Why Use Touchstone (SnP) Modeling for Signal Integrity and Power Integrity Analysis?

Kathleen Love
2 days ago
4 min read

Touchstone Models and Scattering Parameters: Fundamentals and Importance in SI/PI

Touchstone® (SnP) models are a key asset in the fields of signal integrity (SI) and power integrity (PI) when analyzing high-data rate, high-speed designs. These frequency-domain models characterize the linear behavior of passive interconnects and components such as PCB traces, vias, connectors, and packages by describing how signals are reflected and transmitted at various ports across a wide range of frequencies, then compile these complex electromagnetic interactions within a standardized, measurable, and efficient black box format.

Touchstone models use scattering parameters, or S-parameters, to relate incident wave and reflected wave behavior at each port, which is essential to understanding signal transmission and reflections. S-parameters are typically measured at the input port and output ports of a two port network, which is a common structure in RF and microwave analysis. This method allows engineers to accurately simulate and predict signal degradation due to impedance mismatches, reflections (return loss), signal loss (insertion loss), and crosstalk, as well as analyze the performance of power delivery networks (PDNs) by characterizing their impedance profile. The characteristic impedance of transmission lines plays a key role in determining how much of the signal is reflected. Mismatches in characteristic impedance lead to an increased reflection coefficient and greater signal loss. Leveraging Touchstone models can help design engineers to identify and mitigate these potential SI/PI issues early in the design process, and ultimately reduce the number of design cycles by enabling engineers to optimize components and layouts sooner.

When you combine these models with other simulation tools and behavior models, such as IBIS models, you can simulate and evaluate high-speed signal integrity channels for die-to-die interconnect, such as UCIE, or for other channels such as PLLs, clocks, PCIe, SerDes, DDR4, DDR5, LPDDR4, LPDDR5, USB, MIPI, HDMI, LVDS, etc. In addition, Touchstone models can be used in power integrity applications to simulate and evaluate power distribution networks (PDN) for advanced packages as well as printed circuit boards (PCB). At Signal Edge Solutions, we regularly use Touchstone models with our Sandler State-Space Average VRM models to analyze PDN crosstalk between high-current core power rails and sensitive power domains or sensitive RF signals.

Figure 1 illustrates the measurement results for a complete DDR channel insertion and return loss, and Figure 2 shows the results from a near-end crosstalk (NEXT) and far-end crosstalk (FEXT) simulation.

Figure 1 - Total Channel S-parameter – 1 byte Lane - DQ[7:0] + DQS0 – Insertion Loss & Return Loss - Results

Figure 2 - VCK190 DQ4 Channel Far End and Near End Crosstalks Results

Value of Touchstone Models

Touchstone (SnP) files are the industry-standard format maintained by the IBIS Open Forum, of which Signal Edge Solutions is a proud member, and these files are used both by the EDA industry for simulation and by measurement equipment. They document n-port network parameter data and contain S-parameters, which characterize the network performance of high-frequency systems. S-parameters are useful for a variety of engineering designs, including printed circuit boards (PCBs), packages, radio frequency (RF) circuits, RF components, and communication systems. S-parameters can also be used to analyze the response of a transmission line or a PDN.

S-parameters or Touchstone models are easily measured and can enable simulation-to-measurement correlation. Touchstone models take the raw data and provide a causal and passive representation of it. Unlike in the raw data, both the reflection and transmission terms displayed in the Touchstone model are accurate enough to calculate impedance. For further insights on the importance of simulating PCB effects on your design, we have gone in depth on the subject in the linked blog post.

Causality and Passivity in Touchstone Models: Why They Matter

The importance of using causal and passive Touchstone models in signal and power integrity simulations cannot be overstated. Causality dictates that a system's output cannot precede its input. In the context of S-parameters, this means that the impulse response of the interconnect must be zero for all negative time. If a Touchstone model is non-causal, it implies that the system is predicting future behavior, which is physically impossible. Simulating with non-causal models can lead to erroneous and unphysical results, such as signals appearing at the receiver before they are launched at the transmitter, or gain at frequencies where loss should occur, which distorts time-domain waveforms and makes timing analysis unreliable.

Similarly, passivity ensures that the interconnect or component does not generate energy; it can only dissipate or store it. A passive system will never have a power gain. If a Touchstone model is non-passive, it means the model is implying gain within a passive structure, which can cause simulated responses become unstable. This can manifest as oscillations in time-domain waveforms or infinite gain at certain frequencies in frequency-domain analyses, which makes it impossible to predict real-world performance. Ensuring causality and passivity in Touchstone models is crucial for obtaining physically realistic, stable, and reliable simulation results that accurately reflect the behavior of the actual hardware.

Touchstone Modeling Services: Bridging the HPC Gap

While Touchstone (S-parameter) models are an incredibly helpful tool for comprehensive signal integrity (SI) and power integrity (PI) analysis, generating these models often demands substantial computational resources. This can present a significant hurdle for engineers and design teams who may lack access to dedicated high-performance computing (HPC) infrastructure. Our specialized in-house HPC cluster is engineered to support the complex computations required for generating high-quality Touchstone models. We create these models using advanced 3D field solvers, and we ensure that they are both passive and causal. If you want the benefits of Touchstone models for your SI and PI analysis without incurring the overhead costs of managing your own HPC environment, reach out to us about our modeling services.

If you'd like a quote for the generation of Touchstone (SnP) models for one of your designs, try our EZ quote generator on our website (HERE)

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