Resolution Bandwidth (RBW)

This tutorial is part of the National Instruments Measurement Fundamentals series. Each tutorial in this series, will teach you a specific topic of common measurement applications, by explaining the theory and giving practical examples. This tutorial covers an introduction to RF, wireless and high-frequency signals and systems.

For the complete list of tutorials, return to the NI Measurement Fundamentals Main page or for more RF tutorials refer to the NI RF Fundamentals main subpage. For more information on National Instruments RF products, visit www.ni.com/rf.

Two measurements that are fundamental to spectral analysis are resolution bandwidth and dynamic range. In general, the resolution bandwidth is determines the frequency precision of a measurement. On the other hand, the dynamic range determines the amplitude precision of a spectral measurement. These two concepts are discussed below.

Resolution Bandwidth (RBW)

The resolution bandwidth (RBW) determines the FFT bin size or the smallest frequency that can be resolved. The following graphs represent the same signal with varying RBW.



The smaller RBW on the right has much higher resolution and allows the sidebands to be visible. This requires a higher acquisition time. When acquisition time is a factor and the display needs to be updated rapidly, or when the modulation bandwidth is fairly wide, a higher RBW can be used. Often, there is a trade-off between RBW and Acquisition time.


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In FFT-based (digital) spectrum analyzers and vector signal analyzers (VSA) the resolution bandwidth is inversely proportional to the number of samples acquired. By taking more samples in the time domain, or making the acquisition time longer while keeping the sampling rate the same, the RBW will be lowered. You will have more bins in the same span and thus improve frequency resolution.

The FFT process is equivalent to passing your 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. For wide sweeps a wide resolution bandwidth is required to shorten acquisition times and for narrow sweeps a narrow filter is used to improve frequency resolution.

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Conclusion

For the complete list of tutorials, return to the NI Measurement Fundamentals Main page or for more RF tutorials refer to the NI RF Fundamentals main subpage. For more information on National Instruments RF products, visit www.ni.com/rf.

Reader Comments | Submit a comment »

I want numbers, formulae
This very brief tutorial clearly demonstrates the concept of resolution bandwidth. But where are the numbers? There should be a link to a tutorial on window functions, and a table of relative resolution bandwidth for several different window functions. And, there should be a formula for calculating RBW for an N point FFT and either no window or a specified window. Finally, there should be a numerical definition of what RBW is - the idea that a human can resolve two peaks is too vague.
- Joseph Schachner, LeCroy Corp. joes@lecroy.com - Apr 18, 2008