#597 Spectrum Analyzer Basics (part 1)

this video is intended for somebody who's never touched a spectrum analyzer before and there's a little bit of uh strange things that are about spectrum analyzers that you need to know and hopefully this this will help you answer some of those questions that you have so i'm assuming that you've never laid eyes on a spectrum analyzer it's your first time you're going to power one up and you're just not sure how to use it so this particular spectrum analyzer is an hp 8591e and it's good from about 10 kilohertz up to 1.9 megahertz 1.8 hertz 1.8 gigahertz um and so what is a spectrum analyzer looks like an oscilloscope well oscilloscopes give you a picture like this one but it's time versus amplitude and spectrum analyzers give you a picture that's frequency versus amplitude so it's it's kind of like a radio that's constantly tuning all the time and if there is some frequency content somewhere it'll show up in the in the display and i'll and i'll show that so what do you need to know before you even start to use it well the first thing is you know does the spectrum analyzer cover the frequency range that you're interested in and uh is it uh going to be able to input the data that you need so there's some connectors on the front and there's sometimes a bnc sometimes this is an n-type connector and connector sometimes there's different connectors maybe an sma so first thing you have to do is figure out whether the connector is going to mate with what you're measuring so i have an adapter here i'm going to put the adapter in so i can use bnc because that's much more convenient for me all right so like i said before the spectrum analyzer gives you a graph of frequency versus amplitude so so we want to just put a frequency right in but what about amplitude what is the range of amplitudes that you can measure well there's a warning here on the input and it says uh less than plus 30 dbm and less than 25 volts dc now as a general rule i say never ever ever put any dc into a spectrum analyzer so just i would change the label and say zero volts just don't put dc into the instrument if you do have some circuit that you think has dsc on it then put a blocking capacitor in between you can buy these uh that screw on and it's a capacitor and so the dc can't go through but the ac can go through all right the other thing you have to worry about is you want to make sure you're below this plus 30 dbm uh so my rule of thumb is always zero dbm i always like to start at zero i'm gonna make sure i never have more than zero dbm that's one milliwatt and that's what's going to go inside the instrument all right so just because we want to look like we're actually doing something here this spectrum layer has a calibration output so it'll give us something to look at so i'm going to hook up a cable that goes between these two and now we have something to look at okay so this gives you an idea of what type of picture you'll get it's going to be these constant peaks where there's a where there's a frequency so the we see that there's many frequencies it's almost like looking at radio stations like many radio stations and so the reason that we have so many is this is a 300 megahertz calibrator but it's a square wave so we get the first harmonic the second harmonic the third harmonic the fourth harmonic so this peak's going to happen around 300 600 900 1200 1500 and so on right and so that's what is being displayed here and then the amplitude of each so each harmonic is not the same intensity they they go up and down all right so what is the x-axis and what is the y-axis well we can have a start and a stop on the on the on the x-axis or we can have a center and a span so there's two ways for us to to tell the instrument what we want to do we can say start stop or center and then how big of a span right so we have a 300 megahertz signal let's go ahead and say we have a center we're going to press the frequency button that says center we're going to say 300 megahertz now that's going to put us right in the middle here right at 300 okay now we have a whole bunch of them now we have some to the left and some to the right well that doesn't really make any sense how how can that be okay so we're going to get to it later but just for now i'm going to show you how a marker works if i push the marker button i get this little this little marker and i can move it around with the wheel and i can put it here on this peak so remember this peak this peak's gonna give me at 300 megahertz it's going to tell me the amplitude of 300 megahertz what's this one over here okay this one over here is at zero hertz what about this one over here this one is at minus 300 megahertz what's going on there so right away spectrum analyzers are strange um they work above zero hertz of course but there's a reflection that happens inside the instrument and you get negative things so if you zoom out in a particular case like this you have to ignore all these negative ones okay these aren't real just you just have to ignore those right and so that's that's one of the big big problems a lot of times you don't have any input at all let me disconnect the input and you say oh no no we have something there it's right there i can see it of course there's something right there well let's go take a look at it oh it's zero hertz so the instrument itself always puts something at zero hertz and it's fictitious it doesn't exist okay so that one there is just its starting point right so you have to ignore everything right at zero and you have to ignore all the minus ones right okay so that's the first weird thing about spectrum analyzers is that is that weird reflection okay so let's say that we want to zoom in on this so remember we can say start and stop so we can say frequency we can say start let's start at 100 megahertz let's stop at 500 megahertz so now we can see right at the bottom here 100 megahertz is the start 500 megahertz is the stop and we're in the center let's say we want to start at 200 megahertz megahertz there's 200 there's 500 so 300 is going to be somewhere over in here right and so we can set some range let's say we set our our start at 200 okay and we set our stop at 600 and now we have this range oops start that's start at 200 and the stop i forgot to push the stop button stop at 600 and there now we have a range now remember i said we can move this marker around right well we can