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Resolution Bandwidth (RBW)Publish Date: Nov 05, 2014

Overview

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

For the complete list of tutorials, return to the , or for more RF tutorials, refer to the NI Measurement Fundamentals Main page NI. RF Fundamentals main subpage

Two measurements that are fundamental to spectral analysis are resolution bandwidth (RBW) and dynamic range. In general,RBW determines the frequency precision of a measurement. Because RBW affects dynamic range, we can also compare dynamicrange. The dynamic range determines the amplitude precision of a spectral measurement. RBW is discussed below.

Table of Contents

Resolution BandwidthRelated ProductsConclusion

1. Resolution Bandwidth

The resolution bandwidth (RBW) determines the fast Fourier transform (FFT) bin size, or the smallest frequency that can beresolved.The following graphs represent the same signal with varying RBW.

Figure 1. The Same Signal With Different RBW.The smaller RBW, on the right, has much finer resolution which allows the sidebands to be visible. Finer resolution requires alonger acquisition time. When acquisition time is a factor and the display needs to be updated rapidly or when the modulationbandwidth is wide, a larger RBW can be used. RBW and acquisition time are inversely proportional. The following table shows theadvantages and disadvantages of both larger and smaller RBWs.

Table 1. RBW FactorsLarger RBW Smaller RBWSmaller FFT size; fewer samples; requires less acquisition andcomputation time; often unable to resolve two closely spacedtones in a spectrum.

Larger FFT size; more samples; requires more acquisition andcomputation time; tones are easily resolved.

In FFT-based (digital) spectrum analyzers and vector signal analyzers (VSAs), RBW is inversely proportional to the number ofsamples acquired. By taking more samples in the time domain, or making the acquisition time longer while keeping the samplingrate the same, the RBW is lowered. The result is more bins in the same span and improved frequency resolution.The FFT process is equivalent to passing a time-domain signal through a bank of bandpass filters with center frequenciescorresponding to the frequencies of the FFT bins. For wide sweeps, a wide RBW is required to shorten acquisition times. Fornarrow sweeps, a narrow filter improves frequency resolution.Carefully consider which FFT window type to use. As an example, a Flat Top window minimizes amplitude measurement and isrecommended for amplitude measurements even though it has non-optimal selectivity. Refer to The Fundamentals of FFT-Based

for more information about FFT windowing.Signal Analysis and Measurement in LabVIEW and LabWindows/CVI2. Related Products

NI PXIe-5663 6.6 GHz RF Vector Signal AnalyzerThe National Instruments PXIe-5663 is a modular 6.6 GHz RF vector signal analyzer with 50 MHz of instantaneous bandwidthoptimized for automated test.

NI PXIe-5673 6.6 GHz RF Vector Signal GeneratorThe National Instruments PXIe-5673 is a 4-slot 6.6 GHz RF vector signal generator that delivers signal generation from 85 MHz to6.6 GHz, 100 MHz of instantaneous bandwidth, and up to 512 MB of memory.

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NI PXI-5660 2.7 GHz RF Vector Signal AnalyzerThe National Instruments PXI-5660 is a modular 2.7 GHz RF vector signal analyzer with 20 MHz of instantaneous bandwidthoptimized for automated test.

NI PXI-5671 2.7 GHz RF Vector Signal GeneratorThe National Instruments PXI-5671 module is a 3-slot RF vector signal generator that delivers signal generation from 250 kHz to2.7 GHz, 20 MHz of instantaneous bandwidth, and up to 512 MB of memory.

NI PXI-5652 6.6 GHz RF and Microwave Signal GeneratorThe National Instruments PXI-5652 6.6 GHz RF and microwave signal generator is continuous-wave with modulation capability. Itis excellent for setting up stimulus response applications with RF signal analyzers.

NI RF SwitchesThe National Instruments RF switch modules are ideal for expanding the channel count or increasing the flexibility of systems withsignal bandwidths greater than 10 MHz to bandwidths as high as 26.5 GHz.

NI LabVIEWNational Instruments LabVIEW is an industry-leading graphical software tool for designing test, measurement, and automationsystems.3. Conclusion

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