hello and welcome to lecture five where we're going to talk about three different types of amplitude modulation dsb ssb and vsb and we'll look at what these three things stand for in a few minutes so last week we spoke about how to demodulate am after previously speaking about amplitude modulation so these are three more variants of amplitude modulation each with their own unique properties we're going to talk briefly about that and next week we'll talk about how to demodulate how to recover how to detect these signals and together these six lectures will all be included in your class test on march the 17th so watch out for that and in terms of our schedule we are now still in week four but this is the lecture that would have normally been delivered in week five i brought it earlier so that we can use that for the content for wednesday's problem class just a quick reminder that before sunday make sure you complete the weekly progress check also just another reminder that the session on the 3rd of may isn't going to happen because it's a bank holiday so that is a bank holiday i've moved the content or the the monday session to wednesday and i've moved the wednesday session to the following monday so this is the updated schedule you should find that on canvas and it is correct now okay so recap from last lecture after speaking about amplitude modulation we introduced amplitude demodulation or the demodulation of am we introduced this simple envelope detector consisting of a diode rectifier and a resistor capacitor low-pass filter that would trace the envelope and recover our message and even though that message or that envelope isn't equal to our original message it is close to our message so it's a distorted version of our message and we're happy to accept that slight distortion and that that's dependent on the time the choice of the time constant rc so provided rc is chosen so that 1 over rc is between the modulating frequency that's the frequency of the message and the carrier frequency fc we should have a slight rippling that is generally acceptable so remember the envelope detector is a form of asynchronous demodulator or a non-coherent demodulator that just means that there isn't a local oscillator generating a local version of the carrier at the demodulator that will become clearer next week when we speak about synchronous or coherent demodulation of dsp now we also spoke about the power efficiency of am and we said am isn't particularly efficient in terms of power because most of the power ends up in the carrier so the um power efficiency is generally less than 33 percent so that means more than 66.7 of the energy is wasted in the carrier and we said that even though we call it wasted it's still a feature of am that allows low-cost receivers simple receivers to be able to recover the message without having to rely on a local power supply and complex electronics so the low power efficiency of am is a feature rather than necessarily a a reason to avoid it now today we're going to talk about three different variants of amplitude modulation that um are different that don't suffer the same disadvantage they come with a range of different advantages and disadvantages so we're going to speak first about dsb sc now if you compare that with dsb lc that's the am that we're familiar with the l in dsb lc stood for large so dsp lc is dsp with a large carrier component now the s in dsb sc doesn't refer to small it stands for suppressed and suppressed means no carrier so dsb sc means double side band without a carrier okay so we're talking about no carrier not small carrier but no carrier sometimes we just call it dsb so dsb means dsp suppress carrier and ssb the s refers to single as opposed to double so that means we only have one sideband an upper or a lower sideband and again ssb actually means ssb suppressed carrier but we often just drop that and say ssb and it's understood that it's ssb suppress carrier and the same with residual sideband now vestigial you can understand that to mean partial and that doesn't mean that vestigial means partial vestigial means that the remainder or the remains of the leftover off so vestigial sideband means you have the remainder part of a second sideband so you have one sideband and a part a section a segment that remains off a second sideband so it's not single and it's not double it's somewhere in between so that was a summary of today's lecture so um this is am as we know it this is your baseband signal your baseband signal starts at zero hertz and it has a finite bandwidth fm so that's your bandwidth fm after modulation we now have three important components we have a carrier component that's this here at fc we have an upper sideband that's this here and we have a lower sideband that's this here so these three components together form am or dsb with a large carrier this is just the negative frequency reflection of that the carrier is there the lower side band is there and the upper side band is there but you can ignore that and just focus on that side of the frequency axis so that's double sideband and the important thing to note here is that the bandwidth is twice fm why is it twice fm because we have an upper sideband and a lower sideband and that's why we call it double sideband so when we talk about dsb lc we have a large carrier when we talk about dsb sc as we will in a minute that carrier component disappears so now we're going to talk about suppressed carrier mathematically remember