A SerDes channel typically is a differential signal transmission channel. A hardware SerDes channel is typically characterized by measuring its N-port S-parameters and is typically a 4-port. The 4-port differential input ports are typically port 1 (+) and port 3 (-). The associated differential output ports are typically port 2 (+) and port 4 (-). The differential characteristic ( Port 1 – Port 3 vs. Port 2 – Port 4) is the channel transmission characteristic and the S-Parameter data is versus frequency.
See S-parameter detail in References > S-Parameter Channel Examples.
S-Parameters inherently have problems when used in a time domain simulation. S-parameters, though measured on actual hardware, actually deviate from the contraints for physical realizability such as passivity, reciprocity, and causality or include noise in the measured S-parameters for various reasons. For physical realizability, the S-parameters should ideally be measured continuously from 0 Hz to infinity and with no noise or distortion. For practical reasons, the S-parameters are band limited, are tabulated only at discrete frequencies, and are corrupted by measurement noise. These measurement limitations typically cause the S-parameters to be non-causal, non-reciprocal, and non-passive. Thus, to achieve a physically realizable transmission characteristic, the S-parameters must have corrections applied. These corrections must be achieved when converting S-Parameters to equivalent impulses in the time domain.
All SerDes system channel simulators convert the frequency domain S-parameter characteristics into time domain impulse characteristics. While doing this, all such simulators attempt to correct for all impairments in the S-parameters. However, some are less successful than others.
In fact, all of the top 6 EDA channel simulation tools have problems in doing this. As has been observed by many and especially reported by Romi Mayder of Xilinx Inc. at the 2015 DesignCon conference, Jan 27-30, Santa Clara Convention Center, Santa Clara CA, the top 6 EDA Channel Simulators in the industry give widely varying impulse modeling of S-parameters as well as widely varying channel BER performance. See this link: > DesignCon 2015 Technical Papers.
The key to obtaining consistent S-Parameter use with any time domain based tool, including any channel simulator, is to convert S-Parameter data to what is called on this web site “Causal S-Parameters”. Causal S-Parameters for an N-port are based on the frequency domain equivalent of the NxN causal impulse responses generated from the original NxN S-Parameters. Causal S-Parameters will typically provide better eye and BER results while also provide close agreement with the frequency domain response of the original S-Parameters. Causal S-parameters will typically provide faster simulations. Causal S-parameters typically result in impulse responses that have a smaller number of data points. A 2x shorter impulse length results in 4x faster convolution times.
To generate causal S-Parameters, use the web site: > https://www.serdesdesign.com/home/generate-causal-sparameters/
Do this by entering your data in the fields for 1. Define System and 2. Setup Analysis. Upload your S-Parameter file and identify the number of ports that it has. Identify your intended usage bit rate and samples per bit. These define the time domain sample rate ( BitRate * SamplesPerBit). Identify if these S-Parameters are for a differential channel and if so list the + and – channel input and output port numbers.
To generate the Causal S-Parameters select Run in field 3. Generate Causal S-Parameters for SParamFile. The Generate Causal S-Parameters Tool achieves S-Parameter corrections in a proprietary way to meet the constraints for physical realizability and for noise reduction. The corrections applied include meeting the mathematical aspects of the Kramers-Kronig relations applied to linear time invariant (LTI) systems.
Download the Causal S-Parameter data file in field 4 Download Causal S-Parameter Results.
Field 5 enables you to run an analysis to compare the Causal S-Parameters to the original S-Parameters.
Within Field 6, you can select to Open graphs for any of the reflection, transmission, and coupling S-Parameter paths. Additionally, you can observe the reflection and transmission impulse responses for the original S-Parameters and the generated Causal S-Parameters.
For more detail on the data and graphs available from this web site, see the article: > Typical Causal SParameters Characteristics and Displays…