CTLE Transfer Function from a File

Alternatively, define the CTLE transfer function over a appropriate range of frequencies, then put the tabulated data in a file. The format of the CTLE data file is described later in this topic.

Nexxim reads the data on the file specified by the file parameter in the Eye probe, and fits a rational function to the data. The reltol option allows you to specify the fitting error. The resulting rational function is applied in the simulation (QuickEye, VerifEye, Transient, or LNA).

The transfer function with a rational function has the following form. Let:

Transfer function with a rational function.

(4)

A rational function generally provides the best fit to CTLE data: the CTLE transfer function data may represent a lumped RC implementation with known poles and/or zeros. If the data is from a rational function originally, a pure rational function finds the poles and zeros exactly.

The data file can contain multiple transfer functions for the same set of frequencies, controlled by the Eye probe tf_num parameter. Transfer functions in the file are numbered from 1 to N. By default (tf_num =0), Nexxim tries each transfer function in turn, finally using the one that produces the maximum eye height. Setting parameter tf_num to an integer from 1 to N directs Nexxim to use only that one transfer function in the analysis. If the tf_num is less than zero or greater than the number of transfer functions in the file, an error occurs.

Here is a summary of the format for the CTLE transfer function data. The format is similar to the Touchstone 2.0 file format.

The file must be in text format, and the file name must have a .ctle extension.
A comment is any text after an exclamation point (!). Whitespace and blank lines are ignored.
A line beginning with [Complex format]may be included to specify the data format. Options are RI (Real/Imaginary) and MA (Magnitude/Angle in degrees). If no [Complex format] line is present in the file, RI is the default interpretation. All transfer functions in a given file must use the same data format (RI or MA).
A line beginning with [Number of frequencies] must be present to specify the number of frequency points in the file.
A line beginning with [Number of transfer functions] must be present to specify the number of transfer functions given for each frequency point in the file.
A line with [Data] alone introduces the lines of data.
Each line of data has the frequency in Hz as the first entry. The frequencies must be in ascending sequence, but do not need to be evenly spaced. There must be no repeated frequencies (same convention as Touchstone 2.0).
After each frequency, the transfer function values appear, separated by commas, tabs, or spaces. For Real/Imaginary and Magnitude/Angle data formats, each transfer function uses two entries.

Note:

To guarantee that high frequencies remain unique after rounding, use high precision (e.g., for frequencies in the 10GHz range, a precision of 10 digits is recommended).

For Magnitude/Angle data, angles are in degrees (as in Touchstone 2.0).

Here is an example:

! CTLE Transfer Function Data in Real/Imaginary Format

[Complex format] RI

[Number of frequencies] 30

[Number of transfer functions] 2

! frequency,r:1,i:1,r:2,i:2

[Data]

5.00E+07,1.3708,-0.0096,1.3708,-0.00849

1.00E+08,1.3708,-0.0196,1.3708,-0.0175

1.50E+08,1.3708,-0.0297,1.3708,-0.0265

2.00E+08,1.3708,-0.0397,1.3708,-0.0355

2.50E+08,1.3708,-0.0498,1.3708,-0.0445

3.00E+08,1.3708,-0.0599,1.3708,-0.0535

3.50E+08,1.3708,-0.0700,1.3708,-0.0626

4.00E+08,1.3708,-0.0802,1.3708,-0.0717

4.50E+08,1.3708,-0.0903,1.3708,-0.0809

5.00E+08,1.3708,-0.1002,1.3708,-0.0901

5.50E+08,1.3708,-0.1112,1.3708,-0.0992

6.00E+08,1.3708,-0.1212,1.3708,-0.1092

6.50E+08,1.3708,-0.1312,1.3708,-0.1182

7.00E+08,1.3708,-0.1422,1.3708,-0.1272

7.50E+08,1.3708,-0.1522,1.3708,-0.1372

8.00E+08,1.3708,-0.1622,1.3708,-0.1472

8.50E+08,1.3708,-0.1732,1.3808,-0.1562

9.00E+08,1.3708,-0.1842,1.3808,-0.1662

9.50E+08,1.3708,-0.1942,1.3808,-0.1762

1.00E+09,1.3708,-0.2052,1.3808,-0.1862

1.05E+09,1.3708,-0.2162,1.3808,-0.1962

1.10E+09,1.3708,-0.2272,1.3808,-0.2072

1.15E+09,1.3708,-0.2382,1.3808,-0.2172

1.20E+09,1.3708,-0.2492,1.3908,-0.2282

1.25E+09,1.3708,-0.2602,1.3908,-0.2382

1.30E+09,1.3708,-0.2722,1.3908,-0.2492

1.35E+09,1.3708,-0.2832,1.3908,-0.2602

1.40E+09,1.3708,-0.2942,1.3908,-0.2722

1.45E+09,1.3708,-0.3062,1.3908,-0.2832

1.50E+09,1.3708,-0.3182,1.3908,-0.2952