what s of t looked like so this is s of t for am so let me just make that clear for am s t looked like that what we're going to do we're not going to suppress the carrier by adding a carrier component by adding dc and then removing it although that's what it looks like when we illustrate it like this it looks like we're just going to remove that and end up with that that's what it looks like but in fact we're going to generate our dsb suppress carrier now i can actually call this dsb suppressed carrier by multiplying the carrier times the message so that's your carrier and that's your message so by simply multiplying the two we end up with our dsb signal notice with our dsp signal your message looks something like that so what do you see immediately you see that it's not possible to trace the envelope because you have this phase reversal here so you have distortion i.e the envelope isn't equal to the message so what does that mean it means we can't use an envelope detector we can't use an envelope detector to recover dsb sc we'll talk about the recovery of dsb sc demodulation of double sideband suppress carrier next lecture so for now let's just focus on the modulation so the modulation in the case of dsb sc is simply multiplication nothing else we're not adding any dc we don't have a modulation index because there's no dc so these are the things that have changed some things haven't changed so the bandwidth hasn't changed we still have dsb so the bandwidth is still twice the original bandwidth that hasn't changed the power efficiency has changed because we don't have a carrier component so therefore we don't have wastage and power so the power efficiency is much higher you could say it's a hundred percent because there is no wasted power all the power is useful power so this is our familiar spectral plot where this is m of t and this is the spectrum of mft our baseband signal this is s of t and this is the spectrum of our band pass signal we still have an upper sideband we still have a lower sideband the bandwidth is still twice fm so it's still double sideband no surprise there but what's missing where's the carrier component we're expecting to see something happening at fc but there's nothing there because it's suppressed there is no carrier component so that's what i wanted to show here this is where our message is some arbitrary signal with a spectral representation like that now if our message is a single tone this is a single turn message what does single tone mean it simply means that we have a single cosine here so therefore we have a single frequency so for a single tone message the spectrum looks very similar so remember previously it looked like that now it just looks um like this it's very similar we have a low frequency fm we have a high frequency carrier so this is your high frequency carrier and this is the spectral representation so remember these spectral representations we can call them spectra a spectrum is a representation where we have frequency as our horizontal axis we'll use something called a spectrum analyzer to look at these spectral representations so just like an oscilloscope will give you a representation where the horizontal axis is time a spectrum analyzer or a frequency analyzer will give you a spectral representation or a trace where the horizontal axis is frequency so when you multiply these two signals so here if i multiply the message times the carrier i end up with something that looks like this so this is your original message there the envelope isn't very helpful but look at the spectrum we have a spectrum a component at fc plus minus plus fm and fc minus fm but we don't have a component at fc that doesn't exist because this is no longer the sblc that's dsp sc so what's the difference between what we previously referred to as am or dsb lc and dsb sc well there's um no carrier component that's one the power efficiency is a hundred percent and just to be clear we're talking about this the bandwidth is still twice the baseband bandwidth but nothing has changed in terms of the composition of the sideband they're still the same and the upper sideband and the lower sideband both contain the same information so why are we transmitting the same information twice why can't we suppress one of the sidebands so instead of having two sidebands instead of having two sidebands why can't we just transmit one now that is where ssb comes in single sideband now a single sideband transmission rather than transmitting an upper sideband and the lower side band we filter out one of the side bands and just transmit for example the upper side band or the lower side band so it's this or that okay it's not both it's either the upper sideband or the lower sideband what happened to the carrier component why is there no carrier component well for dsb and ssb we eliminate the carrier there is no carrier we're talking only about the side man why do we do that even though dsb suppressed carrier had 100 power efficiency we can still save power by using less of the frequency axis so using less spectrum therefore which would we can put all of the power that we put in there we can put into that or we can simply save power so there's two ways of thinking about it we can either use the same power and have twice as powerful a signal or we can use half the power because in either way we're using half the frequency so that allows other channels or other users to use the remaining frequency so you're making space for