Abstract:
A method for simulating an analog display in a digital display spectrum analyzer (or other digital display test instrument) comprises the steps of defining a plurality, N, of traces of the signal to be displayed, assigning coordinate values to points along each trace, and plotting and simultaneously displaying each of the N traces by illuminating corresponding pixel locations on the CRT as indicated by the assigned coordinate values. The method permits effective simulation of trace persistence as commonly found in analog display test instruments. According to one embodiment of the invention, a color graphics controller/color CRT may be provided, in which case gray scales may be generated and displayed to simulate variations in intensity levels as commonly found in traces plotted on analog display test instruments. An apparatus for practicing the method is also disclosed.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to electronic test instruments that display characteristics of AC signals. More particularly, this invention relates to a method and apparatus for simulating an analog display on a digital display spectrum analyzer. 
     BACKGROUND OF THE INVENTION 
     Digital display spectrum analyzers are well known in the art. One example of a digital display spectrum analyzer is the model 71000A modular spectrum analyzer manufactured by Hewlett-Packard Company (&#34;HP&#34;), Palo Alto, California. Analog display spectrum analyzers are also well known in the art. The model 8558 and 8559 spectrum analyzers manufactured by HP are exemplary. 
     Analog and digital display spectrum analyzers each have their own advantages and disadvantages, but digital display spectrum analyzers have gained greater popularity as a result of their greater bandwidth and flexibility as well as their display stability and consistency. Nonetheless, analog display spectrum analyzers have several desirable features which heretofore have not been realized in digital display spectrum analyzers due to inherent limitations of the latter. 
     For example, analog display spectrum analyzers exhibit a characteristic known as &#34;overwriting&#34; wherein the CRT beam can retrace part or all of a prior trace or traces to provide highly varying intensities along the X-Y axes. Both the relative intensity level of any portion of a trace, as well as the intensity variation along a trace, can be significant and may convey important information, for example, relative amount of Z-axis modulation of the displayed signal. Known digital display spectrum analyzers are generally incapable of displaying a waveform in this manner. 
     The problem is best exemplified by reference to FIGS. 1 and 2. FIG. 1 illustrates traces of a signal waveform plotted on a conventional analog display spectrum analyzer such as the HP model 8558 or 8559. (As used herein, the term &#34;signal waveform&#34; is not limited to a time vs. amplitude characterization of the signal, but as used in connection with spectrum analyzers also includes a frequency vs. amplitude characterization of the signal.) A peak 100, as well as subsidiary peaks 110 which may represent, for example, noise peaks or harmonics, are clearly visible. Noise 120 under the peaks is also clearly visible. Of particular interest, however, is the varying intensity of the peaks 100, 110 which would be clearly visible on an actual display of an analog display spectrum analyzer. (Actual intensity variations cannot be effectively illustrated herein due to limitations in reproducing an actual trace with solely black ink on a white medium.) Noise 120 along the X-Y axes is also of interest. As mentioned, the relative intensity level, as well as the intensity variations of both the peaks 100, 110 and the noise 120 may convey important information. 
     FIG. 2 illustrates a trace of the same signal waveform plotted on a conventional digital display spectrum analyzer such as the HP 71000A. As can be seen, not only are the subsidiary peaks 110&#39; and noise not readily ascertainable, but in an actual display the intensity level of the trace would also be relatively constant. Thus, the information normally conveyed by relative intensity levels across the X-Y axes in an analog display spectrum analyzer is lost. 
     FIGS. 1 and 2 also illustrate another important advantage of analog display spectrum analyzers and corresponding disadvantage of digital display spectrum analyzers. Referring to FIG. 1, an underpeak 140, as well as &#34;ripples&#34; 160 along the base line of the signal waveform are clearly visible in the analog display. However, referring to FIG. 2, the corresponding underpeak 140&#39;, as well as the corresponding &#34;ripples&#34; 160&#39; are not readily ascertainable from the digital display, and indeed might easily be overlooked without first having had the benefit of seeing the display of FIG. 1. 
