Abstract:
A system for displaying waveforms representing an input signal includes a display subsystem, coupled to a source of the input signal, for displaying a waveform representing the input signal in response to a display control signal. A trigger circuit is coupled to the input signal source and detects a trigger event. A time displacement circuit is coupled to the trigger circuit and generates a time displaced trigger signal a controllable amount of time after detection of a trigger event. A switch is coupled between the trigger circuit, the time displacement circuit, and the display subsystem. The switch selectively generates a display control signal in response to either a detected trigger event or a time displaced trigger signal.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to user controls for an oscilloscope, and in particular to control of horizontal display modes in an oscilloscope.  
         BACKGROUND OF THE INVENTION  
         [0002]    Earlier analog oscilloscopes included a trigger circuit which analyzed an acquired input signal for a user selected event, e.g. a negative-going or positive-going transition at a user selected voltage level, and in a specified channel if the oscilloscope was a multichannel oscilloscope. When the trigger event occurred, a waveform representing the acquired input signal occurring from that point forward in time was displayed on the display screen of the oscilloscope. The trigger point was displayed as the leftmost point in the displayed waveform. The timing of the displayed waveform was controlled by the user, and could be changed. As the timing changed, the time (horizontal) magnification of the waveform also changed. The change was made relative to the trigger point, which remained fixed at the leftmost point of the displayed waveform.  
           [0003]    [0003]FIG. 1 is a combined waveform and oscilloscope display diagram illustrating the trigger and horizontal magnification characteristics of an analog oscilloscope. In FIG. 1, a waveform  10  represents e.g. the voltage of an acquired input signal supplied to the oscilloscope via an oscilloscope probe. A user has set the trigger to the illustrated trigger point  12  in a known manner. A first display  11  is a waveform display on the display screen of the oscilloscope in which the time base is set to display the waveform  10  from the trigger paint  12  to the point  14 . A second display  13  is a waveform display on the display screen of the oscilloscope in which the trigger point is the same as that in the first display  11 , but the time base has been adjusted to display the waveform  10  from the trigger point  12  to the point  16 .  
           [0004]    It is apparent from FIG. 1 that the trigger point  12  is displayed in the waveform display, regardless of the setting of the time base, and that the trigger point  12  is always displayed as the leftmost point of the waveform display. Furthermore, it is apparent that the displayed waveform is magnified from the trigger point  12  on. That is, the trigger point is co-located with the magnification point. Consequently, the waveform display may be magnified horizontally to any desired degree without losing the display of the trigger point  12 .  
           [0005]    As digital oscilloscopes were developed, it became possible to display portions of the waveform occurring before the trigger point, as well as after. The trigger could be set by a user in the same manner as in analog oscilloscopes. However, instead of displaying the trigger point as the leftmost point in the displayed waveform. The trigger point could be displayed at any point in the displayed waveform. Also as with analog oscilloscopes, the timing of the displayed waveform was controlled by the user, and could be changed, thus changing the time (horizontal) magnification of the waveform. Again, as with analog oscilloscopes, the change was made relative to the trigger point, but in digital oscilloscopes, the trigger point was not constrained to be the leftmost point in the displayed waveform.  
           [0006]    [0006]FIG. 2 is a combined waveform and oscilloscope display diagram illustrating the trigger and horizontal magnification characteristics of a digital oscilloscope. In FIG. 2, again a waveform  10  represents e.g. the voltage of an input signal supplied to an oscilloscope via an oscilloscope probe. A user has set the trigger to the illustrated trigger point  12 ′ in a known manner. This trigger point  12 ′ is different than that set in FIG. 1, however. A first display  21  is a waveform display on the display screen of the oscilloscope in which the time base is set to display the waveform  10  from the point  22  to the point  24 , and includes the trigger point  12 ′ within the displayed waveform. A second display  23  is a waveform display on the display screen of the oscilloscope in which the trigger point  12 ′ is displayed in the same position as that in the first display  21 , but the time base has been adjusted to display the waveform  10  from the point  26  to the point  28 .  
           [0007]    It is apparent from FIG. 2 that, as in FIG. 1, the trigger point  12 ′ is displayed in the waveform display, regardless of the setting of the time base. However, in FIG. 2, the trigger point  12 ′ is displayed within the waveform display, not at the leftmost edge, as in FIG. 1. It is also apparent that, as in FIG. 1, the magnified signal in display  23  is magnified about the location of the trigger point  12 ′. Thus, the trigger point is co-located with the magnification point. That is, the trigger point remains in the same place in the waveform display, while the displayed waveform expands or contracts around that point as the time base is changed by the user. Consequently, the waveform display may still be magnified horizontally to any desired degree without losing the display of the trigger point.  
