Patent Publication Number: US-6985017-B2

Title: Method and system for pulse waveform variable edge control

Description:
BACKGROUND OF THE INVENTION 
   For many years now, one common form of electronics test equipment used by electronics designers has been signal sources. One type of signal source of particular importance is the pulse generator. Typically, pulse generators are designed to produce an electronic pulse signal consisting of a series of one or more pulses, such as a voltage pulse V P  shown in  FIG. 1 . Normally, various parameters describing the pulse signal, such as its amplitude V pp , positive edge (rise) time T pET  associated with a rising edge  1 , negative edge (fall) time T nET  associated with a falling edge  2 , frequency (not denoted), and duty cycle (also not denoted), are selectable by the pulse generator user. Additionally, pulse generators typically operate in several different voltage amplitude ranges, such as zero to 0.1 volts, zero to one volt, zero to ten volts, and so on, with the smaller ranges generally providing greater voltage resolution than larger ranges. 
   Specifically with respect to the programmability of the pulse rise and fall times, user selection of the positive and negative edge times T pET , T nET  of the pulse V P  allows variable edge control of the pulse generator. Typically, the positive edge time T pET  of the voltage pulse V P  is defined as the time required for the voltage to rise from ten percent to ninety percent of its amplitude V pp  (i.e., 0.1 V pp  to 0.9 V pp ) as shown in  FIG. 1 . Similarly, the negative edge time T nET  of the pulse V P  is commonly defined as the time require for the voltage to fall from ninety percent to ten percent of its amplitude V pp . 
   Users of pulse generators currently desire at least two characteristics of user-selectable variable edge control. First, a pulse generator should have the capability to maintain balanced, or substantially equal, positive and negative edge times T pET , T nET  as specified by the user so that less user manipulation of the pulse generator is required. Furthermore, the positive and negative edge times T pET , T nET  selected by the user should be held substantially constant while the amplitude V pp  is altered through a large dynamic range so that less user manipulation of the generator is required when changing the amplitude V pp  of the pulse V P . 
   Currently, some pulse generators employ variable edge control that exhibits these attributes. Those generators that provide these advantages normally require a calibration process whereby two different edge times within each voltage range are used. The edge times are normally set by way of a digital value resident in a digital-to-analog converter (DAC) for each of the negative and positive edge times. These four DAC values (a negative and positive edge time value for each of the two edge times used) are then employed to calculate a gain and offset value for the particular voltage range. The gain and offset values are then used to determine DAC values for other positive and negative edge times within that voltage range to maintain balanced variable edge control. 
   Given some electronic circuit differences between each voltage range, such a method of calibration normally must be performed separately for each voltage range of the pulse generator. Additionally, since the calibration is performed independent of the actual voltage amplitude of the pulse being used, such a calibration is effective only if pulse amplitude and edge times are independent. In other words, if altering the pulse amplitude also alters the positive or negative edge times without a change in the DAC value associated with the edge times, pulse amplitude and edge time are interrelated, and the aforementioned calibration procedure will not operate effectively on such a pulse generator. 
   From the foregoing, then, a need exists for an improved variable edge control method that applies to a wider range of pulse generation circuits. 
   SUMMARY OF THE INVENTION 
