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USER GUIDE
NI Spectral Measurements Toolkit
This document explains how to use the NI Spectral Measurements Toolkit
™ ™
(SMT) in LabVIEW and LabWindows /CVI for frequency-domain
measurements.
Contents
Conventions ............................................................................................ 2
Using the Spectral Measurements Toolkit .............................................. 3
Integrating the Spectral Measurements Toolkit............................... 4
Locating the Spectral M
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Averaged Cross Spectrum ................................................................28 Averaged Frequency Response ........................................................29 Spectral Domain Measurements..............................................................29 Unit Conversion................................................................................29 Peak Search and Amplitude/Frequency Estimation .........................31 Power in Band ..........................................
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Using the Spectral Measurements Toolkit The Spectral Measurements Toolkit contains LabVIEW VIs and LabWindows/CVI functions that perform the following operations: � Zoom frequency analysis—Zoom fast Fourier transform (FFT) functions and VIs allow you to zoom in on a narrow frequency range in a spectrum. � Spectrum averag ing—The Spectral Measurements Toolkit supports averaging types such as root-mean-square (RMS) averaging, vector averaging, and peak-hold averaging. � Spectral measurements
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Integrating the Spectral Measurements Toolkit You can use the Spectral Measurements Toolkit for the following applications: � Frequency-domain measurements such as: – Adjacent channel power ratio (ACPR) – Channel spectrum – Power-in-band measurements – Average and peak power – Power spectral density – Spectrum limit and mask testing � Modulation-domain measurements such as: – Frequency deviation – AM modulation index � Component-level measurements such as characterization of oscillators, mixe
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SMT Programming Flow Diagram Programming flow diagrams are flowcharts that depict the most effective order for programming Spectral Measurements Toolkit VIs. Use the programming flow diagram in the SMT Programming Flow VI as a visual guide for the order in which you should call VIs. This VI is located in the \examples\Spectral Measurements Toolset\ Simulation folder. The example is not intended to be executable, but rather to supply an informative block diagram depicting general
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2. Enter the output power spectrum into an SMT measurement VI and/or use the SMT Spectrum Unit Conversion VI as follows: a. Enter the output power spectrum into an SMT measurement VI: the SMT Power in Band, the SMT Adjacent Channel Power, or the SMT Occupied Bandwidth VI. These VIs accept a power 2 spectrum with units V and return the requested measurements. rms Perform the measurements on only an unscaled power spectrum. You can specify the units in which to view these measurements. b. Us
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For an averaged power spectrum with zoom, use the SMT Zoom Power Spectrum VI. For an averaged FFT spectrum, which has a complex output for magnitude and phase calculations, use the SMT Zoom FFT VI first and then the SMT Averaged FFT Spectrum VI. If you have two channels of input time-domain data and want cross power spectrum or frequency-response measurements, use the SMT Zoom FFT Spectrum VI first and then the SMT Averaged Cross Spectrum VI or the SMT Averaged Frequency Response VI. Usi
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The averaging parameters cluster specifies the following settings: � Averaging type, such as vector averaging, RMS averaging, or peak hold � Weighting type, such as linear or exponential � Averaging size The linear weighting mode parameter specifies a type of linear weighting. The SMT Zoom Power Spectrum VI, located on the SMT Advanced 2 palette, returns the spectrum in units of V . rms The unit conversion settings parameter specifies the units in which to display the spectrum. For example,
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The channel specification parameter specifies the center frequency, bandwidth, and spacing for the ACP measurement. The bandwidth parameter specifies the width of each channel. The spacing parameter specifies the separation between the center frequencies of each channel. The Units [rms] parameter specifies the units for the ACP measurement. The Power Spectrum parameter is a waveform graph that shows the power spectrum with the three channels, or bands, and the power in each channel. The A
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Cross Power Spectrum Measurement The Cross Power Spectrum Measurement example is located at samples\ smt\simulated\smtcrspwr\smtcrspwr.prj. This example demonstrates how to configure the zoom FFT, then calculates the averaged power spectrum of a stimulus and response signal from a device under test and the averaged cross spectrum between these two signals. The example uses the continuous zoom FFT technique and demonstrates how you must create a unique handle for the stimulus data (handle1)
