National Instruments Toolkit - Manual

© National Instruments Corporation 5 NI Spectral Measurements Toolkit User Guide 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 ...

NI Spectral Measurements Toolkit User Guide 6 ni.com 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 S...

© National Instruments Corporation 7 NI Spectral Measurements Toolkit User Guide 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...

© National Instruments Corporation 9 NI Spectral Measurements Toolkit User Guide 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 sepa...

NI Spectral Measurements Toolkit User Guide 10 ni.com 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...

© National Instruments Corporation 11 NI Spectral Measurements Toolkit User Guide 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...

NI Spectral Measurements Toolkit User Guide 12 ni.com 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 FF...

© National Instruments Corporation 13 NI Spectral Measurements Toolkit 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. The FFT operation itself c...

NI Spectral Measurements Toolkit User Guide 14 ni.com 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 origin...

© National Instruments Corporation 15 NI Spectral Measurements Toolkit 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 s...

© National Instruments Corporation 17 NI Spectral Measurements Toolkit User Guide 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 dis...

NI Spectral Measurements Toolkit User Guide 18 ni.com 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 r...

NI Spectral Measurements Toolkit User Guide 20 ni.com 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 entir...

© National Instruments Corporation 21 NI Spectral Measurements Toolkit User Guide Figure 8 shows the shape of the equivalent filter corresponding to a 7-Term Blackman-Harris window. The cursors are placed at the 3 dB points of the filter response, and the resolution bandwidth is the distance between...

NI Spectral Measurements Toolkit User Guide 22 ni.com The acquisition size comes from the following basic relationship: df = f s / N = 1/ T where N is equal acquisition size and RBW is the frequency resolution df multiplied by the window spectral leakage correction factor of 3 dB bandwidth. If the s...

NI Spectral Measurements Toolkit User Guide 24 ni.com Spectral Domain Averaging Averaging is an important part of spectrum-domain measurements because of the effects of noise on a signal and its spectrum. The Spectral Measurements Toolkit includes averaging VIs that average several records of data t...

© National Instruments Corporation 25 NI Spectral Measurements Toolkit User Guide The averaging VIs require that you enter an FFT spectrum as a complex array. You can perform spectrum unit conversion before or after averaging. Averaging Conventions For Spectral Measurements Toolkit VIs, averaging re...

NI Spectral Measurements Toolkit User Guide 26 ni.com Averaging Options Figure 10 illustrates the options available for spectrum averaging. Figure 10. Spectrum Averaging Options The averaging processes apply weighting to the < > operator in both RMS and vector averaging as shown in the followi...

© National Instruments Corporation 27 NI Spectral Measurements Toolkit User Guide emphasis on the most recent data. The averages sofar parameter stops incrementing at N while the averaging continues. Linear weighting includes the following modes: • One-shot linear averaging—Average one time for the ...

NI Spectral Measurements Toolkit User Guide 28 ni.com Averaged Power Spectrum The following equations describe the averaging methods you can apply to a complex FFT spectrum to yield an averaged power spectrum. The No averaging method converts the complex FFT spectrum to a real power spectrum. The av...

© National Instruments Corporation 29 NI Spectral Measurements Toolkit User Guide Averaged Frequency Response If you have a stimulus to a system with spectrum X and the system response Y , the frequency response H of the system is shown by the following equation: You can use the equations shown in t...

NI Spectral Measurements Toolkit User Guide 30 ni.com Spectrum scaling options are combinations of the following options: • RMS or peak —An FFT returns an amplitude spectrum scaled such that a frequency bin represents the RMS value of a sine wave at that frequency. A bin can also represent the peak ...

© National Instruments Corporation 31 NI Spectral Measurements Toolkit User Guide Peak Search and Amplitude/Frequency Estimation The SMT Spectrum Peak Search VI uses interpolation to precisely locate frequency peaks in the amplitude or power spectrum and to estimate the amplitude of each peak. You c...

NI Spectral Measurements Toolkit User Guide 32 ni.com Power in Band The SMT Power in Band VI, located on the SMT Measurements palette measures the total power within some frequency range or band. X is the input power spectrum in V 2 rms . Perform this measurement before performing unit conversion. E...

© National Instruments Corporation 33 NI Spectral Measurements Toolkit User Guide Figure 12 illustrates a typical ACP measurement and the three parameters that specify the channels. Figure 12. ACP Measurement Occupied Bandwidth (OBW) The SMT Occupied Bandwidth VI, located on the SMT Measurements pal...

NI Spectral Measurements Toolkit User Guide 34 ni.com Figure 13 shows an example of an OBW measurement. The logarithmic amplitude scale gives the appearance of a significant amount of power outside the channel, but only 1% of the signal power is actually located there. Figure 13. Occupied Bandwidth ...