Measuring spectral features using a cursor and a marker

An improved marker and cursor system for spectral waveform measurement permits the operator to link or unlink a cursor and a marker, allowing them to be used separately or in coordination to make bandwidth measurements. A second reference marker permits measurement of the frequency and amplitude differneces between it and the marker at the same time that the marker is being used control the linked cursor. The cursor can be fixed or linked in a constant positive or negative delta-amplitude relationship to the marker. The points on the cursor where the cursor and spectral waveform intersect are intensified for identification and a readout is provided of the difference in frequency values between these points. Measurements continue even during unstable conditions of the instrument or spectral waveform and fluctuations show up as only minor movements of the intensified points along the cursor. If there is no intersection of the cursor and spectral waveform in one direction, the absolute frequency value of the other intersection is displayed instead of a delta-frequency value.

BACKGROUND OF THE INVENTION 
This invention relates to the field of spectrum analysis, and more 
particularly to the field of measurements of spectral features in a 
spectrum analyzer display using a cursor and a marker. 
Spectrum analyzers are electronic instruments that convert time domain 
information at their input into frequency domain information at their 
output. Time domain information describes how the amplitude of an 
electrical signal varies with time, while frequency domain information 
describes how the power of the signal is distributed over frequency. 
Prior art spectrum analyzers have used markers and/or cursors to assist in 
spectral measurements. Typically, one marker is placed on the peak of a 
signal and the frequency and amplitude of the signal at that point are 
displayed somewhere around the screen. Some spectrum analyzers provide two 
or more markers that can be used to measure differences between the 
amplitude and frequency at one point and the amplitude and frequency at 
another point. 
The 49X series of Spectrum Analyzers from Tektronix, Inc. has markers that 
can be linked together to perform bandwidth measurements. In one mode of 
operation, these markers automatically go to the -6 dB points on either 
side of a peak, and a readout displays the frequency difference between 
the locations of the two markers. Markers can be moved individually or as 
a linked pair automatically by NEXT PEAK .fwdarw. and .rarw. NEXT PEAK 
controls. The instrument locates the center of the next peak in the 
indicated direction and moves the single marker to the top of it or the 
pair of markers to the -6 dB points on either side of it. 
The R3261/3361 Spectrum Analyzers from Advantest also have two markers, 
which may be linked, and a horizontal cursor. A readout associated with 
the cursor indicates the amplitude level at the cursor's location and the 
difference in amplitude levels between the cursor's location and the 
location of the primary marker. The two markers are linked for bandwidth 
measurements by a "X dB DOWN" softkey. When the cursors are linked, they 
must be positioned on the peak of interest and a button pressed to 
activate a measurement. Each activation produces a single measurement. 
When one marker is used in conjunction with the cursor, the difference in 
amplitude levels between the location of the marker and the location of 
the cursor is displayed. This instrument also has automatic peak searching 
via .rarw. NEXT PEAK .fwdarw. controls, but there is no way to link the 
cursor to the markers or to make the linked markers move from peak to peak 
to perform a sequence of bandwidth measurements. 
Also, in the Advantest R3261/3361 Spectrum Analyzers, if the frequency span 
or center frequency is changed, the markers do not track the moving peak. 
There are prior art spectrum analyzers from Tektronix, e.g. the 2710 or 
the 49X analyzers, that automatically track a peak when it or the frame of 
reference moves, and perform a new calculation as soon as the real or 
apparent motion of the peak stops for a second or two. However, in all of 
these spectrum analyzers with markers that can be linked to perform 
bandwidth measurements, fluctuations in the spectrum being measured can 
cause the locations of the markers to move up and down vertically by a 
considerable amount that depends on the steepness of the peak being 
measured and the resolution of the display. 
Because prior art spectrum analyzers place markers based on analysis of 
amplitude axis information, they must have relatively stable conditions to 
locate a peak, find the X dB down points and put the markers on them, and 
then make the frequency axis measurement. It would be preferable to find 
an approach that would allow measurements to be made continuously, even 
under more dynamic conditions, such as when signals are fluctuating or 
instrument settings are being adjusted. 
What is desired is an improved marker and cursor system for spectral 
waveform measurement that permits the operator to link or unlink a cursor 
and a marker, allowing them to be used separately or in coordination, and 
that provides a second marker so that additional measurements can be 
performed when the first marker and the cursor are being used linked 
together, and that performs bandwidth measurements in a way that keeps the 
X dB down points stable in the vertical axis despite fluctuations in the 
shape of the peak being measured, and that continues measuring even under 
dynamic conditions. 
