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USER GUIDE NI Spectral Measurements T oolkit This document explains how to use th e NI Spectral Meas urements Toolkit (SMT) in LabVIEW and LabWindows ™ /CVI ™ for freq uency-domain measurements. Contents Conventions .............. .............. .
NI Spectral Measurements T oolkit User Guide 2 ni.com Averaged Cross Spectrum ................. .............. .............. ................. .. 28 Averaged Frequency Response ............... .............. ................. ..........29 Spectral Domain Measurements .
© National Instruments Corporation 3 NI Spectral Measurements T oolkit User Guide Using the Spectral Measurements T oolkit The Spectral Measurements Tool kit contains LabVIEW VIs and LabWindows/CVI f.
NI Spectral Measurements T oolkit User Guide 4 ni.com Integrating the Spectral Measurements T oolkit You can use the Spectral Meas urem ents Toolkit for the fo llowing applications: • Frequenc y-dom.
© National Instruments Corporation 5 NI Spectral Measurements T oolkit User Guide SMT Programming Flow Diagram Programming flow diag rams are flowch arts that depict the most effective order for programming Spectral Meas urem ents Toolkit VIs.
NI Spectral Measurements T oolkit User Guide 6 ni.com 2. Enter the output power spectrum into an SMT measurem ent VI and/or use the SMT Spectrum Unit Con version VI as follo ws: a. Enter the output power spectrum into an SMT measurem ent VI: the SMT Po wer in Band, the SMT Adjacent Channel Power , or the SMT Occupied Bandwidth VI .
© National Instruments Corporation 7 NI Spectral Measurements T oolkit User Guide For an a veraged power spectrum w ith zoom, use the SMT Zoom Po wer Spectrum VI. For an a veraged FFT spectrum, which has a complex output for magnitude and phase calcu lations, use the SMT Zoom FFT VI first a n d then the SMT A veraged FFT Spectrum VI.
NI Spectral Measurements T oolkit User Guide 8 ni.com The averaging parameters cluster specifies the following settings: • A v eraging type, such as vector av eraging, RMS averaging, or peak hold • W eigh ting type, such as linear or exponential • A v e raging size The linear w eighting mode parameter specifies a type of linear weighting.
© National Instruments Corporation 9 NI Spectral Measurements T oolkit User Guide The channel specification parameter specifies the center frequency , bandwidth , and spaci ng for the A CP measurement. The bandwidth parameter specifies the width of each c hannel.
NI Spectral Measurements T oolkit User Guide 10 ni.com Cross Power Spectrum Measurement The Cross Power Spectrum Measur ement example is located at samples smtsimulatedsmtcrspwrsmtcrspwr.
© National Instruments Corporation 11 NI Sp ectral Measurements T oolkit User Guide Figure 2. Zoom FF T T echnique FFT algorithms usually pe rform baseband analysis by displaying the spectrum from zero to the Nyquist frequency .
NI Spectral Measurements T oolkit User Guide 12 ni.com Continuous Zoom FF T Continuous zoom FFT is a technique for quickly analyzing data as it arrives. A decimation process reduces th e sample rate in real time. After the process acquires all the data and d ecimates it in time T , a relatively small FFT remains.
© National Instruments Corporation 13 NI Sp ectral Measurements T oolkit User Guide 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.
NI Spectral Measurements T oolkit User Guide 14 ni.com Block Zoom FF T Use a block zoom FFT in situations when you cannot access data until the data acquisition is complete.
© National Instruments Corporation 15 NI Sp ectral Measurements T oolkit User Guide 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.
NI Spectral Measurements T oolkit User Guide 16 ni.com Figure 5. Sp ectrogram Process Example The SMT Config Zoom STFT VI specif ies the spectrogram in terms of its center frequency , frequency span, and time span. The frequency span controls the FFT zoom.
© National Instruments Corporation 17 NI Sp ectral Measurements T oolkit User Guide ef fectiv e band specification. If you lea ve the default adv anced parameters, the configuration VI calculates the co rrect parameters for a spectrogram with ev enly distributed time and frequency resolution on a square display area.