     Still further, analog display spectrum analyzers provide trace &#34;persistence&#34; since trace data on the CRT is not immediately lost from trace to trace. Thus, multiple traces may be simultaneously plotted and displayed even though the input signal may have been digitally processed by the spectrum analyzer prior to display. Known conventional digital display analyzers generally do not plot multiple traces of a given input signal, and hence do not provide trace persistence. 
     The following examples illustrate the importance of the information that is &#34;lost&#34;, or otherwise not readily ascertainable, in a conventional digital display spectrum analyzer. In a standard NTSC television signal, there is occasionally interference in the frequency area between the video carrier and the color carrier. If this interference is not frequency stable, or is weak, it may be difficult to detect on a conventional digital display spectrum analyzer due to the lack of intensity variation and persistence. Similarly, so-called &#34;black-burst&#34; television signals may exist when a first weak signal exists below a second stronger signal in the frequency spectrum. The first (weaker) signal exhibits an &#34;underpeak&#34; 140 readily ascertainable in an analog display spectrum analyzer (FIG. 1); however the underpeak is not readily ascertainable in a conventional digital display spectrum analyzer (FIG. 2). The so-called &#34;Hannover Blind&#34; effect often seen on a television screen (as the result of displaying a striped or checkered pattern) can be indicated by &#34;ripples&#34; readily ascertainable in an analog display spectrum analyzer. For example, &#34;ripples&#34; 160 (albeit not due to the Hannover Blind effect) are clearly ascertainable in FIG. 1, but not in FIG. 2. 
     Pulsed signals, such as those found in radar or FM applications, are not clearly ascertainable on a conventional digital display spectrum analyzer when one of the signals is stronger than another, due to the inability of a conventional digital display spectrum analyzer to plot the varying intensities of the trace of the signal waveform. Both signals would be readily visible and ascertainable on an analog display spectrum analyzer. Another example is the examination of television gray scales. It is possible to determine the amplitude of gray levels with an analog display spectrum analyzer because it is capable of displaying varying intensities, but not on a conventional digital display spectrum analyzer. Other examples where an analog display spectrum analyzer provides superior visual signal information include gated applications such as wideband local area networks, intermod distortion of television signals, differentiation of carrier feed-through power for radar signals, and situations in which it is necessary to differentiate between a plurality of signals that are superposed. 
     Notwithstanding the foregoing limitations, digital display spectrum analyzers offer important advantages over analog display spectrum analyzers, and further, digital display spectrum analyzers readily interface with other types of digital test equipment. It is therefore desirable to provide a digital display spectrum analyzer that retains all of the benefits of conventional digital display spectrum analyzers, but also provides the advantages of analog display spectrum analyzers discussed above. The present invention achieves this goal. 
     SUMMARY OF THE INVENTION 
     A method of simulating an analog display on a digital display spectrum analyzer (or other electronic test instrument having a digital display circuit including a CRT for displaying signal waveforms representing the characteristics of an AC signal) comprises the following steps: defining a plurality, N, of traces of the signal to be displayed; assigning coordinate values to points along each trace, each coordinate value corresponding to a pixel location of the CRT; and, plotting and simultaneously displaying each of the N traces by illuminating the pixel locations indicated by the assigned coordinate values. The plot is continuously updated by sequentially receiving traces of the signal and eliminating the oldest one of the N sequentially received traces when a subsequent trace of the signal is received. The subsequent trace is included as the newest one of the N traces, and the previous N-1 traces continue to be displayed while the display is updated with the newest one of the N traces. 
     According to one embodiment of the invention, the spectrum analyzer is operated in a &#34;dots&#34; mode wherein the illuminated pixel locations (&#34;dots&#34;) are unconnected and the traces appear as a plurality of unconnected dots on the CRT. According to another embodiment of the invention, the spectrum analyzer is operated in a &#34;lines&#34; mode wherein the illuminated pixels (&#34;dots&#34;) are connected and the traces appear as a plurality of lines on the CRT. 
