VBW and VBW Averaging

Video filtering determines the bandwidth of the video amplifier (VBW) and is used to average or smooth the trace seen on the screen. By changing the video bandwidth (VBW) setting, the peak-to-peak variations of noise in the spectrum analyzer can be decreased, making signals otherwise obscured by noise easier to find.

Video averaging can also be used to smooth variations in the envelope-detected amplitude. With video averaging, when the video filter’s cutoff frequency is reduced, the video system no longer follows the more rapid variations of the envelope of the signal passing through the IF chain. The amount of smoothing that takes place is determined by the ratio of VBW to RBW Resolution Band Width (RBW or ResBW): specifies the minimum frequency bandwith that two separate frequency spectra can be resolved and viewed seperately. For FFT (digital) based VSA's the process is equivalent to passing a time-domain signal through a bank of bandpass filters, whose center frequencies correspond to the frequencies of the FFT bins. For a traditional swept-tuned (non-digital) spectrum analyzer, the resolution bandwidth is the bandwidth of the IF filter which determines the selectivity.. Ratios of 0.01 or less provide good smoothing.

The 89600 software power spectrum measurement emulates a traditional VBW filter by averaging blocks of FFT Fast Fourier Transform: A mathematical operation performed on a time-domain signal to yield the individual spectral components that constitute the signal. See Spectrum. results to achieve a similar variance reduction for the same VBW value that a traditional swept analyzer would have.

Power Spectrum measurements support the following types of VBW averaging:

Power Averaging

In this average type, all filtering and averaging processes work on the power (the square of the magnitude) of the signal, instead of its log or envelope voltage. This scale is best for measuring the true time average power of complex signals. This scale is sometimes called RMS because the resulting voltage is proportional to the square root of the mean of the square of the voltage.

In the equation for averaging on this scale (below), K is the number of averages accumulated. (In continuous sweep mode, once K has reached the Average/Hold Number, K stays at that value, providing a running average.). The New data, Old avg, and New avg in the following equation are power (magnitude squared) values.

Summary

With power average (RMS), all measured results are converted into power units before averaging and filtering operations, and only converted to decibels for displaying.