           [0008]    Other digital oscilloscopes used a different technique for triggering the waveform display, termed a viewport technique. In such a digital oscilloscope, the displayed waveform is displaced by some user controlled amount of time from the trigger point. This display is termed a viewport. The displayed waveform in the viewport does not, necessarily, contain the trigger point. The magnification of the displayed waveform takes place within the viewport and the displayed waveform expands and contracts around a magnification point, which may be any point on the display screen, such as the leftmost point of the viewport. In a preferred embodiment, however, the magnification point is the center point of the display screen.  
           [0009]    [0009]FIG. 3 is a combined waveform and oscilloscope display diagram illustrating the trigger and horizontal magnification characteristics of a digital oscilloscope. In FIG. 3, again a waveform  10  represents e.g. the voltage of an input signal supplied to an oscilloscope via an oscilloscope probe. A user has set the trigger to the illustrated trigger point  12  in a known manner. This trigger point  12  is the same that set in FIG. 1. In FIG. 3, a user has set a time displacement to point  32  of the input signal. This is a point after the trigger point, and is referred to as a positive time displacement. It is also possible for a user to specify a negative time displacement to a point before the trigger point. A first display  31  is a waveform display on the display screen of the oscilloscope in which the time base is set to display the waveform  10  starting from the time displacement point  32  to the point  34 . The center point of the display screen corresponds to point  35  of the waveform  10 . A second display  33  is a waveform display on the display screen of the oscilloscope in which the time base has been adjusted to display the waveform  10  from the point  36  to the point  38 . The center point of the display screen continues to correspond to point  35  of the waveform  10 . The point  35 , thus, is the magnification point.  
           [0010]    It is apparent from FIG. 3 that the trigger point  12  is not necessarily displayed in the waveform display, depending upon the settings of the time displacement and the time base. Also, in FIG. 3, the magnified signal in display  33  is magnified about the center point of the waveform display. Thus, in FIG. 3, the trigger point  12  is not co-located with the magnification point  35 . Instead, the displayed waveform expands or contracts around the magnification point  35 , which corresponds to the display screen center point, as the time base is changed by the user.  
           [0011]    One skilled in the art will understand that more sophisticated triggering techniques exist. For example, there may be what it termed a main trigger, which analyzes an acquired input signal for a user specified main trigger event, and a secondary, or delayed trigger. These operate in the following manner: after the main trigger detects the main trigger event, the secondary trigger analyzes the acquired input signal for a user specified secondary or delayed trigger event. This secondary trigger event, then initiates the waveform display. The oscilloscope may be controlled to display the waveform after, or surrounding the main trigger event; or after, or surrounding, the secondary or delayed trigger event.  
           [0012]    Alternatively, the oscilloscope may include what is termed an A and a B trigger. The A trigger may be set by the user in the known manner, e.g. a positive going or negative going signal through a user set voltage level on a selected channel. The B trigger may be similarly set. The combination of the A trigger and the B trigger initiates the waveform display. Either or both of the A and B triggers may also have a time displacement associated with them. Additional such triggers are also possible, e.g. C trigger, D trigger, etc. in any case, using the waveform display techniques illustrated in FIG. 1 or  2 , the displayed waveform contains the trigger event, and the magnification occurs around the display point representing the time of that event. Using the waveform display technique illustrated in FIG. 3, the trigger point is not necessarily contained in the displayed waveform, and the displayed waveform is time magnified about the magnification point, which is the center point of the display screen in a preferred embodiment.  
           [0013]    The trigger-centric techniques of FIGS. 1 and 2 are useful for a user to observe phenomena surrounding the trigger point because, regardless of the time magnification, the trigger point is never removed from the waveform display. For example, in manufacturing or production environments, in which signals containing trigger events can be reliably and accurately supplied to and detected by the oscilloscope, the trigger centric technique allows observation of a signal near such trigger points. On the other hand, the viewport technique of FIG. 3 is useful to observe phenomena located at some time distance from the trigger point. For example, a location in the middle of a pulse train, can be easily observed by adjusting the trigger to detect the start of the pulse train, and adjusting the time displacement to the location of interest in the middle of the pulse train.  
