Abstract:
A time domain measurement instrument and method are provided. The instrument comprises an acquisition circuit for acquiring a signal in a time domain and a processor for performing a Fast Fourier Transform (FFT) processing on the acquired signal to generate an FFT of the acquired signal. A frequency domain analysis tool is also provided for analyzing and manipulating the FFT, wherein the frequency domain analysis tool instructs the processor to automatically determine the frequency of a peak amplitude of the FFT, and to determine the amplitude at the determined peak frequency.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates generally to time domain measurement instruments, such as an oscilloscope, and more particularly to such an instrument that employs a frequency domain analysis feature.  
         [0002]     Spectrum analyzers are well known devices that receive an input signal, and sample and display the signal in the frequency domain. While dedicated Spectrum Analyzers have been available from any number of sources, they can often be expensive, and require a user to buy such a unit in addition to other test equipment. However, with these drawbacks, Spectrum Analyzers provide a great level of control of viewing the frequency domain signal.  
         [0003]     Oscilloscopes and other time domain analysis measurement instruments are used generally to view an input signal in the time domain. Various functions and measurements may be performed on the time domain displayed signal. Additionally, a number of oscilloscopes allow for the viewing of a signal as if it were on a Spectrum Analyzer that is in the frequency domain. Rather than sampling the signal in the frequency domain as dedicated Spectrum Analyzers might do, such oscilloscopes typically perform a Fast Fourier Transform (FFT) on the time domain data to generate the frequency domain information. While the resulting signal can be displayed as a frequency domain signal, more advanced Spectrum Analyzer features have traditionally been lacking in such a setup, primarily because the oscilloscope has not been optimized to perform the Spectrum Analyzer functions.  
         [0004]     An example of such an oscilloscope is described in U.S. Pat. No. 6,681,191 issued to Pickerd et al. While a frequency domain analysis system is described as being incorporated into a time domain measurement instrument, the features described that are associated with the frequency domain analysis system are primarily designed to provide integrated time base and frequency domain controls. For example, a user is able to control center frequency, frequency span, and resolution bandwidth by selecting an appropriate setting for each feature, using a front panel dial. Thus, the user is limited to manual control of the basic parameters of the frequency domain analysis system. However, such manual control, relying on the sight of a user, is often inaccurate.  
         [0005]     Therefore, it would be beneficial to provide a control setup for a frequency domain analysis system implemented on a time domain measurement instrument that allowed for more precise measurement and automatic functioning.  
       SUMMARY OF THE INVENTION  
       [0006]     Therefore, in accordance with the invention, a more robust set of Spectrum Analyzer functions is provided in accordance with a frequency domain analysis system employed in accordance with a time domain measurement instrument. These features include a peak detect function that automatically determines the largest frequency peak of the FFT, and also includes functionality to make this automatically defined frequency the center frequency on the display. This automatically defined frequency can also be selectively placed at any location on the spectrum display. The defined frequency can also be defined by a user placing a cursor on a selected frequency of the FFT, and this frequency can be designated the center frequency, or can be placed at any other position along the spectrum. The response output can be rescaled both horizontally and vertically. Noise can be reduced and/or resolution increased in the acquisition and display.  
         [0007]     The invention therefore allows for an enhanced set of analysis tools to be applied to a frequency domain analysis system residing in a time domain measurement instrument.  
         [0008]     Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification and the drawings.  
         [0009]     The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others, and the apparatus embodying features of construction, combination(s) of elements and arrangement of parts that are adapted to affect such steps, all as exemplified in the following detailed disclosure, and the scope of the invention will be indicated in the claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     For a more complete understanding of the invention, reference is made to the following description and accompanying drawings, in which:  
         [0011]      FIG. 1  is a screen shot depicting an oscilloscope screen before implementation of the invention;  
         [0012]      FIG. 2  is a screen shot depicting selection of a peak search feature of the invention;  
         [0013]      FIG. 3  is a screen shot depicting a marker to center frequency feature of the invention;  
         [0014]      FIG. 4  is a screen shot depicting repositioning of a cursor while the center frequency remains the same;  
         [0015]      FIG. 5  is a screen shot depicting the marker to center frequency of the invention utilizing the marked frequency of  FIG. 4 ;  
         [0016]      FIG. 6  is a screen shot depicting implementation of an up/down feature of the invention (in this example, horizontal center frequency positioning);  
         [0017]      FIG. 7  is a screen shot depicting a horizontal resealing feature of the invention;  
         [0018]      FIG. 8  is a screen shot depicting a vertical rescaling feature of the invention;  
         [0019]      FIG. 9  is a screen shot depicting a resolution bandwidth feature of the invention used to increase frequency resolution.  
         [0020]      FIG. 10  is a screen shot depicting waveform averaging to reduce noise in the frequency spectrum, in accordance with the invention; and 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]     The invention will now be described, making reference to the accompanying drawings.  
         [0022]     Referring first to  FIG. 1 , a display  100  is shown presenting a frequency domain signal  110  in a rendering portion  115  of display  100 . Display  100  further comprises a main portion  120  which includes various information and settings regarding the state of the oscilloscope, and the displayed signal. A menu  130  presents various settings that are employed by a user to implement the various features of the current invention. As shown at  130 , no parameters are included as the features of the invention have not yet been activated.  
