#339: Basics of a Super-heterodyne Receiver - how it works, and a peek at the signals

in today's video we're going to take a look at the basics of a super heterodyne receiver and there's really two key elements to any super hat design one is frequency conversion using mixers so we'll talk about that first and then the second is filtering in the if or the intermediate frequency stage so let's get started now i've talked about mixers in previous videos and i'll link one or two of those down below so we'll just do a quick review here so a mixer is a device that can take two input signals and then produce an output that's related to those two inputs and the rules are basically this the output will consist of a number of different frequency components one of which is the sum of the input frequency and the local oscillator frequency as well as the difference products the frequency in minus flow or flo minus fn depending on which frequency is higher and the reality is you can also get harmonics and things like that as well but we're just going to concern ourselves with those particular components now typically a super heterodyne receiver is going to take advantage of really just one of these and in most cases it is the lo minus fn meaning that the local oscillator frequency is designed to be higher than the input frequency but that's not always the case there are plenty of radios that are just the other way around so if we consider the case where the local oscillator frequency is higher than the input frequency what we're going to get out of that is the difference flo minus fn so a signal sitting here as well as the sum which is way out here now this is very far away from this component here so that's easy to filter away now one of the things that's always essentially an issue that can be concerned about when designing a super head receiver is the image response because while a signal sitting here with respect to flo will produce a component at the output in this location a signal equidistant on the other side of the local oscillator frequency called an image frequency will also produce an output here we'll also produce a sum output way out over here so we don't worry about that one but this image frequency can land right on top of the signal we're really interested in so you generally have to have ways of dealing with that and it's usually done with some front end filtering to keep that signal from getting into the mixer in the first place so here's a highly simplified version of a single conversion superhector dyn receiver or actually just a portion of it typically you'll have the antenna and then some front end which will maybe include some amplification so maybe some attenuation some limiting maybe some filtering to get rid of images etcetera and also the oftentimes the gain control in this front end is part of the automatic gain control or automatic uh volume control of the particular radio and that's typically what's in the front end so the rf signal after being processed from the front end is presented as f in or a whole range of frequencies input into the mixer mixer is also fed with a local oscillator and when you're tuning a radio up or down when you're tuning the vfo or the main tuning knob what you're really tuning is the frequency of this local oscillator because that's going to change essentially what the mixing products are that are coming out so this is what you're changing when you're adjusting the tuning settings on your radio now as we described the output of that mixer is going to consist of a number of different frequency components some indifference frequencies of the local oscillator and then all of the signals present at the input so there's going to be a whole pile of signals appearing here it is the job of the if or intermediate frequency filter to select just the particular frequency of interest that you actually want to go receive and that's going to reject everything else on either side of it we'll take a look at this graphically in a moment and actually go probing around in the receiver to go take a look at that then finally the output of that if filter generally goes through some amplification maybe some additional filtering and then off to the demodulation stages whether it's a amplitude detector for am or a product detector for a single sideband or cw or maybe even a ratio detector for fm but it's really the job of the if filter to provide the selectivity of the receiver in terms of being able to select and receive the signal of interest and reject signals on either side of it so we only are listening to one particular station at a time so here's what's going on kind of graphically inside that receiver this is say the signals that are being present at the input of the antenna now i'm showing just three signals present across some frequency range with frequency going up in this direction so these are the signals are effectively presented to the input of the mixer so here's the ideal spectrum of the local oscillator it's just a frequency a single frequency here and in this case again we're doing what we call high side injection which is creating an lo that is higher than the signals of interest so that's the local oscillator input into the mixer and in this case the product that we're interested in is the local oscillator frequency minus the fn and because we have minus f in here what that means is the spectrum that's coming out of the mixer is inverted from the spectrum going in so while the frequency is going up in this direction in terms of the frequency of each of these stations there go it's going in the opposite direction at the output of the mixer this is the lowest frequency station this is the one in the middle and this is the highest frequency station this is simply called spectral inversion and it happens when you're using the difference product that's a result from the lo being higher than the signal of interest now in many cases many different modulation types like am and cw and things like that it really doesn't matter that the spectrum is inverted so we can detect it the same way whether it was inverted or not so in this sense it