Class D Amplifier Testing - AES17 Filters

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Overview

Audio testing of class D or type D amplifiers (also known as switching amplifiers) including frequency response (FRF), total harmonic distortion (THD), and other factors often requires filtering to remove out of band noise. The filters provided here in LabVIEW example VIs are for AES-17 filtering on class D amplifiers.

 About Class D Amplifiers

Class D amplifiers use high-frequency switching, typically a pulse width modulated (PWM) signal, which provides a very power efficient design, allowing for smaller size than other amplifiers.  In audio equipment and consumer electronics, that high frequency signal is then passed through a low-pass filter before connecting to speakers.  This filter removes the high frequency signal components leaving only the audio data resulting in an analog-like signal just like in class A and other traditional amplifiers.  Because of the PWM and other modulation schemes used in class D amplifiers, they are sometimes mis-characterized as digital amplifiers.  This article on switching amplifiers contains additional details.

Testing Class D Amplifiers

Class D amplifiers are tested and validated like other types of amplifiers, performing frequency response, distortion, and dynamic range measurements.  However, to perform these tests, a low pass filter for the high frequency switching components is often used on the output of the class D amplifier before analyzing the signal for measurement.  Standard filters for this purpose are the AES-17 low and high pass filters.  One implementation of this filter is to use an active front-end low pass analog filter in hardware before the analyzer.  However, this approach adds hardware complexity and cost to the overall test system.  Another implementation is to use a digital low pass filter that is compliant with AES-17.  Digital filtering can be reliably used provided that the acquired signal is alias free, which it is when using measurement systems with anti-aliasing filters.   The VIs included with this document implements digital filters that are compliant with the AES-17 standard.

An alternative way to test class D amplifiers is to examine high frequency signal directly from the amplifier without use of a low pass filter.  To directly measure the PWM signal, a higher speed signal acquisition device is needed, with much higher sampling rates than traditional audio analyzers.  National Instruments provides a variety of high speed digitizers for direct measurement of the high frequency signal.

AES17 Filters

Three VIs are included with this document that implement and test the standard low-pass and high-pass filters specified in AES 17. The tests pass for sample rates between 44.1k Hz and 204.8k Hz:

• AES17 Low Pass Filter
• ASE17 High Pass Filter
• An example VI that uses the AES17 Low Pass Filter and ASE17 High Pass Filter VIs together in an application with limit testing.  
  • The specified limits for the high pass filter in this VI are valid for selected sample rates between 42k and 204.8k. Stop band attenuation is defined relative to the upper band-edge frequency (attenuation should be better than 40 dB for frequencies less than or equal to the upper band-edge frequency).  For upper band-edge frequency = 20 kHz, HP filter verified for sample rates between 42k and 204.8k.

  • The specific limits for the low pass filter in this VI are valid for selected sample rates between 42k and 51.2k. Stop band attenuation is defined relative to the upper band-edge frequency (attenuation should be better than 60 dB for frequencies higher than 1.3*f_upper_band-edge).  For upper band-edge frequency = 20 kHz, LP filter verified for sample rates between 42k and 204.8k.

Related Links

• NI Sound and Vibration Portal
• Audio Test Technical Library
• Audio Test Case Studies