something which is called frequency domain multiplexing and we'll talk about that in our final lecture okay so this is good because it allows us to save on bandwidth and improve snr remember snr signal to noise ratio now why do we improve the signal to noise ratio because we're using less bandwidth now because we're using less bandwidth there's less opportunity for noise to affect your signal so if noise is attacking your signal here or here then our signal won't be affected because we're using less of the bandwidth so less of an opportunity for noise to affect our signal now these advantages come at a cost and the cost is the complexity the complexity comes at a cost and also there is the issue of stability okay so this comes at the expense of the complexity in order to actually filter out one of the two sidebands and that complexity translates into the cost of the devices so what does it look like in the frequency domain so if you have a spectrum that for the baseband signal that looks like that then you'd have an upper side band that looks like that by removing the lower side band you could remove the lower side band or you could remove the upper side band in either case you would have an a bandwidth of f m rather than two fm so this is dsb and this is ssb so you can block either upper or lower sideband how do you block it or you need a filter what kind of a filter so here the the shape of the spectrum has changed but that's fine you need to filter what kind of a filter would block all frequencies higher than the carrier frequency you'd need something called a high pass filter sorry well i meant was what kind of filter would block frequencies if you wanted to keep the upper sideband and block the lower sideband you would need a high pass filter that would block the low frequencies alternative if you wanted to keep the lower sideband and block the high the upper sideband you would need a low pass filter or a band pass filter now here what i'm trying to show you is that for an ideal filter you would be able to get this sharp transition but in real life your filter will always have this roll-off and this roll-off here means that you will always have some leakage from here you'll always pick up some of the other sideband now the closer the closer your filter gets to this the more complex and expensive it becomes okay so what you want is an ideal high pass filter or a filter with a very high q factor very sharp filter that requires a higher order and higher complexity now the third member of the amplitude modulation family we wanted to talk about today is vestigial sideman now vestigial sideband is where rather than having two sidebands or one sideband we have one sideband so that we have one sideband and the remains of the other sideband so here we were talking about leakage happening when you have a non-ideal filter so that some of the lower sideband or let's say the other sideband is retained the same happens here so for vestigial sideband you keep one sideband you block the other sideband but not completely so you need use a non-ideal filter and you end up with one sideband and a little bit the remainder off or the leftovers the remains off another sideband and we use this mainly or almost exclusively for analog television okay so analog television is on its way out there not many places in the world that still use analog television but it's still important for an electronic engineer to understand what vestigial sideband is and what the spectrum looks like and what applications it has so the terrestrial analog tv signal consists of audio using fm and video using vsp and it's at a particularly high frequency even though it's amplitude modulation it's still a particular very high frequency so because we're talking about a message bandwidth or a message signal of um 4.5 megahertz that's not kilohertz that's megahertz so twice that would be nine megahertz and that's a lot so to reduce that 9 megahertz we use one sideband and only part of the other so this sideband for example the lower sideband is 4.5 megahertz for the upper sideband we wouldn't retain the full 4.5 we could retain for example two megahertz so you'd have a total bandwidth of 4.5 plus two which is less than nine okay so how much is retained will depend on the filter and on the application and on the particular tv signal and whether we use the upper or the lower sideband will also depend on several factors but the point we need to understand about vestigial sideband is that we have one sideband and a partial second sideband so this is the uh television signal if you look carefully you'll see that there's an audio section this is the audio with an audio carrier but this is actually frequency modulated and then you have your upper sideband here and your lower sideband here the upper sideband is transmitted in full and this lower sideband is transmitted partially so the total bandwidth in this case is limited to seven megahertz so that's starting from here which is almost halfway in the middle of the lower sideband up to the end of the audio signal so you include the opposite band part of the lower sideband and here we're actually including the the audio which is frequency modulated so this is just an example to illustrate what a vestigial sideband spectrum will look like it's impossible to visualize this in the time domain that's why we're looking at it only in the frequency domain same with ssb we're interested in the spectral representation not the time domain