My favorite part of electronics design is circuit analysis and simulations. During my time working on metal oxide optoelectronics, I would routinely use a customized numerical solver for fitting theoretical circuit and component models to test devices.
Whether you are designing precision imaging systems, high frequency electronic systems, or other high-speed systems, simulation tools are critical for designing, analyzing, and validating the functionality of your circuits for use in your next PCB. Most novice and veteran designers take advantage of a few standby simulations and analyses, but more complex systems require more complex analytical techniques. The function of your circuits can become as complex as your schematic and layout, so it is important to understand which simulation tools to use in different situations. Unfortunately, the marketing copy you'll find on many PCB design software company websites does not communicate this in the best way.
I've compiled a list of typical analyses you can find in most PCB design simulation packages, or that you can program on your own using SPICE or some other language. I've also listed what each of these tools can determine and why they are important in high speed design. If you're interested in learning more about these different analysis techniques, then read on…
Before getting started, I think that it's important to note that one should distinguish between time domain and frequency domain simulations. First, each type of simulation will tell you different pieces of information about the behavior of your circuits. However, this does not mean that time domain simulations are exclusively used in circuit analysis for high speed PCB design, nor does this mean that frequency domain simulations only tell you something about analog PCB design.
Each type of simulation has its place in each domain. Various PCB design simulation tools are designed to operate in specific domains, but they can be used to analyze circuits in either domain. In other words, using a frequency domain simulation for a digital circuit tells you something about the frequency components that make up a digital signal, while time domain simulations can tell you how the signal changes over time.
This topic deserves its own article, both for digital signals, analog signals, and modulated signals. In general, a linear circuit is composed of purely linear circuit elements (e.g., resistors, capacitors, and inductors), while a nonlinear circuit contains at least one nonlinear element (e.g., diodes, transistors, and other semiconductor devices).
This circuit will act nonlinear once the input amplitude exceeds a certain threshold thanks to the presence of the transistor.
Nonlinear circuits will have a nonlinear relationship between the voltage and current in a circuit. Think about a diode; the current is an exponential function of voltage. This simple distinction between linear and nonlinear circuits is critical to understanding the right circuit simulation to use in different designs.
Certain PCB design simulation tools are designed to work with one or both types of circuits. Understanding this distinction and how different types of circuits relate to your simulation goal is very important for determining the right analytical technique to use. In some cases, nonlinear circuits can act like linear circuits, and understanding when this is the case can help you avoid drawing inaccurate conclusions about your design. Your PCB design simulation tools can typically be applied at the component level, as long as parasitics are properly considered.
Circuit design and analysis tools are generally intended for use at the schematic level. This is an important point to understand in that circuit schematics and PCB layouts do not communicate the same information. Your PCB layout will determine parasitics in your board, which will affect the behavior of your board. Some effects produced in a real layout include crosstalk, conducted/radiated EMI, and transmission line effects.
From here, we want to focus on circuit and system-level simulations, meaning PCB design simulation tools that are applied at the schematic level. Certain specialized simulation and analysis tools are designed to account for a 2D or quasi-2D PCB layout. More powerful simulation tools, such as 3D field solvers that run in the frequency and/or time domain, are designed to account for the real geometry of your layout.
As you start building successively more complex devices, you'll soon find that most circuit simulation tools are just not equipped to address issues like crosstalk, radiated EMI, power integrity, and other problems that arise due to parasitics. If you want to account for parasitics at the schematic level, then you need to include these parasitic circuit elements in your schematic simulations. EMWorks provides a decent guide on determining parasitics in your board in certain special situations. These processes help with certain aspects of transmission line design, including crosstalk due to coupling.
As much as I would like to get into depth on each of these subjects, it is beyond the scope of this article. My intention here is to give you a baseline for choosing the right circuit analysis technique to use in different situations. Hopefully, this will help you determine the best course to pursue for validating the functionality of your circuits.
In general, the response of any circuit in the time-domain can be calculated with finite difference methods. However, there are some specific tasks that can be performed, as shown below.
Results from a Monte Carlo sensitivity analysis simulation for an AC signal.
This set of techniques is fundamental in linear time-invariant circuit analysis:
High speed PCB design simulation results showing timing jitter.
Here are some important nonlinear analyses:
As high speed and high frequency devices continue to become more complex and functionality expands, working with a simulator package can help you better understand your system's performance. Simulation packages cannot build your board for you, but the results can provide some valuable insights into design changes that can boost performance and help you meet your design requirements. Once your simulator returns the results you're looking for, you can decide on the best design changes to improve the performance of your device.
SPICE models help with quick circuit analysis for larger circuits, but you'll have to manually create models that address issues that are important for circuit analysis for high speed PCB design. Unless you can find models for every IC that appears in your board, you'll have to manually recreate logic circuits for use in your SPICE models. Instead, you'll want to use a simulator that goes beyond simpler SPICE models and incorporates a broad range of topologies, components, and structures.
At NWES, we help innovators get their new designs into production and plan marketing campaigns to support product releases. If you're looking for a cutting-edge agile PCB design service bureau and thought leadership marketing services, contact NWES for a consultation.