           [0014]    In existing oscilloscopes, to switch from a trigger centric waveform display to a viewport waveform display requires resetting of trigger controls on the oscilloscope, which can involve properly setting several switches and making appropriate adjustments to user controls for trigger voltage levels and time displacements for each trigger. It is desirable to provide a simple and fast way for a user to switch between the trigger centric and the viewport modes of operation so that the user can observe the acquired input waveform both in the location of the trigger to ensure it is operating reliably and accurately, and easily switch to observe the acquired input waveform in the time location of the delayed phenomenon.  
         SUMMARY OF THE INVENTION  
         [0015]    In accordance with principles of the present invention, a system for displaying waveforms representing an input signal includes a display subsystem, coupled to a source of the input signal, for displaying a waveform representing the input signal in response to a display control signal. A trigger circuit is coupled to the input signal source and detects a trigger event. A time displacement circuit is coupled to the trigger circuit and generates a time displaced trigger signal a controllable amount of time after detection of a trigger event. A switch is coupled between the trigger circuit, the time displacement circuit, and the display subsystem. The switch selectively generates a display control signal in response to either a detected trigger event or a time displaced trigger signal.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0016]    In the drawing:  
         [0017]    [0017]FIG. 1 is a combined waveform and oscilloscope display diagram illustrating the trigger and horizontal magnification characteristics of an analog oscilloscope;  
         [0018]    [0018]FIGS. 2 and 3 are combined waveform and oscilloscope display diagrams illustrating the trigger and horizontal magnification characteristics of a digital oscilloscope;  
         [0019]    [0019]FIG. 4 is a block diagram of a portion of a digital oscilloscope in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0020]    [0020]FIG. 4 is a block diagram of a portion of a digital oscilloscope in accordance with the present invention. FIG. 4 illustrates only those elements necessary to understand the design and operation of the present invention. One skilled in the art will understand that other elements are necessary in a digital oscilloscope, and will understand what those elements are, and how they are interconnected with the elements illustrated in FIG. 4.  
         [0021]    In FIG. 4, an input terminal  5  is coupled to a source (not shown) of an input signal. For example, input terminal  5  may be an oscilloscope probe. Input terminal  5  is coupled to respective input terminals of an acquisition circuit  102  and a trigger circuit  108 . An output terminal of the acquisition circuit  102  is coupled to an input terminal of an acquisition memory  104 . An output terminal of the acquisition memory  104  is coupled to an input terminal of a rasterizer  106 . An output terminal of the rasterizer  106  is coupled to an output terminal  15 . Output terminal  15  produces a signal representing a raster displaying the acquired input signal, and is coupled to utilization circuitry (not shown). The utilization circuitry receives the raster representative signal from output terminal  15 , and generates a image on a display device, such as the oscilloscope display screen, of the waveform display, as represented by the raster, all in a known manner.  
         [0022]    A first output terminal of the trigger circuit  108  is coupled to a first input terminal of a switch circuit  112 , and a second output terminal of the trigger circuit  108  is coupled to an input terminal of a time displacement circuit  110 . An output terminal of the time displacement circuit  110  is coupled to a second input terminal of the switch circuit  112 . Respective output terminals of the switch circuit  112  are coupled to corresponding control input terminals of the acquisition circuit  102 , the acquisition memory  104  and the rasterizer  106 .  
         [0023]    A user input terminal  25  is coupled to a source (not shown) of user oscilloscope control inputs. For example, the user input terminal  25  may be coupled to receive signals indicating the positions of various switches, and settings for variable controls, such as dials, which are placed on the control panel of the oscilloscope. The user input terminal  25  is coupled to an input terminal of a control circuit  114 . Respective output terminals of the control circuit  114  are coupled to corresponding control input terminals of the time displacement circuit  110 , the trigger circuit  108  and the switch circuit  112 .  
         [0024]    One skilled in the art will understand that the various user controls described above could be incorporated directly into the circuit being controlled, e.g. trigger circuit  108 , time displacement circuit  110  and/or switch circuit  112 , instead of passing through the control circuit  114 . Specifically, the switch circuit  112  could be implemented as a user controllable, physical, single pole double throw (SPDT) switch  116  having a pole (movable contact) coupled to the respective control input terminals of the acquisition circuit  102 , the acquisition memory  104  and the rasterizer  106 . A first throw (non movable contact) of the SPDT switch  116  is coupled to the output terminal of the trigger circuit  108  and a second throw of the SPDT switch  116  is coupled to the output terminal of the time displacement circuit  110 , all as illustrated in phantom in FIG. 4. In a preferred embodiment, however, this switch is an electronic switch controlled by a signal from the control circuit  114 , also as illustrated in phantom in FIG. 4.  
