#51: Basic Spectrum Analyzer Do's and Dont's ...

a couple of my YouTube viewers and some friends on some forums I belong to asked me about doing a video on the do's and don'ts of using a spectrum analyzer particularly how not to blow it up since it is at a expensive piece of equipment so we'll talk a little bit about you know some of the precautions and some of the basic operating practices to not only use it safely so you don't hurt it but also to use it effectively so several weeks ago you may remember that I did a video on this on the basics of spectrum images using this little Tektronix 1401 spectrum analyzer module that I found at a ham fest I kind of went through this block diagram to describe the way a spectrum analyzer works you may want to review that again if you've got some questions about that but typically when it comes to you know how not to blow up the spectrum analyzer you really want to pay attention to what's going on at the input most spectrum analyzers have some kind of variable attenuator which can be dialed down to zero sometimes there may be a filter or a preamp or something here as well not always and then you typically got a mixer that does the first conversion it's this mixer typically that is the most susceptible to damage and maybe some of the other components as well so the thing you want to be really careful of is what can we do with the input and what can we apply there safely most of the spectrum analyzers will have a warning right on the front panel to tell you the maximum signal level that's available or that you can put in there excuse me like this one says minus 30 DBM max okay so I'll tell you the maximum RF level but will also tell you the maximum DC voltage level typically so in some spectrum analyzers in fact most of the microwave spectrum analyzers the maximum DC voltage is actually use zero volts they will tolerate no DC voltage at the front end so in those cases you want to ensure that your signal is AC coupled you may even want to put a coaxial DC block you know right on the the input connector to ensure that that's the case now the spectrum analyzer I'm using up here this agilent 44:11 is a little bit unusual in two respects one is that it has a 75 ohm input impedance most spectrum analyzers i've got a 50 ohm input impedance I got this one inexpensively because it has this less popular less desirable in my mind 75 ohm input also what's unusual with this one is that it will tolerate up to 100 volts DC maximum again many spectrum analyzers are just a couple of volts and in the case of most microwave spectrum analyzers zero volts DC max so you may want to make sure you put a DC block in series I've got this 50 ohm - 75 ohm in loss pad here to give me a 75mm I'm assuming a 50 ohm input impedance for most of my work so a little bit of a compromise but it works the other thing you may want to note is what is the maximum RF input level see this one says plus 30 DBM that's one watt okay that's kind of typical in terms of spectrum I don't exceed that don't exceed that so you may want to ensure that you can attenuate your signal down below that level and that can be done with you know an external attenuator something like that guy there or if you're dealing with more higher power higher power devices like you know transmitters and things like that you may want to use something like this guy here this is a little bird RF sampler they basically put your RF set your high-power RF signal through this in one side out the other the other side might go to an attenuator and then you basically can adjust the value of this tap and you might get a 30 40 50 DB down version of your signal or sample of your signal coming out here that you can then send to the spectrum analyzer safely so something to you may want to consider if you're dealing with high-power RF devices so so that the main thing in terms of safety is just ensuring you don't overload the front end after that it's just basically using it and the main controls that you'd use to set up the analyzer are these guys right here the amplitude span and frequency controls those are the big three if you will the amplitude control is one we'll talk about first and that what that does when you hit that it basically brings up this menu typically where you're setting the reference level you can also adjust the 10 you Aitor you'll notice this that the attenuator set to auto here get that thing to focus you're right so that says auto ok and what that's doing is setting this attenuator value here because there's an actually there's an optimum signal level that you want to actually inject into this mixer if you go above that 2 things could happen if one is that you could cause the mixer to operate in a nonlinear fashion and generate distortion components you don't want that but also you could damage the mixer if you drive it too hard so the attenuator is typically going to no keep that signal level in check from being too high but also you don't want the signal level to be too low because then you'll just suffer in terms of signal-to-noise ratio so typically that attenuator is under automatic control and tied to the reference level ok the reference level is basically what is the power level that is being represented by the top of the screen so the idea typically is that when you put a signal up a signal in here like I have is you want the peak of that signal to be right at or just below the reference level okay so you would set that reference level if I hit amplitude and hit reference level I would set that reference level to something that you know makes sense for the signal level that I'm dealing with okay so that's the first thing and by leaving the attenuator in Auto ok what we'll see is that if I adjust the reference level up for example we'll see that at some point that attenuation kind of clicks up so have a 16 DBM reference level the attenuators of 25 dB I work my way down so at a 3 DBM reference level my attenuators at 10 DB okay and you let that thing adjust automatically now you can take manual control over that attenuator and there are applications where that is where you can do that but I caution you against doing it until you know you know that you need to do that that's a subject for another you know video typically leave the input attenuator set to auto and let the reference level drive it adjust the reference level so that it's just above the signal level you want to deal with okay so that that's really enough to ensure that you operate the front end of it safely rest of it is just setting up the parameters to display the signal that you want typically you'd set Center frequency okay by hitting the the frequency button here and the first thing you typically set is center frequency optionally you can set a start/stop frequency here which is kind of instead of setting a center frequency and a span okay by using the span key it gives hit a start and a stop frequency instead so for example I could dial in in this case we dial in center frequency I could dial in I got a hundred megahertz signal coming in here $1.00 in 100 and then hit megahertz I got a hundred megahertz center frequency and I hit the span and maybe I'll set my span to Ellis picks a fifty megahertz okay so now I've got a fifty megahertz span centered at 100 megahertz just by using those two keys okay and the other thing that you'll notice is that we have a sweep speed here and we haven't done anything to adjust that typically the sweep speed is automatically set okay that's typically tied to something else it's automatically set which is the bandwidth or the resolution bandwidth will notice the resolution bandwidth here of 300 kilohertz a resolution bandwidth has also been tied typically to the span okay so if I hit the bandwidth key we can see the resolution bandwidth is kind of set to auto alright I can take manual control over that if I hit manual and I start bumping down the resolution bandwidth you'll notice the sweep speed okay now it sees now at 12 and 1/2 milliseconds if I drop down the rvw again now it's at 138 milliseconds typically the sweep speed is tied to the resolution bandwidth okay because it's ensuring that you don't sweep too fast to not allow that resolution bandwidth filter to charge ok the narrower the resolution bandwidth the longer the response time so therefore you have to sweep slower so the analyzer knows that so by leaving the sweep control which is over here leaving that in Auto which is kind of the default way it is it will be tied to the resolution bandwidth resolution bandwidth is typically tied to the span so you can leave all these things under automatic control it's when you start you know taking them out of automatic control and driving them manually that you've got to start worrying about your consequences but in most cases if you leave things automatically coupled you'll be in good shape until you know you want to do you know something else beyond that so that's the real basics of it you know how to drive it safely how to not you know get yourself into trouble where you might damage the front end of the analyzer and you've kind of start off with your the attenuator control kind of set to auto maybe a resolution bandwidth control set to auto your sweep speed set to auto and just drive the center frequency span and amplitude controls to look at the signal that you want and then you can start gating from there so it's a good place to get started and it will do some more advanced videos at some point in the future but any questions that you have about how to kind of set up and use your spectrum analyzer would be very happy to try and answer thank you

Channel: w2aew

Views: 229,825

Keywords: W2AEW, HP, Agilent, SA, Spectrum, Analyzer, Basics, Basic, How-to, how, to, do's, dont's, tutorial, attenuation, DC, max, maximum, RF, power, impedance, tap, level, signal, auto, automatic, couple, coupling, RBW, sweep, span, frequency, center, start, stop

Id: E6P-R0C9ro0

Length: 9min 41sec (581 seconds)

Published: Tue Jun 19 2012