representation again i've shown you this illustration or something similar to this before just so that you appreciate that whenever we speak about am we're talking about um frequencies in the kilohertz or the very low megahertz as we start talking about fm we're now talking about hundreds or well 88 to 108 megahertz we're talking about megahertz and as we increase our frequency towards gigahertz and beyond we now look at different types of applications including cellular and bluetooth and wi-fi 5g gps etc so we started lectures one two three four here and as we progress lectures 6 7 8 we'll be increasing the frequency range that we'll be looking at so now a quick comparison to end our lecture um of the different applications of amplitude modulation so amplitude modulation includes the whole family so it includes dsb lc dsb sc ssb and vsp so we'll be looking at all of these so we looked at this in some detail we looked at this in some detail ssb and vsp we haven't looked at it in much detail but that's all we need to appreciate is why it exists what it's used for and what it looks like in the frequency domain so for am we've spoken about it a lot we've spoken about the bandwidth we spoke about the fact that it's used for am broadcast for voice and music we spoke about the simplicity to demodulate it all you need is a very simple circuit we've said that the carrier is transmitted and we we've spoken about how wasteful it is so where will you see it well the am masts you'll see if you look carefully and am radios if you have one of those in the house or if you you notice almost all cars have radios they'll have digital dab radios but they'll also have long wave medium wave am radios and you can also always notice the antenna at the top so dsb sc so suppressed carrier this is different it operates at a higher frequency and there's less transmitted power than am why is there less transmitted power because there is no carrier component now all of the power is useful so it has a power efficiency of a hundred percent where do you find it you often find this in applications like air traffic control so at the airport the air traffic controllers use dsb but you also find this for in the remote controls for garage doors and for uh car fobs so in situations like this where you have short range where power is important and where you have limited power at the transmitter you don't have the ability to waste or you don't have the capacity to waste power in the carrier you um you use the sbsc so i wanted to share with you this little um uh youtube clip it's about an anecdote i spoke about in a previous lecture it's about something that happened in 2019 we have new information on a mystery in north olmsted we have been telling you how residents even engineers have been stumped over why key fobs and garage door openers just stopped working in one neighborhood tonight we now have the answer ray strickland shows us how it took weeks to crack this case bill camarada lives at the end of virginia avenue it's the street where more than 10 people's key fobs and garage door openers suddenly stopped working i really didn't think it was real because i have no problems on this end of the street it was hard to believe at first until he drove up his street and was left scratching his head like everyone else and sure enough i could not open up my car door unless i was like barely right on top of my car for weeks no one knew why this was happening not the city not the utility companies and even after using a fifteen thousand dollar spectrum analyzer we also couldn't find the problem the signal was bouncing off other houses bouncing off the wires the signal was so powerful north olmsted city councilman chris glasbern says it was incredibly hard to zero in on one area their detection devices were being thrown off that was until he and a volunteer used the amplifier to mute the signal it allowed for them to pick up the signal when they were close to it and after going door to door of about 40 homes they finally found the problem it was a man-made custom device that was designed to notify that resident when someone was in their home glasburn says the device was the size of a shoebox he says the reason why it caused so many problems was because the person didn't design it the right way it was putting out a signal continuously and only when the signal was interrupted would it tell them that an individual was in another part of the house now that the issue has been resolved i'm glad they found it comrada says he is happy things can go back to normal and councilman chris glasbern says the person who did this had no idea his device was causing these issues in that north olmsted neighborhood the councilman said this was not malicious just an honest mistake reporting in the newsroom ray strickland channel 3 news so here it's an example of something we spoke about before which is interference so someone had had a diy set up at home for his home security and was putting out a 315 megahertz signal and that signal at 315 megahertz it was a dsb signal dsp suppress carrier but it was interfering with the dsb suppressed carrier signal from the key fobs and all the garage openers and the the car keys in that village stopped working did you notice in the video um the engineers were referring to this device here a spectrum analyzer or a frequency analyzer and that's what i referred to before it has a horizontal axis of frequency so what those traces you were seeing on the