Generating Signals with DC on ATS-2 Analyzers

Created on 2009-05-07 17:35:00

Question:

I need to test my device with a combined ac/dc signal. Can I generate this signal with my ATS-2 audio analyzer?

Answer:

Note: This article only applies to the ATS-2 analyzer with the ATS software. The APx Series analyzers have the ability to generate ac + dc built into the APx500 software (since version 2.8). The 2700 Series analyzers can not 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 how to generate 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.

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

For ATS-2 audio analyzers, 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. The generator buffer length must be a power of 2, with a minimum value of 256 samples and a maximum of 16,384 samples.

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 1). The resulting waveform measured with an ATS-2 is shown in Figure 2.

Fig 1 ATS-2 panel settings.

Fig 2 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.

For additional help, please contact Audio Precision Technical Support.

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Sine_1k_DC_51_24_256S.wav