Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Cathode ray
Reexamination Certificate
1998-06-25
2001-06-19
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Cathode ray
C324S088000
Reexamination Certificate
active
06249115
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a method of controlling brightness and contrast in a raster scan digital oscilloscope.
An oscilloscope presents the activity of an electrical signal to its user. In the traditional analog oscilloscope, the waveform display is generated by an electron beam incident on a phosphor coating on the face plate of a cathode-ray tube. The point of incidence of the electron beam on the face plate sweeps horizontally (X direction) at a uniform speed across the face plate and is deflected vertically (Y direction) as a function of the magnitude of the signal being observed.
Two variables that influence the appearance of a waveform display are the brightness of the display (the overall light intensity emitted by the display) and the contrast of the display (the ratio of the light intensity emitted by the most brightly illuminated elements of the display to the light intensity emitted by the dimmest illuminated elements of the display).
The user interface of the traditional analog oscilloscope has a single knob for controlling the brightness with which a waveform is displayed. This knob, which is typically labelled as an intensity control, controls the current supplied to the electron gun of the cathode ray tube. For a given setting of the intensity control knob, the intensity with which a segment of the waveform is displayed depends on the slope of the segment and the frequency with which the event represented by the segment occurs. At a low setting, the user might not be able to see the waveform features of an infrequent event or a particularly fast edge. Accordingly, the user may increase the setting until a complete waveform can be seen, and at that point there will generally be a significant contrast between the slower edges and the faster edges, and between more frequent events and less frequent events. By increasing the intensity further, the intensity of all segments of the waveform is increased, until when maximum intensity is approached, the intensity of the more frequent events is limited by the maximum current value and the intensity of the less frequent events continues to increase and so the contrast is reduced.
The traditional analog oscilloscope is a common measurement instrument. Many technicians and engineers are familiar with the manner in which the display changes in response to turning of the intensity control knob.
In a multi-channel oscilloscope, different hues may be associated with the signal channels respectively: green may be associated with one channel and red with another channel. When the intensity control knob is turned in the counterclockwise direction, associated with decreasing intensity, the effect is to reduce the saturation with which a waveform of a given hue is displayed.
In a color temperature display, different hues may be associated with different intensity ranges: redder hues may be associated with higher intensity ranges and bluer hues with lower intensity ranges. When the intensity control knob is turned in the clockwise direction, the effect is to translate the hue with which a given intensity is associated toward the red end of the spectrum.
Referring to
FIG. 1
, a conventional raster scan digital oscilloscope includes a display panel
10
having a two-dimensional array of pixels, with each pixel location being uniquely defined by a row number and a column number. The oscilloscope also includes a raster scan memory
14
having a two-dimensional address space. The memory locations in the raster scan memory map on a one-to-one basis with the pixel locations of the display panel
10
. The state of each pixel depends on the contents of the corresponding memory location in the raster scan memory
14
.
In the case of the oscilloscope shown in
FIG. 1
, the raster scan memory stores n bits of information for each pixel, where n is an integer greater than one, which allows each pixel to have 2
n
illumination states. One of the states is off, and in the other 2n
−1
states, the pixel is illuminated at different respective intensities. Thus, for example, a 4-bit deep raster scan memory can support fifteen levels of partial to maximum illumination (gray scale levels) as well as the dark or off state.
Depending on the nature of the signal and the settings of the oscilloscope, a given column of pixels may contain one or more illuminated pixels (hereinafter referred to as dots). Each column displays a vector, defined as the segment of the column between the uppermost dot in the column and the lowermost dot in the column.
The brightness of a vector depends on the sum of the intensities of the dots in the vector. If the intensity of a dot is proportional to the value of the data word stored in the corresponding location of the raster scan memory, the brightness of the vector is proportional to the sum of the data words of the dots in the vector. The brightness of the display is the sum of the brightnesses of all the vectors, and therefore is proportional to the sum of the data words of the dots in all the vectors.
The digital oscilloscope shown in
FIG. 1
also includes an A/D converter
18
having an input terminal for acquiring an electrical signal at a test point in an electronic circuit. The A/D converter samples the signal during an acquisition interval and quantizes the samples to generate a sequence of digital data words. The data words generated by the A/D converter and having values D1−DN are stored as a linear waveform record in an acquisition memory
22
having a one-dimensional address space A1-AN.
When the acquisition is complete, the linear waveform record stored in the acquisition memory is supplied to a rasterizer
26
which generates a rasterized waveform record and stores it in a rasterizer memory
30
having a two-dimensional address space (X1−XN, Y1−YN). (The common suffix N is used for economy and is not intended to indicate that the number of elements in the set {Xi}, for example, is the same as the number of elements in the set {Ai}.) The X component of the address of a data word in the rasterized waveform record stored in the rasterizer memory
30
is derived from the address Ai of at least one word of the linear waveform record and the Y component of the address is derived from the value Di of at least one word of the linear waveform record.
Each combination of addresses (Xi, Yi) at which a data word is stored in the rasterizer memory
30
represents an event, characterized by a unique combination of time (dependent on Xi) and signal level (dependent on Yi).
The rasterized waveform record may be added to an existing display record stored in the raster scan memory
14
to control the state of the display panel
10
. Referring to
FIGS. 2A-2C
, in which the numerical values designate decimal values of data words,
FIG. 2A
represents the original display record for three adjacent columns of the display panel prior to addition of the rasterized waveform record for a new acquisition,
FIG. 2B
represents the rasterized waveform record for the corresponding interval of the new acquisition, and
FIG. 2C
represents the updated display record obtained by adding the rasterized waveform record of
FIG. 2B
to the display record of FIG.
2
A. Thus, if the same event occurs during multiple acquisitions, the value of the data word representing that event in the raster scan memory
14
increases.
As also shown in
FIG. 1
, the contents of the raster scan memory may be influenced by a decay process
34
, which reduces the value stored at each location in the raster scan memory by a selected amount per unit time, so that events that occur only infrequently will be shown with reduced intensity as compared with events that occur more frequently.
The oscilloscope includes a controller
38
, which controls operation of the other components shown in
FIG. 1
, and operator controls
42
which allow the user to adjust the settings of the oscilloscope.
In one known technique of rasterizing, referred to as the dot mode, the address (Xi, Yi) of a data word of th
Gerlach Paul M.
Yost Jeff W.
Lenihan Thomas F.
Metjahic Safet
Smith-Hill John
Sundaram T. R.
Tektronix Inc.
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