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Typically in the block scheme of a spectrum analyzer a video filter, with a bandwidth VBW, is located after the envelope detector and before the screen.

As pointed out in this document, the VBW may (and sometimes should) be less than RBW (Resolution BandWidth, that is the bandwith of the IF filter): the resulting signal will be less affected by noise (page 28). But if the VBW is smaller than RBW, this means that part of the RBW output is "wasted", which is undesirable.

1) Isn't it?

The envelope detector itself is a low-pass filter applied after the RBW and before VBW, so (even without the VBW) the signal displayed on the screen has a lower bandwidth than RBW. But the bandwidth of the envelope detector is nowhere specified. So:

2) How can I become sure that no useful signals are cut off by a VBW < RBW? Should I display the signal with VBW > RBW and then lower VBW till all the harmonics are still preserved and stop when a harmonic is cancelled, in a trial-and-error procedure?

Note: the document was just an example, the only useful I found so far. I am not referring to a particular model, but to the (theoretical) reason why a video filter is used in a spectrum analyzer.

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  • \$\begingroup\$ The VBW filter seems to implemented very differently in all kinds of spectrum analyzers, modern realtime scopes even display what essentially is a color coded histogram. You seem to have a specific device in mind, it might help understanding what you ask for if we could talk about that specific device. \$\endgroup\$ – PlasmaHH Jun 23 '15 at 9:41
  • \$\begingroup\$ @PlasmaHH I don't have a specific device in mind, that document was just an example of what I read and found. I am talking about the principle of the Video filter and the reason why it is usually implemented in a spectrum analyzer. \$\endgroup\$ – BowPark Jun 23 '15 at 10:00
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Do you have any hands-on experience with a spectrum analyzer ? If not then maybe you should as it will give you a better understanding of the effect of changing the RBW and VBW.

Ad 1) Indeed the VBW setting can be used to filter out noise and "smooth" the graph. The information is not so much "wasted" but "averaged". Most spectrum analyzers also have an option to average but this averages over multiple traces, this takes longer time and is inconvenient for observing a short event.

Ad 2) Normally you would leave the analyzer on auto settings and then it will take care of the RBW and VBW settings for you based on the frequency span you select. There are no guaranteed settings so that you don't miss anything ! Depending on the signal you're looking for you determine what settings you should use. This requires some practice.

I have been using spectrum analyzers for 20 years already without really thinking too much about how they work. I consider them basically a tunable radio receiver with a visual output.

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  • \$\begingroup\$ No, I don't have any experience, but thank you for your answer. It is very clarifying. \$\endgroup\$ – BowPark Jun 24 '15 at 15:44
  • \$\begingroup\$ Happy to help ! No doubt that experience will come later then :-) \$\endgroup\$ – Bimpelrekkie Jun 24 '15 at 17:27

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