         [0025]    In operation, the acquisition circuit  102  operates to reproduce an exact replica of the input signal at the input terminal  5 , and then convert this signal to a series of multi-bit digital samples representing the input signal. This series of samples is stored in the acquisition memory  104 . The rasterizer  106  retrieves these samples from the acquisition memory  104  and generates a raster representative signal at output terminal  15 . This raster, when displayed by the utilization circuitry (not shown), produces an image of the waveform of the acquired input signal on the oscilloscope display screen, all in a known manner.  
         [0026]    The control circuit  114  receives user control inputs through the user input terminal  25 . Among other things, these user control inputs select the trigger event criteria and (if necessary) the location of the trigger point within the displayed waveform, or the time displacement of the waveform display from the trigger point. Control signals representing these user control inputs are supplied to the trigger circuit  108  and the time displacement circuit  110 . These control signals condition the trigger circuit  108  and time displacement circuit  110  to operate in the manner specified by the user to set the trigger event criteria, trigger point location and time displacement, all in a known manner.  
         [0027]    The display of the waveform by the acquisition circuit  102 , the acquisition memory  104  and the rasterizer  106  is controlled by display control signals from the switch circuit  112 . These display control signals control, among other things, which part of the input signal is acquired, rasterized and displayed. These control signals, in turn, are derived from either the trigger circuit  108  for trigger centric waveform displays, or from the time displacement circuit  110  for viewport waveform displays.  
         [0028]    In a first mode of operation, the trigger centric mode, the switch  116  in the switch circuit  112  is in a first position as illustrated in FIG. 4. In addition, a user controllable dial on the oscilloscope control panel is set by the user to specify the location within the displayed waveform at which the trigger point is to be displayed. In a preferred embodiment, this dial is calibrated from 0 to 100, in which 0 represents the left hand edge of the displayed waveform and 100 represents the right hand edge of the displayed waveform. In this case, the trigger circuit  108  analyzes the input signal from input terminal  5  and detects the trigger event according to the criteria specified by the user as described above. When the trigger event is detected, a trigger signal is generated by the trigger circuit  108 . The trigger signal from the trigger circuit  108  is used to generate a display control signal, which is coupled to the control input terminals of the acquisition circuit  102 , the acquisition memory  104  and he rasterizer  106  through the switch circuit  112 .  
         [0029]    In a second mode of operation, the viewport mode, the switch  116  in the switch circuit  112  is in a second position, in which the time displacement circuit  110  is coupled to the respective output terminals of the switch circuit  112 , opposite to that illustrated in FIG. 4. The same user controllable dial on the oscilloscope control panel used in the first, trigger centric, mode of operation to set the trigger point location in the displayed waveform, is set by the user to specify the positive or negative time displacement from the trigger point to the displacement point. In this case, the trigger circuit  108  analyzes the input signal from the input terminal  5  and detects the trigger event according to the criteria specified by the user, as described above. This trigger signal from the trigger circuit  108  is supplied to the time displacement circuit  110 . The time displacement circuit  110  provides a time displaced trigger signal, displaced by a positive (delay) or negative (advanced) time period as specified by the user. The time displaced trigger signal from the time displacement circuit  110  is used to generate a display control signal, which is coupled to the acquisition circuit  102 , the acquisition memory  104  and the rasterizer  106 . Thus, a single switch, used in conjunction with a single user controlled dial, can allow a user to easily switch between the trigger centric mode and the viewport mode.  
         [0030]    One skilled in the art will understand that several display control signals may simultaneously need to be switched between the trigger circuit  108  and the time displacement circuit  110 . Several electronically controlled switches, such as switch  116  illustrated in FIG. 4, one for each such signal, may be included in the switch circuit  112 , all controlled by the same control signal from the control circuit  114 . In this case, a single switch on the oscilloscope control panel is coupled to the control circuit  114  through user input terminal  25 . The control circuit  114  generates a trigger centric/viewport (T/V) control signal. The T/V control signal simultaneously controls all of the electronically controlled switches in the switch circuit  112 .  
         [0031]    An oscilloscope as illustrated in FIG. 4 can provide simple switching between the trigger centric waveform viewing mode, in which the waveform in the vicinity of the trigger point can be observed, and the viewport waveform viewing mode, in which the waveform at some time displacement from the trigger point can be observed. This switching between these operating modes can be provided through the operation of a single switch on the oscilloscope control panel. In addition, a single user controlled dial on the oscilloscope control panel can control both the location of the trigger point within the displayed waveform when the switch is in the ‘trigger centric’ position, and the time displacement of the displayed waveform when the switch is in the ‘viewport’ position.