Spectrum analyzers are essential tools in the field of signal analysis, helping engineers and technicians measure the spectral composition of electrical, acoustic, or optical waveform.
They can also identify the frequency, power, distortion, harmonics, bandwidth, and other spectral components of a signal.
A Spectrum analyzer has two critical settings—resolution bandwidth (RBW) and video bandwidth (VBW).
RBW determines how well the analyzer can resolve distinct frequencies and manage noise, while VBW primarily affects how the analyzed signal is displayed
This article delves into the details: RBW and VBW, their roles in spectrum analysis, and how they impact measurement accuracy and speed.
What is Resolution Bandwidth (RBW)?
Resolution bandwidth (RBW) refers to the bandwidth of the filter that is used to select the frequency component(s) of the input signal. This filter is essentially a band-pass filter that determines the range of frequencies that can pass through at any given time. It therefore directly affects the spectrum analyzer’s ability to resolve closely spaced signals.
Key aspects of RBW:
- Signal Resolution: Lower RBW settings allow for better resolution of closely spaced signals, making it easier to distinguish between signals that are close in frequency.
- Sensitivity and Noise: Lowering the RBW improves the sensitivity of the analyzer by reducing the noise floor. This enhancement allows weaker signals to be detected alongside stronger signals.
- Sweep Time: The sweep time increases as RBW decreases because the analyzer takes more time to process the finer details of the signal.
What is Video Bandwidth (VBW)?
Video bandwidth (VBW) is used in a spectrum analyzer to filter the output of the detector, which processes the signals after they have passed through the RBW filter. The VBW filter smooths the displayed trace by averaging adjacent points in the frequency sweep to reduce display noise and variability.
Key aspects of VBW:
- Trace Appearance: The main function of the VBW is to affect the appearance of the trace on the display. A lower VBW smooths out the trace, reducing the variance and making it easier to read.
- Sweep Speed: Unlike RBW, VBW does not typically affect the sweep speed. It acts more on the aesthetic representation of data.
- Signal Accuracy: Reducing VBW can lead to more accurate signal representation by averaging out the noise, although excessive smoothing might obscure short-term signal variations.
Comparison Table for RBW and VBW
Here’s a closer look at how RBW and VBW interact and affect the performance of a spectrum analyzer:
| Feature | Resolution Bandwidth (RBW) | Video Bandwidth (VBW) |
|---|---|---|
| Primary Function | Determines the frequency resolution of the analyzer | Smoothes the displayed trace of the analysis |
| Impact on Noise | Reduces received bandwidth noise, improving sensitivity | Averages display noise, affecting trace readability |
| Effect on Sweep | A smaller RBW increases sweep time due to finer filtering | Does not directly affect sweep time, but modifies trace smoothness |
| Usage Scenario | Essential for viewing closely spaced signals and measuring small signal details | Used for a clearer view of the signal trace, especially in noisy environments |
Summary
Resolution bandwidth (RBW) and video bandwidth (VBW) serve distinct but complementary functions in a spectrum analyzer.
RBW determines how well the analyzer can resolve distinct frequencies and manage noise, while VBW primarily affects how the analyzed signal is displayed.
Effective use of these settings is key to maximizing both the accuracy and efficiency of spectral analysis, whether for troubleshooting, research, or design applications. Understanding the relationship between RBW and VBW allows users to tailor their spectrum analysis to specific needs, enhancing both the utility and functionality of their measurements.
