E68 Broadband Communication Circuits - Lecture 5

so this is e68 5 blsi Broadband communication circuits so before going into the introduction for the class so I'll just uh say a few words about the tablet I chose this mainly because I could also record the lectures and put it on the web particularly because there is no book for this course but uh I don't recommend that you stop taking notes completely because you also learn something when you write down uh you don't learn by passively reading even when you're reading a book it's best if you work through the steps it's not like reading a story book or something so by watching Baywatch you won't become a lifeguard similarly by just listening to the lecture you can't absorb everything that's going on but it will reduce hopefully the drudgery of uh reproducing every diagram that I write because you all can always refer back to it or at least you have a record of the whole thing so and also remember that this is an experimental stage and Technology can fail at any time so keep keep a backup also right now what I'm doing is using the tablet like more or less like the traditional Blackboard so if you can think of some other ways of using it which is very different from how it is used now then of course I would be welcome to all suggestions so what is this course all about so it says Broadband communication circuits so basically what we will talk about in this course are circuits that are used for digital Communications and digital Communications basically means say sending digital data bits from some place to some other place so I'll just across the channel and the channel can be anything so as you know Digital Data is transmitted in uh mobile phones in Ethernet in USB in a variety of uh applications and this uh the channels that we consider in this course also include uh cases where you are not particularly interested in telecommunication meaning if you have a PCB and you want to send data from one side to the other you may not think of it as a communication link but it is right the idea is not to send messages from one person to another it's just to uh have Digital Data on one end appear at the other side now what about Broadband what do we mean by Broadband so if you look at the spectral density of any transmitted signal it will occupy some extent some finite extent we can't deal with infinitely large bandwidths so let's say it is something like this what this implies is that in this range the uh there is a lot of spectral content outside there it has negligible spectral content and the reason of frequencies over which it has significant Energy is known as the band width and the signal is centered around some Center frequency FC so we use the ratio FB by FC to distinguish between narrow band and Broadband signals so if FP by FC is very small so let's say much smaller than one it's a narrow band signal and if it's of the order of one or even more it's a Broadband signal so the spectral density of a Broadband signal might look like this it may extend all the way up to DC or something close to DC and up to some high frequencies so in this case if you think of this as the center frequency the ratio of bandwidth to Center frequency is two where do you see narrow band signals have you seen examples of either of these cases where have you seen narrow band signal where do you see broad band signals mobile application should be narrow band signals why would use narrow band signal why would use Broadband signals broadcast radius are narrow band so it is related to that so when you have a common medium across which you have to transmit multiple channels you have to support multiple channels then you will op for narrow band because using different frequencies is one way of distinguishing between different channels so if you have a common medium like air and all signals have to be transmitted across the same medium then the one of the ways and the most obvious way to distinguish between them is to use different frequencies for different signals so these things necessarily will be narrow band so you have a certain usable range of frequencies and you divide it further into channels and you use that for communications so this is typically the case for transmission over a common medium like air now the medium may not always be common meaning for every signal that you need to transmit you can have a pair of wires between point a and point B wherever you want to transmit in that case you are not constrained by this you can occupy the entire range of usable frequencies I not Define what usable frequency is it depends on the situation okay it basically means the range of signals that you can uh conveniently process that's all so if you have transmission over a confined medium for instance a pair of wires right so if you have two separate pair of pairs of wires and you transmit signals on both of them they don't interfere with each other at least not to a significant degree but then if you transmit two different signals in air they'll completely interfere with each other right one will affect the electromagnetic field just as much as the other one will right so in this case maybe you have a Broadband signal so when do you transmit signal over wires what are the examples that you know of telephones yes ethernet what what other say yeah Optical also in the optical domain it is narrow band because Optical frequencies are necessarily very high and the bandwidth of the signals that you transmit is small but yeah actually we will deal with some of those things also what else where else can you think of digital signals being transmitted over pair of wires TV channels actually TV yeah now cable TV is transmitted over cable but they're also you have the same medium for various channels so you have to have some narrow band requirement so there is a huge variety of examples one is sending signals over telephone lines telephone lines of course traditionally carried voice in an analog fashion but now there is Digital Data if you look at DSL it is digital and various flavors of DSL right and another common example is ethernet you all seen and are familiar with ethernet and you have Digital Data transmitted at various rates depending on how old your modem is so you see things like 1000 and 1,000 Bas T it refers to whether the data is being transmitted at 10 megabits per second 100 megabits per second or or 1000 mb per second and people are currently working on uh making ethernet modules for 10,000 mb per second or 10 GB per second okay over almost the same pair same kind of cable so the speeds are increasing constantly then what else we have USB you have all kinds of devices working off USB and it's not necessarily something like a mouse which needs only a uh low speed uh link it can be a high speed link also now it is up to 480 MBS per second then you have Optical links now if you look in the optical domain these are narrow band signals but we'll need Broadband circuits to process these signals that we'll see afterwards okay when we translate the signals from Optical to electrical domain you'll end up with Broadband signals and you'll need Broadband circuits to process them okay so these are all examples where you it is like a communication link meaning you are explicitly thinking of sending data from one side to the other and the examples where that's not the case but you do have some similar Behavior or you have printed circuit boards and you have to transer data across printed circuit boards I mean that may be simply because you have two chips so this is a printed circuit board you have chip number one and Chip number two so this could be the processor and this could be the memory and you have to transmit data between the two and as you know the speeds are increasing every day right so anytime you have to transmit the data transmit Digital Data between these you will have to deal with the same issues as in the as in these cases where you think of these being explicitly as communication links okay so this is one example and there are what are known as back planes back plane is nothing but a large PCB which holds a large number of small pcbs there are they are known as line cards so finally all of your let's say ethernet traffic goes through some switches and so on so you can see some of these back Lanes in those large switches so you have many links which are put together and then you have to connect between various links and you'll have back planes so it's nothing but a large PCV with PCV traces up to 1 m okay so the kind of issues that you have to deal with in PCB TR are similar to what you have to deal with in telephone lines maybe the frequencies are different maybe uh the exact uh magnitude of signals are different but the kind of things that you will see the kind of problems that you will see in transmitting data will be the same okay so these are also Broadband communication links now you can go even further inside the chip itself so on the chip I don't know how many of you are familiar with how how an IC is made an IC is nothing but a piece of silicon on which with lithography you fabricate the transistors and the interconnects and the chip is packaged inside some plastic or ceramic enclosure and the pins are brought out you definitely seen chips in your lab and so on so typically you have a piece of ceramic or plastic with pins on the sides or it may be at the bottom and inside the package is a cavity in which you place the Silicon chip and the chip is connected to the package through water are known as Bond wires so you have small wires about 25 microns thick connecting the chip to the package and on the chip you have traces just like you have on a PCB connecting the bond wires to the pins so this is how you make connections between uh the outside world and what is going on on the Silicon right you have the connections on the package and bond wires and if you have to transmit Digital Data across these again you will encounter