DESIGN CREATION In the design-creation stage, engineers will move into final component selections and creation of their libraries, which in turn facilitates schematic creation. They'll undertake the task of constraint definition and capture as well.
"In this stage, designers are evaluating and selecting building blocks," says Mistry. They'll also head for manufacturers' Web sites in search of datasheets and specs.
"A more advantageous way to approach this," says Mistry, "is to move part selection directly into the schematic capture process." By performing schematic capture in this manner, the process can be used as an experimental canvas of sorts (Fig. 2).
In schematic capture, it's important for designers to be able to quickly add, subtract, or change components, or even the entire design topology. For instance, designers who are developing a speed filter for a mobile handset should set up the passband and other filter parameters during schematic capture by experimenting with various capacitance or inductance values.
While creating the schematic, the PCB design tools also automatically create a netlist for the circuit in the background. That netlist describes how the circuit's components are interconnected and how they will be used by downstream placement and routing tools for board layout.
This is when designers will create symbols and footprints for so-called "megacomponents," such as FPGAs or other programmable devices. It's also when design constraints are captured—a critical step that requires considerable thought, particularly in terms of downstream processes.
"Everything is constrained in PCB design now," says David Wiens, business development director in Mentor Graphics' System Design Division. "It used to be confined to manufacturing issues, but now everything has advanced constraints as we attempt to squeeze boards into small space while still making it manufacturable."
Design requirements may indeed lead to an abundance of constraints, yet it's important not to overconstrain your design. "It's preferable to rely more on simulation and analysis than to simply constrain your design," says Zuken's Ed Duranty. "I've heard engineers say, ‘I know this has worked before so let's do it.' They don't really know if they need a given constraint or not."
During design creation, engineers need to mind signal-integrity concerns that will crop up later in the process. "Signal integrity requires some tackling at the design-capture stage as well as during board layout," says Phil Loughhead, director of product knowledge at Altium. "The design flow has to support that process. You can't tune out the issue of impedance mismatches during design capture."
SIMULATION IS KEY Once the circuit is designed and a schematic finalized, then comes functional verification. This is usually accomplished through the use of simulation tools. Also, there are various reasons for undergoing a thorough simulation of your circuit. First and foremost, it'll give you a good indication of your circuit's behavior.
"There are misconceptions about simulation," says NI's Mistry. "It's not intended to replace physical prototyping, but rather to eliminate iterations in prototyping." That's because simulation enables designers to pick up on design flaws that typically wouldn't be caught until prototyping.
Simulation makes it easy to experiment with "what-if" scenarios. You can experiment with various design topologies and substitute parts from various vendors to examine their effects on the circuit's performance.
The ever-present rub with simulation, though, is the availability of models, as well as their validity. All of today's commonly used PCB design suites come with expansive model libraries, but there may be times when a given part isn't represented. However, component vendors are increasingly picking up the slack in this regard by making Spice models available on their Web sites, so it's a good idea to check for them.
SIDESTEPPING LIMITS Spice-based simulation does have its limits—it can produce somewhat idealized simulated signals that aren't necessarily representative of real-world conditions. "A real signal may have elements of noise- and phase-shifting that alter its fidelity," says Mistry.
National Instruments' board-design flow includes virtual instruments, which can be used with the company's array of PXI instrumentation to generate real signals with the attendant nonlinearities intact. Those signals can be captured in a native file format for use in Spice simulation to validate the circuit's behavior. Furthermore, virtual prototyping can help provide feedback into the selection of components (Fig. 3).
It's also crucial that simulation be performed at system level. "It's not just about modeling a signal across the PCB," says Mentor's Wiens. "You also have to examine what the signal does inside of components and even across multiple boards."
A complicating factor is the proliferation of multi-gigabit signaling on PCBs. Serial bus architectures are gaining favor over traditional parallel bus schemes. This requires board designers to cope in simulation with lossy, coupled transmission lines as well as detailed via models.
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