Abstract:
A device and method for producing Inter Symbol Interference (ISI) scaling of S-Parameter Touchstone files for the generation of ISI scaling effects on serial data patterns by direct digital synthesis is described. The features of the present invention allow user to set parameters such as data rate, voltage amplitude, encoding scheme etc. as per requirements for the serial data patterns. An ISI scaling value is selected and applied to an S-Parameter Touchstone file representing transmission path effects. The serial data pattern parameters and the ISI scaling value used with the S-Parameter Touchstone file are compiled to generate a digital data waveform record file. The digital waveform record file is applied to a waveform generation circuit for converting the digital data into an analog serial data pattern with ISI scaling effects.

Description:
TECHNICAL FIELD 
     The present invention relates generally to signal generators and more specifically to a signal generator that scales the effects of Touchstone files which characterize the transmission paths of serial data. 
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority of Indian Provisional Application No. 227/MUM/2008, filed Feb. 1, 2008. 
     BACKGROUND OF THE INVENTION 
     High speed serial data designers are required to design robust and reliable serial data receivers to meet required Bit Error Rates (BER) for specific conditions. Physical channels between serial data transmitters and receivers, such as Printed Circuit Board (PCB) traces, cables, connectors and the like, cause loss of high frequency components in a high speed serial data signal due to skin effects, dielectric loss and the like. The net effect of this leads to Inter Symbol Interference (ISI). The transmission path of physical channels are preferably characterized by S-Parameter responses acquired using a Vector Network Analyzer (VNA) or the like. The S-Parameter characteristics of physical channel transmission paths are sets of S-Parameters for a series of discrete frequencies which are stored as Touchstone files. Based on the number of ports in a network representing a physical channel transmission path, various combinations of S-Parameters are measured. 
     An equivalent response filter can be derived from the S-Parameters in a physical channel transmission path Touchstone file to generate the effects of the physical channel transmission path. Serial data may be generated using this filter to measure the performance of the serial data receiver. It may be necessary for a designer to change the physical channel or modify the parameters of the physical channel, such as the length of the PCB trace, cable length, connector type and the like, to increase the performance of the serial data receiver. This requires a designer to measure the modified physical channel to characterize the S-Parameters of the modified transmission path. Characterizing the S-Parameters of a physical channel transmission path using a VNA is generally a time consuming activity. Moreover, the desired physical channel may not be available due to the non-availability of the components for the modified physical channel (e.g. a modified PCB with a new trace length). 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is to a signal generating device, such as a signal generator, and a method of setting serial data pattern parameters and an intersymbol interference scaling parameter for a Touchstone file to generate a serial data pattern representing varying characteristics of a signal path. A signal generating device has a display and a central processing unit that generates a user interface on the display for setting parameters of a serial data pattern, selecting an S-parameter file representing a measured signal path and an intersymbol interference parameter for scaling magnitude values in the S-parameter file. The central processing unit generates a waveform record file using the serial data pattern parameters and the S-parameter file modified by the intersymbol interference scaling parameter. A waveform generation circuit receives the waveform record file and generates a serial data pattern analog output signal having the serial data pattern modified by the S-parameter file having magnitude values scaled by the intersymbol interference scaling parameter. 
     The display may be an external display coupled to the signal generating device. An external controller, such as a personal computer, is coupled to the signal generating device with the external controller generating the user interface on an external display for setting parameters for the serial data pattern, selecting an S-parameter file representing a measured signal path and an intersymbol interference parameter for scaling magnitude values in the S-parameter file and generating a waveform record file using the serial data pattern parameters and the S-parameter file modified by the intersymbol interference scaling parameter for use by the waveform generation circuit. 
     The intersymbol interference scaling factor has a range from 0.0 to 10.0 with the central processing unit incrementing the intersymbol interference scaling factor in units of 0.001. The S-parameter file is preferably a Touchstone file. The S-parameter file may be a 1-port, 2-port, 4-port single ended or a 4-port differential S-parameter file. The method of generating a serial data pattern representing varying characteristics of a signal path has the steps of generating a user interface on a display device for setting serial data pattern parameters, selecting an S-parameter file representing a measured signal path and an intersymbol interference parameter for scaling magnitude values in the S-parameter file and selecting the serial data pattern parameters, the S-parameter file representing a measured signal path and the intersymbol interference parameter for scaling magnitude values in the S-parameter file. A waveform record file is generated using the serial data pattern parameters and the S-parameter file modified by the intersymbol interference scaling parameter and a serial data pattern analog output signal is generated from the waveform record file having a serial data pattern modified by the S-parameter file having magnitude values scaled by the intersymbol interference scaling parameter. 
