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The envelope detector is followed by the video filter which defines the
video bandwidth (VBW). The video filter is a first order lowpass configuration
used to free the video signal from noise, and to smooth the trace that
is subsequently displayed so that the display is stabilized.
Video Bandwidth - change smoothness of signal (when it high , not appearing noise level and became a singal smooth line)
Resolution Bandwidth - Change Resolution of bandwidth that you selected (When it high your signal spread on your monitor)
To make it in the simplest way:
Resolution bandwith: the bandwith of the "filter" that is swept across the rquired band of the SpAnal.
Video Bandwith: after the swept "filter" (as above) an amplitude detection takes place. followed by a lowpass filter. The VBW is the bandwith of this afterdetection filter.
I feel a little confused, the Video filter is applied to the spectrum waveform ( not the time domain), right? then how can you tell " the video bandwidth is 100kHz" (for example) ? What is the physical meaning of the video filter's output?
See App Note 150:
Page 16 describes the IF filter. This is the important "resolution bandwidth" filter.
Page 34 describes the video filter. This is usually a simple display plot smoothing filter.
Page 10 shows the IF filter located before the detector, and the video filter located after the detector.
Does that help a little?
Do you have a spectrum analyzer to play with, so you can see how these filters behave?
lets take an example, if the resolution bandwidth is 100khz, video bandwidth is 10khz, then how to tune the video filter to make this bandwidth? I know the Video filter is to make the spectrum smooth, but how this filter be related to a bandwidth? I need to know why the video bandwidth inverse proportional to the smoothness of the spectrum.
The IF filter sets the instrument's resolution bandwidth. That's the important one.
The video filter is usually a dumb little thing that simply smooths the display, and may not be adjustable. In many instruments, it is a low-pass filter applied to the CRT vertical deflection circuit, so it isn't really related to bandwith. It's more related to the sweep speed. Most of the time I simply turn it off.
The IF filter sets the instrument's resolution bandwidth. That's the important one.
The video filter is usually a dumb little thing that simply smooths the display, and may not be adjustable. In many instruments, it is a low-pass filter applied to the CRT vertical deflection circuit, so it isn't really related to bandwith. It's more related to the sweep speed. Most of the time I simply turn it off.
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