Simulation Driven Design Can Solve Your PCB’s Signal Problems and More

July 13, 2018 Cody Stetzel

High density trace routing on a PCB

 

If you work in the electronics industry or in research, there’s a chance that simulations are a regular part of life. While working in optics, we would simulate everything  from wavefront aberration corrections to microscope images. I always fell back on simulations to better understand the complex systems we were building. Simulation-driven design is taking the idea of this same process comfort and applying it throughout the whole design process.

 

With today’s PCB devices running at higher speed and requiring better signal-to-noise ratio, simulation software has taken a more prominent role in PCB design. Circuit board simulations should be performed early in the design process to identify signal and power problems before a PCB is sent for production. This way you can communicate board needs more effectively to mechanical and manufacturing teams, as well as keeping your focus on the engineering.

Why PCB Simulation?

Design engineers used to rely heavily on their mathematical modeling skills to design the circuit (in its theoretical form), build a prototype (perhaps on breadboards), then run the physical prototype through its trials. The lead time for these designs was lengthy and expensive. Add on top of that multiple iterations of the prototype and testing became both expensive and a heavy time investment.

 

This would slow down the process of communication between departments and team members as well as make the entire journey from schematic to product wrought with the frustrations of unmet deadlines. Simulation technology can resolve these frustrations, though.

 

Simulation-driven designs enable you to circumvent this weighty and costly process by keeping data available for use and analysis, as well as being adaptable enough to be able to run new simulations based on minor adjustments. Whether you’re an engineer, designer, in-between or wearer-of-many-hats in your workplace, a simulation-driven workflow will enable you to communicate results and analysis with easy-to-explain data and visual displays of your product.

 

Simulations enable better and more informed decisions with quicker turnarounds. But even more importantly, simulations are easier to transport, display, and explain to people with any range of expertise over designs. So how can you make the jump into the field?

Power Delivery Simulations

Delivery of power is as its name implies: you need to ensure that the 5 V from your power supply is delivered as 5 V to your components. In digital systems, this makes the difference between bringing logic gates up to full power and being able to distinguishing between high, low, and undefined signals. Even though the rest of your device may meet basic specifications, it can still fail if critical components do not receive the required power level.

 

You’ve probably been told that the resistance of copper conductors is so small that it can be neglected in power distribution calculations. But in a PCB, parasitic capacitance and inductance both become significant and limit the current over a broad range of frequencies. Power losses due to voltage drop (also called “IR drop”) across PCB traces can reduce power levels below the required level before signals reach your components.

 

So how do you identify whether your downstream components are likely to receive the right voltage? This is where power delivery network analysis (PDNA) comes in. This lets you identify how power is distributed throughout your board, and the best PDNA tools will present this information in a visual plot. This makes it easy to identify which elements on your PCB are consuming excess power and helps you redesign the board as necessary.

 

PDNA also tells you how current moves throughout your board, and this lets you deduce which portions of the board are likely to overheat. Since current is proportional to heat dissipation, an excellent PDNA tool will calculate the heat dissipation throughout the board. You’ll want to identify which components are likely to overheat before your PCB goes up in smoke.

 

Heat dissipation in an IC on a blue PCB)

Heat dissipation in an IC

Analyzing Signal Integrity in Digital Circuits

When working with low speed signals on short traces, the digital signals with decently long rise time won’t behave as transmission lines as long as the line delay is short. As digital signal speeds increased into the MHz and GHz levels, line delays became longer than signal rise times. If there is an impedance mismatch between the trace and the downstream component, resonance can occur due to signal reflection, which leads to ringing.

 

Rather than stare at your PCB layout and estimate impedance mismatches between traces and components, ringing can be diagnosed with a signal analysis tool. Ringing appears as a decaying oscillation that is superimposed on top of a digital signal on a voltage versus time graph.

 

Your signal analysis tool should also tell you the level of impedance mismatch. This will allow you to compensate for ringing with a series resistor. You can also increase the natural ringing oscillation frequency by changing the LC time constant of your terminating inductor and capacitor.

Eye Diagrams and Bit Error Rates

When logic ICs switch between high and low output states, they draw power from the power supply. Due to output capacitance on the power supply, the logic states tend to switch faster than the power supply can respond. This momentarily affects the power distribution in the rest of the PCB. When an entire bank of ICs or an entire address bus switches states, the momentary power drop can be significant.

 

Decoupling capacitors can be added to the IC power pins to compensate for the momentary power drop. At the moment the power drops, the capacitors discharge and help correct the drop in supply voltage. The power drop due to switching manifests itself as significant noise in an eye diagram, which leads to higher bit error rate. Compensating for power drop due to switching can tighten your eye diagram and reduce the bit error rate.

 

Eye ball on PCB background

Eye diagrams have nothing to do with optometry

 

See the Value, Sell the Value of PCB Simulation Software

Outside of the obvious benefits to the security of your designs, simulation technology and simulation driven design are helpful, too, from a workplace perspective. Whether it is through easier, visual displays of information or access to files that can be sent to whomever, it is easy to display the benefits of incorporating simulations into your workflow from any angle of your team. For example:

  • Designers and engineers will exhibit a greater confidence in their early designs which will lead to better decision making in design variations, improving the optimization of their designs.
  • Analysts may be able to focus more on complex simulations with the time that is freed up with the use of simulation software.
  • Test engineers can even use simulations to better their testing plans by optimizing sensor placement which could lead to a reduction in testing verification time.

Spreading the word on how the company as a whole can benefit, rather than just a sole entity, will ensure a team-wide appreciation for the process shift. A team that sees benefits together, will win together.

 

As with any large shift in process, you should take simulation adoption with baby steps. Let someone trustworthy learn and find snags and hang-ups before implementing it for everybody. This will give you a chance to prove the technology, define your own best practices, and relay the success out to the next teams in the simulation path.

 

After, the next hurdle will be in acquiring training resources for incorporating PCB simulation into your team. Luckily, training for much of the simulation software out there will be in an easily digestible format with cloud-based support. Long gone are the days of massive textbooks and CD tutorials. Hello, days of seamless team-wide incorporation of new processes.

 

This learning curve again will be a large consideration for companies, especially if the curve is steep and long. Again, be sure to demonstrate the return on investment you’ll be seeing post-simulation incorporation.

 

Your signal integrity simulation and analysis tool should allow you to generate eye diagrams based on your PCB layout. Compatibility between simulation programs is also extremely important and gives you the ability to simulate just about any PCB design issue under the sun. Pursue simulation driven design and you’ll assuredly be soaring afterwards.

 

Altium Designer 18.1 includes a variety of advanced design and simulation tools can help you diagnose and avoid signal integrity problems. Now you can download a free trial and find out if Altium Designer is right for you. To learn more about what great benefits simulation-driven design can offer you, talk to an Altium expert today.

 
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