it turns out it will encount you will encounter the same problems as you do in any other case Okay so even that falls under in way under Broadband Communications okay and on top of it you can have a single chip and you have two different parts of the Chip And you have to transmit data from one one part to the other and even there it's the same thing everything is scaled down in size that's all the wire is much shorter but it's also much thinner okay so it turns out that you will have the same kind of issues so a link between two parts of the chip also classifies as a broadband communication link now depending on the situation it may be easy or difficult to transmit data across any of these things so first thing is what are the what's the what is the potential issue that you see we have a pair of wires and you want to transmit data so what is the big deal you put some voltage on this side and the same voltage appears on the other side doesn't it what is the problem what are the problems in community ating data across a pair of wires because a Wire by definition is a good conductor and you apply a voltage on one side and the same voltage should come to the other side so why are why do we even have a course about the whole thing okay parastic capacitances so what happens where is the parastic capacitance now in this situation so this is the kind of scenario that we are discussing right every scenario that I outlined before whether it's Ethernet or connections on a chip is something similar to this you have two wires and you have to transmit data across it so what is the problem so this this serves as a good model for everything that we saw before now what is the problem with this why do we have to study this or why do we have to even uh think think about transmitting data right you apply voltage on one side and it should appear on the other side what are the problems that you see so okay where is the capacitance in this between the two wires okay so what is the problem now so what happens is that that is true basically a wire is not a perfect conductor and firstly a pair of wires are in proximity to each other so there is capacitance between them and since the wire is not a perfect conductor it has some series resistance and because it has some finite extent it also has series inductance okay so you have the wire really is made up of these resistive inductive and capacitive elements okay and in reality it is distributed it's not like you can identify some particular part as being resistor or some other part as being an inductor okay and a different part as being a capacitor the capacitor resistor and inductor Are all uh present continuously so that is known as a distributed system okay so you can think of the wire as being made up of RLC sections like this and so on okay so this is the reality of the wire it's not a perfect conductor it has parastic resistance parastic inductance and parastic capacitance the next question is so what what happens because of this what is that that's okay what is the problem with that okay firstly there is a lag in voltage meaning the output doesn't appear instantaneously yeah actually the problem is that it is dependent on frequency okay we'll go into details of that later in fact lot of these things I've just introduced without uh describing in detail but the whole course is about that this is just the introduction okay so let's take a very simple case where we'll say that the inductance is insignificant okay and also we'll make a further approximation that we don't need this distributed structure we'll simply consider one resistor and one capacitor okay so let's see what happens in that case let's say our perfect wire was replaced by something like this and as I mentioned earlier I am interested in transmitting Digital Data okay so let's consider a case where I'm sending across a bunch of bits and minus1 volt stands for my symbol zero and + one volt stands for symbol one okay can you see properly on this or should I write bigger so what do you get out is there a problem so the idea of any communication link is that you send something that you don't know on the other side obviously that's why you sending it and you should receive exactly what you sent that is the that's an ideal communication link or that's what you aim for okay now this is my message I have sorry 1 0 0 1 1 something like that okay some arbitrary bit pattern what do I receive depends on the bandwidth okay so assume something and tell me what happens so it depends on the values of R and C but we know the step response so we firstly can recognize that this digital bitstream can this bit the digital bitstream here can be broken up into a number of steps and we know the step response of the RC filter RC system so we can reconstruct the output signal so depending on let's say R and C are very small to start with in that case what you get is let's say you send this this is what you send and what you receive will be so it starts from here and the edges won't be sharp right because the capacitor voltage cannot change abruptly it can only change gradually so it will do something of this sort this is for some particular value of RC let's say now I go on increasing the value of uh R and C what will happen happen the time constant will become longer and longer so maybe it will take it'll do this and it can even become worse than this in that even in two even in the time required to transmit two symbols it has not gone all the way to the steady state value okay so this figure is a little cramped so let's reexamine the same thing and this time what I'll do is I'll just transmit single one this is 0o which is - 1 Vol and this is 1 which is + 1 volt so firstly how do we distinguish zeros and ones at the output so one thing that we have to recognize here is that what you get at the output is an analog waveform see what is a digital waveform it consists of a discrete number of levels that is y- axxis is quantized whereas what we get at the output is something like this in this case the y- axis is definitely not quantized so firstly how do we distinguish between once and zeros of the output what is the threshold here 0 volts because we are we are using symmetrical voltages above zero for our 0 and one symbols our threshold is 0 volts so let's say I look here and if it is above 0 Vols if it's positive I say it is 1 and if it is negative I say it's a zero so I don't care about the exact voltage I'll just say that if it's greater than this it's 0o one and otherwise it's zero okay so that gets rid of the problem of having an analog output meaning having a continuous time output like this uh with continuous values right so even if the output voltage is only9 volts we still recognize it to be a One S Syble one but what happens as we increase the time constant RC so it can do this okay if the time constant is large enough it can do this where it has barely the input goes from 0 to one and back to zero the output starts from a negative value because it has reached steady state with minus one volt and then it barely reaches the positive values and comes down and it can be even worse in that it may not even reach 0 volts okay you can always I make R and C large enough that this happens so here it doesn't attain positive values at all okay so anywhere you look you only see negative voltages so you would obviously determine that uh the transmitted sequence was all was a string of zeros okay whereas there was a one in it and you couldn't detect the one so that's a gross error now this case it does reach positive values but you can see that the positive voltage that is reached is small so in this case of course with a perfect detector you can always detect this one but the detectors also have some error so if the error happens to be greater than this value you may not detect it correctly okay so this definitely gives you errors this may give you errors depending on how robot how accurate the detector is and this is a very good case so this is the fundamental problem with transmitting data across a pair of wires the wire is not perfect and you think you are sending Digital Data the first thing that happens is even if you send quantize levels that change abruptly like this right that is the meaning of a Digital Data it has discrete levels and it also changes at discrete some instance of time I mean some usually some periodic instance of time but the output will not be like that the output shows a continuous voltage uh that is a continuous amplitude meaning the output amplitude is not discrete it doesn't consist of two values or any number of finite values okay so that is one problem that itself is not a big issue because you can always map a range of output amplitudes to one symbol okay it's not like you're saying if I receive exactly one volt I'll say it's say one no that's not the case right you're saying if it is more than zero it's a one so that problem the first problem is got rid of by appropriate thresholding right so you m range of output amplitudes to one symbol so if you have a number of symbols and you have infinite number of output amplitudes infinite number of levels but you map a range to a particular symbol and you choose the ranges appropriately now the problem is even with appropriately chosen ranges if your wire is bad enough meaning your if your R and C are large enough you can have a case where it's very difficult to distinguish which range it is in or where it's impossible simply because the signal at the output doesn't reach the amplitude it's supposed to reach okay so this is the basic problem with digital Communications so how do we get rid of it is the topic of the course okay so now here we saw that how bad the problem is depends on how much the RC time constant is in or alternatively how much the cut off frequency of the filter is okay the Bas we will see that it is related to high frequency attenuation okay so if you have a very large time constant or a