SUMMARY OF THE INVENTION 
Accordingly the present invention is an improved marker and cursor system 
for spectral waveform measurement that permits the operator to link or 
unlink a cursor and a marker, allowing them to be used separately or in 
coordination to make bandwidth measurements. A second reference marker 
permits measurement of the frequency and amplitude differences between it 
and the marker at the same time that the marker is being used to control 
the linked cursor. The cursor may be fixed or linked in a constant 
positive or negative delta-amplitude relationship to the marker. The 
points on the cursor where the cursor and spectral waveform intersect are 
intensified for identification and a readout is provided of the difference 
in frequency values between these points. Measurements continue even 
during unstable conditions of the instrument or spectral waveform and 
fluctuations show up as only minor movements of the intensified points 
along the cursor. If there is no intersection of the cursor and spectral 
waveform in one direction, the absolute frequency value of the other 
intersection is displayed instead of a delta-frequency value.

DETAILED DESCRIPTION 
Referring to FIG. 1, a sequence of spectral peaks 1-7 rise above a 
background level of noise 8. A reference marker 10 is positioned at the 
top of the first peak 1. A marker 12 is positioned at the top of the 
second peak 2. Cursor 14 is linked to the marker 12, as can be determined 
by the intensified dots 16 and 18 at the points where the cursor 
intersects the second peak 2. The bandwidth value measured by the cursor 
appears at the top right of the display. At the top left of the display, 
the frequency difference between the marker and the reference marker is 
provided. Here, these two markers 10 and 12 are shown as having two 
different shapes, but they may also have the same shape and be 
distinguished from each other by being made different colors. 
FIG. 2 is a simplified block diagram of a spectrum analyzer. An input 
signal is analyzed for its frequency domain content by the spectrum 
analyzer front end 23. The resulting analog spectral information is 
converted to corresponding digital information by analog-to-digital 
converter 24 and stored in memory 25. A control processor 26 performs 
procedures according to the present invention to make operator controlled 
measurements of the spectral information in the memory 25 and presents the 
cursors and markers of the present invention to the operator via display 
27. 
FIGS. 3A and 3B are a logic flow diagram of the procedure used to support 
the cursor and marker bandwidth measuring system of the present invention. 
This procedure assumes that the operator has already entered the mode of 
operation that links cursor 14 to the marker 12. The first step 30 
involves operator input of one type or another. The marker may be moved 
using a knob or other manual means, or may be moved automatically using a 
peak finding facility of the instrument. 
The second step 32 and third step 34 shown in FIG. 3A are optional, 
depending on whether an "absolute" or "relative" amplitude level is 
desired for the cursor position. If the operator selects "absolute", he 
must position the cursor himself, i.e., supply the amplitude level 
directly that will be the reference amplitude threshold of step four 36. 
If the operator chooses "relative", a default offset of 6 dB is supplied 
as the delta amplitude of step two 32. Means are also provided in the user 
interface for allowing the operator to change the amplitude level 
difference between the marker 12 and the cursor 14, that is, to change the 
delta amplitude of step two 32. Operation of the cursor and marker system 
is further described in the 2782 Spectrum Analyzer Operator's Manual from 
Tektronix (part number 070-6794-00) which is hereby incorporated by 
reference. 
Referring to FIG. 4, it should be noted that the relationship between the 
marker 12 and cursor 14 need not be as shown in FIG. 1, but rather they 
can be linked for "relative" operation with a negative value of delta 
amplitude 32 as shown in FIG. 4. And, in the "absolute" mode, the marker 
12 and the cursor 14 also frequently get in the relationship shown in FIG. 
4 simply because the cursor stays still while the marker is being moved. 
In either event, the brightened dots 16 and 18 appear at the nearest 
places in each direction that the spectrum crosses the cursor 14 and the 
bandwidth readout 20 displays the difference in frequency between these 
two points. 
The third step 34 shown in FIG. 3A is one of subtracting the delta 
amplitude value from the amplitude level of the location marked in step 
one 30 to obtain a reference amplitude level against which other amplitude 
levels are compared later. The fourth step 36 is to draw the cursor 14 at 
this amplitude level on the screen. This fourth step 36 of drawing the 
cursor on the screen is also optional in the following sense. The spectrum 
analyzer could be under remote control and be "faceless", that is, not 
have a local screen. The methods of this invention could then take place 
in a "virtual display" locally and the results sent to the remote 
controller over a bus. 
The next three steps 38, 40, and 42 of FIG. 3A, describe the process of 
moving point by point along the spectral waveform in one direction, while 
comparing the amplitude value at each point with the reference amplitude 
level. This process continues until the sign of the result of the 
comparison changes, step 40, or an edge of the displayed data is 
encountered, step 42. If the sign of the result of the comparison changes, 
the spectral waveform has just crossed the reference amplitude threshold. 
