vbw spectrum

Hi,

Is there any rule of thumb what kind of video bandwidth I should use for a certain resolution bandwidth?

Does this rule apply to all measurement cases or is there multiple rules (for example when measuring with zero span etc.)

Any good references in this issue?

on Agilent SA, the default ratio VBW/RBW is close to 1.
In order to smooth the trace, You may decrease the ratio down to 0.1 but scan time will rise.
Extremally rare the need to use VRB/RBW<0.1

In the case You are using Zero-span, the SA act as tuned-oscilloscope; if You are looking at slow phenomena, You may increase sweep time and decrease VBW.
The VBW act as low pass filter; fast phenomena cannot be seen if the VBW is set at low freq.

what do you mean by slow / fast phenomena ?

Video bandwidth is a simple low pass filter unlike the 1st, 2nd and 3rd IF which have a fixed bandwidth but very steep skirts.

In a pure RC circuit, the output risetime (10% to 90%), as shown above, is approximately equal to 2.2* RC and BW= 1/(2pi*RC) for a 1st order filter.
So video filter is on the baseband video signal to filter out jitter or noise to display average whereas the log amp before this is designed to detect "quasi-peak" or peak & hold with droop for an unspecified time constant. ( perhaps secretly held by the FCC and the SA supplier ;)