screen that was actually the uh the spectrum and that's how they actually picked up that 315 megahertz uh signal okay so that was dsb for ssb it's less common less likely you'll be using it remember we said that it has very good power efficiency because not only is there no pat no carrier there's also no lsb you know it could be no usb there's one sideband transmitted one sideband is blocked so it's also less noise and less bandwidth so these are all benefits low power efficiency good power efficiency less noise less bandwidth the fact that you need a complex modulation and demodulation is a disadvantage so both at the transmitter you need a high order filter and at the receiver you need complex electronics to recover it um so it is um it comes at that cost where do we use it we use it for um long-distance voice signals and two-way radios so the kind of radios that um you might have seen taxi drivers use so these two-way signals or two-way radios are very bandwidth conscious they use very little frequency and the radios that are licensed for such use are ssb okay and finally vestigial sideband we said the only application really where this is commonly used is analog television and while i say it is commonly used it won't be used for much longer because analog television is on its way out so what you need to know is that the bandwidth for vsp is somewhere between that of ssb and dsb but because vsb refer is typically used for television then its bandwidth will still be so the bandwidth of vsb if it's for television will still be greater than the bandwidth for am if it's for audio simply because this is going to be in kilohertz and this will be in megahertz so even though here we have 2fm and here we have maybe 1.3 fm or something like that this will still win out but if we have an equal fm or the same message modulated using am and vsb then vsb will have a smaller bandwidth so in summary this is what i was talking about so vsp will be somewhere between fm and 2fm but it just happens that for tv the fm here will be much bigger than the fm used here okay so this is just a summary of what we've spoken about in the previous four slides now finally a few examples a few questions before we end the lecture so i'll go through these quickly with you so for a single turn message what kind of modulation are you looking at so what kind of modulation does that look like now we won't know unless we know whether this is a single turn message or not i'm telling you that the message is single tone so that means that this must be the upper sideband this must be the lower sideband and there should be a carrier somewhere in here but there isn't so clearly we're looking at something with two sidebands and no carrier so two side bands no carrier that's what we're looking at double sideband suppressed carrier now we're not told what the message looks like we're simply told what kind of modulation scheme is this so all we have is these two components now is this the upper side band is this the lower side band well the upper side band the lower side band should have the same the same power they should be symmetric because they're not then no is either entirely the upper side band or entirely the lower side band so what we what we probably think is that this is all one sideband we don't know whether it's the upper sideband or lower sideband but we can tell it's single sideband could it be vsb not really for vsp you need a continuous spectrum not a single component could it be dsb lc not really because well where's the carrier there's no carrier here and if that's the carrier if this was the carrier and this is the lower sideband where's the upper sideband could it be dsb sc well not really not not with these asymmetric um sidebands so the correct answer is it's single sideman okay what are we looking at here the question sort of tells us that there's a carrier frequency right there and there isn't a carrier component so immediately we can eliminate dsb lc and that isn't even an option so there's no carrier component we have an upper sideband and a lower sideband so it's not ssb it's dsb so we can eliminate that now how many components are there there's one two three so it's a three tone signal we don't count these because these are simply reflections of these so it's a three tone dsb sc so we can eliminate that because there's no carrier we can eliminate that because there's only three components so it's a three tone ssb sorry three tone message dsb very similar question here but this time it's asking for the carrier frequency so what is this frequency here how would you find this frequency do we look at this do we look at this no these are completely irrelevant the only frequencies that are relevant here are this frequency and this frequency so your carrier frequency has to be right in the middle symmetrically placed between the 80 and the 90. so your frequency your carrier frequency will be 70 plus 80 over 2. that's your carrier frequency what's the band pass bandwidth it's a band pass so we're talking about the actual modulated signal and not the message so this is your bandpass bandwidth so it's simply the difference of the highest frequency and the lower frequency so it's 100 minus 50. so that's your band pass bandwidth i think this is very similar but now it's asking for the message bandwidth so the message bandwidth another way for asking for that it's the base band bandwidth so we're not asking for this bandwidth in total we're asking before