# Correction Transfer Function Files

De-embedding operators generate correction transfer functions from two user-defined circuits. A correction transfer function represents the ratio (in the frequency domain) between voltage waveforms defined by the measurement circuit and the simulation circuit. For any given application, the correction transfer function can be calculated from circuit models you create or load directly from a transfer function file.

When you create the transfer function via circuit models, you first create the measurement and simulation circuits in the application. The response of the two circuits due to a stimulus from the same voltage source is calculated at their specified observation nodes. The transfer function is then calculated from the ratio (in the frequency domain) of the simulation node's response divided by the measurement node's response. This ratio can then be convolved (in the time domain) with the measured waveform in order to convert it into the desired simulated waveform.

In 4-port mode, there are actually two sources and observation nodes in each circuit. In this case, the ratio of node voltages becomes a matrix division operation generating a set of four frequency domain functions that are convolved with the two measured waveforms to produce the desired simulated waveforms.

De-embedding operators can be used on differential signals as well as single-ended signals. In single-ended applications, the measurement and simulation circuits are constructed from 2-port network model blocks and the analysis is performed on each oscilloscope channel independently. In differential applications, you must decide whether to use 2-port analysis or 4-port analysis. 2-port analysis can be used on differential signals or common-mode signals independently, but it does not include cross-coupling components between the differential and common mode signals. You would use 2-port analysis on a differential signal if that signal is being converted to a single oscilloscope channel input by a differential probe.

### Transfer Function Files

Transfer function files describe the relationship between two sets of time varying node voltages as a tabular list of complex transfer function values at specific frequencies. Formatted as ASCII text, transfer function files are easily viewed or edited using any text-based editor. De-embedding operators support a single proprietary transfer function file format called ATF (version A.01.00) derived from the Touchstone 1.1 format. ATF files are identified by the file extensions .tf2 or .tf4.

All transfer function files must include values at DC (0 Hz).

De-embedding operators require transfer function values that are uniformly spaced in frequency with a spacing or resolution that is small enough to accurately represent the slowest time constants and time delays exhibited by the modeled device. However, this does not mean that all transfer function files must contain uniformly spaced frequency data. Uniformly spaced file data is usually preferred, but not required if you are knowledgeable about how de-embedding operators interpret file data.

In the absence of the special keyword `DEFAULT_FREQUENCY_RESOLUTION`, de-embedding operators check the data to ensure that it is uniformly sampled in frequency (with exception to the interval between DC and the next highest frequency point) and generates a warning if the data is not uniformly sampled. If the `DEFAULT_FREQUENCY_RESOLUTION` keyword is accompanied by a numeric value, the data is resampled using linear interpolation and the frequency resolution specified by that value. De-embedding opertors resample the file data using a frequency resolution equal to the frequency interval between the last two frequency points in the file.

De-embedding operators expect transfer function files to list their values in monotonically increasing frequency order and ignores those values at which frequencies are less than or equal to the frequencies of previously listed values. De-embedding operators also truncate all file data to a maximum of 100,000 frequency points.

### Keywords

The following are the currently supported optional keywords you can place in your transfer function files. De-embedding operators scan transfer function files for special keywords which communicate how the data within the file was generated or how it should be interpreted. Special keywords are case-insensitive and permit additional comments on the same line.

#### ! #DSO ATF_FILE_VERSION <string>

The value of <string> represents the format version of the ATF file.

#### ! #DSO DEFAULT_FREQUENCY_RESOLUTION <value>

The argument <value> may be a numeric value in Hz or the text strings `AUTO` or `AUTOMATIC`. In the absence of this keyword, de-embedding operators check the data to ensure that it is uniformly sampled in frequency (with exception to the interval between DC and the next highest frequency point) and generates a warning if the data is not uniformly sampled. If <value> is a numeric value, the data is resampled using linear interpolation and the frequency resolution specified by <value> . Otherwise, de-embedding operators resample the file data using a frequency resolution equal to the frequency interval between the last two frequency points in the file. If <value> is equal to either of the text strings (`AUTO` or `AUTOMATIC`), de-embedding operators do not report a warning about non-uniformly sampled data.

#### ! #DSO TRANSFER_FUNCTION_DEFINITION_STRING <string>

De-embedding operators use this keyword to include a complete description of the circuit models from which it calculates the transfer function file. When loading transfer function file data, de-embedding operators check for this keyword and, if present, uses this information to re-populate the circuit model descriptions.