Abstract:
A system and method for configuring an automatic test system to produce a plurality of clocks from a reference clock includes a user interface and software. The user interface receives a plurality of inputs that specify desired frequencies of the plurality of clocks. In response to a command from the user interface, the software calculates values for dividers coupled to the reference clock, for deriving each of the desired frequencies from the reference clock. According to one embodiment, the desired frequencies form ratios that must be met to satisfy coherence. In calculating the divider values, the software minimizes frequency errors while precisely preserving the required ratios.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
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     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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     Reference to Microfiche Appendix 
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     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to automatic test equipment (ATE) for electronics, and, more particularly, to techniques for programming multiple clocks in an automatic test system. 
     2. Description of Related Art Including Information Disclosed under 37 C.F.R. 1.97 and 1.98 
     ATE systems typically use multiple clocks for testing electronic devices. In a common testing scenario, three or more clocks may be used—for example, a digital clock, a source clock, and a capture clock. The digital clock defines the rate at which digital I/O applies digital signals to the DUT (device under test), or samples digital signals from the DUT. The source clock defines the rate at which an analog source, such as an AWG (arbitrary waveform generator), varies the output levels of analog signals applied to the DUT. The capture clock defines the rate at which a digitizer samples analog signals from the DUT. 
     Device testing generally requires that different clocks of a test system be coordinated. When testing a DAC (digital-to-analog converter), the digital clock is coordinated with the capture clock to ensure that the digitizer samples the DAC&#39;s output at known points referenced to the DAC&#39;s input codes. Similarly, when testing an ADC (analog-to-digital converter), the digital clock is coordinated with the source clock to ensure that digital readback from the ADC is referenced to known points along the analog input signal. 
     It is particularly important to coordinate clocks for achieving “coherence.” A signal is coherent when each tone in the signal completes an integer number of cycles within the sample window. Mathematically, coherence is achieved when F s /N=F ik /M k , where
         F s  is the instrument&#39;s sampling rate;   N is the number of samples in the sample window (sourced or captured);   F ik  is the frequency of interest (output or expected) of the k th  tone; and   M k  is the number of cycles that the k th  tone completes within the sample window.
 
The need for coherence arises when two or more clocks are involved in a single test, for example, when one clock controls the sourcing of an input signal and another clock controls the capture of an output signal that contains the same frequencies as the input signal. Without coordination, it would be impossible to guarantee that F s /N=F ik /M k , and thus to guarantee coherence.
       