     In either embodiment, a color graphics controller/color CRT may be substituted for the conventional monochrome graphics controller/monochrome CRT commonly found in digital display spectrum analyzers. In this embodiment, intensity variations may be displayed according to the following method: determining, for each coordinate value assigned, the number of traces having a corresponding coordinate value, the number of traces determined to have a corresponding coordinate value being a count of the number of times that the corresponding pixel location will be overlapped by the N traces; assigning a gray scale level to each coordinate value based upon the count determined in the immediately preceding step; and, illuminating the pixel locations at an intensity level determined by the gray scale levels assigned in the immediately preceding step. 
     An apparatus for practicing the method of the present invention is also disclosed. According to the invention, the apparatus comprises a spectrum analyzer (or other test instrument) equipped with a color graphics controller/color CRT and a digital display circuit, and means for (i) plotting and simultaneously displaying a plurality, N, of traces of a signal waveform to be displayed; (ii) determining and assigning gray level intensities to pixel locations of the CRT based upon the number of the N traces passing through each pixel location; and, (iii) illuminating each pixel location at its assigned gray level intensity. The apparatus thus permits different pixels of the CRT to be illuminated at different intensity levels to reflect a characteristic (such as Z-axis modulation) of the signal, and further permits trace persistence to be simulated as a consequence of simultaneously displaying a plurality of traces of the signal. Thus, the apparatus effectively permits simulation of an analog display in a digital display spectrum analyzer. In a preferred practice of the invention, the spectrum analyzer comprises a local oscillator module including a controller and ROM for controlling the digital display, and the aforementioned means comprises firmware stored in the ROM. The spectrum analyzer may comprise a Hewlett-Packard Company model 71000A digital display spectrum analyzer, and the local oscillator module may comprise a Hewlett-Packard Company model 70900A local oscillator module. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration of a signal waveform displayed on a prior art analog display spectrum analyzer. 
     FIG. 2 is an illustration of the same waveform displayed on a prior art digital display spectrum analyzer. 
     FIGS. 3A-3D conceptually illustrate the manner in which a trace is plotted and displayed in a prior art digital display spectrum analyzer. 
     FIG. 4 is a block diagram of a spectrum analyzer according to the present invention. 
     FIG. 5 is a block diagram of a local oscillator module for use in the spectrum analyzer of FIG. 4 according to the present invention. 
     FIG. 6 conceptually illustrates a set of indices into a color map table as employed according to the practice of the present invention. 
     FIG. 7 is a flow chart illustrating the basic steps performed in accordance with the practice of the present invention. 
     FIG. 8 illustrates a signal waveform as displayed on a digital display spectrum analyzer in accordance with one embodiment (&#34;dots&#34; mode) of the invention. 
     FIG. 9 illustrates the same signal waveform as displayed on a digital display spectrum analyzer in accordance with another embodiment (&#34;lines&#34; mode) of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will best be understood by first considering the manner in which a conventional digital display spectrum analyzer plots and displays a trace of a signal waveform. FIGS. 3A-3D are provided for this purpose. 
     The digital display portion of a digital display spectrum analyzer is a bit-mapped device wherein each element (bit) in an array corresponds to a pixel in the display. In the prior art, a selected number of data points, e.g., 800, is employed to construct a trace. The trace is displayed by illuminating the pixels corresponding to the data points (&#34;dots&#34;) and also illuminating the pixels falling along lines that connect the &#34;dots&#34;. For purposes of simplicity and explanation only, a hypothetical display may be considered as a 2×2 array as illustrated in FIG. 3A. Each element a-d represents a pixel along the X-Y axes of the display. The following X-Y coordinate values are assigned to each element of the array: a=(0,0); b=(1,0); c=(0,1); and d=(1,1). Each pixel represented by the elements a-d may assume one of two states, either on or off, signified respectively by &#34;1&#34; (on) or &#34;0&#34; (off). Before any traces are plotted on the display, each pixel a, b, c and d is off, and thus each element in the array has a &#34;0&#34; entered into a corresponding location, i.e., in a display buffer. 