   Embodiments of the invention, to be discussed in detail below, provide a method and system for providing variable edge control of pulse waveforms produced by pulse generators and similar devices. According to embodiments of the method, a first amplitude for a first pulse waveform is selected. A positive edge DAC number is then set so that the positive edge time of the first pulse waveform is substantially equal to some initial edge time. Similarly, a second amplitude for a second pulse waveform is selected. Accordingly, a second positive edge DAC number is set so that the positive edge time of the second pulse waveform is substantially equal to the initial edge time. A positive gain factor and a positive offset factor are then generated for calculating a third positive edge DAC number so that the positive edge time of any pulse waveform of some amplitude substantially equals a desired edge time. A negative edge DAC number for the third pulse waveform may then be calculated in a similar manner so that the positive and negative edge times of the third pulse waveform may be balanced. Alternately, identifiable mathematical relationships between the positive and negative edge DAC numbers may allow a simplified calculation for the negative edge DAC number. Furthermore, calculation of the negative edge DAC number may occur prior to that for the positive edge DAC number. 
   Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a waveform diagram of an example voltage pulse signal generated by a pulse generator. 
       FIG. 2  is a block diagram of a pulse generator upon which embodiments of the invention may operate. 
       FIG. 3  is a flow diagram of a method for pulse waveform variable edge control according to an embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Before describing the method embodiments of the invention herein disclosed, a block diagram of a portion of a pulse generator  100  upon which the method embodiments may operate is shown in  FIG. 1 . A pulse circuit  110  is primarily responsible for generating an intermediate pulse waveform V A  based on several inputs received. For example, the amplitude of the intermediate pulse waveform V A  is determined by an amplitude DAC voltage V aDAC , which is generated by an amplitude DAC  120  and is based on an amplitude DAC number N aDAC . The higher the amplitude DAC number N aDAC , the higher the amplitude of the intermediate pulse waveform V A . 
   The pulse circuit  110  also accepts two inputs that allow modification of the slew rate of both the rising and falling edges of the intermediate pulse waveform V A . The slew rate is the rate at which the voltage changes per unit time. The slew rate of the rising edge of that pulse is controlled by a positive edge DAC voltage V pDAC , which is generated by a positive edge DAC  130  and is based on a positive edge DAC number N pDAC . The higher the positive edge DAC number N pDAC , the higher the slew rate. Likewise, the slew rate of the falling edge of the intermediate pulse waveform V A  is controlled by a negative edge DAC voltage V nDAC , which is generated by a negative edge DAC  140  and is based on a negative edge DAC number N nDAC . Opposite to the response of the pulse circuit  110  to the positive edge DAC number N pDAC , the higher the negative edge DAC number N nDAC , the lower the slew rate. 
   The DAC numbers N aDAC , N pDAC  and N nDAC  may be represented within the pulse circuit  100  by any of a number of typical digital formats, such as unsigned binary, one&#39;s complement, two&#39;s complement, and so on. Any such format may be employed successfully in various embodiments of the invention. 
   Typically, other characteristics of the voltage pulse V P , such as frequency and duty cycle, are controlled by other circuits within the pulse generator  100 . However, since such circuits do not directly affect the method embodiments of the present invention, which concern pulse variable edge control, those circuits are not shown in  FIG. 2 , nor are they described herein. 
   Other pulse circuits that exhibit the same or similar aforementioned characteristics of the pulse circuit  110  of  FIG. 2  have been produced in the prior art. In pulse generators in which such pulse circuits have been employed, one feature common to many is that the amplitude and the edge times of the intermediate pulse waveform V A  are interrelated (i.e., not independent). For example, as the amplitude of the intermediate pulse waveform V A  is increased, and the slew rates of the rising and falling edges are held constant, the positive and negative edge times of the intermediate pulse waveform VA are expanded proportionally. This complication typically makes control of the DAC values N aDAC , N pDAC , and N nDAC  more difficult when maintaining balanced edge times than in pulse generators in which amplitude and edge time are independent. 