     Coherence is important for preventing “leakage.” Leakage is the mathematical consequence of sourcing or sampling truncated tones. Leakage adds harmonic distortion to sourced signals and distorts the DFT&#39;s (Discrete Fourier Transforms) of sampled signals. Therefore, leakage impairs a test system&#39;s ability to test device characteristics accurately. 
     Test engineers conventionally apply their knowledge of a test system&#39;s clocking system and instruments ascertain clock frequencies manually. Clocking systems and instruments can be extremely complex, however, imposing constraints that limit the flexibility with which timing can be prescribed. Therefore, test engineers must conventionally have a high degree of expertise to configure the clocks of a test system successfully. 
     Difficulties in prescribing a test system&#39;s clocks are compounded when a test setup requires coherence. Coherence imposes the additional constraint that two or more clocks in a tester must have a fixed frequency ratio. Oftentimes, constraints in the clocking system or instruments make it impossible to achieve this ratio at the specific frequencies that the test engineer wishes to use. To work around these constraints, the test engineer generally searches for nearby frequencies that the test system can produce, which also meet the required ratio. This task often comes down to a time-consuming process of trial and error. In addition, nothing about this manual technique ensures that the frequencies ultimately used are the closest realizable frequencies, which meet the ratio constraint, to the originally desired frequencies. Therefore, the current technique does not necessarily minimize frequency errors. 
     What is needed is a simpler, more accurate technique for programming clocks in an automatic test system. 
     BRIEF SUMMARY OF THE INVENTION 
     With the foregoing background in mind, it is an object of the invention to prescribe a configuration of multiple clocks in an automatic test system conveniently. 
     To achieve the foregoing object, as well as other objectives and advantages, a technique for configuring an automatic test system to produce a plurality of clocks from a reference clock includes a user interface for receiving a plurality of inputs that specify desired frequencies of the plurality of clocks. In response to a command from the user interface, software calculates values for dividers coupled to the reference clock, for deriving each of the desired frequencies from the reference clock. 
     According to one embodiment of the invention, the desired frequencies form ratios that must be met to satisfy coherence. In calculating the divider values, the software minimizes frequency errors while precisely preserving the required ratios. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Additional objects, advantages, and novel features of the invention will become apparent from a consideration of the ensuing description and drawings, in which— 
         FIG. 1  is a simplified block diagram of the clocking system with which the system for configuring clocks may be used; 
         FIG. 2  is a simplified block diagram showing a tester environment in which a user can employ the system for configuring clocks; 
         FIG. 3  is an illustration of the Clock Manager Form, which forms part of a graphical user interface of the system for configuring clocks; 
         FIG. 4  is an illustration of the Clock Manager Form of  FIG. 3 , after the system is operated in variable ORC mode; 
         FIG. 5  is an illustration of the Clock Manager Form of  FIG. 3 , after the system is operated in fixed ORC mode; 
         FIG. 6  is an illustration of the Coherency Grouping Form, which can be accessed via a control on the Clock Manager Form; 
         FIG. 7  is an illustration of the Instrument Form, which can be accessed via one or more controls on the Clock Manager Form; 
         FIG. 8  is an illustration of the Instrument Form of  FIG. 7 , showing the selection of a different instrument from the one selected in  FIG. 7 ; 
         FIG. 9  is an illustration of the Instrument Form of  FIG. 7 , showing a calculator portion for computing frequency dependencies; 
         FIG. 10  is a flowchart showing an algorithm for determining divider values for a clocking system using variable ORC mode; and 
         FIG. 11  is a flowchart showing an algorithm for determining divider values for a clocking system using fixed ORC mode. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a simplified, equivalent block diagram of a clocking system  100  with which the system for configuring clocks may be used. The clocking system  100  corresponds to that of the Catalyst™ test system, which is manufactured by Teradyne, Inc., of Boston, Mass. Within the clocking system, a 10 MHz crystal oscillator  110  is frequency-multiplied, by a phase-locked loop  112 , to produce a 100 MHz reference clock. The reference clock is frequency-multiplied by a K Multiplier  114  to produce an ultra-high resolution, optical reference clock (ORC). The ORC has both a fixed mode and a variable mode. In fixed ORC mode, K=500,000,000, which corresponds to an ORC frequency of 50,000 THz (1 Terahertz=10 12  Hz). In variable ORC mode, K is allowed to vary between 2 28  and 2 29 , which corresponds to an ORC that ranges between approximately 26,844 THz and 53,687 THz. Because it uses round numbers, fixed ORC mode is generally easier to program than variable ORC mode. Because it is more flexible, however, variable ORC mode more often produces coherent solutions precisely at desired frequencies. 
     The ORC effectively branches to a digital clocking system  116  and to one or more analog clocking systems  122 . The digital clocking system  116  consists of a timing path that includes a first frequency divider  118  for dividing the ORC by an integer “D” to produce a first digital output clock C 0 . (also designated as c 0  in  FIGS. 3  to  9 ). A second divider  120  divides the first digital output clock C 0  by an integer “J” to produce a second digital output clock T 0 . (also designated as t 0  in the  FIGS. 3  to  9 ) Digital clocks C 0  and T 0  can then be used to control the timing of digital vectors applied to or sampled from the DUT. The analog clocking system  122  includes four timing paths. Each timing path includes a first frequency divider  124  (DMC 0 -DMC 3 ) for generating a respective analog master clock (AMC 0 -AMC 3 ), a multiplexor (any of  126   a-d ) for directing the output of the first frequency divider, and a second frequency divider  128  (DA 0 -DA 3 ), for generating a respective output clock (A 0 -A 3 ). (designated as a 0 -a 3  in the  FIGS. 3  to  9 ). The output clocks A 0 -A 3  are transmitted over an instrument bus, where they are accessible for use by individual instruments. 