     The intersymbol interference scaling parameter is preferably incremented in units of 0.001 over a range of 0.000 to 10.000. The generation of the waveform record file includes the step of compiling the serial data parameters to generate serial data pattern digital data modified by the S-parameter file having magnitude values determined by the intersymbol interference scaling parameter. The selecting of the S-parameter file further has the step of selecting at least a first port configuration for the measured signal path, where the port configuration may be a 1-port, 2-port, 4-port single ended or a 4-port differential configuration. 
     The objects, advantages and other novel features of the present invention are apparent from the following detailed description when read in conjunction with the appended claims and attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front view of a signal generator providing ISI scaling to Touchstone files according to the present invention. 
         FIG. 2  is a representative block diagram of a signal generator according to the present invention. 
         FIG. 3  is an initial user interface for providing ISI scaling to Touchstone files according to the present invention. 
         FIG. 4  is representation of a base pattern pop-up window in the user interface for providing ISI scaling to Touchstone files according to the present invention 
         FIG. 5  is a representation of a Transmitter pop-up window in the user interface for providing ISI scaling to Touchstone files according to the present invention. 
         FIG. 6  is a representation of a channel/cable pop-window in the user interface for providing ISI scaling to Touchstone files according to the present invention. 
         FIG. 7  is a representation of the channel/cable pop-up window for a 1-Port S-Parameter Touchstone file in the user interface for providing ISI scaling to Touchstone files according to the present invention. 
         FIG. 8  is a representation of the channel/cable pop-up window for a 2-Port S-Parameter Touchstone file in the user interface for providing ISI scaling to Touchstone files according to the present invention. 
         FIG. 9  is a representation of the channel/cable pop-up window for a single ended 4-Port S-Parameter Touchstone file in the user interface for providing ISI scaling to Touchstone files according to the present invention. 
         FIG. 10  is a representation of the channel/cable pop-up window for a differential 4-Port S-Parameter Touchstone file in the user interface for providing ISI scaling to Touchstone files according to the present invention. 
         FIG. 11  is a graphical representation of the frequency response for different scaling factor values in the user interface for providing ISI scaling to Touchstone files according to the present invention. 
         FIG. 12  is a representation of a compiles setting pop-up window in the user interface for providing ISI scaling to Touchstone files according to the present invention. 
         FIGS. 13A and 13B  are a flow chart representing the generation of a serial data pattern having scaled ISI effects. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , there is shown a signal generator  10 , such as the AWG7102 manufactured and sold by Tektronix, Inc. Beaverton, Oreg. The signal generator  10  has a front panel  12  with controls  14 , such as buttons and knobs or the like, and a display device  16 , such as a liquid crystal display, cathode ray tube or the like. The signal generator  10  also has a CD or DVD/CD drive for storing waveform data, executable programs and the like. User defined signals are generated and output by the signal generator  10  using the front panel controls in conjunction with a user interface displayed on the display device  16 . 
