same here this circuit is similar to the foster sleeve detector but here there is a minor change that is that D2 is connected in the reverse bias condition rest of it is same and here we are considering a point O which acts as a operating point for the circuit. The working generally depends upon this operating point which is connected after the RC circuit and which is across which the output is taken that is V0. The frequency response is same as foster sleeve here also the useful range generally it lies between FC minus del F and FC plus del F where FC is the carrier frequency and del F is the frequency deviation. This frequency deviation can be found out by using the modulation index. So for this we have to consider two cases in this circuit as we are considering point O as the operating point of the circuit. In the case one we are considering that the diode is in reverse bias hence the point O is positive with respect to point B which is connected at the RC circuit hence the output which is given by VA is the sum of voltages but the C5 which is connected across the RC circuit at the output side keeps this voltage sum of voltages constant. In case two we are considering that D1 is in reverse bias hence the circuit operates in vice versa mode that is the output which we are taking at VA is generally the difference of voltages but here the C5 at this case also the C5 keeps the sum constant hence the output voltage V0 is generally given by the difference between the two output voltages from two different diodes D1 and D2. Hence the voltage is given by V0 is equal to V01 minus V02 divided by 2. Here the amplitude limiting action is generally provided by the capacitor C5 where how that we are going to see now in these three stages here the input which we are giving FM it tries to increase the voltage due to which the secondary voltage increases and which leads to a high current flow through the diodes D1 and D2 hence this increases the load current across the point O. This load voltage remains constant due to capacitor 5 as capacitor 5 is connected across the RC circuit and the output voltage terminal. Here the capacitor C5 keeps the load voltage constant across the point even though the FM varies from high to low. What are the disadvantages? The disadvantage of this ratio detector is the circuit is complicated hence it requires more components and the circuit becomes bulky. Here it becomes difficult to trace out the another disadvantage is that here it becomes difficult to trace out in which part the error has occurred or the circuit has shorted. So give comparison between Foster's Lee and a ratio detector depending upon the circuit diagram and the frequency response which we have seen for the both Foster's Lee and ratio detector and the points which we have seen in advantages and disadvantages. So you can pause the video at this point and you can write down the comparison between Foster's Lee and a ratio detector. So these are some of the references which are used for making this PPT and you can use these books for the further study for frequency detectors part 2. Thank you for watching the video.