     Consider three exemplary traces having the following coordinate values: 
     trace 1=(0,0), (1,1); 
     trace 2=(0,1), (1,1); and 
     trace 3=(0,0), (1,1). 
     FIG. 3B illustrates trace 1. Thus, trace 1 is plotted by placing a &#34;1&#34; in the (0,0) pixel location and a &#34;1&#34; in the (1,1) pixel location. The c and b pixel locations remain at 0, signifying that these pixels remain off for this trace. In the prior art, pixels falling along the line that connects elements a and d would also be illuminated. FIGS. 3C and 3D illustrate traces 2 and 3 plotted in similar manner. FIGS. 3B-3D demonstrate a significant drawback of conventional digital display spectrum analyzers, i.e., the prior trace is erased each time a new trace is plotted and displayed. Thus, these figures demonstrate that trace 1 is lost when trace 2 is plotted and displayed, and trace 2 is lost when trace 3 is plotted and displayed, since only the last plotted trace can be displayed on the CRT at any time. The prior trace is erased in the prior art because overwriting a pixel with a series of &#34;1&#34;&#39;s and/or &#34;0&#34;&#39;s in a conventional digital display spectrum analyzer has no effect other than turning the pixel on and off. Since prior traces are erased, spurious signals and random noise, and also non periodic and unstable signals, are not displayed on the CRT with regularity. As mentioned, this occurs since conventional digital display spectrum analyzers do not provide multiple traces (i.e., persistence) as found in analog display spectrum analyzers. 
     According to the invention, the foregoing problem is overcome by providing a digital display spectrum analyzer with means for plotting and displaying multiple traces on a CRT. In one embodiment, the multiple traces are displayed in a &#34;dots&#34; mode wherein the pixels corresponding to the data points for each of the multiple traces (&#34;dots&#34;) are illuminated, but intervening pixels falling along the lines connecting the &#34;dots&#34; of each trace are not illuminated. In other words, the &#34;dots&#34; are unconnected. In another embodiment, the multiple traces are displayed in a &#34;lines&#34; mode wherein the intervening pixels are illuminated. In other words, the &#34;dots&#34; are connected. In both modes, persistence is simulated, and the previously &#34;lost&#34; information is recovered. In either embodiment, a color graphics controller/color CRT may be provided (for operation of the digital display spectrum analyzer in either the &#34;dots&#34; or &#34;lines&#34;  mode) so that gray scales, and hence intensity variations of the signal waveform, may be displayed. In a preferred embodiment of the invention, the &#34;dots&#34; or &#34;lines&#34; mode traces are generated on a conventional digital display spectrum analyzer (such as the HP 71000A modular spectrum analyzer equipped with a HP70900A local oscillator module) according to firmware hereinbelow described that may reside in a controller section of the local oscillator module. If display of intensity variations is desired, a color graphics controller/color CRT may be substituted for the conventional monochrome graphics controller/monochrome CRT so that generated gray scales can be displayed as hereinbelow described. 
     Generally illustrated in FIG. 4 is a so-called &#34;swept-tuned&#34; digital display spectrum analyzer 200 according to the invention. The spectrum analyzer 200 may be a HP 71000A modular spectrum analyzer but is not limited thereto. As shown, a signal to be analyzed is input at 210 and provided to a mixer 220 which mixes the input signal with another signal from a local oscillator module 310. The local oscillator module 310 may be a HP 70900A local oscillator module as described and modified hereinbelow, but the invention is not limited to use of this particular local oscillator module. As is generally known in the art, when the mixer output signal frequency equals the intermediate frequency of an IF filter 240, the signal passes through to a peak detector 280. The output of the peak detector 280 is amplified by video amplifier 300 and then input to local oscillator module 310. The local oscillator module 310 processes the output of video amplifier 300 and feeds the processed output to a graphics controller/CRT 320 for display. Except as noted, the system and components thus far described are conventional and are present in commercially available digital display spectrum analyzers such as the HP 71000A modular spectrum analyzer. The exceptions are as follows. In one embodiment, the local oscillator module 310 is modified as described herein. In another embodiment, a color graphics controller/color CRT display is provided in substitution for the conventional monochrome graphics controller/monochrome CRT 320 generally provided therein, and the local oscillator module 310 is modified as described herein. In the latter embodiment, gray scales are generated, and intensity variations can be displayed. 
