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 display needs 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 advantages and disadvantages of both larger and smaller RBWs.
| Larger RBW | Smaller RBW |
|---|---|
| Smaller FFT size; fewer samples; requires less acquisition and computation time; often unable to resolve two closely spaced tones in a spectrum. | Larger FFT size; more samples; requires more acquisition and computation time; tones are easily resolved. |
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 wide RBW 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 has non-optimal selectivity. Refer to The Fundamentals of FFT-Based Signal Analysis and Measurement in LabVIEW and LabWindows/CVI for more information about FFT windowing.
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 MB of 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 RF signal analyzers.
NI RF Switches
The 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 as 26.5 GHz.
NI LabVIEW
National 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 NI Measurement Fundamentals main page or for more RF tutorials refer to the NI RF Fundamentals main subpage. For more information about National Instruments RF products, visit www.ni.com/rf.