Keysight U8903A User Manual

Also see for U8903A: Demo guide How to Do Quick start guide Quick start guide

Contents

Keysight U8903A User Manual Manual pdf 118 page image

5 Audio Analyzer Measurement Functions118 Keysight U8903A User’s GuideDC voltage level (Vdc)DC voltage levels are often encountered in audio equipment although they are notpart of the audio signal, for example, bias voltages and outputs from AC-to-DCconverters. DC voltage is expressed in volts (V).DC level is one of the U8903A measurement functions.Coupling must be set to DC for DC level measurements.THD + N Ratio and THD + N LevelHarmonic distortion on a spectrally pure signal is created by nonlinearities in thecircuit through which it passes. The nonlinearities can arise in the transfercharacteristics of an active device or by running the active device into saturationor cutoff. In most cases, distortion can be reduced by decreasing the signal level,applying filtering, or adding negative feedback.In communication devices, THD + N indicates how well a signal is beingreproduced at the receiver after going through the communication medium. Tolimit the bandwidth to the band of interest, input filters like the C-Messageweighting filter can be applied. Use the appropriate input range to avoid clippingand also to make full use of the ADC dynamic range to get the most accurateresults.According to Fourier mathematics, the nonlinear terms in the circuit transferfunction give rise to harmonics of the signal. Therefore, the THD + N functionprovides a quantitative measurement of the quality of an audio signal or in otherwords, the purity of a signal. THD + N Level is defined as the square root of thesum of the squares of all the signal harmonic components and noise amplitude,and is expressed in Vrms. The THD + N Level can be computed as follows.THD + N Level = rms value of noise and distortion