EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: w2aew on November 24, 2013, 02:07:33 am
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Did you ever wonder what the RBW (resolution bandwidth) and VBW (video bandwidth) controls do on a spectrum analyzer? My latest video should answer these questions for you.
Basics of Resolution Bandwidth and Video Bandwidth in a Spectrum Analyzer (RBW VBW) (https://www.youtube.com/watch?v=Ffhs9Ny03lM#ws)
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Very good video as always :-+
One question though, what is the practical difference between more averaging vs. more VBW filtering?
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I really liked this video when you first posted it and I would have commented and thumbs upped it but since I closed my channel and told Google to shove their G+ nonsense up their wazoo I am not allowed. Heck, I can't even sub to anyone new I am allowed, for now, to just keep the ones I already was subbed to.
Keep up the good work and like that one comment said your videos should be shown to under grad EE.
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Very good video as always :-+
One question though, what is the practical difference between more averaging vs. more VBW filtering?
One advantage of VBW vs. averaging is speed, since it occurs in one sweep. VBW is a technique that was put in on old analog SAs long before storage and averaging was possible. Averaging can ultimately give you more smoothing of the traces after many averages.
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The video is kind of OK for a very basic or homebrew analyser designed 35 years ago but the stuff people need to understand today about VBW, RBW and the detector technology in a good quality old school swept analyser isn't covered. Also many modern analysers do all of this in DSP anyway.
Also, a typical old school swept spectrum analyser doesn't/can't measure power with its envelope detector so the references to power detection in the video are going to confuse a few people.
Also, depending on the way the analyser is designed and what mode the detector is placed in, you can often reduce the average (displayed) noise level when reducing the VBW or by selecting video averaging. The benefit can sometimes be several dB less average level displayed on some analysers. This can be due to the detector mode selected by the operator and also due to automatic changes in detector mode when video averaging is selected by the operator.
So some operators 'will' often see the (displayed) average noise level fall when the VBW is reduced or when video averaging is selected. It depends on the way the analyser is designed and used.
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I'd like to add my input.
VBW is not the same as averaging. RBW, VBW, and sweep speed, are all related. As the video showed (great video BTW), RBW allows separation of closely spaced signals. There is a relationship between RBW, VBW and sweep speed. You must have a sufficiently SLOW sweep speed to allow the detectors to property charge up and output the correct amplitude. If you uncouple RBW or go manual, and you select a sweep speed that is too fast, most analyzers will alert you with a "uncal" notification. The same will happen with VBW. The VBW is a low pass filter, if you have a certain sweep speed, the video applied to the screen has certain high frequency components. If you turn down the VBW, you will start to filter out those high frequency components.
The video may be somewhat misleading in stating the VBW will not change noise amplitude. It does not change the noise floor, but it does filter the noise and it will change signal amplitude. Both the RBW and VBW will change signal amplitude if not set properly. If you independantly change, and go too far in turning down (narrowing) the RBW, VBW, or sweep speed, you will start to filter out the sharp peaks. You don't care about the noise, but if there is a very narrow signal peak, whether the signal of interest or another signal off to the side, the VBW will prevent the signal from reaching it's maximum peak. As in the RBW, the analyzer will notify you when your VBW setting is compromising the amplitude accuracy. This applies whether it's an analog or digital analyzer. Leaving the RBW, VBW, and sweep speed in "auto" or "coupled will keep you out of trouble.
Now for averaging. Averaging does not "filter out" the noise, it mathematically adds them together, and as we all know, "true" noise is random, and thus mathematically they eventually disappear after a sufficient number of sweeps. But what does it do to the signal of interest? Nothing. If the signal is stable and there continuously, it will be mathematically reinforced. This includes the most thinnest and sharpest peaks. So averaging does not remove the high frequency components that are really there, only the ones that are random and not there all the time. It's very good at looking at the signal of interest, but it can mask intermittent interfering signals. One note of caution, the analyzer's frequency drift puts a limitation of the amount of averaging that is possible. If the swept bandwidth is moving due to high oscillator drift (as in the older HP8590), the averaging will destroy your signal. This happens by adding sweep copies that are not in the same place; the peak in the current sweep, is moving relative to the location it occupied in the previous sweeps. Note that the analyzer will probably NOT alert you of this condition. Most likely in analyzing a signal, you will be changing all of these parameters at one time or another, this is why you need to know how to operate your instrument, as our video author is trying to convey. Again, well done on the video.
Bob
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I thought that a good basic overview of the subject.
It might be better if the detector was shown as a log detector rather than just a diode with a smoothing capacitor which would give a video bandwidth that is very dependent on the signal level.
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Maybe I'm being too critical as an experienced user of modern spectrum analysers? I think maybe the video is OK for a very basic 40 year old or homebrew analyser but this thread mentions video averaging and this implies a fairly modern analyser architecture.
Now for averaging. Averaging does not "filter out" the noise, it mathematically adds them together, and as we all know, "true" noise is random, and thus mathematically they eventually disappear after a sufficient number of sweeps. But what does it do to the signal of interest? Nothing. If the signal is stable and there continuously, it will be mathematically reinforced. This includes the most thinnest and sharpest peaks.
The classic lab analysers from HP/Agilent will revert to sample detection mode when averaging and in some situations this can severely upset (i.e. cause misreading of) the signal of interest even if it is stable and continuous. i.e. it can look fine in the +peak detection or Normal detection modes but the moment averaging is selected then the analyser can lose amplitude information of the wanted signals because of the way the sample mode operates.
This is a classic failing of sample mode and averaging. It's great for looking at noise but you have to be very careful it doesn't lose amplitude information on the wanted signals.
So video averaging has to be selected with some caution even on stable and continuous signals.
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G0HZU, KJDS, bob11746... Really good info, thanks for that. There was only so much I could put into an intro video without making it too long. I appreciate all the comments and discussion.