Spectrum Analyzer Terminologies: A Glossary

This article provides a breakdown of common spectrum analyzer terminologies. Understanding these terms is crucial for accurately using and interpreting data from these powerful test and measurement instruments.

Understanding the Basics

A spectrum analyzer, as illustrated in the block diagram below, takes an incoming RF signal and converts it down to a lower intermediate frequency (IF). This IF signal then passes through amplification and filtering stages before being detected. The detected signal is further processed with a video filter before being displayed. The horizontal axis, which represents the frequency, is determined by the set span and the sweep time.

Key Spectrum Analyzer Terms

Here are some essential spectrum analyzer terms explained in detail:

RBW - Resolution Bandwidth

The Resolution Bandwidth (RBW) is a configurable setting, usually found on the front panel of a spectrum analyzer. It corresponds to the bandwidth of the IF filter, a band-pass filter that acts as a window for signal detection.

• Impact of RBW:
  • A lower RBW value results in higher selectivity. This means that the analyzer can more easily distinguish between two closely spaced frequencies.
  • Therefore, the RBW should be adjusted based on the desired frequency resolution.

VBW - Video Bandwidth

The Video Bandwidth (VBW) is another adjustable setting on the spectrum analyzer. It relates to the bandwidth of the video filter, located after the diode detector.

• Purpose of VBW:
  • The VBW filter is particularly useful when measuring signals that are close to the noise floor.
  • It smooths out the noise, making low-amplitude signals more visible.
• Important Note:
  • Changing the VBW does not affect the frequency resolution of the spectrum analyzer, unlike adjusting the RBW.
  • Lowering the VBW will simply smooth the noise and make the signal clearer, but it will not improve the analyzer’s sensitivity.

Span

The Span setting dictates the frequency range that will be displayed on the horizontal axis. It’s determined in conjunction with the RF center frequency.

• Configuration Options: Spectrum analyzers usually provide ways to set the span via a center frequency and span, or directly by specifying start and stop frequencies.
• Span and Viewing Range: A wider span allows for viewing a broader range of frequencies.

Reference Level

The Reference Level is related to the IF gain and attenuator stages of the analyzer, specifically after the RF mixer. It helps the analyzer correctly measure a DUT (Device Under Test) within a suitable dynamic range.

• What it Defines: The reference level represents the power level of the uppermost vertical line on the analyzer’s display.
• Measurement Reference: All amplitude or magnitude readings are taken in relation to this level.
• Adjustability: There is a control to set the reference level, with common units including dBm, dBV, Watts, or volts.