         [0023]      FIG. 2  depicts display  100  after a peak search function has been employed. In this particular embodiment peak search designation  132  comprises a touch screen, so that the user can implement the feature by touching the screen at the location corresponding to the peak search designation  132 . Of course any other type of selection device, such as a mouse pointer or the like may be employed for not only this selection, but for all other selections in accordance with the invention. Upon implementation of the peak search function, a peak search algorithm is employed to determine the largest frequency peak in signal  110 . The algorithm loops through the entire array of waveform values and selects the largest vertical value in the waveform. A cursor  140  and peak label  141  are then automatically placed corresponding to this determined peak  111 , and the frequency and amplitude of the peak are measured when the peak is determined. After determination of these frequency and amplitude values, they are displayed inside the graticule at  141  along with cursor  140 , and also at location  136  and  135 , respectively, in menu portion  120 . An indication that the marker has been placed at the detected largest frequency peak is provided at  137 .  
         [0024]     In  FIG. 3 , the user then selects a Marker to Center Frequency button  133 . This feature of the invention repositions the frequency which is currently marked by the cursor, the detected largest frequency peak  111 , to be the new center frequency on the frequency display. Thus, the user can be assured that the actual largest frequency peak is positioned at the center of the display. Because no user interpretation is required to determine the frequency value of the largest frequency peak, this frequency value is reliably repositioned to the center of the frequency display, and the frequency value corresponding to the location of the cursor is displayed at  142 .  
         [0025]     Referring next to  FIG. 4 , a user is able to reposition cursor  140  at any desirable frequency location  112  along signal  110 . When such a repositioning is performed, the new frequency location corresponding to the cursor is noted at  142 . The values  141  of the determined peak frequency  111  remain displayed on display  100 , and are still noted in locations  135  and  136 . Thus, while the user has moved the cursor to designate another portion of signal  111 , the information regarding the determined largest frequency peak is retained. A user may then select the Marker to Center Frequency button  133  again, thus moving the cursor  140  and designated frequency location  112  to be the center frequency on display  100 . The frequency of the cursor  140  is still noted at  142 , and the information regarding the previously determined peak frequency  111  are still retained. Even after repositioning the frequency signal the determined peak frequency  111 , and values associated therewith, are still indicated.  
         [0026]     Instead of selecting a new frequency by using the cursor, in accordance with the invention, a user may also incrementally adjust the center frequency by a predetermined amount (in the depicted embodiment, looking ahead to the next  FIG. 6 , at  142  it can be determined that the center frequency has shifted 2 MHz to 87 MHz). By first selecting a Center button  151  (indicating that the center frequency is to be acted upon) and then selecting up or down buttons  155  or  156 , the signal  110  can be shifted to the right or the left, as desired. This may be desirable when a user has designated a portion of signal  110  to be at the center of display  110 , but the user was slightly inaccurate. This feature allows for incremental adjustments of the positioning of the signal so that the precise portion desired by a user may be located at the center of display  100 . During these adjustments, the information relating to the previously determined peak frequency is retained.  
         [0027]     In  FIG. 7 , a user is able to select Span button  152 , and by using up and down buttons  155  and  156  is able to incrementally rescale the width  101  of display  100 . The use of the up and down buttons changes the MHz per division shown on the display (in the example embodiment from the 2.00 MHz per division of  FIG. 6  to the 4.00 MHz per division of  FIG. 7 ), thus allowing for more or less of the signal to be shown on display  100 , while still maintaining the previously determined peak frequency information. If such rescaling causes any indicated frequency data to scroll off of the display, any tag previously shown in the display will still be displayed, the lead line from the lead line from the tag to point off of the screen. Similarly, in  FIG. 8 , the selection of a Marker to Reference Level button  134  rescales the height  102  of display  100  so that the use of the graticule is maximized, and the signal is displayed covering as much of display  100  as is practicable. Reselection of the button returns the display to its original view.  
         [0028]      FIG. 9  depicts a user selection of a Resolution Bandwidth button, and through the use of the up and down buttons  155 ,  156 , allows the user to incrementally change the size of the acquisition memory used to accumulate a next acquired waveform. Thus the user can increase or decrease the frequency resolution of the display of the next-acquired signal, resulting in a displayed signal with more or less calculated values per unit frequency  114 . Thus, when a bandwidth increase or decrease is requested, a next, new input waveform data is acquired by the oscilloscope to generate the resultant FFT data.  
         [0029]     Referring next to  FIG. 10 , a user may select a video bandwidth button  160 , and then using up and down buttons  155 ,  156  allows the user to incrementally designate the amount of averaging of waveforms should be performed in order to reduce noise in the displayed frequency spectrum, thus generating a smoother-looking displayed signal  113 . While in  FIG. 10 a  different peak value is shown than that in the earlier figures, in a preferred embodiment various scaling and other manipulation of the displayed data has no bearing on the determined and labeled values, such as the detected peak frequency.  
         [0030]     Therefore, in accordance with the invention, a user is provided with a wide array of FFT signal manipulation features that are not found on other frequency domain packages implemented on time domain analysis instruments. These features allow for a user to more precisely identify and manipulate particular portions of the FFT signal more easily than could previously be achieved.  
         [0031]     It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, because certain changes may be made in carrying out the above method and in the construction(s) set forth without departing from the spirit and scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.  
         [0032]     It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.