Old measurements, new techniques: DSP drives speed and accuracy; coherence saves the day

摘要

To engineers who are used to worrying about the quality and precision of signal generators, the cross-spectral technique for measuring frequency response should come as a welcome relief, because it relaxes the requirements on the signal generators used in the measurements. Drift, amplitude stability, flatness, noise, and distortion become quite irrelevant. The only thing that really matters is that the generator be able to create some semblance of the desired signal, whether it is noise, a chirp, or a transient. As the background material for this article shows (see box below), the cross-spectral technique measures the amplitude and phase of the spectra of both the excitation and response signals and thereby corrects for signal-generator errors. The distortion or noise that a poor-quality generator creates becomes just another part of the excitation signal - a part that the mathematics can handle. Recall that several types of noise, including white noise and transients, are perfectly legitimate forms of excitation, so if the excitation signal contains them, there is no issue. The only type of signals that the math can't handle are alias components (signals that the sampling process shifts from one frequency to another). It's unimportant whether the source of these components is the generator, the UUT (unit under test), or the measuring instrument's ADC, which can create aliases if it lacks suitable input filters. Therefore, you must pay attention to prevent out-of-band signals from reaching the ADC.