AUX-0025 / AUX-0100 Measurement Filters

AUX-0025 and AUX-0100 Switching Amplifier Measurement Filters
Providing signal preconditioning for measurement accuracy in Class-D amplifier test

The AUX-0100 is an eight-channel passive low-pass filter specifically designed to minimize switching amplifier carrier components while passing a broad audio spectrum. This filter provides the signal preconditioning necessary to accurately measure switching amplifier outputs using a wide-range audio analyzer. It uses a passive design for optimal performance, with custom inductors designed for high power handling and minimal low-frequency distortion. Balanced XLR inputs and DB-25 outputs make it ready for easy connection to an APx58x Series multichannel audio analyzer. A rack mount option is available with the RAK-AUX-0100 kit.

The AUX-0025 is a dual-channel passive filter with same the high quality components as the AUX-0100. It is designed to be used with two-channel analyzers, including the APx555, APx52x Series and 2700 Series test systems.

Manufacturers of power amplifiers are moving to switching designs (also called Class D or switch-mode) to make their amplifiers lower cost and more efficient compared to conventional linear power amplifier designs. This technology has particular appeal in automotive and personal stereo applications where efficiency, low battery consumption, small size, and low cost are desired. However, amplifiers using this technology present new measurement challenges.

The switching process adds fast rising edges at the switching frequency to the audio output signal. These fast edges are of no consequence to the typical load (a loudspeaker), but present a difficult signal for measurement instruments. The fast switching edges present high energy content and will introduce slew rate limiting when presented to the input stage of most measurement instruments. When stressed by these fast edges, the analyzer input amplifier will usually slew rate limit and will not be able to function effectively in its normal mode. Auto ranging will be affected and the signal under test will be misrepresented to the following measurement circuits. The result is that noise and distortion measurements of switching amplifiers with almost any analyzer without preconditioning will yield inaccurate and unpredictable results.

AUX-0025/0100 overall response showing attenuation beyond the passband

AUX-0025/0100 overall response showing attenuation beyond the passband

The solution to this problem is to precondition the signal before presentation to the analyzer. The way to do this is in the form of a low pass filter that will soften the fast edges while passing the primary audio signal intact. The best approach to this filter is a passive design, as it will handle the fast edges properly, is relatively low cost, and will not require power. A well-designed passive filter will also not compromise the audio signal passing through it by adding noise or distortion as an active design might.

UX-0025/0100 passband response showing flatness out to 20 kHz

AUX-0025/0100 passband response showing flatness out to 20 kHz

Number of Channels
2 (AUX-0025)
8 (AUX-0100)

Frequency Response
±0.05 dB, 20 Hz to 20 kHz

Insertion Loss
Typically 0.05 dB

High-Frequency Rejection
Typically > 50 dB, 250 kHz to 20 MHz

Maximum Input
±200 Vpk; 140 Vrms, 20 Hz to 5 kHz, decreasing to 90 Vrms @ 20 kHz

Interchannel Crosstalk
>90 dB at 20 kHz (AUX-0025)
>82 dB at 20 kHz (AUX-0100)

< -110 dB harmonic (measured at 70 Vpp, 1 kHz)
< -100 dB IMD (at 70 Vpp with 18 kHz and 20 kHz dual-tone test signal)