small cut off frequency then you will have bigger problems transmitting Digital Data right so this will be a problem for everything to transmit data over any pair of wires this is this can be an issue now it could be that in most of the case in many cases that you see the RC time constant is small enough when I say small it's compared to what these are Dimension quantities so I have to compare it to something what is when I say small so in this particular case if this is the symbol rate if to is much smaller than TS I don't have any problem and this is the kind of case that you encounter normally let's say in the lab when you connect uh each time you connect a pair of wires to take some signals from the let's say the signal generator to the output you don't think of all this you don't have to think of all of these impairments right so that is simply because the time constant in that case is much smaller than the time period that you are interested in okay but you will see cases where the time constant is uh comparable to are much larger than the time period that you interested in okay and that is true for the cases that we saw earlier so if you look at uh any of these cases let's say transmitting DSL data over a phone line right the phone line has a very low bandwidth and you try to transmit DSL data and even the traditional ADSL has a bandwidth of I think 384 kilobit per second compare this to voice which has a 4 khz bandwidth okay so it's almost like 50 times larger bandwidth or something so voice goes through the cable without any problem voice is not digital signal but I'm just uh concentrating on the frequency content of the signal but DSL when it goes through the if you transmit this one Zer kind of binary Digital Data through a telephone line on the other side you'll absolutely not be able to recognize the one Zer data so this whole DSL modem is about transmitting it in the correct way so that you can recover it properly right similarly for ethernet if you look at the output of even a 100 megab per second ethernet line you connect it to an oscilloscope and C you will not see anything like this okay firstly it's not a twole Digital Data one thing I have to say here is that Digital Data does not have to have just two levels we don't need only binary Digital Data you can have uh five levels or seven levels or whatever you use whatever is convenient and it turns out that in Ethernet you in gab ethernet you use five levels okay but if you look at the output at the cable you will not see five different levels you will see a whole mess of things okay and similarly for USB or any other case now I think you all use pcbs in the lab again when you are using pcbs to let's say transmit data from one digital chip to another you don't think of all this again that's because the time constant in that case happens to be much smaller than the period of the signal now there are cases where the time constant is much more than the period of the signal so what is happening is the amount of data you transmit has been increasing constantly as you know the hard disk capacity is increasing the data rates has been increasing the amount of stuff that you want to download is increasing everything is increasing right so the way to accomodate that is to either have more wires to accommodate all this data or you use the same number of wires but send more data in the same amount of time that is you send the data faster and faster now you can use more wires that is a possible solution but that's an expensive solution because you have to actually change the infrastructure right so let's say I wanted uh I didn't have this gigabit Ethernet card right many of you have G ethernet card in your computer but I have had I had only 100 megabits per second cards and I had to transmit 1000 megabits per second then I had to have 10 cards and 10 cables I mean you can already see that that's an impossible thing to do so what what is what we attempt to do is to transmit 1 GBS per second over the same wire right so naturally with progress in Communications and the amount of data storage and everything the time periods of the signals that you want are decreasing so even though wires are not getting worse it's not like the wires are getting worse it's not like the time con of the wire is getting any worse but it's just that you the period of the data that you want to send is getting smaller and smaller okay the wires are in fact getting better but not at the same rate that the periods have been decreasing okay so it's not like use 10 times better wire for this than this one okay you use almost the same wire it may be slightly improved but you want to send 10 times faster data so you have 10 times smaller amount of time to send a bit and detect a bit and so on okay so that is the problem so the amount of data you want to send is increasing so because of that the data rates have been increasing the data rates have been increasing because you can't multiply the infrastructure so many times if you want to send 10 times the data you can't put 10 times as many wires or 10 times as many traces on a PCV you can try to do that but then it becomes big and becomes expensive and so on so that's why you try to use the same infrastructure and send data faster and faster okay the same thing on an on a package again the same so you are sending all this data and that comes out of some chip right so the data that the chip is processing is also getting faster so if you put it in the same package what was a good package 5 years ago for that data rate is not good enough now right and the package also has been improving but again not at the same rate as the demand for data rate similarly that goes for uh uh links on a chip and so on okay and one thing one set of wires that has been getting worse is the wires on a chip right the geometry the size of transistors on Chip have been decreasing but it's useless if you just shrink the geometry of the transistors but if you had the same size of wires to shrink the size of the Chip and pack more functionality into the chip you also have to shrink the size of wires so it turns out that the wires have been becoming poor and poor because you have thinner and thinner wires that means more and more resistance and so on so on a chip the problem is particularly compounded because you want higher higher speeds but the wires have been getting worse and worse okay so the data rates are increasing you can't multiply the infrastructure so the infrastructure is used Loosely here it's not some big installation it is in case of ethernet and so on but I'm even talking about having a larger PCB and things like that okay this is because it's too expensive so you multiply the speed instead so this naturally gives you this problem so if T is much less than TS you have no problems but TS is constantly reducing so we will always run into this scenario okay and you can open up real systems or look it up on Wikipedia or something and see so if you look at 10 Bas 10 10 megab per second ethernet you don't have very sophisticated transmitters and receivers because you don't need it whereas 100 it's more sophisticated and 1,000 is very sophisticated and 10,000 it's on the border line of doable it's really difficult to do okay so the higher speeds give you data impairments and you have to correct them okay so this is basically the topic of the course how do you send highs speeed data over a pair of wires and particularly over a pair of wires that may not have sufficient bandwidth to carry the signal that is the period of the signal is much smaller than the the the time constant associated with the pair of wire okay not all wires can be described with a single number like R * C it also has inductances and so on but you get the idea okay so that's the topic of this course and it's called Broadband because why is it called Broadband how do you get a narrow band signal how do you get the signals to be narrow band in a radio no that is after you how do you generate the signal why does it become a narrow band signal see this narrow band and Broadband are not some absolute uh uh limits right a wireless land signal can have 54 megabit per second bandwidth that is still an arand signal whereas let's say your uh telephone signal or maybe some low speed the older versions of USB have a few kilobit maybe a few megabits per second that is still Broadband why is that exactly so you modulate the uh stream onto a high frequency carrier okay again high and low are not absolute values here they are in Rel they are in relation to the bandwidth of the signal so what do you do in Wireless you start with a baseband signal very simply you multiply it by sinusoid at FC so what comes out is center around FC and has a bandwidth of 2 FB okay this is a very BAS basic up conversion that you do okay now you have this carrier right so you have a carrier with which you multiply and turn this into an narrow band signal at a center frequency FC it's not that the bandwidth of the signal has become smaller right it's just that in relation to the center frequency of this here it was wide and here it is narrow okay that's all so what we mean by Broadband signals here is that when transmitting data over wires we don't use modulation like this the reason we use modulation in a radio is because it is being transmitted over a common medium okay so if I want to transmit over air and you want to transmit over Air at the same time either we have to stand far apart from each other or maybe we like each other too much and we want to stay here then we use you use fc1 I use fc2 and they go to different frequencies okay so that's how that's how somebody will be able to distinguish the signals from me and the signals from you whereas on a wire we don't need to do that it's a confined medium so it will always almost always be a Broadband signal like this so that's about Broadband signals and the title says vsi Broadband communication circuits what we mean