RBW

Resolution bandwidth is the frequency span that a spectrum analyzer's filter passes at any given moment. When you measure radio signals, power supplies, or electromagnetic emissions, the RBW controls how finely the instrument can separate two frequencies that sit close together. A narrow RBW lets you see distinct peaks; a wide RBW blurs them together. The trade-off is time: narrower settings require longer sweep periods to build up a complete picture across your frequency range.

RBW is set in hertz, kilohertz, or megahertz depending on the instrument and measurement task. Common values on general-purpose analyzers range from 1 kHz to 1 MHz or wider. EMC (electromagnetic compatibility) testing often mandates specific RBW settings, typically 1 kHz for emissions below 1 MHz and 10 kHz or 120 kHz above that, according to regulatory standards. The actual filter is usually a Gaussian or brick-wall shape; the name RBW refers to the 3 dB bandwidth of this filter response.

Choosing the right RBW requires balancing resolution against noise floor and measurement time. Too narrow an RBW and thermal noise dominates; too wide and real signals merge into a single lump. Closely spaced harmonics or sidebands demand tight RBW settings. Conversely, if you only care about peak energy across a broad band, a wide RBW gets you done faster. Many technicians start wide to locate energy, then zoom in with a tighter RBW for detail.

RBW interacts with the analyzer's video bandwidth (VBW), which is a separate filter applied after detection. Mismatched RBW and VBW settings can yield misleading results. Some modern analyzers offer automatic RBW selection based on span and measurement type, but manual control remains essential for reproducible, standards-compliant testing. Confusion between RBW and VBW causes more measurement errors than almost any other single setting.