#45: Sweep Generator Basics and Receiver Alignment

hey one of my youtube subscribers have asked me to talk a little bit about actually sweep generators and using them to tune like the if' or do an alignment on a receiver for example so that's we're going to talk about here and just show one example so we're going to talk a little bit about you know kind of how to tune you know typical if' transformer because you'll typically find in a receiver chain and basically it's just a little bit of a resonant circuit that basically rejects signals that are above or below the frequency that it's tuned to and that's typically how you get selectivity and a receiver at least part of it anyway and here's a typical example there's a shortwave receiver schematic and we can kind of see here I've got a signal coming out here actually out of a mixer getting amplified going in through a bandpass filter then anion amplifier here's one of the tuned if' transformers and other if' amplifier to the tune transformer and part of the alignment process for this receiver is to tune these various the resonant frequencies of these transformers to kind of peak the response okay so that's kind of what we would have here and sometimes you can certainly do it with just a an RF signal generator you know inject a signal in you know into the right point and then just kind of monitor the response as you tune the various scans call them cans because they kind of look like little cans you kind of see that one there in my vise we're going to go look at okay but sometimes you want to take a look and see what the shape is and that's not so easy to do just by tuning it I mean you can kind of get a feel for it but let me kind of show you what I mean so I'm using this like this leader function generator here as my signal source okay that's being coupled down into this little eye f kin that I've got clamped in here I got the probes on here going up to the scope okay so I got the scope kind of dialed in at a fairly low sweep speed about a millisecond or so so we're just looking at the RF envelope okay in fact if I kind of changed my triggering and way out here and kind of see is just a sinusoid okay it's a 50 nanoseconds of division so this is about a three maker it signal let's go back down to about a millisecond where I was okay and I've also got an output from that signal generator on the back gives me a sync output so I've got that going into a frequency counter so I can actually see them running at about 3.15 megahertz here okay so what we can do is I can tune you know the the frequency here and watch the response of the the if' can and watch its frequency and I kind of see its response here so if I kind of do this when you reach over here and see if I get both a frequency counter and the scope and a shot now if I move the frequency down or actually that's up so you know I know it's at 3.3 megahertz I can see my response went down okay if I move the frequency down okay so now I'm at 3 megahertz and I can see the frequencies down and I keep going lower and they kind of pull this out so you can see both we keep going low and frequency we see the response goes way down and if I come back up as I slowly bring the frequency up I could see it Peaks if I keep going and it goes back down again so you can kind of see that response you can almost picture it I'm just wiggling back and forth now between no through its resonant frequency okay and then the idea is on the can there's a little core that you can adjust with a little screwdriver type device to change essentially the resonant frequency to kind of move that resonant frequency back and forth okay and that's the process of doing the alignment now of course you know I could just set the signal generator up to the desired if' frequency okay and then just peak all the responses and everything low fine but but sometimes it's fun to look at things to see what that shape is as I was moving this the frequency back and forth here we can kind of see that you know the signal is kind of going other signals going up and down but I can't get an idea of you know how steep is that skirt on that filter right now how how steep is a kind of the spirit of this here and that's part of what the sweep ring using a sweet boss later we'll help you do so what is it so sweep generator is basically a generator that basic over time starts off at one frequency slowly and linear linearly or logarithmic Li sweeps all the way up to a higher frequency and then goes back and repeats itself again so I kind of just over time it just kind of starts slow and then go fast again all right so we can set up the sweep oscillator 2 or sweep generator to sweep over a given frequency range and ideally it set up the low frequency side and the high frequency side to be you know below the resonant frequency of the of the can or the if' and then the other end to be above that vif frequency so they will sweep through the peak alright so looking at it say in frequency versus time we basically would get this ideally kind of either a linear or logarithmic we're going to use a linear there kind of rate change from low frequency to high frequency okay so it's what will set up the generator to do that a lot of the sweep generators will also have some some other signals okay that may be helpful okay and in terms of you know being able to look at the signals on a scope they may have a trigger or a synchro sync output that will give you a pulse each time the sweep starts that's a really handy one to have it might also give you a gate which goes hi say during the sweep goes low when it returns back to the original frequency then goes high again or they might even give you a ramp voltage that kind of ramps along with the frequency okay so depending on which generator you have with sweet boss later you have you might have one or more of these signals available and they'll be kind of handy okay so we take a look at what they look like on this one okay so I'm going to turn off channel two and turn off channel turn