     The Catalyst clocking system employs an internal architecture (not shown) that enables it to operate as if the ORC was running at K * 100 MHz. In its physical implementation, however, the reference clock remains fixed at 100 MHz. This detail is operationally transparent, however. Programming the digital clocks C 0 , T 0  and the analog clocks A 0 -A 3  involves merely prescribing divider values for the first and second divider values for each respective timing path, as if the ORC were simply being divided by the divider values. 
       FIG. 2  illustrates a tester environment in which the invention may be used. A user  210 , for example a test engineer, operates a workstation or other computer  212 . The workstation  212  runs a computer program  213  for configuring clocks in a tester  220  and a test program  218 . The tester  220  includes a clocking system  222 , which corresponds to the clocking system  100  of FIG.  1 . The tester  220  also includes digital I/O  224 , for sourcing and sampling digital signals from a DUT  228  under control of clocks C 0  or T 0 , and analog instruments  226 , for sourcing and sampling analog signals under control of the analog clocks A 0 -A 3 . 
     The computer program  213 , herein referred to as the “Clock Manager,” includes a graphical user interface (GUI)  214  that communicates with software  216 . In the preferred embodiment, the Clock Manager is implemented on a PC-compatible computer or workstation using Microsoft® Visual Basic®. The GUI  214  includes fields and controls for receiving user input, displaying output, and receiving commands. The software  216  operates in response to input and commands to generate clocking configuration data. The clocking configuration data specifies how the clocking system (i.e., the dividers and K Multiplier) should be configured to meet the desired clock frequencies specified by the user. 
     The GUI  214  preferably includes forms with which the user can interact.  FIG. 3  shows the Clock Manager Form  300 , which is preferably displayed when the user first launches the Clock Manager program. The Clock Manager Form  300  presents the user with a graphical model of the test system&#39;s clocking system. Within this model, a representation  310  of the 100 MHz reference appears to be frequency-multiplied by a modeled “K Multiplier”  312  to produce an ORC  314 . The ORC  314  appears to branch into a digital section  316  and an analog section  318 . 
     Within the digital section, fields  320  and  324  respectively correspond to the first and second frequency dividers  118  and  120  of the clocking system  100 , and fields  322  and  326  respectively correspond to the frequencies of the digital clocks C 0  and T 0 . Within the analog section, the ORC branches to four timing paths. Each timing path has fields for the first and second dividers,  328  and  332 , which correspond respectively to dividers  124   a-d  and  128   a-d  of the clocking system  100 . Each timing path also has fields  330  and  334  that correspond to the Analog Master Clock (AMC 0 -AMC 3 ) and the Analog Clock (A 0 -A 3 ), respectively. 
     The analog section  318  also includes radio buttons  336  and checkboxes  338  for designating how the AMC&#39;s are to be distributed to second dividers  332  for generating the analog clocks a 0 -a 3 . These radio buttons and checkboxes represent the switching functions of multiplexors  126   a-d  of FIG.  1 . By selecting a radio button  336  and checking any number of checkboxes  338 , the user effectively “connects” the selected AMC to the input of the second divider  332  of each timing path whose checkbox is selected. Once established, connections remain in place, even when different radio buttons and checkboxes are selected. Newer connections overwrite older connections. 
     Inputs to the Clock Manager Form of  FIG. 3  are designated as unshaded fields, and outputs are designated as shaded fields. To operate the Clock Manager Form  300 , the user inputs desired clock frequencies to any of the fields for C 0 , T 0 , and A 0 -A 3  (fields  322 ,  326 , and  334 , respectively). The user then clicks either the “Find Dividers” button  352  or the “ORC Fixed Find” button  354 . In response to either of these button clicks, the software  216  automatically calculates divider values for all dividers required to produce the clocks for which the user has entered desired frequencies. If the user clicks the “Find Dividers” button, the software  216  computes the divider values and K Multiplier for variable ORC mode. If the user clicks the “ORC Fixed Find” button, the software  216  computes the divider values for fixed ORC mode (i.e., K=500,000,000). 
       FIG. 4  shows the output of the Clock Manager Form  300  for variable ORC mode. The user has previously entered 100 MHz in the T 0  field ( 326 ), 500 KHz in the A 0  field, and 150 KHz in the A 1  field ( 334 ). The user has then clicked the “Find Dividers” button  352 . In response, the software  216  has computed and outputted values for the K Multiplier  312 , the first and second dividers (for the digital clocks, A 0 , and A 1 ), and C 0   322  (C 0  can be either an output or an input. If no input is prescribed, the software assumes J equals 1 and sets C 0  equal to T 0 ). 
     It is important to note that the software  216  has successfully determined values for the K Multiplier and dividers to provide the precise desired frequencies that the user has requested. Because the software was able to precisely hit the desired frequencies, the ratios between T 0  and A 0 , T 0  and A 1 , and A 0  and A 1  are precisely what the user prescribed. If these ratios are required for coherence, the ratios are preserved and coherence is assured. 
       FIG. 5  shows the output of the Clock Manager Form  300  for fixed ORC mode. The user has previously entered the same frequencies as in  FIG. 4 , but has clicked the “ORC Fixed Find” button  354 . As before, the software  216  has computed and outputted values for the first and second dividers for T 0 , A 0 , and A 1 . The software has operated under the constraint that the K Multiplier be fixed at 500,000,000, however. This constraint limits the flexibility of the timing system and, in this case, has prevented the software from precisely hitting the target frequencies. Rather than abandoning its search or simply generating an error, the software  216  finds the frequencies closest to the desired target frequencies that meet the prescribed ratio. Since the ratio of the displayed frequencies for T 0 , A 0 , and A 1  are the same as the ratios of the originally prescribed target frequencies, coherence is still assured. 
     Note that the frequency error generated by fixed ORC mode is exceedingly small, less than 0.1 parts per million, in this case. This error is much smaller than other errors typically found in test systems, and can generally be ignored. The Clock Manager Program therefore enables the test engineer to derive coherent clocks for fixed ORC mode, without significantly sacrificing frequency accuracy. This is a great benefit to the test engineer, because fixed ORC mode is generally easier to use than variable ORC mode. 