     FIG. 5 illustrates in block diagram form a HP 70900A firmware driven local oscillator module that may be employed as the module 310 when modified as described herein. As shown, module 310 comprises a controller 460, including a main processor and ROM. The ROM contains firmware that controls other modules that may be placed in the HP 71000A; the firmware also controls the front panel interface and the digital display of the spectrum analyzer. According to the invention, this firmware is modified as described hereinbelow. Module 310 also contains a video processor 470 which processes video (analog) data from the video amplifier 300. The video processor 470 bidirectionally communicates with controller 460. The module 310 further includes a YIG-tuned oscillator (YTO) 410 that is preferably swept from 3 to 6.6 GHz under control of a frequency control section 400. The frequency control section 400 is in turn responsive to commands from controller 460 and timing signals from sweep timers 480. A 300 MHz reference signal derived from a 100 MHz reference 430 is provided to an idler phase locked loop 440, and 12.5 MHz and 50 MHz reference signals similarly derived are provided to a YTO phase locked loop 420. A fractional-N frequency synthesizer 450 responsive to commands from the controller 460 controls the sweep of the YTO 410. The sweep timers 480 receive commands from the controller 460 and supply control signals to frequency control section 400 and video processor 470. The graphics controller/CRT 320 is responsive to commands from controller 460 to display waveforms thereon. 
     Firmware provided according to the present invention (for use in the controller 460 of the model 70900A local oscillator module 310) permits plotting of multiple traces to provide persistence on the display. If a color graphics controller/color CRT has been provided, the firmware may also plot traces or portions of traces in one of a plurality of gray scales on the color CRT display 320 so that they exhibit intensity variations. 
     FIG. 7 illustrates, in flowchart form, a preferred method of carrying out the instant invention. As illustrated at 500, the display of the spectrum analyzer is first initialized. Initialization clears the display of all prior traces. In the event that a color graphics controller/color CRT is provided (including well known color generating circuitry for assigning, by means of a color map table, gray scale levels to pixels of the CRT as described below), initialization also initializes the indices of the color map table to particular gray scales. 
     As illustrated at step 510, the number of traces, N, to be displayed is next obtained from the firmware. In the preferred embodiment, N equals 7. As illustrated at 520, an item number, 1 to N, is assigned to each trace for identification purposes. The first trace is assigned item number 1; the second trace is assigned item number 2; and so on with each subsequent trace being assigned the next number. Assignment of the item numbers &#34;wraps around&#34; after the Nth trace has been assigned item number N, so that when the next new trace (N+1) is received, the Nth trace receives item number 1 while the Nth plus 1 trace (i.e., the next new trace) receives item number 2, etc. Thus, it is only necessary to identify a trace by specifying its item number; its actual trace number in real time need not be considered. 
     As illustrated at 530, if a color graphics controller/color CRT has been provided, then the indices of the color map table for each of the points (i.e., pixels) of the N traces are next determined. This is performed in accordance with the principles hereinbelow described. Next, as illustrated at 540, if a color graphics controller/color CRT has been provided, then the display is placed in a graphics mode so that each of the N traces can be displayed according to the indices of the color map table. 