is we are looking at IC design for transmitting and receiving Broadband signals so basically we'll concentrate on how to make circuits on a chip that will transmit these things effectively and receive them properly so we discuss we discussed one major source of impairment the wire itself has parastic elements and that uh modifies the signal in a way that you can't distinguish between different digital symbols there are other things as well so we said that this is a confined medium so uh when you send a signal in a wire no other signal interferes with it now that is not entirely true right so you have let's say one pair of wires here and another pair of wires here you transmit V1 on this you transmit V2 on this one now ideally V3 should be related only to V1 and V4 should be related only to V2 but if these two pairs of wires are close to each other there will be some interaction between the two for instance there will be a parastic capacitance between this wire and these two wires and so on okay so there will be what is known as cross talk that is because you do have multiple pairs of wires in proximity you do have communication between one channel and a totally different channel so this is known as cross talk so this is another problem that you may have to combat or at least minimize so that you can uh use the link without being to affected by this so for instance if you look at an ethernet cable what do you see how many such pairs are there five four there are four pairs okay there are eight wires right on the RJ45 connector so there are four pairs and there is no isolation between them they're all twisted together so there is a huge amount of cross talk in ether so that is another reason why if you look at the output of uh if you just like connect one side to the computer put the other one of the other pairs to an oscilloscope you'll see total garbage Okay so firstly it's because the wire itself is so long and whatever you transmit on that particular pair itself is not being received properly and secondly what is being transmitted on the other three pairs also gets coupled to this particular pair so this is one of the problems so basically the problems associated with the wires are as a high frequency attenuation basically this is related to large time constants now this is not the only kind of impairment on a Wire so we only took a very simple case where the wire had a resistance and a paretic capacitance now it will also have inductance and it will also have all kinds of connectors and other impedance discontinuities okay it's not a uniform transmission line right so it turns out they also cause many problems whenever you have an impedance discontinuity you have a reflection okay this is you know this from the basic transmission line scores so for instance if you have a uniform transmission line that is terminated by an open circuit right this has an impedance Z KN on the other side it has Infinity so if you send a pulse What Do You See it'll keep going back and forth right so in the lossless transmission line it will go back and forth forever and ly transmission line it will die out but at least what you see won't be just a step you'll see some more things okay you'll see some things like this and these Reflections do occur because simply you can't make any un you can't make a uniform medium for instance I mean the ethernet cable it's no use having just a cable firstly it's not uniform at all even if the cable were uniform you have to connect it somewhere the moment you make a connector that becomes a impedance discontinuity okay so there are various such things which impair the signal so for instance let's take an extreme case where a transmission line is terminated by an open circuit and you drive it with a ideal voltage SCE at one end let's again take our uh favorite test signal it's min-1 volt and it becomes + one for one symbol period and then goes back to minus one what do you see at this end if you send a pulse like this or let's take an even simpler case let's say it's just a step let's say it's a step like this what do you see here let's say it is an ideal loss transmission line ringing what is the frequency of ringing huh TS firstly there is no TS in this at all right it's just a step let's say the delay of this line is TD what you see is the output will rise up if you apply a step after a delay and then it'll go to twice the steady state value okay because it's an open circuit it comes the wave comes back here and gets reflected and goes back so 2 TD later you will see another reflection so you will see ringing like this the period of the ringing will be four * TD so every 2 TD it will alternate okay and if it will if it's a lossy transmission line it won't go all the way up and it won't last indefinitely it'll do something of the sort basically for a signal at every reflection in this at this end it'll change I mean it will uh uh experience a step for the next step to occur the signal has to go to the other end and get reflected back so that delay is twice the delay of the transmission line so clearly here also you can see that you can run into problems if the ringing is so much that it takes you to the it reverses the polarity of the signal okay okay and the second one we already discussed it is cross talk what else can be a problem how do you transmit Digital Data you know that is actually a big problem that is we are happily saying that we'll transmit ones and zeros and the symbol period is TS now TS is known even if if it is known it's not exactly known there always there is always some parts per million type of error but that can give you significant errors okay because you don't know exactly where the bit starts where the bit ends or how long the bit is in many cases okay so the third issue with transmitting Digital Data is timing that is knowing the bit interval and also knowing where in the bit the sample right not not every place may be ideal you may want to sample just before the transition transition or at least you want to sample where the probability of error is the smallest okay how do you find that that is also a problem okay we can discuss this a little more in the next class but that is a big problem what is the simplest way of uh solving this how can I make sure that I always know the frequency of the transmitter how do you generate a digital signal how do you generate the timing of a digital signal from a clock right there is a clock that typically clocks a flip-flop or something and you get the Digital Data out so the easiest thing you can do is provide the clock in addition to data okay so when whenever you have a communication link you don't just have one link for the data you have one link for the data another one for the clock that is always possible what's the problem with this clocks Q okay that's a problem but what's an even lower yeah that is one of the problems meaning the delay for the clock may not be exactly same as the delay for the data so even though when you Senter it the clock loock is at the exact position required when you receive it it may not be at the exact position required to receive the signal now you have solved the problem of knowing what the bit interval is right because the clock frequency will not change the bit period will not change so the at the on the other side when you receive the clock you know exactly the interval of the clock and you know exactly the bit interval but uh you may not know the clock may have got delayed by different amount compared to data so that can be a problem what is another problem with this no that is all like more detail I'm talking about more of Economics type of things I mean it's the same thing as initially before I said that I can't use two wires if I want to double the data rate because it's too expensive that is always an option so here it's the same thing right to transmit the same data reliably I have to send both the data and the clock so that may be too expensive so maybe I choose not to do that so if I have some way of recovering the clock from the data I would prefer that compared to sending the data separately okay so all this is a question of whether it makes sense economically in a particular system or not so in some cases you do use parallel uh parallelizing right instead of sending one signal at 1 gbits per second maybe it makes sense to send have four wires each operating at 250 megabits per second so in the in the same way maybe making the clock recovery circuit is too expensive or it's too much effort in that case you send the clock separately so you have both cases you have you have cases where the clock is sent separately and you have cases where it is not sent if it's not sent you have to generate both the timing both the uh frequency of the uh data and the phase of the clock if it is sent you may have to correct for the phase so similarly for multiplexing or Dem multiplexing whether you want to have one wire operating at a high frequency or many wires operating at low frequency it's also a matter of cost and how complex it is and so on so if you choose the highest data rate and choose only one wire it uses the smallest amount of infrastructure but it's also most complicated to uh do signal processing and recover what was being sent now if you send like you have a whole bunch of wires at a very low data rate it's very easy to recover the data but then it becomes too expensive because you have to have transmitter and receiver for each one of them and that may be too expensive so there are both cases okay so in practice also

