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Resolution Bandwidth (RBW)Publish Date: Apr 09, 2012 | 67 Ratings | out of 53.81

Overview

This tutorial is part of the National Instruments Measurement Fundamentals series. Each tutorial in this series teaches you a specific topic of common measurement applications by explaining theheory 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 , or for more RF tutorials, refer to the .NI Measurement Fundamentals Main page NI RF Fundamentals main subpageTwo 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 dynamic range. The dynamic range determines the amplitude precision of a spectral measurement. RBW is discussed below.

Table of ContentsResolution Bandwidth

Related Products

Conclusion

1. Resolution Bandwidth

The resolution bandwidth (RBW) determines the fast Fourier transform (FFT) bin size, or the smallest frequency that can be resolved.

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 a longer acquisition time. When acquisition time is a factor and the displayneeds to be updated rapidly or when the modulation bandwidth is wide, a larger RBW can be used. RBW and acquisition time are inversely proportional. The following table shows the advantagesand disadvantages of both larger and smaller RBWs.

Table 1. RBW FactorsLarger RBW Smaller RBWSmaller FFT size; fewer samples; requires less acquisition and computation time; often unable toresolve two closely spaced tones in a spectrum.

Larger FFT size; more samples; requires more acquisition and computation time; tones are easilyresolved.

In FFT-based (digital) spectrum analyzers and vector signal analyzers (VSAs), RBW 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 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 frequencies corresponding to the frequencies of the FFT bins. For wide sweeps, a wideRBW is required to shorten acquisition times. For narrow 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 is recommended for amplitude measurements even though it hasnon-optimal selectivity . Refer to for more information about FFT windowing.The Fundamentals of FFT-Based Signal Analysis and Measurement in LabVIEW and LabWindows/CVI

2. Related Products

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

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

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

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

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

NI RF SwitchesThe National Instruments RF switch modules are ideal for expanding the channel count or increasing the flexibility of systems with signal bandwidths greater than 10 MHz to bandwidths as high as26.5 GHz.

NI LabVIEWNational Instruments LabVIEW is an industry-leading graphical software tool for designing test, measurement, and automation systems.

3. Conclusion

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

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