The time-domain representation gives the amplitudes of the signal at the instants of time during which it was sampled. However, in many cases, you need to know the frequency content of a signal rather than the amplitudes of the individual samples.
The fast Fourier transform (FFT) provides a method for examining a relationship in terms of the frequency domain. Fourier’s theorem states that any waveform in the time domain can be represented by the weighted sum of sines and cosines. The same waveform then can be represented in the frequency domain as a pair of amplitude and phase values at each component frequency.Figure 1. Time Domain Versus the Frequency Domain
In the frequency domain, you can separate conceptually the sine waves that add to form the complex time-domain signal. Figure 1 shows single frequency components, which spread out in the time domain, as distinct impulses in the frequency domain. The amplitude of each frequency line is the amplitude of the time waveform for that frequency component. The representation of a signal in terms of its individual frequency components is the frequency-domain representation of the signal. The frequency-domain representation might provide more insight about the signal and the system from which it was generated.
The samples of a signal obtained from the data acquisition device constitute the time-domain representation of the signal. Some measurements, such as noise or harmonic distortion, are difficult to quantify by inspecting the time waveform. When the same signal is displayed in the frequency domain by an FFT, you easily can measure the harmonic frequencies and amplitudes.
If you have a noise in your measured signal, you can move from the time domain into the frequency domain to isolate the disturbance in your measurement.
Using LabVIEW and NI data acquisition devices, you can acquire data and present it on the user interface in minutes to analyze and explore the measured signal. Using NI myDAQ, you can configure the device on the PC and acquire an analog signal as follows:
To find the noise on the measured signal, you need to see the signal in the frequency domain. Using the Spectral Measurements Express VI, convert the time-domain signal into the frequency domain. To measure the peak, the VI analyzes all of the frequencies of your signal and displays them on your graph.
Once you have identified the unwanted components of the signal, you can remove the noise and keep the signal of interest. A filter removes an unwanted component or feature but does not alter the original frequencies. Using a Lowpass Filter Express VI, you can pass all frequencies below a specified value while removing higher frequency values. While running LabVIEW, you can interact with the user interface and interact with and modify the variable of the cutoff frequency as you acquire the signal.
Right-click on the above VI snippet and select Save Image As... Locate the file on your hard disk and drag the file icon onto your LabVIEW block diagram. LabVIEW automatically generates the code from the VI snippet. Click here for more information on VI snippets.
Learn how you can use NI educational software and hardware to teach fundamental measurements concepts. LabVIEW interfaces directly with NI Educational Laboratory Virtual Instrumentation Suite (NI ELVIS) and NI myDAQ hardware to provide a complete solution for measurement theory, simulation, and physical implementation and validation.
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