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The swept-sine measurement example in this topic measures the frequency response and harmonic distortion of a device under test (DUT). The DUT is a notch filter centered at 1 kHz. An NI PXI-4461 generates the excitation signal and acquires the stimulus and response signals.

The following illustration shows the connection scheme to measure the dynamic response of the DUT using a swept-sine measurement.

The acquired stimulus signal on analog input channel 0, AI0, is the generated excitation signal from analog output channel 0, AO0. The NI PXI-4461 converts the desired stimulus signal from digital data to an analog signal and outputs that signal on AO0. The excitation signal is connected to both the stimulus input channel AI0 and the input terminal of the DUT. The response signal is connected from the output terminal of the DUT to the response input channel AI1.

The following block diagram illustrates the SVXMPL_Swept Sine FRF (DAQmx) example VI, which is located in the labview\examples\Sound and Vibration\Swept Sine directory.

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The While Loop in the previous block diagram controls the synchronized generation and acquisition. Display controls and measurement indicators are updated inside the While Loop. This loop allows for the monitoring of intermediate results.

The following table lists the actions that the VIs in the previous block diagram perform. Some steps are required and must be done for the VIs to function correctly. The optional steps allow you to customize your measurement.

Step Number	Description	Optional or Required
1	Initialize a swept-sine measurement by specifying the hardware device and channel settings.	Required
2	Specify the scaling that will be applied to the acquired stimulus and response data.	Optional
3	Configure the source by specifying the test frequencies, amplitude, and whether or not the sweep automatically restarts after completion.	Required
4	Set the settling and integration parameters to allow sufficient time for the DUT to settle before the measurement is performed at the new test frequency and sufficient integration time to achieve the desired level of accuracy.	Required
5	Set the block duration input terminal for the measurement to be small enough to give a reasonable test time and large enough so it does not put the test computer at risk of being unable to continuously generate and read the signals. The smaller the block size, the faster the swept sine can transition from one test frequency to the next.	Optional
6	Explicitly set the sampling rate for the measurement. The rate is automatically selected if this VI is not used. The same rate is used for input and output channels.	Optional
7	Specify the propagation time terminal input specific to the DAQ device being used for the measurement. You can measure the device propagation time using the SVL Measure Propagation Delay (DAQmx) VI.	Optional
8	Configure the harmonic distortion measurement by specifying the maximum harmonic to use in the computation of the total harmonic distortion (THD). Only those harmonics specified in the harmonics to visualize array return individual harmonic components.	Required only if performing distortion measurements
9	Start the swept-sine to perform the hardware configuration and start the output and input tasks. Channel synchronization is performed internally in this VI.	Required
10	Generate the excitation and acquire the stimulus and response data at each test frequency.	Required
11	Convert the raw data to the specified format in order to display and report measurement results.	Required
12	Stop the swept-sine measurement and clear the output and input tasks to release the device.	Required

Many of the steps in the previous table are configuration steps. Using the Configure Swept Sine VIs, you can specify numerous configuration parameters to achieve fine control of the swept-sine measurement parameters. For many applications, two or three configuration VIs are sufficient.

You must allow for the propagation delay of the data acquisition (DAQ) or dynamic signal acquisition (DSA) device. This delay is specific to the device used to perform the measurement. To determine the device propagation delay, refer to the device documentation or measure the delay with the SVL Measure Propagation Delay (DAQmx) VI.

The following front panel shows the time-domain stimulus and response signals for the 138.49 Hz test frequency.

From the time-domain data, you can see that the notch filter has attenuated the signal and introduced a phase shift.

The following front panel shows the magnitude and phase responses of the notch filter at all the test frequencies in the magnitude and phase spectra in a Bode plot.

In addition to measuring the frequency response, this example simultaneously measures the harmonic distortion at each test frequency. The following front panel shows the graph of total harmonic distortion (THD) versus frequency.

You expect to see a peak in the THD at the notch frequency. The peak occurs because the fundamental frequency is attenuated at the notch frequency. However, the graph indicates that this measurement has failed to accurately identify the power in the harmonic distortion components. For the example in the previous front panel, the number of integration cycles is two. More integration cycles must be specified to perform accurate harmonic distortion measurements. If you change the number of integration cycles to 10 and rerun the example, you obtain the THD versus frequency results displayed in the following front panel.

Now, with a sufficient number of integration cycles specified, you can see the characteristic peak in the THD at the center frequency of the notch filter.