     The operation of variable ORC mode and fixed ORC mode has been illustrated with only two target frequencies. More target frequencies can be prescribed, however, as testing needs require. 
     Certain tests do not require coherent clocks, or do not require that all the clocks prescribed for a test be coherent. Other tests require different sets of coherent clocks. Because coherence is a constraint that limits the software&#39;s ability to hit the target frequencies (especially in fixed ORC mode), the GUI  214  preferably allows the user to remove the constraint if it is not required. By pressing the “Grouping” button  350 , the GUI displays a Coherency Grouping Form  600  that allows the user to group different clocks together, or to separate clocks into different groups. 
       FIG. 6  shows the Coherency Grouping Form  600 . A set  610  of radio buttons, numbered 1-5, is provided for C 0 /T 0  and each of analog clocks shown on the Clock Manager Form (T 0  and C 0  are treated as one clock because they are inherently grouped together). The user assigns a clock to a group by clicking the radio button for a group number. Only one group number can be selected for each clock. The user assigns different clocks to the same group by giving them the same group number, or assigns different clocks to different groups by giving them different group numbers. Upon clicking the “Find Dividers” or “Fixed ORC Find” button, the software computes divider values (and K Multiplier, for variable ORC mode) that preserve the ratios of the desired frequencies within each group. The software does not necessarily preserve ratios among clocks assigned to different groups. 
     Coherency requires not only that inputted ratios among the clocks be preserved, but also that the ratios be correct. To this end, the Clock Manager Program includes an Instrument Form  700  for assisting the user in correctly prescribing coherent captures. The user invokes the Instrument Form  700  for a particular clock by clicking the Clock Assign Button  356  for that clock on the Clock Manager Form  300 . A different Clock Assign Button is provided for each analog clock A 0 -A 3 . 
       FIG. 7  shows the Instrument Form  700 . The Instrument Form  700  includes an aN Clock field  710 , which specifies the frequency of the analog input clock (A 0 -A 3 ). Label  712  indicates the number of the selected clock (here, “0” for clock a 0 ). The user operates the Instrument Form  700  by first selecting an instrument from an Instrument Field  714 , which is preferably implemented as a combo box. When the user clicks the Instrument Field, the field displays a list of all instruments that the form is capable of configuring. The user selects a desired instrument from the list, whereupon the Instrument Field displays the name of the selected instrument. The user can then specify timing characteristics for configuring the selected instrument. 
     Timing characteristics can be specific to each instrument type, but generally include sampling rate (F s ), number of samples (N), frequency of interest (F i ), and number of cycles (M) of the frequency of interest. These timing characteristics apply to both source and measure instruments. For an AWG, for example, F s  specifies the rate at which the AWG generates output samples, N specifies the number of samples generated, F i  specifies the output frequency, and M specifies the number of cycles generated at F i . For a digitizer, F s  is the digitizer&#39;s sampling rate, N is the number of samples acquired during the sample window, F i  is the expected frequency to be measured, and M is the number of cycles at F i  that are completed during the sample window. 
     The Instrument Form  700  includes fields for each of F s , N, F i , and M ( 716 ,  718 ,  720 , and  722 , respectively). It also includes a field  724  for frequency resolution, F res . Frequency resolution depends upon F s  and N, or, alternatively, on F i  and M, based on the equation F res =F s /N=F i /M. For a digitizer, F res  represents the bin width of a DFT taken from its acquired samples. For an AWG, F res  represents the smallest frequency increment in which the AWG can be programmed. The Instrument Form  700  allows the user to configure F res  separately because users often prefer to specify timing in terms of F res . 
     The Instrument Form  700  presents the user with different fields when different instruments are selected, depending on the particular requirements of each instrument. For example, when the user selects the Precision Low-Frequency Source (PLFSRC), the Instrument Form presents only the fields described above (See FIG.  7 ). But when the user selects the Precision Low-Frequency Digitizer (PLFDIG), the Instrument Form displays two additional fields, one for multiplying aN Clock and one for dividing aN Clock to derive F s  (See FIG.  8 ). 
     Returning to  FIG. 7 , the Instrument Form  700  includes arrow buttons  734   a-h  to assist the user in computing values for various fields. When the user clicks any of these arrow buttons  734   a-h , the software computes a value for the field the arrow button points to, based upon one or more of the other fields on the form. Clicking button  734   c  computes F res  based upon previously entered values of F s  and N (F res =F s /N). Conversely, clicking button  734   e  computes F s  based on previously entered values of F res  and N. Clicking button  734   d  finds the best value of N, given the previously entered values of F s  and F res . If F s /F res  is an integer, the software simply assigns N=F s /F res . If F s /F res  is not an integer, the software works in one of two ways, depending on the state of checkbox  732 . If the checkbox  732  is checked, the software holds F res  constant in computing the best value of N. If the checkbox  732  is unchecked, the software holds F s  constant in computing N. In either case, the software selects the value of N that effects the smallest change in the timing characteristic that it is not instructed to hold constant—F s  or F res . Buttons  734   f, g , and  h  work analogously to buttons  734   c, d , and  e , respectively, based on the constraint that F res =F i /M. 
     The Instrument Form  700  also includes a Calculate Button  742 . Clicking the Calculate Button directs the software to calculate F res , M, and N, based on previously entered values of F s  and F i . Given the equation F s /F i =N/M, the software first attempts to calculate N and M by reducing the fraction F s /F i  to simplest terms. The software assigns the numerator of the reduced fraction to N, and the denominator to M. F res  is then computed as F s  * N. If N is larger than allowed (i.e., the inputted values require that the instrument take too many samples), the software successively shifts F i  by small increments in search of the smallest integer values of N and M that place M and N within the allowable range. The resulting mismatch between the entered F i  and actual F i  is generally exceedingly small. 
