Hello, welcome to Analog Arts spectrum analyzer tutorial. Please feel free to download the Demo application software from analogarts.com to help you follow this seminar. For this presentation, we use a 2 channel spectrum analyzer, one of the instruments of SL987. In this instrument, all the user controls unique to CH1 are grouped in the CH1 panel. Similarly, all the controls unique to CH2 are grouped in the CH2 panel. Besides these two panels, there are a frequency panel, a data acquisition mode panel, a center frequency panel, and a utility panel. Each individual button in these panels allows the user to perform a unique task. Together, they control the various features of the spectrum analyzer. In order to illustrate these features, we first provide a real life application. We connect CH1 to a 5V, 20 KHz AM signal with the carrier frequency of 5 MHz using a 10X scope probe. We also connect a 2V, 10 MHz square-wave to CH2 through a 50 Ohms coax cable. The spectrum analyzer originally resets to its default settings. settings might not be suitable. Depending on the application, these In order to have accurate measurements, we must first choose the proper probe setting. Since we are using a 10X probe for CH1, 10X, the default setting, is appropriate. The green buttons below "Ref Level" adjust the input range of the spectrum analyzer. The button marked with the up arrow increases the voltage range, whereas the button marked with the down arrow decreases it. With the 10 X probe setting, this range can be changed from 35dBV to -25dBV. The dotted line at the top of the screen indicates the position of the reference level. If the signal amplitude exceeds the Ref level clipping occurs, causing erroneous harmonics. The data information panel highlights this condition in red. Once, the voltage setting is adjusted for CH1 such that clipping does not occur, the peak amplitude of the signal and its frequency are displayed in their corresponding panels. The data corresponding to the position of the mouse is also displayed near its position in each channel's corresponding color. CH1 vertical resolution can also be adjusted from 1 to 20 db per division. The up arrow increases this scale and the down arrow reduces it. Applying Fast Fourier Transform causes signal leakage. Coherent sampling eliminates this leakage. When coherent sampling is not an option, the digital signal can be filtered by a windowing function. Each of the channels offers various windowing functions that can be applied by the left click of the mouse. Now, let's turn on CH2. Since we are using a coax cable to connect the signal to this channel, in order to have accurate measurements, the 1X scope probe setting must be used.
Notice that the signal range is exceeding the reference level, which causes clipping. Therefore, the reference must be changed to a more appropriate level. We also choose a windowing function for this channel. Similar to CH1, CH2 signal frequency and amplitude are displayed in the panel. Each channel also provides the user with a set of markers. These markers can be used to analyze the spectrum. The markers are turned on and off by clicking on them. To move a marker, left click the mouse near it and while holding it down move the marker to the intended position and then release the mouse. The voltage difference between the horizontal markers are updated and displayed at the screen's top left (for CH1) and top right (for CH2) in their corresponding colors. The data acquisition panel features "Sampling", "History", "Average", and "Peak Hold" modes. The "Sampling" mode, the mode we have been using up to now, performs the FFT process on some portion of the data in the buffer memory and plots its amplitude spectrum on the screen. The information between the selection points is lost. Also, each time the screen is updated the previous data is erased from the screen. This mode offers a uniform sampled data suitable for frequency measurements. In the "History" mode however, the screen retains the data. The number of retained data can be changed by entering the desired value in the sample textbox. This number can range from 1 to 256. To maintain all the previous data simply input an "i" in the sample text box. Notice the older data are displayed with a lesser intensity. "History" tracks the signal changes and displays a collective set of data. It is the method of choice for observing signal variations over time. In the "Average" mode, the displayed signal is the average of a number of consecutively acquired data points. The number entered in the sample text box determines how many sets of data are averaged. This number can range from 1 to 256. The "Average" mode is best suitable for repetitive signals. For these types of signals it removes the uncorrelated noise and makes the signal stand out. The "Peak Hold" mode finds the highest value for each bin in the acquired data and displays it on the screen until a higher value is found. This mode is a suitable for identifying random events and analysis of fast changing signals. To have a better understanding of the different types of acquisition modes, let's review each mode when the signal input to CH1 is replaced with white noise. In the "Sample" mode, the plot correlates with the spectrum of a white noise signal, as expected. Changing the acquisition type to the "History" mode makes the screen maintain the previous data.