really doesn't matter now when you tune the receiver as we talked about you're tuning the frequency of the local oscillator so when that tunes back and forth this difference product also moves back and forth the same way and the whole idea is to move this converted spectrum back and forth so that the particular signal of interest that you want to receive lands inside the if filter it's a fixed frequency filter so whatever signal lands inside that if gets through and everything else on either side of it gets blocked so the only thing that is sent down to the rest of the receiver for demodulation and for you to listen to is the particular signal of interest now the main reason this is done is that you know the critical thing in any receiver oftentimes is selectivity being able to hear the signal you want and not the signals on either side of it it's pretty difficult to design a tunable bandpass filter that's tunable over the frequency range you want to listen to so by converting everything into an if the iaf can be at a fixed constant frequency so you can optimize the filter and the amplification stages of things just for that particular frequency and then by using this mixing process we can put the signal of interest into that fixed if so with that let's actually go take a look at these particular spectral components in an actual receiver now our test subject for this video is my old realistic dx160 uh five band communications receiver i bought this radio in the mid 70s after saving up lawn mowing money for a whole summer we've got a number of probes sitting inside this receiver and we're looking at the rf input coming from the antenna going through some input amplification stages now we're looking at the local oscillator the output of the mixer and then the output of the if filter okay so here we have a couple of selected points in that receiver looking at the spectral content of it the top trace here the yellow one is looking at the rf input so this is a portion of the am band i'm centered at about 830 kilohertz in the center here um starting from 580k up to 1.08 meg so it's about the lower half of the am broadcast band we can see a pretty strong signal here there's another strong signal here i've got some spurs in here just coming from noise here in the lab so that's what's being shown just in terms of the input signals coming into the receiver this next trace here is actually looking at the spectral content of the local oscillator that's going into the mixer now while these two spectral plots have the same span they have a different center frequency so the center frequency of this one here is actually at 1.2 megahertz let's kind of center that up there okay so that's centered at 1.2 meg so we can see this signal here is sitting a bit higher than 1.2 meg all right so it's over you know past this right side edge of the input spectrum now this signal down here shown in red is the output of the mixer and so it does have a bit of a hump right at the i f frequency of 455 kilohertz and the reason for that is because the load of the of this uh output of the mixer is going into a tuned circuit it's going into an if transformer so it's essentially a tuned load so it's going to provide more gain at the if frequency so it's essentially the beginning of the if filtering although it's not the if filter itself now if we look carefully we can see this input signal here which is pretty strong we can see the modulation side bends on it it's an am signal that signal is this guy over here okay you can kind of recognize that if you look at them both at the same time they're doing the same thing so this is that spectral inversion that i mentioned so working my way up you know there's another couple of signals working way up there's one here another one over here those are this signal here and then actually this one over here there's a couple of other strong ones here as well like this guy here is likely that guy all right and then the finally the last output here is taking the signal out of the mixer and going through the actual if filter so we're seeing essentially just the pass band response and we can see i've got you know just a lot of gain at the 455 kilohertz center frequency but all these other signals that are outside of that pass band are really not making it through so that's where we're getting our seal activity so what we're going to do is tune the radio and what we'll see is the local oscillator frequency move and this spectral content move so we can place any of these particular signals into that iaf filter so let's tune down okay so i can i want to tune down to this frequency here because that's about the strongest one i have on the band here so that's this guy so we want to put that guy into the if filter so you'll notice as i'm tuning down and the local oscillator frequency is coming down and the spectrum of the signal uh the output is moving down all those components are moving down here along with that so i want to get down to that strong signal right here i'm just going to ease up on it here and now with that centered in now i can see that signal is making it through the iaf really no other signals are and that particular signal is now being converted down from this lo to the mixer output centered on the if output or the if filter the iaf is stripping away everything else but that signal and that's the one that's going into the demodulator to actually go listen to so i hope you enjoyed this brief look at the super heterodyne receiver and what those signals actually look like inside an actual receiver if you like the video give me a thumbs up if you haven't subscribed to the channel already please consider doing so and if you'd like to get notified when i post a new video be sure to click the bell that's just down below this video in the youtube player thanks again as always for watching we'll see you next time

Channel: w2aew

Views: 25,756

Keywords: W2AEW, Tek, Tektronix, radio, receiver, super heterodyne, mixer, frequency, filter, IF stage, spectrum, local oscillator, oscillator, shortwave

Id: Vf06HSR4LdY

Length: 12min 4sec (724 seconds)

Published: Wed Oct 13 2021