94 U8903A User’s Guide
5Audio Analyzer Measurement Functions
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
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