   To further complicate the matter, the pulse generator  100  normally includes some substantially linear voltage gain associated with the signal path between the pulse circuit  110  and output signal representing the voltage pulse V P  ultimately produced by the pulse generator  110 . This gain is shown as a path gain  150  in  FIG. 2 . As a result, the amplitude of the intermediate pulse waveform V A  is decreased or increased by some factor, resulting in the voltage pulse waveform V P , thereby affecting the edge times T pET , T nET  of its rising and falling edges  1 ,  2 . 
   Given such a pulse generator  100 , a method  200  according to an embodiment of the invention, as shown in  FIG. 3 , begins with selecting a first amplitude V pp1  for a first voltage pulse waveform V P1 , by setting the amplitude DAC number N aDAC  so that the required amplitude is attained (step  210 ). In some embodiments, the amplitude V pp1  should be near the top of the amplitude range provided by the pulse generator  100  to allow the proceeding calculations to be performed over a greater dynamic range of the instrument. At that amplitude, the positive and negative edge DAC numbers N pDAC , N nDAC  are set so that the edge times of the rising and falling edges  1 ,  2  of the first voltage pulse waveform V P1  are substantially equal (step  220 ). Ordinarily, these edge times are set close to the maximum allowed by the pulse generator  100 , although that is not strictly necessary. The resulting positive and negative edge DAC numbers N pDAC , N nDAC  required for this particular operating condition will hereinafter be referred to as N pDAC1 , and N nDAC1 , respectively. Furthermore, the resulting positive and negative edge times T pET , T nET  in this case are T pET1  and T nET1 , which are substantially equal. 
   Similarly, a second amplitude V pp2  is then selected (step  230 ), resulting in a second voltage pulse waveform V P2 , preferably near the bottom of the amplitude range of the pulse generator  100 , by setting the amplitude DAC number N aDAC  so that the required amplitude is attained. The positive and negative edge DAC numbers N pDAC , N nDAC  are then set (step  240 ) to provide positive and negative edge times T pET , T nET  that are substantially equal to each other, and substantially equal to those associated with V pp1 , namely T pET1  and T nET1 . The resulting positive and negative edge DAC numbers N pDAC , N nDAC  are thus termed N pDAC2  and N nDAC2 , respectively. 
   Using the numbers produced up to this point (namely, V pp1 , N pDAC1 , N nDAC1,  V pp2  , N pDAC2,  N nDAC2,  and T pET1  (equal to T nET1 )), a set of gain (GainFactor) and offset (OffsetFactor) factors may then be generated (step  250 ). In one embodiment of the invention, these values allow the calculation of the proper values for the positive and negative edge DAC numbers N pDAC , N nDAC  for a particular amplitude V pp  and balanced edge time T ET  (equal to each of the positive and negative edge times T pET , T nET ) for a voltage pulse waveform V P . In the particular case of first determining a positive edge DAC number N pDAC , a positive gain factor GainFactor p  and a positive offset factor OffsetFactor p  would be generated. More specifically, the slew rate of the intermediate pulse waveform V A  at the output of the pulse circuit  110  is multiplied by the calculated positive gain factor GainFactor p , and then is offset by the calculated positive offset factor OffsetFactor p  to calculate N pDAC . Given that a PathGain is a constant gain factor resulting from the path gain  150 , the slew rate of V A  is determined by the rise in voltage from 0.1 V pp  to 0.9 V pp  at the voltage pulse waveform V P  (equal to 0.8 V pp ), divided by the PathGain (thus giving the voltage rise from 0.1 to 0.9 of the amplitude of V A ), divided by the edge time T ET . As a result, N pDAC  may then be calculated as follows: 
         N   pDAC     =         0.8   *     V   pp     *     GainFactor   p         PathGain   *     T   ET         +     OffsetFactor   p           
 