NI Spectral Measurements T oolkit User Guide 18 ni.com Configuring Zoom FF T VIs When using Spectral Measurements To olk it VIs, you must enter several values to completely specify a z oom FFT.
© National Instruments Corporation 19 NI Sp ectral Measurements T oolkit User Guide The left side of Figure 7 sho ws exam ples of the four combinations of center frequency and span that you can encounter in the case of a real input signal.
NI Spectral Measurements T oolkit User Guide 20 ni.com that is outside the ef fect iv e band, the span changes to the default v alue, which is the center of the ef fectiv e ba nd.
© National Instruments Corporation 21 NI Sp ectral Measurements T oolkit User Guide Figure 8 shows the shape of the equiv ale nt filter corr esponding to a 7-T erm Blackman-Harris window . The cursors ar e placed at the 3 dB points of the filter response, and the resolution ba ndw idth is the distance between the cursors.
NI Spectral Measurements T oolkit User Guide 22 ni.com The acquisition size comes from the following basic relationshi p: df = f s / N = 1/ T where N is equal acquisition size and RBW is the frequency resolution df multiplied by the w indow spectral leakage correction factor of 3 dB bandwidth.
© National Instruments Corporation 23 NI Sp ectral Measurements T oolkit User Guide Figure 9 shows the spectrum of a mu ltiton e signal calculated using two RBW values. The multitone si gnal cons ists of two tones, at frequenci es 1.0 MHz and 1 .1 MHz, separated by 100 kHz.
NI Spectral Measurements T oolkit User Guide 24 ni.com Spectral Domain A veraging Averaging is an important part of spectrum-domain measurements because of the effects of noise on a s igna l and its spectr um. The Spectral Measurements Toolkit includes av eraging VIs that average several records of data to reduce the noise eff ects.
© National Instruments Corporation 25 NI Sp ectral Measurements T oolkit User Guide The av eraging VIs require that you enter an FFT spectrum as a complex array .
NI Spectral Measurements T oolkit User Guide 26 ni.com A veraging Options Figure 10 illustrat es the options av ailable for spectrum averaging. Figure 10.
© National Instruments Corporation 27 NI Sp ectral Measurements T oolkit User Guide emphasis on the most recent data. The averages sofar parameter stops incrementing at N while the averaging continues. Linear weighting i ncludes the foll owing modes: • One-shot linear averaging—A ver age one time for the specified duration of N measurements.
NI Spectral Measurements T oolkit User Guide 28 ni.com A veraged Power Spectrum The following equations describe the averaging meth ods you can apply to a complex FFT spectrum to yield an averaged power spectrum. The No averaging method converts the comp lex FFT spectrum to a real power spectrum.
© National Instruments Corporation 29 NI Sp ectral Measurements T oolkit User Guide A veraged Frequency Response If you have a stimulus to a system with spectrum X and the system response Y , the fre.
NI Spectral Measurements T oolkit User Guide 30 ni.com Spectrum scaling options are com binations of the following options: • RMS or peak —An FFT returns an amplitu de spectrum scaled such that a frequency bin represents the RMS v alue of a sine wav e at that frequency .
© National Instruments Corporation 31 NI Sp ectral Measurements T oolkit User Guide Peak Search and Amplitu de/Frequency Estimation The SMT Spectrum Peak Sear ch VI uses interpolat ion to precisely locate frequency peaks in the amplitude or power spectrum and to estimate the amplitude of each peak.
NI Spectral Measurements T oolkit User Guide 32 ni.com Power in Band The SMT Power in Band VI, located on the SMT Measurements palett e measures the total power within some frequency rang e or band. X is the input power spectrum in V 2 rms . Perform this measurement before performing unit conversion.
© National Instruments Corporation 33 NI Sp ectral Measurements T oolkit User Guide Figure 12 illustrates a typical A CP m easurement and the three parameters that specify the channels.
NI Spectral Measurements T oolkit User Guide 34 ni.com Figure 13 shows an e xam ple of an OBW measurement. The logarithmic amplitude scale gi ves the appearance of a signif ican t amount of power outside the channel, but only 1% of the signal power is actually located there.
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