The importance of noise spectrum analysis_How to choose the best resolution bandwidth when using a spectrum analyzer

The setting of the spectrum analyzer's resolution bandwidth (RBW) must be carefully considered because it affects the separation of spectral components, the setting of an appropriate noise floor, and the demodulation of the signal.

When making demanding spectrum measurements, a spectrum analyzer must be accurate, fast, and have a high dynamic range. In most cases, emphasizing one of these parameters will affect the other parameters. Therefore, these factors need to be weighed when setting the resolution bandwidth (RBW).

The advantage of using a narrow resolution bandwidth (RBW) can be seen by measuring low-level signals. When using a narrow RBW, the spectrum analyzer exhibits lower average noise (DANL), increased dynamic range, and improved sensitivity. In Figure 1, changing the RBW from 100 kHz to 10 kHz will better resolve the -95 dBm signal.

Figure 1. Test results of 100 KHz RBW and 10 KHz RBW

But the narrowest RBW is not always the best. For modulated signals, the RBW must be set wide enough to include the signal sidebands. If this is ignored, the measurement will be extremely inaccurate.

One significant drawback of narrow RBW settings is sweep speed. Wider RBW settings allow for faster sweeps over a given frequency range. Figure 2 compares sweep times for 10 kHz and 3 kHz RBWs over a 200 MHz frequency range.

Figure 2. The sweep time for a 10 kHz resolution bandwidth (RBW) is 7.627 seconds.

Figure 3. The sweep time for a 3 kHz resolution bandwidth (RBW) is 26.79 seconds.

It is important to understand the basic trade-offs that must be made when selecting RBW so that the user can optimize appropriately when they understand which parameters are most important. But when trade-offs are unavoidable, modern spectrum analyzers provide you with ways to mitigate or even eliminate them. By using digital signal processing, spectrum analyzers provide higher speeds while achieving more accurate measurements, even when using narrow RBWs.

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