modulation what was the bandpass bandwidth so you would be interested in the bandwidth from this carrier that doesn't exist to the highest frequency in the upper uppermost sideband that's your um message bandwidth alternatively you could look at that it's exactly the same it should be the same so to find this first we need to find fc we need to find that so we can determine the bandwidth so i think we've already established that fc is 75 kilohertz so what you're looking for is 100 take away 75. so that's your message bandwidth now notice when what we're not doing is finding this we're not interested in that that's not your baseband bandwidth your baseband bandwidth is measured from the carrier to the highest frequency and not from this frequency here to there that is meaningless okay your baseband bandwidth is from the carrier to the highest frequency okay so before we end one final numerical question this time so one example one question answered three different ways so we're told that we have an ssb transmitter and it's transmitting 100 kilowatts and we're going to replace it with a standard am signal what does that mean what does standard am signal mean it means dsb large carrier and it says the same total power so the power will remain 100 kilowatts and it says compare the respective carrier and sideband powers so before and after so what we have is this and we're replacing it with this the total power remains the same so initially we had 100 kilowatts all in this sideband we now have 100 kilowatts in the upper sideband the lower sideband and the carrier all together will be 100 kilowatts so how much of this 100 kilowatts will go into the upper sideband how much will go into the lower sideband how much will go into the um carrier now before answering it mathematically um we can sort of just look at it using a bit of common sense and our gut feeling so we know we know that the maximum efficiency of am that's is 33.3 that means that at best we have 33.3 kilowatts in the upper and lower sidebands at best so at best the power here and here is equal to 33 kilowatts so that means that each sideband will have half of 33.3 so let's just write it as 16.6 kilowatts in this sideband and 16.6 kilowatts in that sideband and the remaining 66.6 kilowatts is in the carrier so that's one way of looking at it you could answer it mathematically remembering the expression for the power and the upper side band and the lower side band obviously we're looking at the best case scenario where m equals one and you could answer it that way and you'd end up with the same solution another way is using this expression remember we said we can relate the efficiency to the modulation index and if you rearrange you can find the power of each sideband and you get the exact same answers so however you choose it so there are three ways this is i call this the simpler of the methods but actually just looking at it and remembering this efficiency you can probably um just answer it that way so that's three ways to answer a numerical question like that so i hope you found that helpful last thing i wanted to show you before the end of the lecture um another short youtube clip so this is a channel one this is the dc side of the lower diff pair there's our modulating signal and we can see that that modulating signal is sitting always above the base of q6 so we're getting essentially amplitude modulation of our carrier in this case i've chosen a fairly low frequency so it's easy to kind of see what's going on with the phase of the carrier as i adjust the dc bias so as i adjust the dc bias of the modulating signal up and down with respect to the bias voltage at q6 we can actually see the modulation depth changing so we've got a shallow modulation depth with a lot of offset as i bring the signal closer and closer to the q6 space voltage we can see the modulation depth getting deeper and deeper until we get to the point where the negative trough essentially equals the voltage at the base of q6 and we get to 100 modulation where the carrier is essentially completely collapsed because they've gone to zero at the lower trough of the baseband signal this is about as deep as you can bring the offset and still have amplitude modulation this gives me essentially 100 amplitude modulation where we completely suppress the carrier at the troughs of the baseband signal we come up we'll go something a little bit less than 100 modulation but if we can continue to go down we're going to get to the point where the carrier starts appearing again at the lower peaks but it appears at a the opposite phase from the upper peaks of my baseband signal as we continue to bring the offset down we can actually see the peak to peak magnitude of the upper trough modulation envelope start to decrease but the we're seeing now an increase during the lower trough we get to the point where the peak to peak value of both of those are the same and this is now a double sideband suppressed carrier modulation where the baseband signal is essentially following the alternatively following the positive peaks the negative peaks positive peaks and negative peaks we can see that going on here in the baseband if we continue to bring the offset down further we'll get to the point where we revert back again to amplitude modulation but now we're just biased the baseband signal down below the bias point or back to an am situation again so you can see that simply changing the bias we can go from amplitude modulation