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Building stimulus-response system with AWG, digitiser

18 Mar 2016  | Arthur Pini, Greg Tate, Oliver Rovini

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Many devices and systems do not need a stimulus signal for you to test them. But, when it comes to amplifiers, filters, transmitters, receivers, digital interfaces, or any system that requires an input signal, you'll have to generate it and measure the output. Modular digitisers and arbitrary waveform generators (AWGs) let you test one or more devices that need input signals. Combining the two products in one system provides very cost effective and efficient way to meet an extensive range of automated test requirements.

Characterise a filter
AWG's let you generate almost any waveform, analogue or digital. Consider a simple test for determining the frequency characteristics of an amplifier or filter. The test requires a signal source with a bandwidth greater than that of the device under test. Furthermore, the source needs to deliver a constant output level over the entire testing bandwidth. A swept sinewave or impulse function waveforms offer broadband output with flat spectral characteristics. Either can be generated by an AWG. The swept sine offers a greater dynamic range for the measurement. Figure 1 shows the result of a swept sine frequency response measurement of a 36MHz low-pass filter.

Figure 1: The frequency response of a 36MHz filter shows the filter's input and output in the time domain (top left & right, respectively, and in the frequency domain (bottom left & right, respectively).

The upper left grid shows the swept sine that we applied to the filter's. For this test, we created the test signal using the equation:

Where X=Sample value

XS=Start period in samples

XE=End period in samples

XMAX=Duration of the waveform in samples

In this example, we used:

The bottom-left grid shows the FFT (Fast Fourier Transform) of the filter. The swept sine was flat out to 100MHz. The upper-right grid shows the filter's time response. The filter's output decreases rapidly above its 36MHz cut-off frequency. The matching frequency response is shown in the grid on the lower right. You can clearly see the filter's bandwidth, in-band flatness, and stop-band attenuation.

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