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What is Sensitivity?
A primary application of a spectrum analyzer is analyzing and measuring low-level signals. The sensitivity of a spectrum analyzer determines its ability to measure small signals accurately. It is crucial to understand the sensitivity capabilities of your analyzer to ensure accurate measurements of low-level signals. An ideal spectrum analyzer would measure a signal without adding noise. Active circuits, including those in spectrum analyzers, unavoidably introduce internally generated noise into the displayed signal. This unfortunate limitation means that achieving a noise-free output is not possible.
Spectrum analyzers characterize the amount of internally generated noise by specifying the displayed average noise level (DANL) in dBm using the lowest resolution bandwidth (RBW) setting. Displayed average noise level is another term for the instrument’s noise floor at a specific bandwidth. When making small signal measurements, a spectrum analyzer’s sensitivity, or noise floor, indicates its limitation and cannot detect input signals below this level.
The input signal in the spectrum analyzer block diagram, as shown in Figure 1, undergoes several processing stages. These components contribute to noise generation, starting with the input attenuator, mixers, and the first gain stage. However, the noise they generate is at or near the absolute minimum of -174 dBm / Hz, so they do not significantly affect the noise level input to the first gain stage, and the amplification is insignificant. However, the same input circuitry affects the analyzer’s ability to display low-level signals because it attenuates the input signal. The spectrum analyzer reduces the signal-to-noise ratio and, at the same time, reduces the sensitivity.
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