Keysight U8903A, User Manual

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5 Audio Analyzer Measurement Functions
118 Keysight U8903A User’s Guide
DC voltage level (Vdc)
DC voltage levels are often encountered in audio equipment although they are not
part of the audio signal, for example, bias voltages and outputs from AC-to-DC
converters. 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 Level
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.
In communication devices, THD + N indicates how well a signal is being
reproduced at the receiver after going through the communication medium. To
limit the bandwidth to the band of interest, input filters like the C-Message
weighting filter can be applied. Use the appropriate input range to avoid clipping
and also to make full use of the ADC dynamic range to get the most accurate
results.
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. THD + N Level is defined as the square root of the
sum 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