Created on 2009-02-24 13:41:00
Question
I am testing a receiver with the speaker outputs connected to the analog balanced inputs of an APx585. When I generate a sine wave signal into the receiver, the APx shows a strangely distorted waveform with a level much lower than expected. What is wrong?
Answer
Most likely, the DUT contains a Class D (switching) amplifier. Switching amplifiers typically have a significant DC voltage relative to chassis ground on both terminals of the speaker connections. When connected to the analog inputs of an APx585, this DC voltage can confuse the analyzer’s auto-ranging circuitry, causing it to select the wrong voltage range. This results in the signal being distorted within the APx585.
For example, Figure 1 shows a waveform acquired by an APx585 from the speaker outputs of a Blu-ray disc player/receiver combo. In this case, a 1 kHz tone was input to the receiver, and the DUT’s volume control was adjusted to an output voltage level of approximately 5 Vrms. The input range of the APx585 was not changed from its default setting of Auto Range. As shown in Figure 1, the sinusoidal waveform is badly distorted, and the voltage level is much lower than the correct value of 5 Vrms (about 7 V peak).
If you measure similarly distorted waveforms with an APx585, check for the presence of a common mode DC voltage on the signal. Using a DC voltmeter, measure the voltage between each side of the DUT’s speaker terminals and chassis ground (the metal frame of the DUT). On the DUT described above, we measured a voltage of approximately 17 VDC. This device probably contains a Class D amplifier, because traditional analog amplifiers typically have almost no DC voltage (usually 100 mVDC or less). Note that if we measure more than 100 mVDC between the speaker terminals, then there is an objectionable DC current going through the speakers and we have a fault in the amplifier.
We can demonstrate that the DC voltage on the analyzer inputs has fooled the auto-ranging circuit by manually setting the Input Range to a higher value. Although we can now make measurements without gross distortion, the range is not properly matched to the AC audio signal level and accuracy is compromised. The correct way to test this DUT with an APx585 analyzer is by adding an AUX-0100 Multichannel Switching Amplifier Filter (Figure 2) between the DUT and the analog inputs of the APx585. The AUX-0100 is AC-coupled and blocks the DC voltage from reaching the analyzer. Figure 3 shows the 1 kHz reference signal from our DUT measured with an AUX-0100 filter in line.
In addition to significant DC voltages, switching amplifiers often present other measurement difficulties. These amplifiers have an output signal that is a high-frequency switching carrier wave modulated by the audio signal. This can introduce frequency components of the carrier wave that are outside the audio band. When the amplitude of these out-of-band components remains high in comparison to the audio signal, the active input circuits of the analyzer may be driven into slew-rate limiting, and the analyzer ranging circuits may respond to the carrier components rather than to the audio signal. Either of these mechanisms will reduce the accuracy of the measurement. The AUX-0100 switching amplifier filter addresses this problem by reducing the amplitude of the switching carrier wave before the signal is input to the analyzer.