Introduction - Audio Precision AUX Series User Manual And Specifications

Audio analyzers are generally designed to have broad mea-
surement bandwidths, broader than a typical audio circuit or
system and much wider than the audio passband. Such
designs enable accurate analysis of fast, high-performance
audio circuits and also allow measurement of any low-level,
high-frequency spurious signals that may accompany the
audio signal.
This design philosophy is based on the assumption that the
audio signal and its overtones are the dominant signal com-
ponents applied to the analyzer; this is the case for the out-
put of conventional audio power amplifiers of Class A or
Class AB design. In such a case the analyzer can range its
circuits to the amplitude of the audio signal for optimum
measurement conditions.
Recent practice, however, has often turned to other ampli-
fier designs for improvements in efficiency and weight as
compared to Class A and Class AB amplifiers. Although
AUX-0025 / 0040 / 0100 Measurement Filters: Introduction

Introduction

these amplifier designs vary, as do the names applied to
them, they have in common an output signal that is a high-
frequency switching carrier modulated by the audio signal.
Many of these "switching amplifiers" or "digital amplifi-
ers" present a difficulty to conventional measurement and
analysis techniques due to the out-of-band switching carrier
components that are in the output signal. When the ampli-
tude of the switching carrier components remains high in
comparison to the audio signal, the ranging functions of an
audio analyzer may respond to the carrier rather than to the
audio signal; also, the slew rate of the analyzer input ampli-
fiers may be exceeded. Either will reduce the accuracy of
the measurements.
The best solution in using a broad range, broad bandwidth
analyzer to accurately measure the output of such an ampli-
fier is to insert a carefully designed low-pass filter between
the output of the device under test (DUT) and the analyzer
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