     By clicking the arrow button  734   a  on the Instrument Form, the GUI transfers the value of F s  to the aN Clock Field  710 . The user can close the Instrument Form  700  by clicking the Cancel Button  746  or the Return Button  744 . Clicking the Cancel Button  746  aborts the Instrument Form and returns control to the Clock Manager Form  300 . Clicking the Return Button  744  stores the state of the Instrument Form  700  and closes the form. It also sends the contents of the aN Clock Field  710  to the Analog Clock Field  344  of the Clock Manager Form  300  for the selected clock, i.e., the clock A 0 -A 3  for which the user originally clicked the Clock Assign Button  356 . 
     As indicated above, instrument configurations are inherently subject to the constraint that F s /N=F i /M. We can rewrite this equation in the form F s /F i =M/N, whereupon it becomes clear that the fraction M/N must be reduced to simplest terms to prevent the instrument from generating or acquiring redundant data. To avoid redundant data, the Instrument Form  700  includes an “N &amp; M” status indicator  738 . The indicator turns green when M/N is in simplest terms (i.e., M and N are mutually prime); otherwise, the indicator turns red. 
     The Instrument Form  700  also includes a Valid Indicator  736  and a Diagnostic Field  740 . Instrument capabilities constrain the flexibility with which timing characteristics can be configured. For instance, an AWG is designed to operate over only a limited range of sampling rates and to receive input signals over only a limited bandwidth. The software includes an Instrument Database to ensure that the inputted timing characteristics for each instrument accord with the instrument&#39;s capabilities. The Instrument Database stores programming constraints for each instrument that is configurable using the Instrument Form  700 . In response to the user entering data in any of the timing fields (e.g., fields  716 - 724 ), the software runs a validation routine. The validation routine compares the inputted timing characteristics with the stored programming constraints in the Instrument Database. If the inputted timing characteristics violate any of the instrument&#39;s programming constraints, the Valid Indicator  736  turns red and a diagnostic message is displayed in the Diagnostic Field  740 . The diagnostic message generally identifies the violating timing characteristic and specifies an allowable range over which the timing characteristic can be programmed (See FIG.  8 ). Sometimes, an instrument configuration is invalid because certain combinations of timing characteristics are incompatible. In these cases, the diagnostic message preferably includes suggestions for modifying the timing characteristics. If the inputted timing characteristics do not violate any of the programming constraints stored in the Instrument Database, the Valid Indicator  736  turns green and the Diagnostic Field  740  is cleared. 
     As discussed above, coherence generally requires coordination among different clocks in a tester. Consider the case where a tester measures the gain of an amplifier at a particular frequency. An AWG applies a stimulus having frequency F i  to the input of the amplifier, and a digitizer captures an amplified response from the amplifier&#39;s output. Since the amplifier&#39;s output frequency is also F i , the digitizer must sample the DUT&#39;s response over a sample window that includes an integer number of cycles of F i . In this case, therefore, F i  of the digitizer must equal F i  of the AWG to ensure coherence. 
     The Instrument Form  700  simplifies the task of ensuring coherence by allowing the user to configure one instrument&#39;s timing characteristics as a function of another instrument&#39;s timing characteristics. Clicking buttons  726 ,  728 , and  730  on the Instrument Form causes the form to display additional fields for configuring timing dependencies for F s , F i , and F res , respectively. 
       FIG. 9  shows the Instrument Form  700  after the user has clicked the button  728 , for F i  dependence. As shown in  FIG. 9 , the Instrument Form displays a calculator region  900 , which was previously hidden. The calculator region  900  includes a numerator field  910  and a denominator field  912 . It also includes an analog branch region  914 , a source region  916 , and a Get Value button  918 . The user specifies a clock dependency for F i  by entering a numerator and a denominator in fields  910  and  912 , selecting a clock from the analog branch region  914 , and choosing one of the timing characteristics for the selected clock from the source region  916 . Regions  914  and  916  include radio buttons that allow only one item to be selected from each region. The settings displayed in the example of  FIG. 9  correspond to an F i  frequency that equals 3/2 the F s  frequency specified for analog clock a 1 . As shown in the calculator region  900 , F i  can be configured based on any of the other clocks in the system, for of the timing characteristics shown. The user can click the Get Value button  918  once the user has set the fields  910 ,  912 ,  914 , and  916 . Clicking this button computes a value for F i  based on the entered settings and returns the computed value to the F i  field  722 . 
     The contents of the calculator region  900  are substantially identical regardless of which of the buttons  726 ,  728 , or  730  has been clicked, with the exception that the title of the calculator region is changed based on the button pressed. The title reads, “F s  Dependence” when button  726  is clicked, “F i  Dependence” when button  728  is clicked, and “F res  Dependence” when the button  730  is clicked. 
       FIGS. 10 and 11  show examples of algorithms according to the invention for configuring a tester&#39;s clocking system for variable ORC mode ( FIG. 10 ) and fixed ORC mode (FIG.  11 ). Preferably, the algorithms are executed by the software  216 , which operates in response to user input and commands from the GUI  214 . The software  216  preferably executes one of these algorithms whenever the user has entered desired frequencies in the Clock Manager Form  300  and has clicked either the Find Dividers button  352  or the ORC Fixed Find button  354 . Referring briefly to  FIG. 1 , we first recognize that the elements of the clocking system  100  are mathematically related as follows:
   C   0 * D=ORC;   (1)   A   L   *DA   L   *DMC   L   =ORC; L =0, 1, 2, and 3; and  (2) 100 MHz* K=ORC.   (3) 
To realize the desired frequencies using frequency dividers (e.g., counters), we search for a solution set for which K and each of the following dividers is an integer: D, DA L , and DMC L ; L=0, 1, 2, and 3.
 