so this is e68 5 blsi Broadband communication circuits so before going into the introduction for the class so I&#39;ll just uh say a few words about the tablet I chose this mainly because I could also record the lectures and put it on the web particularly because there is no book for this course but uh I don&#39;t recommend that you stop taking notes completely because you also learn something when you write down uh you don&#39;t learn by passively reading even when you&#39;re reading a book it&#39;s best if you work through the steps it&#39;s not like reading a story book or something so by watching Baywatch you won&#39;t become a lifeguard similarly by just listening to the lecture you can&#39;t absorb everything that&#39;s going on but it will reduce hopefully the drudgery of uh reproducing every diagram that I write because you all can always refer back to it or at least you have a record of the whole thing so and also remember that this is an experimental stage and Technology can fail at any time so keep keep a backup also right now what I&#39;m doing is using the tablet like more or less like the traditional Blackboard so if you can think of some other ways of using it which is very different from how it is used now then of course I would be welcome to all suggestions so what is this course all about so it says Broadband communication circuits so basically what we will talk about in this course are circuits that are used for digital Communications and digital Communications basically means say sending digital data bits from some place to some other place so I&#39;ll just across the channel and the channel can be anything so as you know Digital Data is transmitted in uh mobile phones in Ethernet in USB in a variety of uh applications and this uh the channels that we consider in this course also include uh cases where you are not particularly interested in telecommunication meaning if you have a PCB and you want to send data from one side to the other you may not think of it as a communication link but it is right the idea is not to send messages from one person to another it&#39;s just to uh have Digital Data on one end appear at the other side now what about Broadband what do we mean by Broadband so if you look at the spectral density of any transmitted signal it will occupy some extent some finite extent we can&#39;t deal with infinitely large bandwidths so let&#39;s say it is something like this what this implies is that in this range the uh there is a lot of spectral content outside there it has negligible spectral content and the reason of frequencies over which it has significant Energy is known as the band width and the signal is centered around some Center frequency FC so we use the ratio FB by FC to distinguish between narrow band and Broadband signals so if FP by FC is very small so let&#39;s say much smaller than one it&#39;s a narrow band signal and if it&#39;s of the order of one or even more it&#39;s a Broadband signal so the spectral density of a Broadband signal might look like this it may extend all the way up to DC or something close to DC and up to some high frequencies so in this case if you think of this as the center frequency the ratio of bandwidth to Center frequency is two where do you see narrow band signals have you seen examples of either of these cases where have you seen narrow band signal where do you see broad band signals mobile application should be narrow band signals why would use narrow band signal why would use Broadband signals broadcast radius are narrow band so it is related to that so when you have a common medium across which you have to transmit multiple channels you have to support multiple channels then you will op for narrow band because using different frequencies is one way of distinguishing between different channels so if you have a common medium like air and all signals have to be transmitted across the same medium then the one of the ways and the most obvious way to distinguish between them is to use different frequencies for different signals so these things necessarily will be narrow band so you have a certain usable range of frequencies and you divide it further into channels and you use that for communications so this is typically the case for transmission over a common medium like air now the medium may not always be common meaning for every signal that you need to transmit you can have a pair of wires between point a and point B wherever you want to transmit in that case you are not constrained by this you can occupy the entire range of usable frequencies I not Define what usable frequency is it depends on the situation okay it basically means the range of signals that you can uh conveniently process that&#39;s all so if you have transmission over a confined medium for instance a pair of wires right so if you have two separate pair of pairs of wires and you transmit signals on both of them they don&#39;t interfere with each other at least not to a significant degree but then if you transmit two different signals in air they&#39;ll completely interfere with each other right one will affect the electromagnetic field just as much as the other one will right so in this case maybe you have a Broadband signal so when do you transmit signal over wires what are the examples that you know of telephones yes ethernet what what other say yeah Optical also in the optical domain it is narrow band because Optical frequencies are necessarily very high and the bandwidth of the signals that you transmit is small but yeah actually we will deal with some of those things also what else where else can you think of digital signals being transmitted over pair of wires TV channels actually TV yeah now cable TV is transmitted over cable but they&#39;re also you have the same medium for various channels so you have to have some narrow band requirement so there is a huge variety of examples one is sending signals over telephone lines telephone lines of course traditionally carried voice in an analog fashion but now there is Digital Data if you look at DSL it is digital and various flavors of DSL right and another common example is ethernet you all seen and are familiar with ethernet and you have Digital Data transmitted at various rates depending on how old your modem is so you see things like 1000 and 1,000 Bas T it refers to whether the data is being transmitted at 10 megabits per second 100 megabits per second or or 1000 mb per second and people are currently working on uh making ethernet modules for 10,000 mb per second or 10 GB per second okay over almost the same pair same kind of cable so the speeds are increasing constantly then what else we have USB you have all kinds of devices working off USB and it&#39;s not necessarily something like a mouse which needs only a uh low speed uh link it can be a high speed link also now it is up to 480 MBS per second then you have Optical links now if you look in the optical domain these are narrow band signals but we&#39;ll need Broadband circuits to process these signals that we&#39;ll see afterwards okay when we translate the signals from Optical to electrical domain you&#39;ll end up with Broadband signals and you&#39;ll need Broadband circuits to process them okay so these are all examples where you it is like a communication link meaning you are explicitly thinking of sending data from one side to the other and the examples where that&#39;s not the case but you do have some similar Behavior or you have printed circuit boards and you have to transer data across printed circuit boards I mean that may be simply because you have two chips so this is a printed circuit board you have chip number one and Chip number two so this could be the processor and this could be the memory and you have to transmit data between the two and as you know the speeds are increasing every day right so anytime you have to transmit the data transmit Digital Data between these you will have to deal with the same issues as in the as in these cases where you think of these being explicitly as communication links okay so this is one example and there are what are known as back planes back plane is nothing but a large PCB which holds a large number of small pcbs there are they are known as line cards so finally all of your let&#39;s say ethernet traffic goes through some switches and so on so you can see some of these back Lanes in those large switches so you have many links which are put together and then you have to connect between various links and you&#39;ll have back planes so it&#39;s nothing but a large PCV with PCV traces up to 1 m okay so the kind of issues that you have to deal with in PCB TR are similar to what you have to deal with in telephone lines maybe the frequencies are different maybe uh the exact uh magnitude of signals are different but the kind of things that you will see the kind of problems that you will see in transmitting data will be the same okay so these are also Broadband communication links now you can go even further inside the chip itself so on the chip I don&#39;t know how many of you are familiar with how how an IC is made an IC is nothing but a piece of silicon on which with lithography you fabricate the transistors and the interconnects and the chip is packaged inside some plastic or ceramic enclosure and the pins are brought out you definitely seen chips in your lab and so on so typically you have a piece of ceramic or plastic with pins on the sides or it may be at the bottom and inside the package is a cavity in which you place the Silicon chip and the chip is connected to the package through water are known as Bond wires so you have small wires about 25 microns thick connecting the chip to the package and on the chip you have traces just like you have on a PCB connecting the bond wires to the pins so this is how you make connections between uh the outside world and what is going on on the Silicon right you have the connections on the package and bond wires and if you have to transmit Digital Data across these again you will encounter it turns out it will encount you will encounter the same problems as you do in any other case Okay so even that falls under in way under Broadband Communications okay and on top of it you can have a single chip and you have two different parts of the Chip And you have to transmit data from one one part to the other and even there