The Art of Precision Measurement: RBW, Detectors, and the Truth of Spectrum Analysis

In the world of RF testing, the Spectrum Analyzer (or Vector Signal Analyzer) is not merely a piece of equipment; it is the RF engineer's "eyes." However, the scene these eyes perceive is not absolute objective reality. It is a result "interpreted" by the myriad filters and processors inside the instrument.

A common fallacy is to believe that the reading on the screen is the absolute truth. In reality, incorrect settings—specifically Resolution Bandwidth (RBW) and Detectors—can make a failing transmitter look perfect, or a healthy system look riddled with interference.

The art of precision measurement lies in understanding how these settings alter our perception of the signal.

Resolution Bandwidth (RBW): The Instrument's "Filter Width"

RBW is the central concept in spectrum analysis. Imagine RBW as the bandwidth of a Bandpass Filter moving across the frequency axis. The instrument observes spectral energy through this "window."

The RBW setting dictates three critical trade-offs:

1. Resolution: The Ability to Distinguish Adjacent Signals

This is the most intuitive function of RBW. If two signals (e.g., a main carrier and a nearby interferer) are close in frequency, say 10 kHz apart:

• If RBW is 100 kHz: This "wide window" will cover both signals simultaneously. On the screen, you won't see two separate peaks; you will only see one merged, wide "hump." The details are blurred.

• If RBW is 1 kHz: This "narrow window" can sweep across each signal individually. The screen will clearly display two distinct peaks.

Conclusion: To resolve adjacent frequency components, the RBW must be significantly smaller than the frequency spacing between them.

2. Displayed Average Noise Level (DANL)

This is the most magical and frequently utilized characteristic of RBW: RBW determines how low the "Noise Floor" appears on your screen.

The instrument's internal thermal noise is uniformly distributed across all frequencies (White Noise). The wider the RBW filter, the more noise energy enters the detector.

• The Physics: Every time you reduce the RBW by a factor of 10, the displayed Noise Floor drops by 10 dB.

This is crucial for hunting weak Spurious Emissions. If a weak interference signal has a power of -100 dBm, but a wide RBW sets the instrument's noise floor at -90 dBm, that interference signal will be "drowned" by the instrument's own noise and remain invisible. Only by narrowing the RBW and pushing down the noise floor will this "fish" rise to the surface.

3. Sweep Speed

There is no such thing as a free lunch. Narrow RBW brings high resolution and a low noise floor, but the cost is Time. Filters have a physical response time (settling time). The narrower the RBW, the slower the response. In traditional swept analyzers, reducing RBW by factor of 10 can increase sweep time by a factor of 100. This is unacceptable in production testing. Therefore, test engineers must find the golden balance between "seeing details" and "test speed."

Detectors: Interpreting the Energy

After the signal passes through the RBW filter, it is still a high-frequency AC waveform. To draw a point on the screen, the instrument must convert this into a DC voltage. This is the job of the Detector.

For simple Continuous Wave (CW) signals, the choice of detector matters little. But for modern Noise-like Signals (e.g., 5G OFDM, LTE, Wi-Fi), the choice of detector is a matter of life and death.

Within one sampling bucket of the instrument, the signal amplitude might vary thousands of times. Which value should the detector report?

1. Peak Detector

• Logic: Always captures the highest voltage that occurred during the sampling interval.

• Use Case: Searching for transient spurious signals or pulsed interference. This is the mode for EMC/EMI regulatory testing because it represents the "worst case."

• Impact on 5G: If you use a Peak Detector to measure 5G signal power or ACLR, the result will be wildly incorrect. Because OFDM signals have a high PAPR (peaks), the Peak Detector will constantly grab those rare high spikes, making the displayed power appear 10 dB+ higher than the actual average power, leading you to wrongly believe the PA is overpowering or failing.

2. RMS (Root Mean Square) Detector

• Logic: Integrates all energy within the sampling interval (square, average, square root) to calculate the True Thermal Power.

• Use Case: Measuring Channel Power and ACLR for modern digital modulation signals (OFDM, QAM).

• Importance: This is the only way to represent "energy" in a physically meaningful sense. When a spec calls for "Average Power," you must use the RMS detector.

3. Sample Detector

• Logic: Randomly captures one instantaneous value from the middle of the sampling interval.

• Use Case: Primarily used for analyzing the character of noise itself. It reproduces the random appearance of noise most faithfully, unlike the Peak Detector which artificially elevates the noise floor.

Video Bandwidth (VBW): The Visual Smoother

After the RBW, there is typically a Video Bandwidth (VBW) filter. This is a low-pass filter applied to the video signal after detection.

VBW does not change the resolution, nor does it change the physical noise energy; it merely smooths the displayed trace.

• Wide VBW: The trace looks fuzzy, thick, and full of noise jitter.

• Narrow VBW: The trace becomes a smooth, stable, thin line.

For observing stable CW signals hidden in noise, lowering VBW helps stabilize the reading. However, for observing fast-changing signals (like AM modulation), a VBW that is too low will "smear" out the signal's own variations, causing distortion.

Conclusion: A Strategy for Truth

Precise spectrum measurement is a strategic game played against signal characteristics:

1. When measuring 5G ACLR/Power: You must use the RMS Detector. RBW should be set to roughly 1% - 5% of the signal bandwidth to ensure accurate integration.

2. When hunting for weak Spurious (Spurs): Switch to the Peak Detector and aggressively narrow the RBW (e.g., 1 kHz or less). This pushes down the noise floor, leaving those "ghosts" hiding in the noise nowhere to hide.

3. When facing production speed pressure: If ultra-narrow RBW is not allowed, try using the FFT Mode of modern instruments (as opposed to Swept Mode), which can significantly boost speed while maintaining a narrow RBW.

The line on the instrument screen is not a sketch of the signal; it is a portrait "rendered" by the RBW and Detector settings. Only engineers who deeply understand these parameters can see through this portrait to the true form of the electromagnetic wave.