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Publication Number: 966099 Rule Parts:  Publication Date: 03/07/2013
Keyword: Linear vs. Log Average Detector

First Category:
Second Category: Measurement Procedures

Question:   Can a log average detector be used to make average measurements on intentional radiators?


Three types of average detection are commonly available in most modern spectrum analyzers:

(1) Power averaging (also known as RMS), in which averaging is performed on the voltage-squared;

(2) Voltage averaging (also known as linear averaging), in which averaging is performed on the absolute value of the voltage;

(3) Log averaging (also known as dB averaging), in which averaging is performed on the log of the power or log of the absolute value of voltage (i.e., averaging of decibels).

Log averaging is not permitted for compliance measurements under any FCC rule part.

Selection of power averaging or voltage averaging is based on the language in the relevant rules and measurement procedures. For example, if a rule specifies a limit on average power or on average power spectral density, power averaging is required. If a rule specifies average detection of a field strength, voltage averaging is required because field strengths are proportional to the voltage induced in the measurement antenna (for example, 15.209 for measurements above 1000 MHz). In cases where voltage averaging is required, procedures may also permit the use of power averaging, because power averaging produces a measurement result that is equal to or greater than that of voltage averaging when the measurement results are converted to the same units.


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