it&#39;s the same thing everything is scaled down in size that&#39;s all the wire is much shorter but it&#39;s also much thinner okay so it turns out that you will have the same kind of issues so a link between two parts of the chip also classifies as a broadband communication link now depending on the situation it may be easy or difficult to transmit data across any of these things so first thing is what are the what&#39;s the what is the potential issue that you see we have a pair of wires and you want to transmit data so what is the big deal you put some voltage on this side and the same voltage appears on the other side doesn&#39;t it what is the problem what are the problems in community ating data across a pair of wires because a Wire by definition is a good conductor and you apply a voltage on one side and the same voltage should come to the other side so why are why do we even have a course about the whole thing okay parastic capacitances so what happens where is the parastic capacitance now in this situation so this is the kind of scenario that we are discussing right every scenario that I outlined before whether it&#39;s Ethernet or connections on a chip is something similar to this you have two wires and you have to transmit data across it so what is the problem so this this serves as a good model for everything that we saw before now what is the problem with this why do we have to study this or why do we have to even uh think think about transmitting data right you apply voltage on one side and it should appear on the other side what are the problems that you see so okay where is the capacitance in this between the two wires okay so what is the problem now so what happens is that that is true basically a wire is not a perfect conductor and firstly a pair of wires are in proximity to each other so there is capacitance between them and since the wire is not a perfect conductor it has some series resistance and because it has some finite extent it also has series inductance okay so you have the wire really is made up of these resistive inductive and capacitive elements okay and in reality it is distributed it&#39;s not like you can identify some particular part as being resistor or some other part as being an inductor okay and a different part as being a capacitor the capacitor resistor and inductor Are all uh present continuously so that is known as a distributed system okay so you can think of the wire as being made up of RLC sections like this and so on okay so this is the reality of the wire it&#39;s not a perfect conductor it has parastic resistance parastic inductance and parastic capacitance the next question is so what what happens because of this what is that that&#39;s okay what is the problem with that okay firstly there is a lag in voltage meaning the output doesn&#39;t appear instantaneously yeah actually the problem is that it is dependent on frequency okay we&#39;ll go into details of that later in fact lot of these things I&#39;ve just introduced without uh describing in detail but the whole course is about that this is just the introduction okay so let&#39;s take a very simple case where we&#39;ll say that the inductance is insignificant okay and also we&#39;ll make a further approximation that we don&#39;t need this distributed structure we&#39;ll simply consider one resistor and one capacitor okay so let&#39;s see what happens in that case let&#39;s say our perfect wire was replaced by something like this and as I mentioned earlier I am interested in transmitting Digital Data okay so let&#39;s consider a case where I&#39;m sending across a bunch of bits and minus1 volt stands for my symbol zero and + one volt stands for symbol one okay can you see properly on this or should I write bigger so what do you get out is there a problem so the idea of any communication link is that you send something that you don&#39;t know on the other side obviously that&#39;s why you sending it and you should receive exactly what you sent that is the that&#39;s an ideal communication link or that&#39;s what you aim for okay now this is my message I have sorry 1 0 0 1 1 something like that okay some arbitrary bit pattern what do I receive depends on the bandwidth okay so assume something and tell me what happens so it depends on the values of R and C but we know the step response so we firstly can recognize that this digital bitstream can this bit the digital bitstream here can be broken up into a number of steps and we know the step response of the RC filter RC system so we can reconstruct the output signal so depending on let&#39;s say R and C are very small to start with in that case what you get is let&#39;s say you send this this is what you send and what you receive will be so it starts from here and the edges won&#39;t be sharp right because the capacitor voltage cannot change abruptly it can only change gradually so it will do something of this sort this is for some particular value of RC let&#39;s say now I go on increasing the value of uh R and C what will happen happen the time constant will become longer and longer so maybe it will take it&#39;ll do this and it can even become worse than this in that even in two even in the time required to transmit two symbols it has not gone all the way to the steady state value okay so this figure is a little cramped so let&#39;s reexamine the same thing and this time what I&#39;ll do is I&#39;ll just transmit single one this is 0o which is - 1 Vol and this is 1 which is + 1 volt so firstly how do we distinguish zeros and ones at the output so one thing that we have to recognize here is that what you get at the output is an analog waveform see what is a digital waveform it consists of a discrete number of levels that is y- axxis is quantized whereas what we get at the output is something like this in this case the y- axis is definitely not quantized so firstly how do we distinguish between once and zeros of the output what is the threshold here 0 volts because we are we are using symmetrical voltages above zero for our 0 and one symbols our threshold is 0 volts so let&#39;s say I look here and if it is above 0 Vols if it&#39;s positive I say it is 1 and if it is negative I say it&#39;s a zero so I don&#39;t care about the exact voltage I&#39;ll just say that if it&#39;s greater than this it&#39;s 0o one and otherwise it&#39;s zero okay so that gets rid of the problem of having an analog output meaning having a continuous time output like this uh with continuous values right so even if the output voltage is only9 volts we still recognize it to be a One S Syble one but what happens as we increase the time constant RC so it can do this okay if the time constant is large enough it can do this where it has barely the input goes from 0 to one and back to zero the output starts from a negative value because it has reached steady state with minus one volt and then it barely reaches the positive values and comes down and it can be even worse in that it may not even reach 0 volts okay you can always I make R and C large enough that this happens so here it doesn&#39;t attain positive values at all okay so anywhere you look you only see negative voltages so you would obviously determine that uh the transmitted sequence was all was a string of zeros okay whereas there was a one in it and you couldn&#39;t detect the one so that&#39;s a gross error now this case it does reach positive values but you can see that the positive voltage that is reached is small so in this case of course with a perfect detector you can always detect this one but the detectors also have some error so if the error happens to be greater than this value you may not detect it correctly okay so this definitely gives you errors this may give you errors depending on how robot how accurate the detector is and this is a very good case so this is the fundamental problem with transmitting data across a pair of wires the wire is not perfect and you think you are sending Digital Data the first thing that happens is even if you send quantize levels that change abruptly like this right that is the meaning of a Digital Data it has discrete levels and it also changes at discrete some instance of time I mean some usually some periodic instance of time but the output will not be like that the output shows a continuous voltage uh that is a continuous amplitude meaning the output amplitude is not discrete it doesn&#39;t consist of two values or any number of finite values okay so that is one problem that itself is not a big issue because you can always map a range of output amplitudes to one symbol okay it&#39;s not like you&#39;re saying if I receive exactly one volt I&#39;ll say it&#39;s say one no that&#39;s not the case right you&#39;re saying if it is more than zero it&#39;s a one so that problem the first problem is got rid of by appropriate thresholding right so you m range of output amplitudes to one symbol so if you have a number of symbols and you have infinite number of output amplitudes infinite number of levels but you map a range to a particular symbol and you choose the ranges appropriately now the problem is even with appropriately chosen ranges if your wire is bad enough meaning your if your R and C are large enough you can have a case where it&#39;s very difficult to distinguish which range it is in or where it&#39;s impossible simply because the signal at the output doesn&#39;t reach the amplitude it&#39;s supposed to reach okay so this is the basic problem with digital Communications so how do we get rid of it is the topic of the course okay so now here we saw that how bad the problem is depends on how much the RC time constant is in or alternatively how much the cut off frequency of the filter is okay the Bas we will see that it is related to high frequency attenuation okay so if you have a very large time constant or a small cut off frequency then you will have bigger problems transmitting Digital Data right so this will be a