     According to the invention, the firmware can be placed in a &#34;dots&#34; mode wherein the points of traces plotted on the display (each corresponding to one of a plurality, e.g., 800, data points used to construct the trace) are not connected. In other words, the intervening pixels between the &#34;dots&#34; (i.e., those lying along lines connecting adjacent dots of a trace) are not illuminated. Alternatively, the firmware can be placed in a &#34;lines&#34; mode wherein the points of the traces plotted on the display are connected, i.e., the &#34;dots&#34; are connected by illuminating the pixels lying along lines connecting adjacent &#34;dots&#34; of each trace. These modes will become more apparent hereinafter. The decision as to which mode has been selected is made at block 540, though not illustrated therein. 
     Next, as illustrated at 550, the N+xth trace, where x is the oldest trace and N+x is the subsequently received (i.e., newest) trace, replaces the xth trace, i.e., the newest trace replaces the oldest trace. The display is then updated with the new information. The method can be analogized to a sliding window that is moved across the actual waveform to determine which portion thereof will be plotted next. In this fashion, the N most recent traces are always displayed, and if a color graphics controller/color CRT has been provided, this is performed after the indices into the color map table are updated as illustrated at 530. Steps 530-550 are repeatedly performed for the duration of the measurement. In general, any number of traces can be employed to obtain a simulated persistence plot of the signal. As mentioned, in the preferred embodiment, seven traces are employed to display the signal. Additionally, in the preferred embodiment, each of the seven traces comprises 800 data points. 
     Any commercially available color graphics controller/color CRT, equipped with suitable color generating circuitry, including a color map table for assigning gray levels to pixels of the CRT, may be employed in the practice of the present invention if display of intensity variations along the waveform is desired. The process of assigning and displaying gray levels is as follows. A number associated with a location corresponding to a point (i.e., a pixel) in a trace is an index into the color map table. The index indicates the number of traces that will pass through each coordinate pixel. Multiple traces can then be plotted on a digital display thereby simulating the persistence characteristics of an analog display spectrum analyzer. 
     The concept is best illustrated by reference to FIG. 6. As shown in FIG. 6, a set of indices into a color map table has been constructed for the traces 1, 2 and 3 discussed previously. The color map indices provided therein are obtained by scanning the coordinates for each trace to be displayed to determine how many times each of the coordinate data points appears in all of the combined traces, i.e., to determine how many times the traces overlap or intersect at each pixel. Thus, for the three exemplary traces, a &#34;1&#34; appears in the coordinate position (0,0) two times so that the number &#34;2&#34; is placed in the (0,0) coordinate location in the color map table. Similarly, a &#34;1&#34; appears in the (0,1) coordinate position, once, while a &#34;1&#34; appears in the (1,1) coordinate position three times. Thus, the number &#34;1&#34; is placed in the (0,1) coordinate position, while the number &#34;3&#34; is placed in the (1,1) coordinate position. Since a &#34;1&#34; never appeared in the (1,0) coordinate position, a &#34;0&#34; is placed therein. In other words, no trace passed through coordinate position (1,0), and therefore this pixel should not be illuminated at all. 
     Each of the numbers entered into the color map table represents a gray level. For example, &#34;0&#34; represents &#34;off&#34;, &#34;1&#34; represents a first, dim gray level, &#34;2&#34; represents a second, brighter gray level, &#34;3&#34; represents a third, yet brighter gray level, and so on. In the preferred practice of the invention, seven gray levels are provided, so that the numbers 0-7 are mapped into the color map table as above described. In this fashion, the number of traces that pass through each coordinate pixel location directly affects the intensity (i.e., gray level) of a plot at those locations when a color graphics controller/color CRT is provided. Thus, the greater the number of traces passing through a pixel, the greater the value of the corresponding color map index, and thus the brighter the display at that point. According to this embodiment of the invention, both persistence and intensity variations of a plot can thus be displayed on a digital display spectrum analyzer. The color map table can be used in digital display spectrum analyzers which employ raster scanning displays or in equipment which employs addressable X-Y plotting displays. 
