AP - The Recognized Standard In Audio Test

Generating Signals with a DC Component on the APx500 Series and ATS-2

Created 07 May 2009

Question:

I need to test my device with a combined AC/DC signal. Can I generate this signal with my APx Series or ATS-2 audio analyzer?

Answer:

Using the Digital to Analog Converter (DAC) based APx500 Series or ATS-2 audio analyzers, it is possible to generate a signal with a DC component (within limits), using the arbitrary waveform generator.

For the SYS-2700 series, it is not possible to generate a DC signal directly, because the generator is transformer-coupled. Refer to the KB article “Generating Signals with a DC Component on System Two Family Analyzers” for a discussion about generating combined AC/DC signals with a 2700 series audio analyzer. It is also not possible with a P1/ATS-1 analyzer, because these units do not support arbitrary waveform generation.

To use the arbitrary waveform generator for this purpose, you must first create a .wav file containing the combined AC/DC signal that you want to generate. You can do this with third party application software such as Matlab, LabVIEW, and Adobe Audition. For example, suppose that you want to generate a signal that is a 1.0 Vrms sine wave at 1 kHz combined with 3.5 VDC. This is represented mathematically as a DC signal with an amplitude of 3.5 units, combined with a sine wave of amplitude 1.414 units. The combined signal is then normalized, so that the maximum absolute value is 1.0 units (1.0 FS in the digital domain), as shown in Figure 1.

 

Fig 1 Generated sine wave signal with DC offset.

APx Series

In the case of the APx Series, the analyzers have a generous buffer length of 32 Mega-samples—enough to hold more than 11 minutes of mono waveform data sampled at 48 kHz. Therefore, the simplest way to proceed is to create a .wav file long enough to complete the test without repeating the waveform.

However, long waveforms take a long time to load, and they make the project file larger, so if the waveform is cyclic and you want to be more efficient, you can design the .wav file to be minimum length. For example, consider the signal described above - a 1 kHz sine wave superimposed on a DC component. This signal repeats itself after one cycle of the sine wave, so the project can be made more efficient by using a shorter .wav file and allowing the generator to wrap when it reaches the end of the file. To avoid “glitches” (discontinuities when the generator wraps), it is necessary to ensure that the waveform is “synchronous”—i.e., that an integer number of cycles of the sine wave fits exactly into the waveform buffer.

The only constraint when using APx500 is that the .wav file loaded into the arbitrary waveform buffer be a minimum of 256 samples. Thus, one combination of number of samples and sample rate that yields a synchronous .wav file is 288 samples at 48 kHz. This corresponds to exactly 6 cycles of a 1 kHz sine wave. Another combination is 256 samples at a 51.2 kHz sample rate (5 cycles of a 1 kHz sine wave). We used the latter, because this combination also works with the ATS-2 (described below). The file is named Sine_1k_DC_51_24_256S.wav.

To enable measurement of the DC component in APx500, be sure to set the analog input configuration to DC coupling in the input settings dialog box. Then, it is necessary to scale the generator output level. In the case of the .wav file included above, the peak of the combined AC/DC signal was set to 1.0 FS in the .wav file. Therefore, the generator level has been set to 4.914 Vp (3.5 VDC + 1.414 Vp = 4.914 Vp). If you route the generator signal back into the analyzer (loopback on the APx52x Series, or cable from output to input on the APx58x Series), you can view the waveform on the APx500 scope monitor, as shown in Figure 2.

 

Fig 2 APx scope monitor showing waveform with DC offset.

ATS-2

For ATS-2 audio analyzers, the process is very similar to the one described above for APx. However, there are two major differences:

1. In the ATS application, the maximum generator buffer length is 16,384 samples (about 0.34 seconds at a 48 kHz sample rate).  Therefore, in ATS, the .wav file must be made synchronous, because the generator will wrap during the measurement.
2. In the ATS application, the generator buffer length must be a power of 2, with a minimum value of 256 samples and a maximum of 16,384 samples. This adds an additional constraint.

The frequency of a sine wave that is synchronous with a buffer n samples long can be determined from the equation below:

                  where:         f_s          is the desired sine wave frequency

                                    f_s’         is the synchronous frequency

                                    n          is the number of samples in the .wav file (256, 512,…16,384)

                                    SR        is the sample rate in Hz

The file Sine_1k_DC_51_24_256S.wav described above is synchronous with a sample buffer of 256 samples. It is therefore very efficient, and because it is 256 samples long, it will work with ATS. To use this file with ATS, import it into the waveform generator buffer (File|Import|WAV File). To measure the DC component, be sure to check the DC coupling check box on the Analog Input panel, and set the coupling to DC-coupled if using the FFT Spectrum Analyzer (Figure 3). The resulting waveform measured with an ATS-2 is shown in Figure 4.

 

Fig 3 ATS-2 panel settings.

 

Fig 4 Waveform FFT show on ATS-2 graph.

Limitations

Each analyzer’s analog generator has a voltage limit and an output current limit. These voltage limits are specified in Vrms. For convenience, these voltage limits have been converted to Vp in the table below. The peak voltage of the combined AC/DC signal can not exceed these levels and the output current can not exceed the specified rating.

Audio Analyzer	Maximum Output Voltage		Maximum Output Current (mA peak)
	Balanced (Vp)	Unbalanced (Vp)
APx585/586	20	10	30
APx520/521	29	14	80
ATS-2	22	11	50

For additional help, please contact Audio Precision Technical Support.

*****
SLX #xxx
JB

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