problem for everything to transmit data over any pair of wires this is this can be an issue now it could be that in most of the case in many cases that you see the RC time constant is small enough when I say small it&#39;s compared to what these are Dimension quantities so I have to compare it to something what is when I say small so in this particular case if this is the symbol rate if to is much smaller than TS I don&#39;t have any problem and this is the kind of case that you encounter normally let&#39;s say in the lab when you connect uh each time you connect a pair of wires to take some signals from the let&#39;s say the signal generator to the output you don&#39;t think of all this you don&#39;t have to think of all of these impairments right so that is simply because the time constant in that case is much smaller than the time period that you are interested in okay but you will see cases where the time constant is uh comparable to are much larger than the time period that you interested in okay and that is true for the cases that we saw earlier so if you look at uh any of these cases let&#39;s say transmitting DSL data over a phone line right the phone line has a very low bandwidth and you try to transmit DSL data and even the traditional ADSL has a bandwidth of I think 384 kilobit per second compare this to voice which has a 4 khz bandwidth okay so it&#39;s almost like 50 times larger bandwidth or something so voice goes through the cable without any problem voice is not digital signal but I&#39;m just uh concentrating on the frequency content of the signal but DSL when it goes through the if you transmit this one Zer kind of binary Digital Data through a telephone line on the other side you&#39;ll absolutely not be able to recognize the one Zer data so this whole DSL modem is about transmitting it in the correct way so that you can recover it properly right similarly for ethernet if you look at the output of even a 100 megab per second ethernet line you connect it to an oscilloscope and C you will not see anything like this okay firstly it&#39;s not a twole Digital Data one thing I have to say here is that Digital Data does not have to have just two levels we don&#39;t need only binary Digital Data you can have uh five levels or seven levels or whatever you use whatever is convenient and it turns out that in Ethernet you in gab ethernet you use five levels okay but if you look at the output at the cable you will not see five different levels you will see a whole mess of things okay and similarly for USB or any other case now I think you all use pcbs in the lab again when you are using pcbs to let&#39;s say transmit data from one digital chip to another you don&#39;t think of all this again that&#39;s because the time constant in that case happens to be much smaller than the period of the signal now there are cases where the time constant is much more than the period of the signal so what is happening is the amount of data you transmit has been increasing constantly as you know the hard disk capacity is increasing the data rates has been increasing the amount of stuff that you want to download is increasing everything is increasing right so the way to accomodate that is to either have more wires to accommodate all this data or you use the same number of wires but send more data in the same amount of time that is you send the data faster and faster now you can use more wires that is a possible solution but that&#39;s an expensive solution because you have to actually change the infrastructure right so let&#39;s say I wanted uh I didn&#39;t have this gigabit Ethernet card right many of you have G ethernet card in your computer but I have had I had only 100 megabits per second cards and I had to transmit 1000 megabits per second then I had to have 10 cards and 10 cables I mean you can already see that that&#39;s an impossible thing to do so what what is what we attempt to do is to transmit 1 GBS per second over the same wire right so naturally with progress in Communications and the amount of data storage and everything the time periods of the signals that you want are decreasing so even though wires are not getting worse it&#39;s not like the wires are getting worse it&#39;s not like the time con of the wire is getting any worse but it&#39;s just that you the period of the data that you want to send is getting smaller and smaller okay the wires are in fact getting better but not at the same rate that the periods have been decreasing okay so it&#39;s not like use 10 times better wire for this than this one okay you use almost the same wire it may be slightly improved but you want to send 10 times faster data so you have 10 times smaller amount of time to send a bit and detect a bit and so on okay so that is the problem so the amount of data you want to send is increasing so because of that the data rates have been increasing the data rates have been increasing because you can&#39;t multiply the infrastructure so many times if you want to send 10 times the data you can&#39;t put 10 times as many wires or 10 times as many traces on a PCV you can try to do that but then it becomes big and becomes expensive and so on so that&#39;s why you try to use the same infrastructure and send data faster and faster okay the same thing on an on a package again the same so you are sending all this data and that comes out of some chip right so the data that the chip is processing is also getting faster so if you put it in the same package what was a good package 5 years ago for that data rate is not good enough now right and the package also has been improving but again not at the same rate as the demand for data rate similarly that goes for uh uh links on a chip and so on okay and one thing one set of wires that has been getting worse is the wires on a chip right the geometry the size of transistors on Chip have been decreasing but it&#39;s useless if you just shrink the geometry of the transistors but if you had the same size of wires to shrink the size of the Chip and pack more functionality into the chip you also have to shrink the size of wires so it turns out that the wires have been becoming poor and poor because you have thinner and thinner wires that means more and more resistance and so on so on a chip the problem is particularly compounded because you want higher higher speeds but the wires have been getting worse and worse okay so the data rates are increasing you can&#39;t multiply the infrastructure so the infrastructure is used Loosely here it&#39;s not some big installation it is in case of ethernet and so on but I&#39;m even talking about having a larger PCB and things like that okay this is because it&#39;s too expensive so you multiply the speed instead so this naturally gives you this problem so if T is much less than TS you have no problems but TS is constantly reducing so we will always run into this scenario okay and you can open up real systems or look it up on Wikipedia or something and see so if you look at 10 Bas 10 10 megab per second ethernet you don&#39;t have very sophisticated transmitters and receivers because you don&#39;t need it whereas 100 it&#39;s more sophisticated and 1,000 is very sophisticated and 10,000 it&#39;s on the border line of doable it&#39;s really difficult to do okay so the higher speeds give you data impairments and you have to correct them okay so this is basically the topic of the course how do you send highs speeed data over a pair of wires and particularly over a pair of wires that may not have sufficient bandwidth to carry the signal that is the period of the signal is much smaller than the the the time constant associated with the pair of wire okay not all wires can be described with a single number like R * C it also has inductances and so on but you get the idea okay so that&#39;s the topic of this course and it&#39;s called Broadband because why is it called Broadband how do you get a narrow band signal how do you get the signals to be narrow band in a radio no that is after you how do you generate the signal why does it become a narrow band signal see this narrow band and Broadband are not some absolute uh uh limits right a wireless land signal can have 54 megabit per second bandwidth that is still an arand signal whereas let&#39;s say your uh telephone signal or maybe some low speed the older versions of USB have a few kilobit maybe a few megabits per second that is still Broadband why is that exactly so you modulate the uh stream onto a high frequency carrier okay again high and low are not absolute values here they are in Rel they are in relation to the bandwidth of the signal so what do you do in Wireless you start with a baseband signal very simply you multiply it by sinusoid at FC so what comes out is center around FC and has a bandwidth of 2 FB okay this is a very BAS basic up conversion that you do okay now you have this carrier right so you have a carrier with which you multiply and turn this into an narrow band signal at a center frequency FC it&#39;s not that the bandwidth of the signal has become smaller right it&#39;s just that in relation to the center frequency of this here it was wide and here it is narrow okay that&#39;s all so what we mean by Broadband signals here is that when transmitting data over wires we don&#39;t use modulation like this the reason we use modulation in a radio is because it is being transmitted over a common medium okay so if I want to transmit over air and you want to transmit over Air at the same time either we have to stand far apart from each other or maybe we like each other too much and we want to stay here then we use you use fc1 I use fc2 and they go to different frequencies okay so that&#39;s how that&#39;s how somebody will be able to distinguish the signals from me and the signals from you whereas on a wire we don&#39;t need to do that it&#39;s a confined medium so it will always almost always be a Broadband signal like this so that&#39;s about