Chapter 6 Data Acquisition and Spectral Analysis System high-speed digitizer card for acquiring time-domain data. The digitizer is used in

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1 Chapter 6 Data Acquisition and Spectral Analysis System 6.1 Introduction This chapter will discuss the hardware and software involved in developing the data acquisition and spectral analysis system. The system uses a National Instruments NI 5112 high-speed digitizer card for acquiring time-domain data. The digitizer is used in conjunction with a software package written in Microsoft Visual Basic 6.0 for interfacing with the card. The software package fetches the data acquired by the digitizer, displays the data in a graphical user interface, and performs spectral analysis on the data. The next section will discuss the fundamental specifications of the digitizer. 6.2 NI 5112 High-Speed Digitizer The National Instruments NI 5112 is a high-speed digitizer with a maximum realtime sampling rate of 100 MHz and random interleaved sampling rates from 200 MHz to 2.5 GHz. The random interleaving function obtains multiple sets of samples over several periods each shifted slightly. The resulting data is used to reconstruct a waveform that appears to be sampled at a rate higher than what is normally capable by the card. To use random interleaving the waveform being sampled must be periodic or the reconstructed signal will not represent the original signal. In contrast, transient signals and periodic signals can be sampled by using the real-time sampling. 53

2 54 The NI 5112 has two channels which can sample simultaneously at 8-bits per sample. Therefore, the digitizer is capable of distinguishing between 256 or 2 8 different voltage levels across the dynamic range. This is a direct limitation of the ADC (analogto-digital converter) within the card. The NI 5112 has an input range from ±25 mvolts to ±25 Volts and a maximum DC offset of ±37 Volts. The input impedance of the digitizer is user selectable and software controlled for either 50 Ω or 1 MΩ. The voltage seen by the digitizer is expressed by equation (6.1): R in V = m Vs (6.1) Rin + Rs where V m is the voltage measured by the digitizer, V s is the voltage of the source, R s is the output impedance of the source, and R in is the input impedance of the digitizer. If the 50 Ω impedance is selected, then the input voltage to the digitizer should be limited to 1 V rms in order to achieve accurate results from the data acquisition. All of the functions and settings of the digitizer are software controlled. The NI 5112 is compatible with Visual Basic, Visual C and C++, Labview, LabWindows/CVI, and Measurement Studio for Visual C++. In this thesis, Visual Basic 6.0 was used to control and interface with the NI Software Functionality As discussed in section (6.1), the software package is used to acquire data either from a data file or a signal source. After the data acquisition phase, the software is used to display the time-domain information of the signal and to perform spectral analysis.

3 55 The frequency-domain information of the signal is acquired from the spectral analysis. These results are then displayed in the form of magnitude and phase plots Data Acquisition The software has the ability to acquire data from the NI 5112 digitizer, retrieve previously acquired data from a binary (.bin) file, or retrieve data from a text (.txt) file. The data acquired from the digitizer is the sampled and quantized, time-domain data from the signal source on channel 0 (ch0). This data is stored in a text (.txt) file and a binary (.bin) file with the directory and filename specified by the user. The user can later view the time-domain data by opening the corresponding text (.txt) file in the specified directory. To acquire data from the NI 5112, the user must select the Acquire waveform and save to file radio button. Figure (6.1) shows the initial screenshot upon program execution. Next, a path and filename with the.bin extension used to store the acquired data must be provided by the user in the File Path field. The software will also store the acquired data in ASCII format in a text (.txt) file with the same path and filename. Before acquiring the data, the user must specify a minimum sample rate and a minimum record length in the User Input frame. The NI 5112 tries to achieve the specified sample rate and record length. If either of these conditions cannot be met, the NI 5112 will use a higher value for that certain parameter rather than a smaller value. The maximum sample rate is 100 million samples/second. The value of the Resource Name field should remain to be the default value DAQ::1 unless multiple NI 5112 digitizers are in the system. This parameter directs the software to the correct digitizer.

4 56 Figure 6.1 Screen Shot upon Program Execution As discussed in section (6.2), the input impedance of the NI 5112 is software controlled. The input impedance can be specified in the Input Impedance drop-down list. The possible values for this parameter are 50 Ω or 1 MΩ.

