 Hello, and welcome to video session on Superheterodyne receiver myself, Mr. S. N. Chamath Gauder, assistant professor in department of electronics and telecommunication, Walshchand Institute of Technology, Swalapur. This is the learning outcome. At the end of this video session, students will be able to describe Superheterodyne receiver and characteristic of receiver. Before we proceed with this video session, let us recall what are the drawbacks of tuned radio frequency. I assume you have recalled the basic problems of TRF receivers. Let us revise one more time the basic problems of TRF receiver because this will help us to understand the next receiver that is Superheterodyne receiver. The basic problems of TRF receivers are instability, tracking of tuned circuit, variable bandwidth. As we know that TRF receivers in the TRF receiver, the initial stages like RF first stage, RF amplifier second stage and the detector circuits are connected to a common knob through mechanical. That we call it as a gang tuning knob. Here the expectation is that the variable capacitance value in all these three stages must be equal, which is not the case. Hence, the resonant frequency is differs from one stage to the other stage. That is the instability. Second one, the tracking of the tuned circuit. We know that we use the multiple amplifier stages to amplify a common frequency signal, which gets a high gain and gives a positive feedback, which is unavoidable. That is nothing but the tracking of circuit. Third one is the variable bandwidth. The bandwidth changes as the selection of frequency changes in the frequency band. This is the Superheterodyne receiver. Here we can see that a slight modification has been done to the TRF receiver by adding the local oscillator, mixer and multi-stage IF amplifier stage. These are the three additional blocks, which are added to the TRF receiver. TRF receiver is already having this RF amplifier stage and detector stage, audio amplifier, audio power amplifier and loads. Here also in this Superheterodyne receiver, we can see that the RF amplifier stage, mixer stage and the local oscillator stages are connected mechanically to a common knob known as GaN, antenna receives all the radio frequency signals available in the atmosphere and feed it to the RF amplifier. Using the GaN tuning knob, we select the desired frequency and the desired frequency is being amplified by the RF amplifier and it rejects all the remaining other frequencies. And here RF amplifier gives some initial gain to the received signal or desired signal and feed it to the mixer. Mixer also gets one more input signal from the local oscillator that is F naught. Let us call the received signal as FS, so that the local oscillator frequency is maintained in sink in such a way that the mixer produces the constant output all the time. If the frequency of the received signal changes accordingly, the frequency of the local oscillator must change that is why the local oscillator and the RF amplifier and the mixers are connected mechanically here. And the mixer accepts receives two input signals FS as well as F naught and it produces the difference frequency that is F naught minus FS which we call it as intermediate frequency. Now, one thing we have to understand that intermediate frequency carries the same modulation has the original frequency and the characteristics of the IF amplifier is independent of the received by the antenna ok. Once the IF signal is been IF intermediate frequency is generated by the mixer that is fed to the multistage IF amplifier which will provide again sufficient gain to the intermediate frequency signal and that will be fed to the detector circuit. Now detector circuit recovers the original signal from this intermediate frequency signal and pass it to the audio amplifier. Audio amplifier and power amplifier boost the signal sufficiently to drive the loudspeaker. Here in this circuit two more circuits are important that is AGC and AFC. AGC stands for automatic gain controller which controls the overall voltage of the circuit here and AFC stands for automatic frequency control which controls the frequency of the local oscillator. The characteristics of receiver the performance of the radio receiver can be measured in terms of following characteristics first one selectivity second sensitivity third fidelity fourth image frequency and its rejection fifth double spotting. These are the performance parameters of the radio receiver to be measured. Let us discuss the characteristics of receiver one by one first one the selectivity. It refers to the ability of the receiver to select a signal of a desired frequency while rejecting all others. Selectivity in a receiver is obtained by using tuned circuit. The quality factor of these tuned circuit determines the selectivity of the receiver. This is the selectivity curve receiver is tuned to 950 kilohertz we can see this curve here ok. Now vertical axis represents the dv and the horizontal axis represents the frequency here and the receiver is tuned to 950 kilohertz that means the zero value reference value represents the 950 kilohertz. Now we have to put a bar over here at 3 dv level so that we can find out the selectivity band of the frequency. Second one is the sensitivity of a communication receiver refers to the receiver's ability to pick up the weak signals and amplify it. It is often defined in terms of voltage that must be applied at the input terminal to give a standard output power measured at the output terminal. Third one fidelity. Fidelity of the receiver represents the ability of the receiver to reproduce all the modulating signals present in the space. The fidelity at a low modulating frequency is determined by the low frequency response of IF amplifier and the fidelity at the high modulating frequency is determined by the high frequency response of the IF amplifier. It is difficult to obtain because good fidelity requires more bandwidth of IF amplifier resulting in over selectivity. Next one is the image frequency and its rejection. In standard broadcast receiver the local oscillator frequency is made higher than the signal frequency by an amount equal to intermediate frequency that is f naught equal to f s plus f i. In other words f i is equal to f naught minus f s. When f naught and f s are mixed f i is obtained as a byproduct in the mixer. Only f i is passed and amplified by the IF amplifier stage. Let us assume if a frequency f si appears at the input of the mixer then it will produce the sum and difference frequencies regardless of the input. Therefore the mixer output will be the difference frequency at the IF value. The term f si is called image frequency. This signal is also amplified by the IF amplifier resulting in interference. Next one is double spotting. The phenomenon of double spotting occurs at high frequencies due to poor front end selectivity. In this the receiver picks up same short wave station at two nearby points on the receiver dial.