   Once the positive edge DAC number N pDAC  is calculated, the negative edge DAC number N nDAC  may be calculated assuming that the sum of the positive and negative edge DAC numbers N pDAC , N nDAC , herein called DacSum, is a constant for all balanced edge times (i.e., T ET =T pET =T nET ).
 
 N   nDAC   =DacSum−N   pDAC 
 
   This equation assumes that the pulse circuit  110  operates in a “balanced” fashion, which means the average of the N pDAC  and N nDAC  remains constant for all cases in which the positive and negative edge times T pET , T nET  are substantially equal. In other words, the positive and negative edge slew rates each change the same amount per increment or decrement in the associated positive or negative edge DAC number N pDAC , N nDAC  about DacSum/2, which represents the average of any two edge DAC numbers that represent a balanced positive and negative edge time. 
   In cases in which the pulse circuit  110  does not operate in such a balanced manner, the negative edge DAC number N nDAC  for a desired amplitude V pp  and balanced edge time T ET  for the voltage pulse waveform V P  may be calculated separately in a fashion analogous to that performed for the positive edge DAC number N pDAC , as described above: 
         N   nDAC     =         0.8   *     V   pp     *     GainFactor   n         PathGain   *     T   ET         +     OffsetFactor   n           
 
   In this case, a separate negative gain factor GainFactor n  and a negative offset factor OffsetFactor n  associated with the negative edge DAC number N nDAC  would be required. In order to calculate (step  250 ) the proper values for GainFactor p , OffsetFactor p , GainFactor n , OffsetFactor n , and DacSum, identifying some operational characteristics of the pulse circuit  110  is useful. For example, assume that for a constant positive edge time T pET  (=T ET ) the amplitude V pp  of the voltage pulse waveform V P  varies substantially linearly with the positive edge time DAC number N pDAC . As a result, the GainFactor p  would represent the slope of a line denoting the relationship between the positive edge DAC number N pDAC  and the slew rate of the positive edge of a pulse at the output of the pulse circuit  110 . The two data points collected for the positive edge DAC number N pDAC  (N pDAC1  at V pp1  and N pDAC2  at V pp2 ) for a particular balanced edge time T ET1 =T pET1 =T nET1 , are used to determine the slope of that line. More specifically, the positive gain factor GainFactor p  is the reciprocal of the difference of the slew rates of the two data points at the output of the pulse circuit  110  (((0.8*V pp1 /T pET1 )/PathGain)−(0.8* V pp2 /T pET1 )/PathGain)) divided by the difference in the positive edge DAC numbers (N pDAC1 −N pDAC2 ). After algebraic manipulation, the equation for GainFactor p  is reduced to the following: 
         GainFactor   p     =         (       N   pDAC1     -     N   pDAC2       )     *     T   pET1     *   PathGain       0.8   *     (       V   pp1     -     V   pp2       )             
 
   Mathematically, OffsetFactor p  is a constant offset value for the line describing the relationship between the positive edge DAC number N pDAC  and the slew rate of the positive edge of a pulse at the output of the pulse circuit  110 . In other words, OffsetFactor p  is an offset applied to the value for N pDAC  derived from the use of GainFactor p  to achieve a final N pDAC  for a particular amplitude V pp  and edge time T ET . To determine OffsetFactor p , a line defined by the two data points mentioned above (N pDAC1  at V pp1  and N pDAC2  at V pp2 ) on an N pDAC  vs. V pp  diagram is assumed, and the point at which the line crosses the axis for N pDAC  serves as OffsetFactor p . Using the first data point as a base and the basic line equation y=mx+b, where m is the slope of the line, and b is the y-axis-intercept of the line, this relationship may be stated mathematically as follows:
 
 N   pDAC1 =(Slope* V   pp1 )+OffsetFactor p 
 
   Given that the slope of the line is determined by the data points ((N pDAC1 −N pDAC2 )/(V pp1 −V pp2 )), the preceding equation may be solved for OffsetFactor p : 
         OffsetFactor   p     =       N   pDAC1     -         (       N   pDAC1     -     N   pDAC2       )     *     V   pp1           V   pp1     -     V   pp2               
 