     Referring to  FIG. 2 , there is shown a representative block diagram of the AWG7102 signal generator  10  providing intersymbol interference (ISI) scaling to Touchstone files. The signal generator  10  has a central processing unit (CPU)  20  that controls the operation of the instrument according to programs stored on electronic media, such as a Compact Disc (CD). A memory  22 , such as RAM memory, is used for a work area for the CPU  20  to read programs from storage device  24 , such as a hard disk drive. A user can set up the signal generator  10  to generate an output test signal via the buttons, knobs and the like  14  on the front panel  12  of the instrument. The display device  16  may display a user interface for setting various parameters for the output test signal and visualizing an output signal as a function of the parameter settings. An external display output circuit  26  provides a video output which may be connected to an external display  28  for providing a larger display area in addition to the built-in display  16  of the signal generator  10 . A waveform generation circuit  30  generates the output test signal based on user defined parameters. In this example, the waveform generator circuit  30  has a trigger input and an event input and two channel outputs. An input/output port  32  is used for connecting an external keyboard  34 , a pointing device, such as a mouse  36 , and the like to the signal generator. The external keyboard  34  and/or mouse  36  may be included as part of the front panel controls of the signal generator  10  for setting parameters. The circuits are coupled together via a signal and data bus  38 . The bus  38  of the signal generator  10  may have a Local Area Network (LAN) interface  40  for connecting the signal generator  10  to an external controller, such as a personal computer (PC)  42  or other testing instruments. The LAN interface  40  allows the user interface to operate on the PC  42  and pass output signal data to the signal generator  10  and also enables the PC  42  to control the signal generator  10  over a network. Alternatively, a General Purpose Interface Bus (GPIB) interface may be used for the LAN interface  40 . 
     Referring to  FIG. 3 , there is shown an initial user interface  50  for setting parameters of a serial data pattern that is modified by ISI scaling of Touchstone files. The initial user interface  50  and subsequent user interfaces operate under program control of the CPU  20  with the user interface programs stored on the storage device  24 . The user interfaces may be displayed on the display device  16  or on the external display  28 . Alternately, the user interface programs may be stored and accessed by the PC  42  with the PC  42  processing the parameters and generating an output file that is coupled to the signal generator  10 . The programs controlling the various interfaces on the signal generator  10  may be accessed via an icon on the display device  16  or by clicking on a start tab and clicking the appropriate program from a list of programs stored in the signal generator  10 . The initial interface  50  includes a number of pop-up windows that are activated by clicking on various tabs  52  or buttons  53  in the initial user interface  50 . The initial user interface  50  has a BASE PATTERN tab  54  that activates a BASE PATTERN pop-up window  56 . The BASE PATTERN pop-up window  56  is automatically activated with the initial user interface  50 . 
       FIG. 4  is a more detailed representation of the BASE PATTERN pop-up window  56  showing four defined regions of the BASE PATTERN pop-up window  56 : BASE PATTERN  58 , SIGNAL  60 , ENCODING  62  and RISE/FALL  64 . The BASE PATTERN region  58  has buttons  66 ,  68  and  70  that allows a user to select a serial data pattern on which the ISI effects are generated. Clicking on the STANDARD button  66  activates a STANDARD box  72  and a PATTERN box  74 . Click on the STANDARD box  72  displays a general serial data pattern and various serial data standards requiring compliance testing. Once a particular serial data standard is selected, clicking on the PATTERN box  74  displays various waveform patterns defined by the selected serial data standard, such as PRBS7. Clicking on the FROM FILE button  68  highlights a FILE box  76  in which a user can enter a previously stored data file. Clicking on the USER PATTERN button  70  highlights a Pattern Box  78  in which a user may enter a serial data pattern. The pattern may be entered as “0” and “1” binary data or hexadecimal data by clicking on appropriate BINARY or HEX buttons  80 ,  82 . 
     The SIGNAL region  60  has a DATA RATE box  84 , an AMPLITUDE box  86 , and an IDLE STATE box  88 . Clicking on the DATA RATE box  84  allows a user to set the data rate of the serial data. The data rate may be adjusted from 10 Mega Bits per second to 20 Giga bits per second. When the STANDARD button  66  is activated, the data rate is automatically selected as a function of the selected serial data standard. Clicking on the AMPLITUDE box  86  allows the user to select the voltage level of the serial data pattern. The user may vary the voltage level from 250 mV to 1V. The IDLE STATE box  88  is active when SATA is selected in the STANDARD box  72  and Idle Pattern is selected in the PATTERN box  74 . The Idle State may be viewed as selectable periods of DC within the Idle State pattern. 
