Thd+N Ratio And Thd+N Level Measurements - Keysight U8903B User Manual

4 Audio Analyzer Measurement Functions

THD+N ratio and THD+N level measurements

Harmonic distortion on a spectrally pure signal is created by nonlinearities in the circuit
through which it passes. The nonlinearities can arise in the transfer characteristics of an
active device or by running the active device into saturation or cutoff. In most cases,
distortion can be reduced by decreasing the signal level, applying filtering, or adding
negative feedback.
According to Fourier mathematics, the nonlinear terms in the circuit transfer function give
rise to harmonics of the signal. Therefore, the THD+N function provides a quantitative
measurement of the quality of an audio signal or in other words, the purity of a signal.
The THD+N ratio is defined as the ratio of the square root of the sum of the squares of all
the signal harmonics components and noise amplitude, relative to the total signal
amplitude. The THD+N ratio can be computed as follows.
THD+N ratio
THD+N ratio measurement function mode is applicable for the analog, digital, and
Bluetooth analyzer. The settings for THD+N ratio measurement are displayed as shown in
Figure 4-18, Figure 4-19, and Figure 4-20 for the analog, digital, and Bluetooth analyzer
respectively.
Figure 4-18
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rms value of noise and distortion
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= 20 Log 10
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rms value of signal, noise, and distortion
Analog Analyzer > Functions > THD+N Ratio menu page 1
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Keysight U8903B User's Guide