Spectrum Analyzer Measurement Guide : Spectrum Analyzer : Resolution Bandwidth
Resolution Bandwidth
Resolution Bandwidth (RBW) determines frequency selectivity. The spectrum analyzer traces the shape of the RBW filter as it tunes past a signal. The choice of resolution bandwidth depends on several factors. Filters take time to settle. The output of the filter takes some time to settle to the correct value, so that it can be measured. The narrower the filter bandwidth (resolution bandwidth), the longer the settling time needs to be, and therefore, the slower the sweep speed.
The choice of resolution bandwidth depends upon the signal being measured. If two closely spaced signals are to be measured individually, then a narrow bandwidth is required. If a wider bandwidth is used, then the energy of both signals is included in the measurement. Thus, the wider bandwidth does not have the ability to look at frequencies selectively, but instead simultaneously measures all signals falling within the resolution bandwidth. Therefore, a broadband measurement would include all signals and noise within the measurement bandwidth into a single measurement.
On the other hand, a narrow‑band measurement separates the frequency components, resulting in a measurement that includes separate peaks for each signal. There are advantages to each. The ultimate decision depends upon the type of measurement required.
There is always some amount of noise present in a measurement. Noise is often broadband in nature; that is, it exists at a broad range of frequencies. If the noise is included in the measurement, the measured value could be in error (too large) depending upon the noise level. With a wide bandwidth, more noise is included in the measurement. With a narrow bandwidth, less noise enters the resolution bandwidth filter, and the measurement is more accurate. If the resolution bandwidth is narrower, the noise floor drops on the spectrum analyzer display. As the measured noise level drops, smaller signals that were previously obscured by the noise can now be measured.