     The ENCODING region  62  has an ENCODING SCHEME box  90  that allows the user to set the type of coding scheme for the serial data pattern. A user may select from NRZ or NONE NRZ. Clicking on the 8B10B box  92  activates an algorithm for the mapping of 8-bit symbols to 10-bit symbols to achieve DC-balance and bounded disparity. The RISE/FALL region  64  has RISE/FALL TIME buttons  96  and  98  for respectively selecting 10/90 or 20/80 percent rise and fall time. A RISE box  100  allows the user to select the rise time of the serial data pattern leading edges. A FALL box  102  allows the user to set the fall time of the serial data pattern trailing edges. The DCD box  104  allows the user to vary the amount of Duty Cycle Distortion in the serial data pattern. The rise, fall and DCD times may be defined in seconds or unit intervals using the respective SETTING boxes  106 . 
     Clicking on the TRANSMITTER tab  110  activates a TRANSMITTER pop-up window  112  as shown in  FIG. 5 . The TRANSMITTER pop-up window  112  has boxes  114  that allow the user to set parameters for variable amounts of peak-to-peak Periodic Jitter and RMS Random Jitter, set Spread Spectrum Clock (SSC) parameters, Noise parameters in Volts (RMS) and Pre/De-emphasis parameters in dB or volts. The user has the option of placing the noise at the near end or the far end of the serial data pattern. 
     Clicking on the CHANNEL/CABLE tab  120  activates a CHANNEL/CABLE pop-window  122  as shown in  FIG. 6 . The user may activate an ISI box  124  and SETTING box  126  by clicking on an ISI button  128 . In the present invention, the user clicks on an S-PARAMETER FILTER button  130  to activate various S-Parameter Filter boxes and buttons. A READ FROM FILE box  132  and associated BROWSE button  134  allows the user to select a previously stored S-Parameter Touchstone file. Clicking on an INVERSE FILTER button  136  activates an INVERSE FILTER box  138  and associated BROWSE button  140  to allow the user to select a previously stored S-Parameter Touchstone file. An ISI SCALING box  142  allows the user to select an ISI scaling factor value which in the preferred embodiment is varied from 0.000 to 10.000 in increments of 0.001. Below the ISI SCALING box  142  is a SELECTION region  144  that displays various selection options for various ports defined by the S-Parameter Touchstone files. 
       FIG. 7  illustrates the selection of a 1-port S-Parameter Touchstone file where the file extension “s1p” identifies the numbers of ports characterized in the Touchstone file. The 1-port S-Parameter Touchstone file, as defined in the present invention, is the S 21  transmission coefficient of a 2-port S-Parameter Touchstone file. The convention is different from the EIA/IBIS Open Forum Draft of the Touchstone File Format Specification, Rev 1.1. Because the 1-port S-parameter Touchstone file has a single S-Parameter, the SELECTION region  144  is left empty. 
       FIG. 8  illustrates a 2-port S-Parameter Touchstone file with a file extension of “s2p”. A 2-PORT display  146  is generated in the SELECTION region  144  for selecting 2-Port S-Parameter coefficients defined in the Touchstone file format. The PORT display  146  has an IDENTIFICATION HEADER  148  identifying the PORT display  146  as a Touchstone 2-Port S-Parameter Selection display. Within the PORT DISPLAY  146 , there is a CHANNEL TRANSMISSION DATA prompt  150  for selecting the location of the channel transmission data in the Touchstone file. Adjacent to the prompt  150  is a 2-Port S-Parameter matrix  152  listing the S-parameter coefficients in the Touchstone file. Next to each S-Parameter is a button  154  for selecting that S-parameter. The S-Parameter coefficients for channel transmission data is the S 21  S-Parameter for forward channel transmission and the S 12  S-Parameter for reverse channel transmission. The S 11  and S 22  S-Parameters coefficients are reflection coefficients. Generally, the S 21  S-Parameter coefficient is selected as shown by the button  154  being selected next to the S 21  S-Parameter transmission coefficient. 
