Introduction to High Speed PCB Designing: Signal Paths

Have you ever planned an airline trip where there isn’t a direct flight to get you to your destination? I have, and as you probably already know, it isn’t a problem. I use the travel planner on my airline’s website and let it figure out every detail of the trip for me. All I have to do is plug in my origin and destination, and the planner determines my itinerary—with enough time to make my connections. It’s a piece of cake.

Can you imagine how much of your time it would take if you had to do all the travel planning yourself? You would have to research the different airlines, flights, departure and arrival times, and then compute all of that information together to figure out the best route.

An airline trip requiring multiple connecting flights is very similar to a signal path on a printed circuit board. A signal path has a start and a finish, with several connections between them, just like the airline trip. And just as you need to time your itinerary, trying to route a signal path on a PCB requires determining the lengths and parameters of all the individual nets—a time-consuming proposition at best.

Fortunately, though, we have PCB layout software tools that can manage signal paths in the same way that airline websites use travel planning tools to set up trips. Let’s take a look at signal paths and how today’s PCB layout software tools can help you work with them. I can’t promise you a trouble-free trip to some exotic desert isle, but I can promise you a better understanding of how you can work with signal paths in your next high speed PCB design.

What is a Signal Path?

A signal path is a path that a signal takes from its origin through multiple components and nets to its destination. For example, consider a simple transmission line between two parts that have an inline terminating resistor. The first net connects the driver pin to one end of the resistor, while the second net connects the other end of the resistor to the receiver pin. In the CAD system this is two different nets, but for high speed design purposes, the two nets form one signal path.

When you design a high speed PCB, you can control the impedance of your routing through the width of the trace and how you designate the board layer stackup. You can also control the length of the trace routing and match those trace lengths with other traces. The next step in developing your high speed PCB design skills is routing signal paths. Not only will you still need to route controlled impedance and matched trace lengths, but you will have to do that for the entire signal path instead of for individual nets. Fortunately, the CAD tools in use today can offer you a lot of help with signal path routing.

Configuring a Signal Path for Routing

To configure a signal path that maintains good signal integrity, you will create a signal path class in your CAD tools and assign the appropriate nets to that class. When assigning the nets to the signal path class, you can assign them manually or, if your software has this option, let the CAD tools automatically assign the nets based on the drivers and receivers that you specify. Whether you assign the nets manually or automatically, you will still need to confirm that they are in the correct sequence. This is important so that the routing topology of the signal path nets is in the correct order. Many designers automatically assign the nets in their signal classes, and then manually re-order them if needed.

Once the nets are assigned to the signal path class, you will then be able to specify other parameters for signal path class. These parameters include the maximum and minimum lengths of the routing that is allowed and the maximum and minimum amount of the vias that can be used.