 Today we will have the second lecture of the series yesterday we had seen how analog systems play a role in the current day electronics in bringing about the required dynamic range and signal to noise ratio for the analog signal that is picked up by the sensors. And for preparing the signal required for getting converted to digital what are the basic requirements of analog these are indicated in terms of the popular products that are in both today in electronics and we will be selecting one such structure of a product namely we will be discussing about radio receivers which are primarily receivers which are used for entertainment purposes FM and AM receivers. So let us consider the structure of an electronic product it all starts with transducers as I have already indicated transducers convert here for example the audio to electrical signals one such transducer is the microphone pickup okay. So that is the first block that becomes necessary in most of the systems this transducer is connected to pre amplifier pre amplifier with filter okay to get rid of or improve the signal to noise ratio and amplify and then bring it to the dynamic range required for the A to D converter for further processing okay in terms of DSPs. So the current day baseband signal processing is mostly done in digital domain using DSPs it can it is also stored thereafter in memory the digital storage and thereafter when necessary for display or indication in the speaker okay outputting to the speaker is done after converting the digital to analog and applying it to the power amplifier required for driving the speaker actuator in this particular case can be a speaker. Now this is generally the structure of an electronic product okay in the case of video the transducer is the camera and the actuator may be the screen okay but rest of the things remain the same in terms of analog components as well as digital components. So electronic products process analog signals most of the transducers available today convert the non electrical signal to electrical signal okay and then this analog signal as to be processed for preparation to the digital domain and then becomes available as a digital data. Now in some cases it may involve transmission of this information baseband information okay and reception after the after a certain coverage is done of transmission in a channel. So the reception of the signal okay is going to require certain basic blocks and it is generally necessary to code the signal for example before it is transmitted in the form convenient for transmission for large distances. So that is normally done at very high frequencies okay and the this kind of coding can be called as modulation at the receiving end this is received okay and then converted back to the baseband and displayed. Transmission can be over wires or it can be wireless processing and storage are efficient normally in current day electronics in digital form. Several human interface technologies are available now we are going to consider for typical products and investigate what is the role of analog signal processing today in these 4 popular products radio receiver and modem these are the 2 topics we will be covering in the today's lecture and thereafter we will be covering cell phone which is the most popular product today and ECG recorder radio receiver let us consider you are familiar with most of you are familiar with 2 types of receivers that are in mode today AM receiver and FM receiver let us consider these radio waves these are classified as low frequency 30 kilohertz to 300 kilohertz medium frequency which is 300 kilohertz to 3 megahertz high frequency 3 megahertz to 30 megahertz very high frequency VHF 30 megahertz to 300 megahertz ultra high frequency UHF 300 to 300 megahertz to 3 gigahertz so super high frequency 3 gigahertz to 30 gigahertz extremely high frequency 30 gigahertz to 300 gigahertz so most of the commercial radio transmission occurs in these 2 frequency ranges this and medium frequency and high frequency range covers the FM AM medium wave short wave bands next the radio broadcasting is spread all over the radio band medium frequency high frequency and VHF VHF these are the regions where the television transmission occurs okay and it is in the gigahertz range that the cell phone transmissions keep occurring now let us consider the major modes of radio broadcasting radio broadcasting as you can see can be done by coding in this form if you consider a single tone or single frequency sine wave VP sine omega t plus phi as indicated that phi is the phase in radiance one can actually code the information the base band information whether it is audio video or biomedical signal in the form of phase variation directly of this frequency then it is called phase modulation it is then analog modulation right phase shift keying you can actually have binary phase shift keying technique so that it is a digital transmission 1 and 0 1 is 1 phase and 0 corresponds to another phase QPSK quadrature phase shift keying okay these are all digital modulation techniques right if you change the frequency on the other hand frequency in terms of radiance per second omega then FM is analog frequency modulation most of the city stations available today are in FM FSK means frequency shift keying again binary 1 and 0 1 corresponds to 1 frequency 0 corresponds to another frequency that is a digital modulation scheme then you can modulate magnitude VP peak magnitude in volts of this VP sine omega t plus phi when VP is modulated by the information if it is analog it is called amplitude modulation or if it is not having the carrier transmitted then it is called double side band modulation the next one is amplitude shift keying again you can have one amplitude for 1 and another amplitude for 0 that is going to be digital you can call it amplitude shift keying right so these are the various modes of coding the sine wave and transmitting either the analog signal or digital signal so these are the ones which are adopted in present day AM broadcasting or FM broadcasting amplitude of the carrier signal is varied in response to the amplitude of the ahh what is that base band signal so we have output equal to VP carrier you can see here this is the carrier amplitude and this is going to be varied as VPM sine omega MT which is the modulating frequency omega M is the modulating frequency this is the amplitude of the modulating frequency so that into sine omega CT this is the mathematical operation of amplitude modulation now this has a carrier amplitude which is VPC sine omega CT that with two side band omega C plus omega M omega C minus omega M are the ones that result because of this multiplication