Broadband signals and the title says vsi Broadband communication circuits what we mean is we are looking at IC design for transmitting and receiving Broadband signals so basically we&#39;ll concentrate on how to make circuits on a chip that will transmit these things effectively and receive them properly so we discuss we discussed one major source of impairment the wire itself has parastic elements and that uh modifies the signal in a way that you can&#39;t distinguish between different digital symbols there are other things as well so we said that this is a confined medium so uh when you send a signal in a wire no other signal interferes with it now that is not entirely true right so you have let&#39;s say one pair of wires here and another pair of wires here you transmit V1 on this you transmit V2 on this one now ideally V3 should be related only to V1 and V4 should be related only to V2 but if these two pairs of wires are close to each other there will be some interaction between the two for instance there will be a parastic capacitance between this wire and these two wires and so on okay so there will be what is known as cross talk that is because you do have multiple pairs of wires in proximity you do have communication between one channel and a totally different channel so this is known as cross talk so this is another problem that you may have to combat or at least minimize so that you can uh use the link without being to affected by this so for instance if you look at an ethernet cable what do you see how many such pairs are there five four there are four pairs okay there are eight wires right on the RJ45 connector so there are four pairs and there is no isolation between them they&#39;re all twisted together so there is a huge amount of cross talk in ether so that is another reason why if you look at the output of uh if you just like connect one side to the computer put the other one of the other pairs to an oscilloscope you&#39;ll see total garbage Okay so firstly it&#39;s because the wire itself is so long and whatever you transmit on that particular pair itself is not being received properly and secondly what is being transmitted on the other three pairs also gets coupled to this particular pair so this is one of the problems so basically the problems associated with the wires are as a high frequency attenuation basically this is related to large time constants now this is not the only kind of impairment on a Wire so we only took a very simple case where the wire had a resistance and a paretic capacitance now it will also have inductance and it will also have all kinds of connectors and other impedance discontinuities okay it&#39;s not a uniform transmission line right so it turns out they also cause many problems whenever you have an impedance discontinuity you have a reflection okay this is you know this from the basic transmission line scores so for instance if you have a uniform transmission line that is terminated by an open circuit right this has an impedance Z KN on the other side it has Infinity so if you send a pulse What Do You See it&#39;ll keep going back and forth right so in the lossless transmission line it will go back and forth forever and ly transmission line it will die out but at least what you see won&#39;t be just a step you&#39;ll see some more things okay you&#39;ll see some things like this and these Reflections do occur because simply you can&#39;t make any un you can&#39;t make a uniform medium for instance I mean the ethernet cable it&#39;s no use having just a cable firstly it&#39;s not uniform at all even if the cable were uniform you have to connect it somewhere the moment you make a connector that becomes a impedance discontinuity okay so there are various such things which impair the signal so for instance let&#39;s take an extreme case where a transmission line is terminated by an open circuit and you drive it with a ideal voltage SCE at one end let&#39;s again take our uh favorite test signal it&#39;s min-1 volt and it becomes + one for one symbol period and then goes back to minus one what do you see at this end if you send a pulse like this or let&#39;s take an even simpler case let&#39;s say it&#39;s just a step let&#39;s say it&#39;s a step like this what do you see here let&#39;s say it is an ideal loss transmission line ringing what is the frequency of ringing huh TS firstly there is no TS in this at all right it&#39;s just a step let&#39;s say the delay of this line is TD what you see is the output will rise up if you apply a step after a delay and then it&#39;ll go to twice the steady state value okay because it&#39;s an open circuit it comes the wave comes back here and gets reflected and goes back so 2 TD later you will see another reflection so you will see ringing like this the period of the ringing will be four * TD so every 2 TD it will alternate okay and if it will if it&#39;s a lossy transmission line it won&#39;t go all the way up and it won&#39;t last indefinitely it&#39;ll do something of the sort basically for a signal at every reflection in this at this end it&#39;ll change I mean it will uh uh experience a step for the next step to occur the signal has to go to the other end and get reflected back so that delay is twice the delay of the transmission line so clearly here also you can see that you can run into problems if the ringing is so much that it takes you to the it reverses the polarity of the signal okay okay and the second one we already discussed it is cross talk what else can be a problem how do you transmit Digital Data you know that is actually a big problem that is we are happily saying that we&#39;ll transmit ones and zeros and the symbol period is TS now TS is known even if if it is known it&#39;s not exactly known there always there is always some parts per million type of error but that can give you significant errors okay because you don&#39;t know exactly where the bit starts where the bit ends or how long the bit is in many cases okay so the third issue with transmitting Digital Data is timing that is knowing the bit interval and also knowing where in the bit the sample right not not every place may be ideal you may want to sample just before the transition transition or at least you want to sample where the probability of error is the smallest okay how do you find that that is also a problem okay we can discuss this a little more in the next class but that is a big problem what is the simplest way of uh solving this how can I make sure that I always know the frequency of the transmitter how do you generate a digital signal how do you generate the timing of a digital signal from a clock right there is a clock that typically clocks a flip-flop or something and you get the Digital Data out so the easiest thing you can do is provide the clock in addition to data okay so when whenever you have a communication link you don&#39;t just have one link for the data you have one link for the data another one for the clock that is always possible what&#39;s the problem with this clocks Q okay that&#39;s a problem but what&#39;s an even lower yeah that is one of the problems meaning the delay for the clock may not be exactly same as the delay for the data so even though when you Senter it the clock loock is at the exact position required when you receive it it may not be at the exact position required to receive the signal now you have solved the problem of knowing what the bit interval is right because the clock frequency will not change the bit period will not change so the at the on the other side when you receive the clock you know exactly the interval of the clock and you know exactly the bit interval but uh you may not know the clock may have got delayed by different amount compared to data so that can be a problem what is another problem with this no that is all like more detail I&#39;m talking about more of Economics type of things I mean it&#39;s the same thing as initially before I said that I can&#39;t use two wires if I want to double the data rate because it&#39;s too expensive that is always an option so here it&#39;s the same thing right to transmit the same data reliably I have to send both the data and the clock so that may be too expensive so maybe I choose not to do that so if I have some way of recovering the clock from the data I would prefer that compared to sending the data separately okay so all this is a question of whether it makes sense economically in a particular system or not so in some cases you do use parallel uh parallelizing right instead of sending one signal at 1 gbits per second maybe it makes sense to send have four wires each operating at 250 megabits per second so in the in the same way maybe making the clock recovery circuit is too expensive or it&#39;s too much effort in that case you send the clock separately so you have both cases you have you have cases where the clock is sent separately and you have cases where it is not sent if it&#39;s not sent you have to generate both the timing both the uh frequency of the uh data and the phase of the clock if it is sent you may have to correct for the phase so similarly for multiplexing or Dem multiplexing whether you want to have one wire operating at a high frequency or many wires operating at low frequency it&#39;s also a matter of cost and how complex it is and so on so if you choose the highest data rate and choose only one wire it uses the smallest amount of infrastructure but it&#39;s also most complicated to uh do signal processing and recover what was being sent now if you send like you have a whole bunch of wires at a very low data rate it&#39;s very easy to recover the data but then it becomes too expensive because you have to have transmitter and receiver for each one of them and that may be too expensive so there are both cases okay so in practice also

Transcript for:E68 Broadband Communication Circuits - Lecture 5

Transcript for:
E68 Broadband Communication Circuits - Lecture 5