In these different methods, the thermal noise, kTB, is subtracted from the measured trace as well as the NFSA so that results below kTB can be displayed. These results can be valid in many, but not all, cases. Problems occur when the measured values are close or equal to the noise floor of the instrument. In fact, if they are equal, the result would be -infinity dB. Practical implementations of noise corrections normally includes a threshold or a graduated level of subtraction close to the noise floor of the instrument.

This report has examined some of the techniques for measuring low-level signals with a spectrum analyzer. It has pointed out that sensitivity is affected by RBW, attenuation, and use of a preamplifier. Noise reduction methods, such as noise corrections and noise floor extension, can be applied to further enhance the sensitivity of the instrument. In practice, ensuring that external path losses to the spectrum analyzer are reduced can aid tremendously.

Additional Information

More information about spectrum analyzer measurements and analysis can be found in the following application notes:

Agilent Spectrum and Signal Analyzer Measurements and Noise, Application Note 1303.

Using Noise Floor Extension in the PXA Signal Analyzer.

Spectrum Analyzer Basics, Application Note 150.