       FIG. 9  illustrates the selection of a 4-Port S-Parameter Touchstone file with a file extension of “s4p”. A 4-PORT display  160  is generated in the SELECTION region  144  for selecting 4-Port data types. The 4-Port display  160  has an IDENTIFICATION HEADER  162  identifying the 4-PORT display  160  as a Touchstone 4-Port S-Parameter Data Type display. Within the 4-PORT display  160 , there is displayed the data types which are “SINGLE ENDED” or “DIFFERENTIAL” with each data type having an associated button  164  for selecting a particular data type. When the SINGLE ENDED data type is selected, an additional PORT ASSIGNMENT display  166  is generated. Within the PORT ASSIGNMENT display  166 , there is a PORT LINK prompt  168  for linking user defined port assignments of transmission lines under test to ports on a measurement test instrument, such as a Vector Network Analyzer (VNA). In a 4-Port Network, there are two transmission paths with each transmission path having two ports. The user assigns port numbers to each transmission line port. The user connects one or more ports of the VNA to the respective ports of the transmission lines under test. The VNA launches a signal into each port and acquires data for generating S-Parameters that are stores in a Touchstone S-Parameter file. The S-parameters are stored in a specific format in the Touchstone S-parameter file irrespective of the user port assignments. The user has to associate each port of the two transmission paths to the VNA ports to associate the S-parameters in the Touchstone S-Parameter file with the correct ports of the two transmission paths. Adjacent to the PORT LINK prompt  168  is a representation of two transmission paths  170 , Tx+ and Rx+ and TX− and Rx−, with each transmission path  170  having PORT ASSIGNMENT boxes  172  associated with each Tx+, Rx+, TX− and Rx−. The user inputs the port numbers assigned to each port by the user in the respective PORT ASSIGNMENT boxes  172 . 
       FIG. 10  illustrates PORT LAYOUT  180  display when the DIFFERENTIAL data type is selected in the 4-Port display  160 . The PORT LAYOUT display  180  includes pre-defined MAPPING ACRONYMS  182  with each MAPPING ACRONYMS  182  having an associated button  184 . Displayed adjacent to the MAPPING ACRONYMS  182  is a 4-PORT DIFFERENTIAL S-PARAMETER MATRIX  186 . The MARTIX  186  displays four 2-Port S-Parameter matrices with the upper left matrix representing a 2-PORT DIFFERENTIAL S-PARAMETER MATRIX  188 . The lower right matrix represents a 2-PORT COMMON MODE S-PARAMETER MATRIX  190 . The lower left and the upper right matrices represent 2-PORT MIXED DIFFERENTIAL AND COMMON MODE S-PARAMETER MARTICES  192  AND  194 . The user selects one of the MAPPING ACRONYMS  182  which maps the 2-Port S-Parameters matrices  188 ,  190 ,  192  and  194  to the formatted S-Parameters in the Touchstone S-Parameter file. 
     As previously stated, clicking on an INVERSE FILTER button  136  (as shown in  FIG. 6 ) activates an INVERSE FILTER box  138  and associated BROWSE button  140  to allow the user to select a previously stored S-Parameter Touchstone file. An inverse filter, such as a pre-emphasis filter, is derived by applying an inverse FFT to the S-Parameter Touchstone file. The pre-emphasis filter removes the physical channel transmission path effects characterized by the S-parameter Touchstone file. 
     In the preferred embodiment, the Touchstone file contains S-Parameter magnitude data as a function of frequency. A transmission path generally attenuates the higher frequencies resulting in a low pass frequency response. The effects of the transmission path as recorded by the S-parameters in a Touchstone file may be represented by a filter derived from the frequency response of the S-Parameters. The user selects an amount of ISI SCALING using the ISI SCALING box  142  which modifies the shape of the frequency response filter derived from the modified S-Parameters in the Touchstone S-Parameter file as follows. The S-Parameters magnitude responses for each of the given set of frequencies are first converted to a linear scale. The minimum linear magnitude response is subtracted from the other linear magnitude responses as represented by Equation 1:
 
 M 1( f )= M ( f )−min[ M ( f )]  EQ (1)
 
where M(f) are the magnitude responses as a function of frequency and the min [M(f)] is the magnitude value of the minimum magnitude value, generally the last frequency point for S 21  and S 12  S-parameters. The linear magnitude values M1 ( f ) are multiplied by the ISI Scaling Value with the results being added to minimum linear magnitude response min [M ( f )] as represented by Equation 2:
 
 M 2( f )= a×M 1( f )+min[ M ( f )]  EQ (2)
 
where “a” is the ISI scaling value. The scaled linear magnitude values M2 ( f ) are divided by the maximum linear magnitude response of the scaled linear magnitude values M2 ( f ) as represented by the Equation 3:
 
 M 3( f )= M 2( f )/max[ M 2( f )]  EQ (3)
 
where M3 ( f ) is the scaled frequency response.