     Referring to FIG. 8, a plot of a trace obtained according to practice of one embodiment of the present invention is illustrated. The trace is for the same signal waveform illustrated in FIGS. 1 and 2 and is illustrated as would appear on a conventional monochrome digital display. In a &#34;dots&#34; mode plot of this waveform on an actual color CRT in accordance with the invention, intensity variations in the plurality of plotted &#34;dots&#34; would be readily apparent. The trace of FIG. 8 is illustrated as plotted on a conventional HP 71000A spectrum analyzer, equipped with a HP 70900A local oscillator module modified as described above. The trace was obtained by plotting the seven traces simultaneously without connecting the plotted data points, i.e., the &#34;dots&#34; are unconnected. The peak of the signal is clearly visible at 100&#34;, while the subsidiary peaks are visible at 110&#34;. The underpeak 140&#34; and the &#34;ripples&#34; 160&#34; are also visible. Importantly, the noise 120&#34;, which was not plotted in the display of FIG. 2, is also clearly visible. Thus, much of the information previously lost in conventional digital display spectrum analyzers, including persistence, can now be displayed. 
     Referring to FIG. 9, there is illustrated the same signal waveform as illustrated in FIGS. 1, 2 and 8, but the plot of FIG. 9 was generated in accordance with the &#34;lines&#34; mode embodiment of the invention, i.e., the &#34;dots&#34; are connected. This trace is illustrated as it would appear on a conventional monochrome digital display. In a &#34;lines&#34; mode plot of this waveform on an actual color CRT in accordance with the invention, intensity variations in the plurality of traces would be readily apparent. The trace of FIG. 9 is illustrated as plotted on a conventional HP 71000A digital display spectrum analyzer equipped with a HP 70900A local oscillator module modified as described above. Note that the plot of FIG. 9 closely resembles that of FIG. 1. Further note that the peak 100&#39;&#34; as well as the subsidiary peaks 110&#39;&#34; are clearly visible. More importantly, the underpeak 140&#39;&#34; and the ripples 160&#39;&#34; are also quite clearly visible. Noise 120&#39;, is also clearly visible, and the subsidiary peaks 110&#39;&#34;, the underpeak 140&#39;&#34; and the ripples 160&#39;&#34; are easily distinguished from the noise 120&#39;&#34;. 
     The traces of FIGS. 8 and 9 were reproduced from actual photographs of the waveform as displayed on a HP 71000A digital display spectrum analyzer equipped with a color graphics controller/color CRT and a 70900A local oscillator module having firmware modified as described herein. Unfortunately, however, due to limitations in illustrating these traces solely with black ink on a white medium, the gray scales, and hence intensity variations, actually produced, cannot be effectively illustrated. Thus, FIGS. 8 and 9 more accurately depict the traces as they would appear on a monochrome CRT. 
     Appended hereto as Appendix A is a source code listing embodying the method of the present invention. This code is suitable for use with the aforementioned HP 71000A modular spectrum analyzer equipped with a HP 70900A local oscillator module. In the practice of the invention, the firmware in the ROM of the HP 70900A local oscillator module may be modified to include the software of Appendix A. In particular, the following line numbers of the appended code differ from the conventional code and thus relate to the present invention: page A1: lines 903-945; pages A2-A8: lines 47-52, 58-68, 122-400, 442-463, 481-509, and 647-648; page A 9: lines 949-951; page A10: lines 806-807, 816-818, and 829-836; pages A11-A16: lines 376-539, 832-836, and 1050-1058; page A17: lines 427-429; pages A18-A20, lines 355-373, 466-495, and 545-570. The conventional monochrome digital display of the HP 71000A modular spectrum analyzer may be substituted with a color digital display if gray scale displays as described herein are desired, or the standard monochrome digital display may be used to produce similar results, as illustrated in FIGS. 8 and 9. 
     The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims rather than to the foregoing specification as indicating the scope of the invention. ##SPC1##