What is the difference between RBW and VBW?

RBW determines frequency resolution (can you see two close signals separately). VBW smooths the display by averaging noise (makes traces cleaner). RBW affects what you can resolve; VBW affects how cleanly you see it.

Why does narrower RBW take longer to sweep?

The RBW filter needs approximately 1/RBW seconds to respond to each signal. Sweeping a wide span with narrow RBW means the analyzer must move slowly enough for the filter to settle at each point, giving Sweep Time ≥ Span/RBW².

How does RBW affect the noise floor?

Noise floor is proportional to RBW. Halving RBW reduces displayed noise by 3 dB. To see weaker signals, reduce RBW (at the cost of slower measurement). Noise Floor (dBm) = Analyzer spec (dBm/Hz) + 10·log₁₀(RBW in Hz).

Resolution Bandwidth

Quick Answer

Resolution Bandwidth (RBW) is the bandwidth of the internal bandpass filter in a spectrum analyzer, determining the minimum frequency separation between two signals that can be independently resolved. Smaller RBW resolves closer frequencies but requires slower sweep speed. For example, two signals 1 kHz apart require RBW ≤ 1 kHz to be seen as separate peaks. The sweep time constraint is: Sweep Time ≥ Span/RBW². Video Bandwidth (VBW) is a separate post-detection smoothing filter that reduces noise on the display without affecting frequency resolution.

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What Resolution Bandwidth Controls

The RBW filter determines the analyzer's ability to distinguish between closely spaced signals. With RBW = 10 kHz, two signals at 100.000 MHz and 100.005 MHz (5 kHz apart) merge into a single peak. Reducing RBW to 1 kHz reveals them as separate peaks. Think of RBW as a magnifying glass for the frequency axis — narrower RBW provides finer detail. However, narrower RBW also makes the noise floor rise (noise power is proportional to bandwidth), so very narrow RBW can actually make weak signals easier to see by reducing the displayed noise.

Key Formulas

RBW and Sweep Time: The Fundamental Tradeoff

The spectrum analyzer sweeps its local oscillator across the frequency span. The RBW filter needs time to respond to each signal as the sweep passes through it — approximately 1/RBW seconds. For a span of 1 GHz with RBW = 1 kHz, the minimum sweep time is roughly Span/RBW² = 10⁹/(10³)² = 1000 seconds (about 17 minutes). This is why narrow RBW measurements take so long. Modern FFT-based analyzers avoid this limitation by capturing the entire span simultaneously, but at the cost of lower dynamic range compared to swept analyzers.

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Video Bandwidth: Smoothing the Display

Video Bandwidth (VBW) is a low-pass filter applied after the detector that smooths the displayed trace. It averages the noise without affecting frequency resolution — reducing VBW makes the trace less noisy and signals easier to identify, but does not change the ability to resolve close signals (that's RBW's job). A common setting is VBW = RBW for general measurements and VBW = RBW/10 or lower for measuring low-level signals near the noise floor. Reducing VBW also increases measurement time proportionally.

Choosing the Right RBW for Your Measurement

For measuring occupied bandwidth of a signal: set RBW to 1-3% of the signal bandwidth for accurate shape representation. For measuring spurious emissions: use narrow RBW (often specified in the standard, e.g., 1 kHz for FCC conducted emissions below 150 kHz). For finding signals in a wide band: start with wide RBW for fast sweeps, then narrow down once signals are located. For phase noise measurements: very narrow RBW (often 1-100 Hz) at close offset frequencies. EMC testing standards specify exact RBW settings — using the wrong RBW invalidates the measurement.

RBW, Noise Floor, and Sensitivity

The displayed noise floor of a spectrum analyzer is directly proportional to RBW. Halving the RBW reduces the displayed noise by 3 dB (half the noise power passes through the narrower filter). This means you can see weaker signals by reducing RBW — a technique called 'trading sweep time for sensitivity.' The analyzer's sensitivity specification is always stated at a reference RBW (typically 1 Hz). To calculate the noise floor at any RBW: Noise Floor = Specified Floor + 10·log₁₀(RBW/1 Hz). For a spec of −160 dBm/Hz and RBW = 10 kHz: noise floor = −160 + 40 = −120 dBm.

Frequently Asked Questions

RBW is the bandwidth of the analyzer's internal bandpass filter. It determines the minimum frequency separation between two signals that appear as separate peaks. Narrower RBW = finer frequency detail but slower sweep.

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