High-Speed Design Techniques: Trace Tuning for Routing Length Matching

November 27, 2017 Altium Designer

A school band playing together

 

 

The first time I heard someone talking about “tuning” their high-speed PCB design, the memory of my middle school band came to mind. Back in those days, “tuning” meant taking kids who lacked experience and who used tired old instruments and trying to turn it all into something that blended together. As you would expect, our first attempts at being in tune were not very successful. After some work, though, we were able to stay in tune together, and we began to sound like a real band.

 

A band tunes up so that each instrument is in sync with the rest of the instruments in the band. When everyone plays a concert B-flat, it should all sound like the same note. On the other hand, when a band is out of tune it can actually hurt to listen to the sound of the different instruments fighting against each other.

 

Like a band, a high speed printed circuit board design also needs to be “tuned up.” There are high-speed nets that must have specific lengths “tuned” to each other to function as intended. Let’s take a look at what trace tuning is on a PCB and discuss some ideas on how to best accomplish it.

What is Trace Tuning?

PCB trace tuning is creating serpentine trace routing map patterns to achieve a specific overall length of the trace, as shown in the picture below. This trace length must be matched with the lengths of other traces so that all of their signals arrive at their destinations at the same time. Matched trace lengths are important in data and clock routing to synchronize all of these signals together.

 

When I first started tuning traces to match lengths, it was more of a guessing game than anything else. We only had a general length that we had to match, and there were even times that we would visually compare the traces on the screen to see if they were close. Now you can use trace length report generators and real-time trace length reporting, while you are routing, to find the exact trace length. You can also use trace length rule constraints and automated tuning functions to help you.

 

 

Close-up of a blue circuit board

Some examples of serpentine routing on a PCB

 

Different Types of Trace Tuning for Trace Length Matching

In the picture above you can see examples of serpentine trace tuning. This tuning can be tight in an accordion pattern or elongated in a trombone pattern. High-speed design experts recommend routing in big trombone patterns to fill up the area and leave space for tuning later. Then when you are ready, come back and fine tune the trombone routing in a tighter accordion pattern to get the exact trace length that you need. You should also use 45-degree corners on the waves of the accordion patterns and space the waves at a minimum distance of 3 times the trace width.

 

In the same way that individual nets need to be tuned to other nets, you should also match the lengths of the two nets in a differential pair. Keep the pair routed together as much as possible and put in the trace length waves on the mismatched ends. It is important to route the two traces of differential pair together around obstacles such as vias or holes when you are matching their lengths. Don’t split the pair up while routing around these obstacles. Differential pairs will also need to be matched in length to other differential pairs as well.

 

 

PCB routing in blue and green

PCB trace routing

 

 

Trace tuning functionality that will help you

There is a lot of functionality in PCB design tools that will help you with your routing and trace tuning. First, you can set up trace length rules in the high-speed routing constraints. These rules will ensure that your routing is created at the correct lengths and appropriately matched to other nets. You can also use processes that will automatically route the tuning patterns to the correct lengths so that you don’t have to manually create the waves. These utilities provide the control you need to regulate the trace width and the height and spacing of the accordion wave patterns. Finally, you can also use differential pair routers to match the lengths of the two traces in the pair together.

 

PCB design software, like Altium Designer, has high-speed design functionality for routing and trace tuning built into it. This will help you to route the high speed traces on your PCB to the correct lengths without having to guess their actual lengths. With this kind of help, you can create a high-speed compliant design in less time and with greater confidence in its accuracy.

 

Would you like to find out more about how Altium can help you with your high-speed design needs? Talk to an expert at Altium.

 

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About the Author

Altium Designer

PCB Design Tools for Electronics Design and DFM. Information for EDA Leaders.

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