     Referring first to  FIG. 10 , the algorithm for variable ORC mode first seeks to determine the least common multiple (LCM) of the desired frequencies and the frequency of the master clock, i.e., the LCM of A 0 -A 3 , C 0 , and 100 MHz (step  1010 ). If the user has entered fewer desired frequencies in the fields of the Clock Manager Form, step  1010  ignores the blank fields and finds the LCM of only the fields for which the user has entered desired frequencies. The resulting LCM is the smallest number into which each of the desired frequencies and the master clock can be evenly divided. Being divisible in this way means that counters or other integer-based dividers can be used to generate each desired frequency from the ORC. It also means that K can be realized as an integer to generate the ORC from the master clock. 
     It is significant that the LCM is the “least” common multiple, rather than some other common multiple, of the desired frequencies and the master clock. Making the LCM the “least” common multiple maximizes the flexibility with which the algorithm can distribute divisions among different tiers of dividers in the clocking system (i.e., first dividers  124  and second dividers  128 ). This is important because the Analog Master Clocks (AMC 0 -AMC 3 ) in the Catalyst test system have an upper and a lower frequency limit. Therefore, the second dividers  128  (DA L ) must be chosen so that the desired frequencies F DES-L  times the second dividers DA L  fall within the allowable range of the AMCs. In addition, if the LCM is greater than the maximum allowable ORC (2 29 ), no solution will enable the clocking system to precisely produce the desired frequencies. Finding the “least” common multiple thus affords the algorithm its highest probability for success. 
     At step  1012 , the algorithm determines the values of the DA L  dividers  128 . This step operates under two constraints. First, the product of each DA L  * F DES-L  must fall within the allowable range of the respective AMC. Second, the value of DA L  should preferably add as few factors as possible (ideally none) to the LCM. This means that, to as great an extent as possible, the factors of the DA L  dividers should be a subset of the factors of the LCM. As before, minimizing factors promotes greater flexibility in configuring the clocking system and improves the chances of attaining a successful setup. 
     Steps  1012   a - 1012   c  expand the detail of step  1012 . For each analog timing path, the algorithm computes a lower limit (LL) and an upper limit (UL) of DA L  that places AMC L  within its allowable range. Thus,
 
