On a spectrum analyzer, differentiate between resolution bandwidth (RBW) and video bandwidth (VBW) and their effect on measurement accuracy.

On a spectrum analyzer, differentiate between resolution bandwidth (RBW) and video bandwidth (VBW) and their effect on measurement accuracy.

• A. RBW sets frequency resolution; VBW sets display averaging; both influence noise floor.
• B. RBW sets time domain sampling; VBW sets amplitude scale.
• C. RBW sets measurement bandwidth; VBW sets maximum input amplitude.
• D. RBW and VBW both define sensitivity; no difference.

Answer: (A)

When you interpret a spectrum on a spectrum analyzer, two filter stages shape what you see: the resolution bandwidth and the video bandwidth. The resolution bandwidth is the width of the RF filter that determines how finely you can distinguish two close frequencies. A narrower RBW gives better frequency resolution, so nearby tones can appear as separate peaks. But because the energy of a signal is confined to that narrow filter, the peak power in any single bin is reduced, and the noise power within each bin scales with RBW. So RBW sets how finely you can resolve frequency components and also sets the per-bin noise level, which affects measurement sensitivity. The video bandwidth comes after the detector and acts as a smoothing or averaging filter on the displayed trace. It doesn’t change the actual RF content, but it changes how noisy the trace looks by averaging out rapid fluctuations. A larger VBW yields a noisier, more responsive trace; a smaller VBW smooths the trace and lowers the apparent noise floor, at the cost of slower updates and potential smearing of fast events. Because of this display smoothing, VBW directly influences the apparent measurement accuracy on the trace. Both settings impact what you observe: RBW defines frequency resolution and per-bin noise, while VBW controls how much the trace is averaged, affecting the displayed noise and stability. The other options misstate the roles—RBW is not a time-domain parameter, VBW is not about maximum input amplitude, and they aren’t simply interchangeable notions of sensitivity.

When you interpret a spectrum on a spectrum analyzer, two filter stages shape what you see: the resolution bandwidth and the video bandwidth. The resolution bandwidth is the width of the RF filter that determines how finely you can distinguish two close frequencies. A narrower RBW gives better frequency resolution, so nearby tones can appear as separate peaks. But because the energy of a signal is confined to that narrow filter, the peak power in any single bin is reduced, and the noise power within each bin scales with RBW. So RBW sets how finely you can resolve frequency components and also sets the per-bin noise level, which affects measurement sensitivity.

The video bandwidth comes after the detector and acts as a smoothing or averaging filter on the displayed trace. It doesn’t change the actual RF content, but it changes how noisy the trace looks by averaging out rapid fluctuations. A larger VBW yields a noisier, more responsive trace; a smaller VBW smooths the trace and lowers the apparent noise floor, at the cost of slower updates and potential smearing of fast events. Because of this display smoothing, VBW directly influences the apparent measurement accuracy on the trace.

Both settings impact what you observe: RBW defines frequency resolution and per-bin noise, while VBW controls how much the trace is averaged, affecting the displayed noise and stability. The other options misstate the roles—RBW is not a time-domain parameter, VBW is not about maximum input amplitude, and they aren’t simply interchangeable notions of sensitivity.