through double sideband suppress carrier modulation all the way back up to am again [Applause] it'll also be helpful to look at the frequency domain content of the resulting modulation output so we turn on the fft of channel 2 and since our carrier frequency here is is 18 kilohertz i'm going to adjust the center frequency fft to be about 18 kilohertz i'll adjust my scale so i can spread things out and now see my 18 kilohertz carrier and the two upper and lower modulation sidebands of that carrier i'm going to adjust the position of some of these waveforms uh up out of the way here so we can look at things a little bit more clearly and just get these guys overlapping up top here now also so you see he's now superimposing a time domain oscilloscope display in the top with the frequency domain spectrum analyzer or a frequency domain plot in red at the bottom so don't be confused these are not both time domain signals you have time at the top and frequency at the bottom take and adjust the center frequency to be just a little bit off i get a little bit make it a little bit easier to see the carrier here if it's not lined up with the center radical and i'm also going to adjust the vertical scale of the fft result to be 10 db per division so now i can actually see the magnitude of the carrier and the upper lower side bands here let's take a look at how these vary as i vary the dc offset on the baseband inputs so of course we're generating am here because our baseband signal is completely above the offset value if i bring that up even further we can see it's still am and all we're seeing now is that the magnitude of the upper and lower side bands is lower with respect to the carrier indicating we've got a shallower modulation depth as i bring the offset back down we can see that the side bands are now coming back up again and the carrier is shrinking still indicating that we've got am but getting closer and closer to 100 modulation which is right about there now for double sideband suppress carrier we'd expect to see both side bends there but the carrier being significantly suppressed so let's bring the offset down we can actually see that carrier now coming down now we're at a point where it's about equal to the strength of the side bands and if we keep going down carefully we should find the point where that carrier goes down pretty deep now of course i mentioned that the uh these transistors are not very well matched uh so there is going to be some offset things aren't perfect but we can see that we've dropped that carrier way down in this case you know about 20 db down from where the magnitude of the side vans are which is about 30 db down from where it was before which is pretty good even considering that there's a lot of offset in this gilbert cell so this obviously is double sideband suppress carrier we've got both side bends there the carrier is suppressed this is often used in uh ham radio transceivers and things like that to generate single side bend by first starting off with this coming out of the balance modulator and then using a filter to select one or the other sidebands to generate upper sideband or lower sideband modulation if we continue to bring my offset further down we'll come back to the point where the carrier is being raised up again and once the input signal is fully below the bias point at the input or the bias point on the other side of the diff pair now we're back to am and we see the familiar am spectre here again now just to illustrate this a bit further i've increased the carrier frequency to 10 megahertz to make it something that's easy for my shorter receiver here on the bench to pick up so if we got this set for amplitude modulation i've got the receiver set to am i can actually hear that modulation tone which i've adjusted to one kilohertz if i adjust that up or down we can actually hear that okay now if i adjust the dc offset to go down to double sideband suppress carrier listen to what happens to the am signal you actually hear a kind of almost like a second tone because what's happening is the am detector is detecting this envelope and we can see the double sideband suppress carrier basically creates an envelope that includes kind of a second harmonic component to it and that's why we hear that other tone come in i've turned on the fft again to look at the spectral content of the carrier we can see that for am we've got our carrier and the upper and lower side bands as we adjust the offset down we can actually see how that carrier gets suppressed and we're just left with the double sideband suppress carrier modulation i hope this video has given you a better feel for amplitude modulation and double sideband suppress carrier okay so i hope you found that helpful um it was useful to see um the time and fft plots both at the same time it was also useful to hear that that distortion caused by the envelope detector struggling to pick up the dsb so that was uh lecture five we spoke about uh dsb suppressed carrier ssb suppressed carrier and vsb now vsb still has a carrier with it and as i said dsp is normally just referred to as dsb and ssb is referred to as dsb so that was today's lecture i hope you found it helpful next lecture we'll be talking about the demodulation of dsb we won't be talking at the demodulation of these two but we'll be looking at the demodulation of dsb after that is your class test on the 17th of march so i hope you found that helpful until we meet again stay home and