 LL=AMC   MIN   /F   DES-L , and  (4)
 
 UL=AMC   MAX   /F   DES-L .  (5)
 
To ensure that LL and UL are within range, LL is rounded up and UL is rounded down. The algorithm then examines each integer from LL to UL and determines its factors. If an integer is found that adds no new factors to the LCM, the algorithm assigns DA L  the value of that integer and stops searching. Otherwise, the algorithm assigns DA L  the value of the integer that adds the fewest additional factors to the LCM. This process is repeated for each analog timing path.
 
     At step  1014 , the algorithm calculates each AMC L  as F DES-L  * DA L . The algorithm also computes an updated LCM, called “LumpLCM,” which includes all the factors of the original LCM, plus any additional factors that were needed to realize the DA L  dividers. 
     At step  1016 , the algorithm finds for another integer, “p,” such that the product p * LumpLCM falls within the allowable range of the ORC, here 2 28 -2 29 . Given this constraint, p can be any integer between ORC MIN /LumpLCM and ORC MAX /LumpLCM. The middle value of p within this range is used for subsequent processing; however, the choice of p is arbitrarily and any of the values that fall within the designated range can be used. 
     Once p is known, the remaining clock settings fall into place (step  1018 ). In particular,
 
 ORC=p*LumpLCM;   (6)
 
 K=ORC/ 100 MHz;  (7)
 
  D=ORC/C 0   ; and  (8)
 
 DMC   L   =ORC/AMC   L .  (9)
 
At the conclusion of step  1018 , the algorithm has finished computing clocking configuration data for variable ORC mode. The algorithm has either precisely hit the desired frequencies, or the setup has failed because the LumpLCM is larger than the greatest allowable ORC.
 
     In variable ORC mode, coherency grouping need not be considered because the flexibility of varying the ORC obviates the need for rounding. Since frequencies are not rounded, frequency ratios are preserved across all clocks, whether coherence is required or not. Referring now to  FIG. 11 , the first four steps  1110 - 1114  for fixed ORC mode are closely related to steps  1010 - 1014  of FIG.  10 . At step  1110 , the algorithm seeks as before to find the least common multiple (LCM) of the desired frequencies and the frequency of the master clock, i.e., the LCM of A 0 -A 3 , C 0 , and 100 MHz. At step  1111 , however, the algorithm also seeks to find a larger least common multiple, “BigLCM,” of the desired frequencies and the ORC, i.e., the LCM of A 0 -A 3 , C 0 , and 50,000 THz. BigLCM is then used at step  1112 , wherein the second dividers are chosen to minimize new factors beyond those already included in BigLCM. The factors of BigLCM are relevant, instead of those of LCM, to allow the algorithm to find, whenever possible, divider values that exactly produce the desired frequencies (i.e., without rounding—see below). Note that BigLCM is used only for minimizing new factors in step  1012 . Once the new factors, if any, are determined, the algorithm calculates LumpLCM as the product of LCM any additional factors needed to realize the second dividers. 
     At step  1114 , the algorithm computes the AMCs as using the second dividers determined at step  1112 . The actions performed at this step are substantially identical to those performed at step  1012  in FIG.  10 . 
     Proceeding now to step  1116 , the algorithm computes a value n=ORC FIXED /LumpLCM, where ORC FIXED  is nominally 50,000 THz. Because n is not necessarily an integer, the algorithm preferably rounds n to the nearest integer n r  to ensure that the clocking system can be configured with counters or other integer-based dividers. At step  1118 , the algorithm computes the DMC dividers  124   a-d  and the D divider  118  as follows:
 
 DMC   L   =n   r   *LumpLCM/AMC   L ; L=0, 1, 2, and 3; and  (10)
 
  D=n   r   *LumpLCM/C   0 .  (11)
 
If n was not originally an integer, then rounding n to produce n r  produces a rounding error, n r /n, which affects the AMCs and the desired frequencies. At step  1120 , the algorithm accounts for this rounding error in computing the actual frequencies as follows:
 
 AMC   L   =ORC   FIXED   /DMC   L   ; L =0, 1, 2, and 3;  (12)
 
 F   DES-L   =AMC   L   /DA   L   ; L =0, 1, 2, and 3; and  (13)
 
 C   0 = ORC   FIXED   /D.   (14)
 
At the conclusion of step  1120 , the algorithm has finished computing clocking configuration data for fixed ORC mode.
 
