RBW vs VBW: Understanding Spectrum Analyzer Parameters

RBW (Resolution Bandwidth) and VBW (Video Bandwidth) are terms commonly used in radio frequency (RF) spectrum analysis, particularly in instruments like spectrum analyzers. While they both pertain to bandwidth, they serve different purposes and are used in different contexts.

What is RBW (Resolution Bandwidth)?

RBW stands for Resolution Bandwidth; it is used to view distinctly two very closely spaced signals in the frequency domain. It is mainly used for phase noise measurement purposes.

Resolution bandwidth (RBW) filters are bandpass filters located in the spectrum analyzer’s final IF stages. They determine how well closely spaced signals can be separated. The narrower the RBW filter, the more clearly two close signals can be seen as separate signals. The RBW filters also determine the analyzer response to pulse signals and to noise. The resolution bandwidth control selects which RBW filter is used.

RBW vs VBW

The shape of a spectrum displayed on the analyzer screen is a combination of the shape of the RBW filter and the shape of the true signal spectrum. The figure depicts filters used for RBW and VBW tuning in a spectrum analyzer.

RBW filters are defined by their bandwidths and shape factors. Bandwidth is specified in Hz either at 3 dB or 6 dB down from the filter peak. The shape factor, which indicates the steepness of the filter, is the ratio of the RBW filter bandwidth 60 dB down from the peak to its normal (3 dB or 6 dB) bandwidth.

The shape factor is important in determining the minimum separation between two signals which have equal amplitudes to be resolved.

A smaller RBW provides better frequency resolution but requires more processing time. RBW settings are typically adjusted based on the specific requirements of the measurement being conducted.

What is VBW (Video Bandwidth) in spectrum analyzer?

VBW stands for Video Bandwidth; it helps improve the resolution of very weak signals in the presence of noise. It is a post-detection filter bandwidth which just smoothes out the signal of interest. It is used to reduce noise on the trace, but it doesn’t reduce the noise floor.

VBW is associated with a video filter. The position of the video filter is after the envelope detector and just before displaying the signal on the screen display. It’s used in the process of displaying the signals and affects the amplitude accuracy of the displayed signals.

VBW is essentially a smoothing function applied to the displayed spectrum, affecting how smoothly the amplitude response is displayed across different frequencies. A larger VBW smoothens out the display, reducing noise but potentially masking narrowband signals.

VBW is particularly important when observing signals with rapidly changing amplitudes or when there’s significant noise in the signal. Unlike RBW, VBW doesn’t affect frequency resolution but rather affects amplitude accuracy and display characteristics in spectrum analyzer based RF measurements.

Difference between RBW and VBW

The following table compares resolution bandwidth vs. video bandwidth and mentions the difference between RBW and VBW.

Conclusion

In summary, RBW and VBW are both parameters in spectrum analysis but serve different purposes. RBW affects frequency resolution, crucial for distinguishing closely spaced signals, while VBW affects amplitude accuracy and display characteristics, crucial for visualizing signals clearly in the presence of noise. Adjusting these parameters correctly is essential for accurate and meaningful spectrum analysis, mainly in RF phase noise measurement using a spectrum analyzer.