also move the the actual peak around right let's say we want to move it to the middle we can hit frequency right and we can hit center frequency and we could type it in or we could use the wheel to move it back and forth all right so we can make it here we can make it here and so right here at 300 we could put it in the middle right or we could type in 300. we could keep going oh wait a minute there's another one what's this one what's that one why do we have two so let's hit the marker and we'll say oh we have our 300 but what's this one over here oh 600 that's right that's my first my second harmonic right this is the fundamental this is the second harmonic so now we can see both on the same screen that's kind of cool so you can see that we can change the span depending on what we want to look at and we can put the start and the stop or the center in the span so we can change the span i showed you the other one let's say the span let's say that we're going to put the we're going to put center at 600 megahertz okay so now that 601 is there but we're going to change the span and we're going to make the span bigger or smaller okay so here i'm making the span smaller and so i'm seeing i'm kind of zooming in and here i'm making this band bigger so i'm i'm widening out so this is like the zoom on a lens right you can go wide angle or you can go narrow a telephoto right you can go back and forth so you can you can use the wheel to to go to go to go in and out or you can type it in anything you can do with typing you can do with the wheel right okay what about this amplitude thing going on here how do we we've kind of looked at we've kind of looked at frequency but what about this amplitude thing how do we read the amplitude well um when i was using the marker it told us right away so if i put the marker on one of these peaks it said right here it said that was at minus 53 dbm okay so it it just told me minus 27 dbm i didn't have it on the peak minus 27 dbm so we can move the cursor around and read it off but what do these lines mean okay so over here it tells me that the display is logarithmic and that it's 10 dbs per step so so every line is 10 db lower right but where's zero on an oscilloscope i have ground i set the zero line someplace well in a spectrum analyzer we don't set zero we set maximum right because we always get lower and lower and lower it gets noisier noisier noisier but we wanna set just the picture we want to say well what's the top of the picture so we could say okay well we want the top of the picture we hit we hit amplitude the top of the picture is called reference level and we can say that's zero or that's minus ten oh everything jumped up that's because uh i shifted it up instead of the top being at zero i said no let's make the top at minus ten i could say let's make the top at minus 20. now everything's shifted up again or i want to make the top at plus 10. now everything shifted down so you get to you get to set the where the level is you can do that with the wheel also right so let's say you're interested in this peak here you can just kind of bring it up bring it up bring it up and and just when it touches that line okay you know that line is at minus 26 dbm because that's where the reference level was set last so i could say okay i'm going to set my reference level to 30 db plus 30. now i want it at minus 30 that's right frequency so you have to remember plus and minus right plus dbms and minus dbms all right so um if you ever get in trouble there's this magic button called preset and the preset button sends you all back to zero and gives you the picture that used to so let's say that you really messed it up you set the frequency over here and you had the amplitude whoa the amplitude got oh no where am i i'm i'm totally lost i'm totally confused you hit the preset button and it brings you back some oscilloscopes have a button called auto that's kind of like that it puts you back puts you at maximum span and some standard it puts you at reference dv at 0 and 10 10 db per step so it automatically automatically changes changes everything okay so we've talked about frequency we've talked about span we've talked about amplitude now the most important thing you're going to be doing with spectrum analyzers is interrogating these these little spikes here right so you want to find them and i was using the wheel to find them but that was a bit dodgy so there's this marker function like i said you can move it around but it has automatic search features so you could say peak search and it finds the one here and that's at the that says five megahertz that's that zero one that's that fictitious one we need to ignore but i could say go right it says next peak right and it finds that one and i can say go right again oh i found that one found that one found that one found that one or i could go left and go backwards so that's an easy way to find the peaks is to do a peak search all right so i think that'll be it for this first video i've given you the basic ideas and you can see that the spectrum analyzer laid out so that you know these are the most important things frequency span and amplitude and so that's uh you know you can set start and stop with frequency you can set the center with frequency you can change the span if you want with span and then you can change the amplitude amplitude by setting what this reference level is you could also change how many instead of 10 db per unit you could say scale i want i want 5 db per unit now everything is magnified right and so it says here 5 db 5 db provision so you can change things with the amplitude button um so those really the basic things frequency span amplitude and then in order to actually measure things the marker the marker is your friend right the marker allows you to go around and do everything so in the next video we'll take a look at actually making some real measurements

Channel: IMSAI Guy

Views: 10,616

Keywords: s100, s-100, cpm, cp/m, cpm80, intel 8080, 8080, 8085, zilog z80, vintage computer, vintage computing, IMSAI, IMSAI 8080, ALTAIRTTL CPU, cpu design, eagle cad, eagle pcb, 8-bit CPU, 8bit CPU, diy cpu, 74181, 74381, 74382, ALUZeta, Zeta2, ROMWBW, Zeta SBCelectronic test equipment, vintage electronics, DIY electronics, electrical engineering, electronic engineering, electronics, PCB design, Circuit design

Id: Ze4V6Qp1mK8

Length: 14min 26sec (866 seconds)

Published: Thu Oct 29 2020