 
       FIG. 11  illustrates the frequency response for different scaling factor values “a”. a=0 corresponds to an all pass filter which totally removes the effect of the transmission path. a=1 corresponds to the frequency response of the Touchstone file. As “a” is increased from 0 to 1, the effect of the transmission path is gradually included in the serial data pattern. With increases greater than 1, the effect of the transmission path is increased on the serial data pattern. 
     Referring back to  FIG. 3 , once the user has set the parameters for the serial data pattern and selected the S-Parameter Touchstone file and the amount of ISI scaling to be applied to the Touchstone file, the user activates a COMPILE SETTINGS pop-up window  200 , shown in  FIG. 12 , by clicking on a COMPILE SETTINGS button  202  on the initial user interface  50  tool bar  204 . The COMPILE SETTINGS pop-up window  200  has a WAVEFORM NAME box  206  for providing a user defined name for a waveform file generated by the signal generator  10  using the user defined parameters for the serial data pattern and ISI scaling value for the selected S-Parameter Touchstone file. The data rate for the serial data pattern entered using the BASE PATTERN pop-up window  56  is displayed next to “Data Rate”. The user can click on a SAMPLE RATE button  208  or a SAMPLES PER UI button  210  to activate respective SAMPLE RATE or SAMPLES PER UI boxes  212 ,  214 . The user may manually set the sampling rate or samples per UI by entering appropriate values based on the serial data pattern and the data rate. The sampling rate or samples per UI may be automatically set based on the data rate and the serial data pattern by clicking on AUTOMATIC box  216 . A BANDWIDTH EXPANSION FILTER region  218  may be activated by clicking on a BANDWIDTH EXPANSION FILTER button  220 . The BANDWIDTH EXPANSION FILTER region  218  allows a user to select or turn off various interleaving parameters. A COMPILE BUTTON PREFERENCES region  222  allows the user to select either COMPILE ONLY or COMPILE AND SENT TO parameters for the compiled waveform record defined by the serial data pattern parameters and the ISI scaling value parameter. Toward the bottom of the COMPILE SETTINGS pop-up window  200  are additional parameters that may be set by clicking on the appropriate boxes  224 . One of the parameters is SHOW GRAPH AFTER COMPILE which displays the serial pattern data in various forms in two graph regions  226  at the bottom of the initial user interface  50 . At the bottom of the COMPILE SETTINGS pop-up window  200  are COMPLE, OK, CANCEL and HELP buttons  228 ,  230 ,  232  and  234  that allows the user to select various options related to the COMPILE SETTINGS pop-up window  200 . One option is to click on the COMPILE button  228  that initiates the compiling of the serial data pattern parameters and ISI scaling value applied to the S-Parameter Touchstone file to generate a waveform record file containing digital data for generating an analog serial data pattern from the signal generator  10 . Another option is to click on the OK button  230  that saves the parameters selected in the COMPILE SETTINGS pop-up window  200  and closes the window  200 . A further option is to click on the CANCEL button  232  that closes the COMPILE SETTINGS pop-up window  200  without saving the parameters selected in the window  200 . Clicking on the HELP button  234  activates a HELP pop-up window from which the user may seek help. 
     The initial user interface  50  tool bar  204  has a COMPILE button  240  (as shown in  FIG. 3 ) that functions in the same manner as the COMPILE button  228  (as shown in  FIG. 7 ) in the COMPILE SETTINGS pop-up window  200  by initiating the compiling of the serial data pattern parameters and ISI scaling value applied to the S-Parameter Touchstone file to generate a waveform record file containing digital data for generating an analog serial data pattern from the signal generator  10 . 