so amplitude modulation can be got by multiplying this signal along with the carrier right so that means the audio information for example is added to a DC voltage and multiplied by the carrier voltage so this amplitude modulation as VPM by VC VPC equal to M which is known as the modulation index percentage modulation is M% now typical modulation occurs this way this is the carrier signal at very high frequency it may be medium wave or short wave and this is the audio for example a typical sine wave okay a single tone that when this is multiplied with this with a DC shift then we get what is called the amplitude modulation which contains the modulating signal as the envelope of this what is the result of this multiplication you have the carrier coming through with two side bands okay and M corresponds to the modulation index here these are called the two side bands now this kind of transmission for general audience is done primarily to facilitate cheap radio receivers okay to be designed this is because the carrier is transmitted so that the receiving end okay the simple diode detector can detect the envelope and thereby recover the modulating frequency or the information so medium wave is this frequency range short wave is this frequency this is a well known thing let us see the various components that make up the AM receiver this is the normal architecture of most of the receivers so the front end consists of the receiving antenna this is an important thing which all of you know about and what is this going to form you have a receiving antenna which is a selective structure which is nothing but an LNC tuned filter right which receives the wanted station you can therefore tune this LC such that you can select the station that you want to receive and that is applied to the RF amplifier okay which is a tuned amplifier at the input tuned to the required frequency and it is having omega C plus or minus omega M max maximum frequency that modulates the carrier. So this is the bandwidth that is twice omega M max is the bandwidth around omega C that kind of filter is what you have to design here and after that it is applied to a mixer mixer is again a multiplier okay so which is capable of translating the frequency okay from the carrier to the intermediate frequency it is shifting the information from the RF carrier to the IF frequency the same information is transmitted okay from the RF to the IF using the mixer therefore you have to have a local oscillator here which is going to multiply it with omega C plus omega IF so that you can have the information transmitted from transferred from the RF carrier to the IF carrier right so that is all that is done this is to facilitate easy amplification of the signal within the IF stage because IF is a fixed frequency in the case of radio receivers the IF frequency chosen is 455 kilohertz so that the carrier now becomes from some arbitrary RF corresponding to station you want to receive to 455 kilohertz so most of the sensitivity of the radio receiver depends upon the IF amplifier sensitivity gain. There after you have an AM detector which is normally a simple diode peak detector which also gives you the strength of the DC here gives you the strength of the IF that is received and therefore that is controlled by the automatic gain control here unit it is controlling the gain of this so that when the incoming strength is low okay the gain is increased when the incoming strength is high the gain is decreased so that the overall gain of the entire stage remains fairly constant so that there is no fading effect that is seen. So this is an important analog path in most of the radio and television receivers AGC automatic gain control or automatic volume control that is a purely analog circuit this automatic gain control and automatic volume control is also there in most of the digital systems also where the digital ones and zeros themselves may vary in amplitude and in order to make them okay the same okay so that the threshold detector does not have trouble identifying ones and zeros. So this AGC is an analog path which is mostly there in almost all receiver systems. Now after the detection of the baseband signal there is a power amplification that is there normally a class D power amplifier in the audio stages and that for purposes of efficiency and when you power amplify efficiency matters a lot okay so would like to have almost nearly 100% efficiency if it is possible and therefore the most of the useful energy is now driving the speaker okay and appearing as output. So the various blocks have been already explained here antenna the RF amplifier okay mixer which is shifting the carrier frequency from RF to IF okay and IF amplifier which is again a tuned amplifier with the bandwidth of twice the maximum signal bandwidth okay. Now AM detector is normally again it can be a multiplier but for the simplicity it is here a diode peak detector AGC automatic gain control is used to remove the effect of fading in the channel that normally occurs because the received signal keeps on fluctuating depending upon the channel ahh properties. Power amplifier feeds the speaker this is normally a class D power amplifier FM broadcasting frequency modulation I have already explained to you the frequency of the carrier signal is varied in response to the amplitude of the signal to be transmitted. Frequency modulation is achieved by designing an oscillator whose frequency is controlled by the modulation of the signal using the voltage controlled oscillator concept that is if you design a voltage controlled oscillator what it means is the output frequency is sensitive to the input DC voltage by changing the DC voltage you can control the frequency at the output. Now FM broadcasting is over this band of 88 megahertz to 108 megahertz in India particularly FM channels are separated by about 0.2 megahertz. Now what is FM FM broadcasting okay ahh the this is the audio signal or base band signal and you can see since the frequency is modulated as the amplitude is high the frequency is increased as the amplitude is low the frequency is decreased and this is the way the FM modulation occurs through the VCO therefore gives an output which is VPC this is the carrier peak amplitude sine omega C carrier frequency plus delta omega D corresponds to frequency deviation and omega M is the modulating frequency base band frequency that into T. Now that kind of signal looks like this again what does FM receiver comprise of we have the antenna at the front end with RF amplifier again with the center frequency and the bandwidth here okay. Now the bandwidth may may be ahh higher than what is required for the AM okay because of frequency