5 57 After the above parameters have been specified, the program is ready to acquire the data from the NI At this point the user must select Acquire Data from the Tools menu or click on the Acquire Data button on the startup screen. The timedomain data will then be displayed on the plot at the bottom of the screen. To retrieve data from a previously acquired data file, the user must select the Read waveform from.bin file and plot radio button. Next, a path and filename with the.bin extension must be specified by the user in the File Path field. After these parameters have been specified, the program is ready to read in the data. The user must now select Read Data from the tools menu or click the Read Data button on the startup screen. The time-domain data will then be retrieved from the binary file and displayed on the plot at the bottom of the screen. To retrieve data from a text file, the user must select the Read waveform from.txt file and plot radio button. Next the user should follow the same procedure for retrieving data from a binary file. The user can create text files for input into the spectral analysis software. To do this, the user must first open a new text file. The first line of the file should contain the record length. The second line of the text file should contain the sample rate. The third line and below should contain the actual time-domain data values for input into the software. After retrieving data from a binary or text file, the File Feedback frame displays the sample rate and record length of the specified file Time-Domain Plot After data acquisition/retrieval the time-domain data is plotted at the bottom of the startup screen. This is shown in figure (6.2). The horizontal axis represents time and

6 58 the vertical axis represents volts. The scales for these two axes can be adjusted by selecting values in the Plot Controls frame. If the entire waveform will not fit in the window, then a horizontal scroll bar becomes visible. This allows the user to scroll in order to view the entire waveform. Figure 6.2 Screen Shot of Time-Domain Plot

7 59 Cursors can be placed on the waveform by left-clicking and right-clicking the mouse at the desired location on the plot. The Cursor Info frame reports the data values for both the left and right cursors. This can be seen in figure (6.2). The first row of text boxes displays the sample number for the left and right cursors. The second row of text boxes displays the value in volts for both cursors. The third row of the Cursor Info frame displays the change in time between the two cursors as well as the inverse of the change in time between the two cursors. If the left and right cursors are placed one period apart, then the frequency of the waveform is displayed as the inverse of the change in time between the two cursors Additional Time-Domain Tools After the data is acquired/retrieved the user can use the tools in the Tools menu to further analyze the data. The user can find the maximum positive value in volts out of the entire data set by selecting Find Largest Value from the Tools menu. To find the largest absolute value, the user can select Find Largest abs(value) from the Tools menu. Window functions as discussed in Chapter 5 can be applied to the entire input data set by selecting Apply Window Function from the Tools menu. The user has the option of selecting one of the following window functions: rectangular, triangular, sine lobe, Hanning, Hamming, sine cubed, sine to the fourth, and Blackman. After a window function is selected, the program then displays the modified data set (the input data set multiplied by the window function). If a window function has been applied, the original data set can be reloaded by selecting Rectangular window function from the Tools/Apply Window Function menu.

8 60 The user can perform spectral analysis on the input data by selecting FFT or Selective FFT from the Tools menu. The FFT function performs a fast Fourier transform algorithm on the entire input data set. The Selective FFT function extracts the data between the two cursors and performs an FFT algorithm on the extracted data. If a window function is applied, then the FFT function performs an FFT algorithm on the modified data set. After the FFT algorithm has been performed on the input data set, a new window will open displaying the frequency content of the input data set Frequency Plots The new window contains plots of both the magnitude and phase of the FFT output coefficients. This is shown in figure (6.3). The upper graph displays the magnitude spectrum while the lower graph displays the phase spectrum. The horizontal axis represents the base-10 logarithm of the frequency. The vertical axis on the magnitude plot can either be displayed in volts or decibels (db) by selecting the Vertical axis in db check box in the Plot Controls frame. The vertical axis on the phase plot has units of degrees ranging from (-180 to +180 degrees). The scales of the plots can be adjusted using the controls within the Plot Controls frame. The Vertical axis in db check box determines the units for the magnitude plot s vertical axis. The drop-down list determines the number of volts or decibels to use per vertical division. The Expand X-axis arrow keys are used to zoom in or out on the magnitude and phase plots x-axes. The Scroll X-axis horizontal scroll bar is used to shift the magnitude and phase spectrum to the left or right in order to view information that is not currently displayed on the plots. The Vertical Shift arrow keys are used to shift the magnitude spectrum up or down along the vertical axis.

9 61 Figure 6.3 Screen Shot of Frequency Plots Left-clicking the mouse button on the magnitude or phase plots will place a cursor at the nearest point. The cursor will appear on both plots corresponding to the same output coefficient. The cursor information is displayed in the Cursor Information frame. The magnitude in volts or decibels is displayed as well as the phase, the sample number and the frequency. If the x-axis is zoomed out, it can be hard to select the exact data point that is desired since the data points will be concentrated in a small region. The Left and Right buttons within the Cursor Information frame are used to increase or decrease the cursor s sample number in order to achieve the exact data point that is desired by the user.

10 62 The DC component magnitude can be displayed by selecting Find DC Component from the Tools menu Help Brief help topics for using the software are available under the Help menu on the startup window during runtime.