up turn on channel one and three so here I can actually see I've got those two I've got my ramp signal let's slow this down you can kind of see there's my the ramp signal and I also have a gate so the gate goes high during the sweep goes low during the when we go down to low frequency so this this voltage here is kind of representing what the frequency is doing going from low to high and then going down real quick low to high okay it's speeded up now I can see I just barely have you know one full sweep in the screen alright so what we'll do is we'll allow that to kind of set up with the frequency is going to do and we'll sweep through this filter okay and watch its response so we just need to set up the start and stop frequency so on this guy here if we go into CW mode okay on this guy that also sets the stop frequency if that thing will cut it yes you can see CW kind of also sets the stop so we'll just adjust this and get the frequency I want let's set it to something that's kind of easy to see let's set it to say two megahertz close to it so if I kind of bring this down to oh we get it close to two megabits that's pretty close to two megahertz then I'm going to tell it I want to set up the stop frequency so I go into sweet move and push this button here and now I can set the stop frequency with this knob here okay and achieving that was the stop frequency so I'm going to go back and do this again okay so let's we'll set the stop frequency to four megahertz alright and we're just about there there's about four megahertz right about there that's close enough now go over and do it you're going to do set the start frequency start frequency at about two megahertz so now we're going to kind of go through this frequency ramp from two megahertz to four megahertz that's kind of handy because note is we know what the two megahertz total sweep I know on the scope I've got ten divisions okay so I know that each division is 200 kilohertz so it kind of gives me a way of kind of getting an idea of where the frequency is okay so once I've got that done I just go back I go into my sweep mode here okay so now I see my sweeping signals if I put the output from my filter back on here okay now I can actually see that response there's the there's that that's sweeping right through the resonant frequency here now you see this little blip right here because what's happening is after we finished this sweep it very quickly returned through that sweep backwards so we get that little blip again now a good handy way to do this is what I do is I'll speed this scope up so I don't really want to just get one one complete sweep across the 10 divisions so I'm going to speed the scope up to kind of expand it so now it's kind of beyond that and then I'll go to my variable control and I'll just vary the time delay back down again I'm just very very light you know the horizontal time base and a kind of this uncalibrated mode until I get just barely one sweep so you can see right there the sweep is just ending okay right at the 10th division and returning you can see the gate is stopping there so now across the 10 divisions I'm sweeping from 2 megahertz to 4 megahertz so I kind of see where I am so now if I take my little tool here to tweak the the if' can all right I'll stick that in here I'm going to rotate this back and forth okay I kind of like this and as I do that we look back up on the scope now okay I can actually see that I'm actually changing the resonant frequency of that if' can okay so you might say well I want this thing to be tuned right at 3 megahertz I can kind of dial that so it's right in the center just like that because I know I started over here you know at two megahertz and I end at four so I'm going to be at 3 megahertz right in the middle so now I know I've got the this can tune to be resonant right at 3 megahertz now why this is handy is if in receiver you may have multiple if's okay one might be tuned one way might might be tuned another and if say you were looking at the output okay you might tune one you know might look at this and look at this response at the output and you might see I've got a little bit of a peak here and then another peak over here or maybe another one over here it might show you that some of the cans are really way off and that as you adjust hif you can kind of bring them all or each can you know just them all so they all line up right up in the middle and peak your whole response it also allows you to see for example if the filter maybe the bandpass filter here's got kind of an unusual shape to it you'd be able to see essentially the shape of that if' its response versus frequency so so that's a pretty simple way of using a sweep generator sweep oscillator to sweep a tuned circuit like an if' or like an if' can and look at his frequency response and see how it varies as we tune it back and forth and see its shape effect on this one you can actually see as I go higher in frequency its response actually goes down it attenuates a little more they go down lower in frequency I got a little more response at the low frequency here okay just again just interesting things that you wouldn't necessarily see just by manually tuning the oscillator and manually tuning the can it would take a lot of back-and-forth to kind of see what's going on so now I hope that was helpful and just more fun with scopes and signal generators and parts and all the kind of stuff you can do with them comments are welcome any videos you'd like to see please let me know thank you

Channel: w2aew

Views: 99,430

Rating: 4.9548693 out of 5

Keywords: W2AEW, Tek, Tektronix, oscilloscope, scope, 2467, 2465, leader, sweep, function, signal, basics, how-to, how, to, generator, IF, alignment, receiver, cans, counter, adjust, tweak, transformer, filter, superhet, superheterodyne, heterodyne, mixer, bandpass, skirt, shape, factor, radio, align, Transformers, core, local, oscillator, LO, RF, tutorial

Id: -A_DxsxPdeI

Length: 11min 39sec (699 seconds)

Published: Sat May 19 2012