Agilent Technologies E5503A Podręcznik Użytkownika Pobierz pdf (Strona 377)

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101

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Agilent Technologies E5500 Phase Noise Measurement System

Evaluating Your Measurement Results

Problem Solving

Small Angle Line

Figure 15-4 L(f) Is Only Valid for Noise Levels Below the Small Angle Line

Caution must be exercised where

L(f) is calculated from the spectral density

of the phase modulation

(f)/2 because of the small angle criterion. Below

the line, the plot of

L(f) is correct; above the line, L(f) is increasingly

invalid and Sf(f) must be used to accurately represent the phase noise of the

signal. To accurately plot noise that exceeds the small angle line, select the

Spectral Density of Phase Modulation (dB/Hz) graph type (

(f)). L(f)

raises the noise floor by 3 dB.

The –10 dB per decade line is drawn on the plot for an instantaneous phase

deviation of 0.2 radians integrated over any one decade of offset frequency.

At approximately 0.2 radians, the power in the higher order sideband of the

phase modulation is still insignificant compared to the power in the first

order sideband. This ensures that the calculation of cal L(f) is still valid.

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