     The ISI scaling of an S-Parameter Touchstone file is used with a selected serial data pattern to produce an analog serial data pattern output from the signal generator  10 .  FIGS. 13A and 13B  represent a flow chart illustrating the steps in generating an analog serial data pattern with ISI scaling of an S-Parameter Touchstone file. The serial data pattern and ISI scaling programs are activated at step  300 . The serial data pattern parameters are set by the user at step  302 , such as a general standard having a PRBS7 pattern having a date rate of 3 GB/s, a voltage amplitude of 1V, an encoding scheme that does not return to zero, rise and fall times of the leading and trailing edges of the serial data pattern of 120 psec at a 10 to 90 percent level and a sample rate of 18 Gs/s. The jitter, SSC, noise and Pre/De-emphasis parameters in the TRANSMITTER pop-up window  112  are set to zero. 
     The user activates the CHANNEL/CABLE pop-window  122  by clicking on the CHANNEL/CABLE tab  120 , clicks on the S-PARAMETER FILTER button  130  to activate the various S-Parameter Filter boxes and buttons and selects a previously stored S-Parameter Touchstone file using the READ FROM FILE box  132  and associated BROWSE button  134  as shown in step  304 . At step  306  a determination is made whether the S-Parameter Touchstone file is a 1-port file. If the S-Parameter Touchstone file is a 1-port file, then the programs proceed to the select ISI scaling step  326 . If the S-Parameter Touchstone file is not a 1-port file, then a determination is made whether the S-Parameter Touchstone file is a 2-Port S-Parameter Touchstone file at step  308 . If the S-Parameter Touchstone file is a 2-port file, then an S-Parameter transmission coefficient is selected using the 2-Port S-Parameter matrix  150  in the PORT display  146  at step  310 . If the S-Parameter Touchstone file is not a 2-port file, then a determination is made whether the S-Parameter Touchstone file is a 4-Port S-Parameter Touchstone file at step  312 . If the S-Parameter Touchstone file is a 4-Port S-Parameter Touchstone file, then a determination is made whether the S-Parameter Touchstone file data type is SINGLE ENDED at step  314 . If the data type is SINGLE ENDED, then port assignments are selected at step  316  using the PORT ASSIGNMENT display  166  and the PORT ASSIGNMENT boxes  170 . If a determination is made that the S-Parameter Touchstone file data type is not SINGLE ENDED, then a determination is made whether the data type is DIFFERENTIAL at step  318 . If the data type is DIFFERENTIAL, then the 2-Port S-Parameters matrices  188 ,  190 ,  192  and  194  are formatted to the S-Parameters in the Touchstone S-Parameter file at step  320  using the PORT LAYOUT display  180  and the MAPPING ACRONYMS  182 . If the S-Parameter Touchstone file is not a 4-Port S-Parameter Touchstone file, then a determination is made whether the INVERSE FILTER button  136  is selected at step  322 . If the INVERSE FILTER button is selected, then the INVERSE FILTER box  138  and associated BROWSE button  140  are used at step  324  to select a previously stored S-Parameter Touchstone file. 
     The user selects the ISI scaling value at step  326  using the ISI SCALING box  142 . The selection of the ISI scaling value can occur when the Touchstone file is a 1-Port S-Parameter Touchstone file, or the transmission coefficient is selected for a 2-Port S-Parameter Touchstone file, or the Touchstone port assignments are selected for the single ended 4-Port S-Parameter Touchstone file, or the 4-Port layout S-parameter mapping is set for the 4-Port S-Parameter Touchstone file, or the inverse filter Touchstone S-Parameter file is selected. Alternately, the ISI scaling value may be selected before or during selection of the S-Parameter Touchstone files and the various selections for the S-Parameter Touchstone files. 
     The serial data pattern parameters and the ISI scaling value used with the selected S-Parameter Touchstone file are compiled at step  328  to generate a waveform record file containing digital data for generating an analog serial data pattern with ISI scaling effects on a transmission path defined by an S-Parameter Touchstone file. The waveform record file is processed by the waveform generation circuit  30  which converts the digital data in the waveform record file to an analog serial data pattern with ISI scaling effects and the signal generator  10  generates an analog serial data pattern with ISI scaling effects output at step  330 . 
     Although the invention has been disclosed in terms of the preferred and alternative embodiments disclosed herein, those skilled in the art will appreciate that modifications and improvements may be made without departing from the scope of the invention.