modulation and we have the mixer here doing the same thing of shifting the RF carrier to IF carrier as far as the information is concerned it is simply shifted from RF carrier to IF carrier using the mixer or the multiplier and ahh we have ahh IF amplifier which is since the FM is occurring at a higher frequency and it is requiring higher bandwidth okay we have this higher frequency chosen as 10.7 megahertz fixed frequency here and ahh we have the FM detector which may be a PLL okay and then we have the amplitude information of the IF carrier okay given to the AGC and AGC is controlling gain of the stage so that the strength of the signal received is remaining fairly constant and ahh the audio recovered from the FM detector is feeding to a stereo power amplifier with stereo speakers here FM transmission is normally ahh high quality music high fidelity which corresponds to about 30 kilohertz bandwidth as far as high fidelity is concerned. So these are the component blocks and you can see almost ahh similar blocks analog blocks are used as AM receiver in FM receiver except for the FM demodulator which can be FAS lock loop or frequency discriminator and the rest of the blocks remain similar. We now consider the modem which is another ahh block that is commonly used along with the computer all of you have seen modems okay. Now modem is a short for modulator demodulator M O D E M modem is a hardware device that enables a computer to send and receive information over for example not necessarily telephone lines it could be hardware or telephone lines or ahh any other channel communication cable okay wireless in case of satellite modems okay. So ahh ADSL modem is normally the one that sends the thing to the telephone line. It converts the digital data used by the computer into an analog signal that means ahh for example one way is one corresponds to 500 hertz 0 might corresponds to 600 hertz over telephone lines. So that is called frequency shift keyed information FS key and converts the received signal analog signal which may be 500 and 600 into digital data once and zeros to be used by the computer. So modulator corresponds to converting the digital data to analog in terms of FSK for example and converting back the FSK signal to digital data once and zeros that is demodulator. Modems are asynchronous devices the device transmits the data in an intermittent stream of small packets this is the packet switching once the received ahh data is got the receiving system converts this data in packets and this assembles it to form what the computer can use right. So this is the function of a modem modem is one of the most common analog blocks used with a digital system computer. So we had already finished this now what is it that a packet contains it contains one or more bytes bytes means 8 bits is transferred within the packet which is equivalent to one character for the computer to receive information each packet must contain a start and stop it this is the ahh way of coding the thing therefore the complete packet would be about 10 bits generic functional block diagram of a modem. So we have the telephone line here okay now to this we have ahh one input coming this input is coming from the DSP that is we have the information storage stored in DSP form right. So digital form and this is a DAC which is converting the digital information to analog information like for example FSK the transmitter has a gain stage here okay to improve the signal to noise ratio right and then the transmitter has a filter so that it does not unnecessarily transmit ahh other frequencies. So then we have a line driver going on to this here this has a ahh ahh double path here that is why it is called the duplexer so to the same line this transmitter as the signal and receiver signal comes from the same thing okay ahh to a receiver pre amplifier and filter and then ahh programmable gain amplifier so that the strength of the signal is maintained constant here okay and it goes to A to D converter to be converted to the digital for the processing that. So this is what the function of the modem is and you will see ahh the analog blocks appearing here okay we have to now ahh also discuss the cell phone and the electrocardiograph in the next lecture and ahh today we have discussed ahh two important ahh products the receivers which are mainly meant for entertainment purposes. So ahh the idea is that the transmitter may be costly so and it may be transmitting power which may not be utilized efficiently because it is going to be transmitting it in general to everybody and some people may be receiving it some people may not be just interested in it. So lot of power is wasted and therefore most of the cost of the transmitter is such that the receiver design is simple and the receiver is cheap this is the purpose behind this kind of general transmitters. So ahh that is the super heterodyne receiver that we have seen what is super heterodyne super heterodyne heterodyne means mixing okay it is a super heterodyne receiver concept which facilitates easy design of the detector AM transmission is like that. So the AM detector is just a simple diode and ahh we have multipliers or mixers to shift the carrier okay from the RF to the IF. It is an important ahh transfer in order to again facilitate ahh good design of IF stages fixed frequency tuned amplifiers later on see that it is nothing but a filter okay it may be a broadband filter okay that is to be designed efficiently we will see how these filters are designed later again. Then we have the AGC which is a feedback system which actually is there because mainly the received signal keeps fluctuating depending upon the atmospheric conditions of the channel communication channel since it is a wireless channel so it is going on changing. So as it is changing the signal is going to either fade or increase in strength so much that it may distort the amplifier. So this should not happen. So if AGC is not there then at times the signal gets too much distorted or at times it may not be received at all. So automatic gain control maintains the input strength constant. So we have this ahh signal okay which is ahh ahh to be strengthened and maintained constant okay so that the output of the received signal is substantial enough to drive a power amplifier okay. This is the case of the architecture of ahh AM receiver as well as FM receiver the only difference is in the modulation scheme at the input and the demodulation scheme at the output okay. Now tomorrow we will be discussing about two other products cell phone and ECG. Again we will see that it is different end and the back end which is analog that we have to take care to learn about.