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Figure 2. Zoom FFT Technique FFT algorithms usually perform baseband analysis by displaying the spectrum from zero to the Nyquist frequency. However, a standard FFT might not be effective if you need to obtain a higher frequency resolution over a limited portion of the spectrum or if you need to zoom in on details of a spectral region. The zoom FFT uses algorithms to avoid the amount of calculation required using a long standard FFT to obtain high-frequency resolution over an entire spect
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Continuous Zoom FFT Continuous zoom FFT is a technique for quickly analyzing data as it arrives. A decimation process reduces the sample rate in real time. After the process acquires all the data and decimates it in time T, a relatively small FFT remains. The term continuous refers to beginning the process while data arrives. With a standard FFT, you must wait until all the data arrives before beginning calculations. The continuous zoom FFT first shifts the spectral region of interest into
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The continuous zoom FFT technique is sometimes called the real-time zoom FFT because it continuously performs the frequency shifting, decimation, and filtering processes on the arriving data. The FFT operation itself cannot proceed until you acquire all the data. The FFT operation then occurs in parallel with the next data acquisition. You can use the SMT Cont Zoom FFT VI to perform the continuous zoom FFT technique. This VI is located on the Zoom FFT palette, which is a subpalette of the
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Block Zoom FFT Use a block zoom FFT in situations when you cannot access data until the data acquisition is complete. The block zoom FFT is a nondestructive zoom FFT because it stores data before processing, so the data is available in its original form if you need it for other operations. The block zoom FFT is an algorithm that calculates a portion of a large FFT. The block zoom FFT also improves the frequency resolution, df, by increasing the number of points that the FFT processes. A bl
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The block zoom FFT is a general-purpose technique that works best as a post-processing method. The block zoom FFT also is useful for real-time applications where the data rate is too high for a continuous zoom FFT to sustain in real time. To process the entire data set, provide enough memory to store the data until the FFT can process it. If processing every data point is not critical, use the block zoom FFT with the latest data available. Zoom Spectrogram A spectrogram is the result of joi
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Time Span 1.0 0.8 0.6 Window 0.4 0.2 Signal 0.0 –0.2 –0.4 –0.6 0 10 20 30 40 50 60 70 75 Time (µs) FFT FFT F T Figure 5. Spectrogram Process Example The SMT Config Zoom STFT VI specifies the spectrogram in terms of its center frequency, frequency span, and time span. The frequency span controls the FFT zoom. If the center frequency is 10 MHz and the span is 2 MHz, the SMT Config Zoom STFT VI calculates the spectrogram from 9 MHz to 11 MHz. The time span specifies the time interval from the
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18 µs 15 µs 12 µs 9 µs 6 µs 3 µs 0 µs effective band specification. If you leave the default advanced parameters, the configuration VI calculates the correct parameters for a spectrogram with evenly distributed time and frequency resolution on a square display area. If the display area is not square, enter an aspect ratio for the display area in the aspect ratio parameter. Figure 6 shows an example of a completed spectrogram with a center frequency of 16 MHz and a span of 16 MHz. Figure 6.
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Configuring Zoom FFT VIs When using Spectral Measurements Toolkit VIs, you must enter several values to completely specify a zoom FFT. The Spectral Measurements Toolkit provides two configuration VIs that select values for each setting and that require you to enter a minimal number of values. The SMT Config Zoom FFT VI configures the block zoom FFT. The SMT Config Cont Zoom FFT VI configures the continuous zoom FFT. These configuration VIs ensure that the input values are compatible and yi
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The left side of Figure 7 shows examples of the four combinations of center frequency and span that you can encounter in the case of a real input signal. The right side of Figure 7 shows the actual coerced values of center frequency and span that the VI sets in each example. Antialiasing Filter Response User Input Coerced Result Effective Band Effective Band a. Span Span f f f f /2 f f f f /2 l c h s l c h s Effective Band Effective Band b. Span Span f f f f /2 f f = (f + f ) f f /2 l h c s
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that is outside the effective band, the span changes to the default value, which is the center of the effective band. Figure 7c demonstrates that if you request a span that is wider than the effective band, the span decreases until it falls entirely within the effective band without moving the center frequency. Figure 7d demonstrates that if you enter center frequency and span values that fall within acceptable limits but a portion of the span falls outside the effective band, the center f