The "Average" mode cleans the spectrum from its spurious contents. Since the input signal is noise, increasing the number of averages reduces the variations of the displayed signal. An averaging number of 100 has a dramatic effect on the signal. In the "Peak Hold" mode a line corresponding to the maximum values of the signal appears on the screen. Higher number of samples specified in the text box makes this line more defined. Each mode offers its own unique advantages and is suitable for certain applications. We now change the signal on CH1 back to the AM signal. Resolution bandwidth "RBW" of a spectrum analyzer is the frequency delta between two adjacent FFT bins. Higher number of FFT points result in a finer resolution. In this spectrum analyzer, the number of FFT points varies from 1K to 512K. In the frequency panel, the green button with a left pointing arrow makes the FFT resolution finer and the button with the right pointing arrow makes it courser. For a screen bandwidth of 51.2 MHz, the spectrum analyzer bandwidth resolution can range from about 195Hz to 100 KHz. Having a small resolution bandwidth enables the user to zoom on a finer portion of a spectrum. The frequency panel also features the "Zoom In" and the "Zoom Out" buttons. The zoom-in feature allows the user to select a portion of the spectrum and zoom on it. To illustrate this, let's select a segment of the signal by the frequency markers and click on the "Zoom In" button. Notice that the screen now displays the selected portion. Zooming would make the spectrum of the AM signal on CH1 more defined. The "Zoom Out" button shows a larger portion of the spectrum. Each time a zooming function is applied, the start, the stop, and the frequency scale of the screen is updated in their corresponding locations. There is also a set of frequency markers. They are displayed by switching on the button "Marker" in the frequency panel. They enable the user to make frequency measurements. The frequency data is displayed at the left bottom corner of the screen. The center frequency panel offers another method to zoom on the spectrum plot. The value in the text box labeled "CF" corresponds to the center frequency of the screen, and the number in the "Span" text box determines the frequency width of the screen. Therefore, by specifying the appropriate values for these text boxes, the user can zoom on his/her desired section of the spectrum. The utility panel on the top of the screen allows the user to perform a number of tasks. Button "Reset" in this panel brings the spectrum analyzer to its default condition. The "Auto Set" button automatically finds an appropriate reference level, center frequency, and span for the present conditions. The "Pause" button freezes the screen and holds the data as long as the spectrum analyzer is in this mode. Clicking the button again, which is marked "Run" now switches back the spectrum analyzer to its normal mode of operation.
The spectrum analyzer settings can be saved in a text file. To do this, simply click on the "Save-Settings" button. The "Recall Settings" button allows the user to load any desired settings. In addition to the settings, the user can save the spectrum analyzer plot and recall it at anytime. The plot can be saved in a variety of formats. The "Save Ref" button enables the user to save a signal plot as a reference for a later use. To load the reference signal, click on the "Recall Ref" button. Notice the reference signal is plotted in white color. Those applications in which the spectrum of a signal is tested against a reference can benefit from this feature. To remove the reference signal from the screen, click on the "Remove Ref" button. The "Calibrate" button allows the user to start the self calibration process of the spectrum analyzer. This process usually takes about 10 seconds to complete. The "Display" button opens a menu with a set of features, which enable the user to easily configure the display to his or her likings. They help the user to personalize the color of each channel, the color of the screen, the order by which the channels are plotted, and customize the screen grid. The vertical unit of the spectrum analyzer can also be changed from this menu. Clicking the "Print" button sends the spectrum analyzer plot to a printer selected by the user. Finally, the "Help" button guides the user to an online Analog Arts information site that hosts a collection of user manuals, specifications, and useful application documentations and videos. We hope you have enjoyed this presentation. For any additional information please send an email to info@analogarts.com.