If that point is not exactly at the crossing, then either that point, the 
first point past the crossing, or the preceding point, the last point 
before the crossing, can be used as the location along the frequency axis 
to be identified on the cursor as the intersection between the cursor and 
the spectral waveform. The difference between two adjacent points is 
generally insignificant in a display of a thousand or more points. The 
identification of the other side point, in the loop of steps 46, 48, and 
50, is accomplished in the same manner. 
One way to identify the points 16 and 18 according to steps 44 and 52, is 
to brighten or enlarge them, as is shown in FIGS. 1 and 3A although other 
means could be used. It should be noted that the point which is 
identified, by brightening or other means, is the point on the cursor 
corresponding to the frequency of the point on the spectral waveform 
either immediately before, at, or after the calculated intersection. 
Because the point identified for the operator is always on the cursor, 
that is at the amplitude value of the cursor along the amplitude axis, 
fluctuations in the underlying spectrum cause the position of the crossing 
points to change slightly along the frequency axis. 
This behavior is in contrast to that of the spectrum analyzers of the prior 
art that use a pair of linked markers to perform bandwidth measurements. 
In those analyzers, especially with sharp spectral peaks, fluctuations in 
the underlying spectra show up as significant and sometimes disconcerting 
motions of the markers up and down on the amplitude axis. These wobble up 
and down independently as they move to the point on the spectral waveform 
that is closest to the amplitude level that they are looking for. With the 
cursor system of the present invention, the intensified points are fixed 
in amplitude so they can not move about vertically. And, because peaks 
generally have steep slopes, any movement of the identified spots 
horizontally is much less dramatic and disconcerting than the larger 
vertical motion of the prior art markers. 
If an edge of the display is encountered by step 42 before a change in the 
sign of the result of the comparison is detected by step 40, the point 
which is identified in step 44 is the point at the edge of the screen at 
the reference amplitude. 
When an edge of the display is encountered instead of a crossing, any 
resulting bandwidth calculation is meaningless. Under these circumstances, 
the most useful value that the instrument can display is the absolute 
frequency of the other crossing point, if there is one. Thus, when step 42 
exits in the "YES" direction, if step 50 always exits in the "NO" 
direction, then step 54 will exit in the "NO" direction and step 56 will 
exit in the "YES" direction, and the frequency of the other side point 
will be displayed according to step 60. Conversely, if step 42 always 
exits in the "NO" direction but step 50 exits in the "YES" direction, then 
step 54 will again exit in the "NO" direction, step 56 will exit in the 
"NO" direction and step 62 will exit in the "YES" direction, so that the 
frequency of the one point is displayed according to step 68. 
The behavior of the cursor as just described permits useful measurements to 
be made with this cursor even if there is only one intersection of the 
spectral waveform and the cursor. For instance, in characterizing a 
filter's shape, one can make a series of measurements of the general type: 
"What is the (absolute) frequency at this particular amplitude level?". 
If the search for a crossing fails to detect one in either direction, that 
is steps 42 and 50 both exit in the "YES" direction, step 54 will exit in 
the "YES" direction and a message such as "No Frequency Data" or "Not 
Active" is displayed instead of a frequency according to step 58. 
The foregoing behavior of the cursor system of the present invention can be 
used to monitor a displayed frequency span for intermittent or spurious 
spectral features that exceed a threshold. The cursor is put at the 
threshold, and because the displayed span does not normally produce a 
spectral event of the amplitude at which the cursor is placed, the two 
identified points are at the sides of the screen. As soon as an 
intermittent or spurious signal exceeds that threshold, one or both of the 
bright spots jump from the side of the display to the point where the 
intermittent or spurious spectral feature crosses the threshold, thereby 
calling the attention of the operator to the occurrence. 
The sweep speed of the spectrum analyzer front end 23 is generally slow 
compared with the refresh rate of the display 27. The calculations and 
other activities of steps 36 to 68 are all updated with every refresh of 
the display screen, but the difference frequency display 20 is only 
updated every time another sweep of the spectrum is completed. The 
difference frequency display 20 could be updated with every refresh of the 
screen instead, but that would make it too unstable and difficult to read 
if the display is changing significantly between successive sweeps. 
While a preferred embodiment of the present invention has been shown and 
described, it will be apparent to those skilled in the art that many 
changes and modifications may be made without departing from the invention 
in its broader aspects. The claims that follow are therefore intended to 
cover all such changes and modifications as fall within the true spirit 
and scope of the invention.