     Note that each of the desired frequencies and C 0  is askew from its originally prescribed value by a factor n r /n (assuming n r ≠n). Because this factor is present in all frequencies that the clocking system produces, ratios among different frequencies are retained. Therefore, coherence is assured, despite the fact that precise target frequencies can not always be attained. 
     Frequency errors produced by this technique are exceedingly small. The emphasis on minimizing the number of factors in LumpLCM generally ensures that n is a large number. Therefore, the factor n r /n is generally close to 1, ensuring that actual frequencies are very close to their originally prescribed values. Because LumpLCM is minimized, the algorithm minimizes frequency errors to the theoretically lowest values that the clocking system can achieve, while still ensuring coherence. These frequency errors are often smaller than other timing errors commonly found in test systems—so small, in fact, that they can generally be ignored. 
     Coherency grouping is significant for fixed ORC mode, because rounding errors distort frequency values. For fixed ORC mode, the algorithm simply treats different coherency groups as if they belonged to different clocking systems. Different groups are thus allowed have different LumpLCMs and different values of n. It may be necessary to round n for some groups but not for others. Therefore, some groups may have frequency errors, while others may not. 
     The foregoing description assumes that each of the multiplexors  126   a - 126   d  passes the AMC for its respective timing path to its output, i.e., multiplexor  126   a  passes AMC 0 , multiplexor  126   b  passes AMC 1 , and so on. As indicated above, however, the system for configuring clocks is not limited to this arrangement. In the preferred embodiment, the algorithm accounts for switching by computing divider values based on actual connections. 
     The system herein described greatly simplifies the task of configuring multiple clocks in a test system. It automatically generates clocking configuration data based on user-specified frequencies, and guides users to specify valid configurations by checking inputs against clocking system and instrument capabilities. The system is also expandable. The system can accommodate new instruments through updates to the Instrument Form  700  and instrument database. In addition, the algorithm can easily handle additional clocks because the time needed to execute the algorithm scales linearly with the number of clocks. 
     Alternatives 
     Having described one embodiment, numerous alternative embodiments or variations can be made. For example, the Clock Manager program has been described herein in connection with the Catalyst test system. The invention is not limited to use with this test system, however. It can be used with any test system having a reference clock, any number of clocks derived therefrom using frequency dividers and/or multipliers. 
     The embodiment described herein includes two tiers of frequency dividers, e.g., a first tier including dividers  128  and a second tier including dividers  124 . It is apparent from the foregoing, however, that the algorithm is scalable for use with clocking systems having greater than two tiers of dividers. In particular, the algorithm could be scaled by repeating steps  1012  and  1014  (or steps  1112  and  1114 ) for each successive tier. Values for each successive tier of dividers would be chosen to meet frequency range requirements, as well as to add as few factors as possible to LCM. 
     The foregoing description employs mathematical expressions for conveying an understanding of the underlying algorithm. The algorithm should not be limited to these particular expressions, however. As known to those skilled in the art, mathematics provides great flexibility in expressing quantitative concepts in a wide variety of forms. The invention should therefore be construed as encompassing algorithms achieving substantially the same result using a wide variety of substantially equivalent mathematical forms. 
     As described herein, the system for configuring clocks is implemented as a Visual Basic® program running on a PC-compatible computer. The invention is not limited to this implementation, however. As known to those skilled in the art, many different software and hardware environments can be used to implement the system according to the invention. 
     The system for configuring clocks need not be provided as a single computer program, per se. It can be packaged in a myriad of ways, such as with different software modules, software libraries, or computer programs that communicate with one another. Moreover, the interface for communicating with the system need not necessarily be a GUI. It can instead be a software interface for communicating with other software components. 
     The system for configuring clocks need not necessarily be run on a tester. It can also be run on a development station or on any computer on which test programs are developed, debugged, or examined. It can also be used for demonstration or training purposes. 
     In addition to generating configuration data, the system for configuring clocks can also be used to program a tester&#39;s clocking system directly. For example, the GUI of the Clock Manager can include an additional control that, when activated, causes the software to set up the tester according to the clocking configuration data. The clocks could thus be run in a “live” mode to examine or troubleshoot particular test scenarios. As yet another alternative, the GUI can include a control for generating test program code. When activated, the control causes the software to generate test program code for inclusion in a test program. The test program code can be in the native language of the tester, or in a universal testing language that is later translatable into tester-specific code. When the test program is later run, the program code will configure the tester in accordance with the clocking configuration data to establish the desired configuration. 
     Each of these alternatives and variations, as well as others, has been contemplated by the inventors and is intended to fall within the scope of the instant invention. It should be understood, therefore, that the foregoing description is by way of example, and the invention should be limited only by the spirit and scope of the appended claims.