   Once GainFactor p  and OffsetFactor p  have been ascertained, the positive edge DAC number N pDAC  for any positive edge time T pET  and amplitude V pp  may be calculated across all voltage ranges of the pulse generator 100. 
   Please note that since the value of the negative edge DAC number N nDAC  does not directly affect the positive edge time T pET , the negative edge DAC number N nDAC  does not appear in the calculations for the positive gain factor GainFactor p  and the positive offset factor OffsetFactor p . Likewise, since the positive edge DAC number N pDAC  does not affect the negative edge time T nET , the positive edge DAC number N pDAC  does not appear in similar calculations for the negative gain factor GainFactor n  and the negative offset factor OffsetFactor n.    
   At this point, the negative edge DAC number N nDAC  may be calculated in varying ways. For example, if no definable relationship is ascertained between the positive edge DAC number N pDAC  and the negative edge DAC number N nDAC  when the positive and negative edge times T pET , T nET  are balanced, the negative edge DAC number N nDAC  may be calculated directly in a manner analogous to that described above for the positive edge DAC number N pDAC . In other words, given the two data points collected earlier for the negative edge DAC number N nDAC  (N nDAC1  at V pp1  and N nDAC2  at Vpp 2 ) for a particular balanced edge time T ET1 =T pET1 =T nET1 , the negative edge DAC number N nDAC  for any amplitude V pp  and balanced edge time T ET  within the capability of the pulse generator is calculated similarly to that described above: 
         N   nDAC     =         0.8   *     V   pp     *     GainFactor   n         PathGain   *     T   ET         +     OffsetFactor   n           
       where   ,     
     ⁢       GainFactor   n     =         (       N   nDAC1     -     N   nDAC2       )     *     T   nET1     *   PathGain       0.8   *     (       V   pp1     -     V   pp2       )               
     and     
         OffsetFactor   n     =       N   nDAC1     -         (       N   nDAC1     -     N   nDAC2       )     *     V   pp1           V   pp1     -     V   pp2               
 
   As noted above, in some pulse generators a definable mathematical relationship exists between the positive and negative DAC numbers N pDAC , N nDAC  for balanced edge times T pET =T nET . In such cases, other embodiments of the invention may be employed to simplify the calculation of those numbers. For example, the pulse generator  100  may be designed such that N pDAC  and N nDAC  are related such that the sum of the two, herein called DacSum, is always a constant in the balanced state:
 
 DacSum=N   pDAC   +N   nDAC 
 
   In such cases, once either N pDAC  or N nDAC  have been determined, the above equation allows the opposing DAC number to be calculated directly by subtracting the known DAC number from DacSum. 
   One of several methods may be employed to calculate DacSum, given the data points collected above. For example, DacSum may be calculated by adding the positive and negative edge DAC numbers associated with those data points:
 
 DacSum=N   pDAC1   +N   nDAC1 
 
or
 
 DacSum=N   pDAC2   +N   nDAC2 
 
   Alternatively, to incorporate both sets of data points collected, an average of the two above equations may be utilized to generate DacSum: 
       DacSum   =         N   pDAC1     +     N   nDAC1     +     N   pDAC2     +     N   nDAC2       2         
 
   Such an equation may help provide a useful DacSum in cases where the sum of N pDAC  and N nDAC  over a range of amplitudes V pp  and balanced edge times T ET  exhibit slight variations. 
   Given the simplistic nature of the calculations involved in implementing the aforementioned methods for calculating N pDAC  and N nDAC , method embodiments of the invention are easily automated, typically by way of software or embedded firmware associated with the pulse generator  100 . Additionally, these methods have been shown to generate gain and offset factors as described above which work well over all voltage ranges provided by the pulse generator  100 . 
   From the foregoing, embodiments of the invention provide improved methods for pulse waveform edge control, such as that performed by pulse generators. However, specific embodiments of the invention other than those shown above are also possible. For example, while embodiments of the invention herein disclosed have been described in conjunction with voltage pulse waveforms, variable edge control of waveforms involving other physical characteristics, such as current, may also benefit from alternate embodiments. Also, embodiments of the invention may be used in conjunction with other forms of electronics equipment, such as general-purpose waveform generators, or electronics equipment residing outside the test and measurement realm. As a result, the invention is not to be limited to the specific forms so described and illustrated; the invention is limited only by the claims.