Once upon a time, length matching guidelines for high-speed signals required a designer with enough skill to remain productive when manually applying different trace-length turning schemes. With today's advanced interactive routing features in modern PCB design tools, designers no longer need to manually draw out length tuning structures in a PCB layout. The remaining choice for a designer is deciding which length-matching scheme to use: trombone, accordion, or sawtooth routing.
So which of these different options is best for your high-speed design? With sufficiently wide traces (i.e., not in the HDI regime) and near-GHz bandlimited signals, you won’t have to worry about the complex resonance issues you’ll find when working with analog signals in the mmWave and sub-mmWave regimes. However, you still need to consider some important points regarding transmission line and signal integrity behaviors when it comes to length matching in high-speed PCB design.
Whether you’re working with a parallel bus that requires length tuning across multiple signals, or you just need to length match two ends of a differential pair, you’ll need to use some method for length tuning. At low speeds, the difference between the different length-matching styles is superficial due to the longer rise time of those signals. The differences between these become more obvious at faster edge rates, where the input impedance looking into the length tuning structure becomes noticeable and begins to create different levels of mode conversion in the various structures at high frequencies.
When selecting a length tuning option, we have to consider two important points:
Length tuning structures will always create three problems: input odd-mode impedance mismatch, NEXT, and mode conversion in differential pairs. Below I've presented three common length tuning options found in high-speed PCB layouts.
The most popular example of length tuning is sawtooth tuning, sometimes also called serpentine tuning. The guidelines included here are a reflection of the original intent of this length tuning structure, which is to limit mode conversion and the appearance of crosstalk between the extended sections.
In the sawtooth tuning example below, there are no smooth bends along the trace. The trace should be precisely spaced, as shown below. First, there is an “S-2S” rule that has been used below; this was originally intended to ensure that 45-degree bends are used along the length of the length-tuned trace. The “3W” rule (not to be confused with the crosstalk prevention rule of the same name!) is really an upper limit; the length of the extended portion of the sawtooth could range from W to 3W, although some guidelines differ on this rule. These dimensions are used to minimize any impedance discontinuities along the length of the trace.
Sawtooth length matching for high-speed signals: the “3W” rule.
Accordion tuning is also often referred to as serpentine length tuning. Rather than use the diagonal extension shown above, an orthogonal extension is used so that the additional tuning length can be fit into a smaller distane along the straight trace.
The layout shown below uses multple trace extensions of different distances. This method is often found in applications involving a parallel bus of many single-ended signals; the typical example is DDR. These signals need synchronization in time, but these traces are not part of a differential bus, so there is no precise phase requirement across pairs of traces. Therefore, it does not matter where we put the length tuning sections as the receiving component does not distinguish between differential-mode and common-mode noise. This is why typical routing for a DDR interface will look something like the routing below.
Accordion length matching for high-speed signals.
If you are working with lower speed or lower frequency signals, you can get away with trombone tuning on parallel buses with minimal NEXT. This technique should not be used to length tune differential pairs. This is another option that is often found in parallel buses, but it will create much more NEXT than accordion or sawtooth length tuning. The reason for this has to do with the multiple 90 and 180-degree turns in this trace configuration.
If this were used in a differential pair, it should be obvious that the trombone portion alternates differential and common mode coupling between each side of the pair as the signal on one end moves back and forth through the trombone. Signals essentially switch between common mode and differential mode driving as they propagate; this is the very definition of mode conversion. Just as with the other two common length matching methods, if you must use trombone tuning, then you should only put it at the end of the differential pair where the mismatch arises.
Trombone length matching for high-speed signals.
In all three of the above methods, you should be careful not to place each section of a serpentine length matching section too close together. The extension away from the straight trace, and the distance between sections, determines two possible signal integrity effects:
The crosstalk effect (NEXT) and the reflections coming into a length tuning section will distort signals as they travel along the length-matching section. The mode conversion effect causes common mode noise received before the length tuning section to appear as differential mode noise at the receiver. Howard Johnson provides an interesting explanation for the crosstalk effect in this article.
The table below outlines when each of the length tuning methods discussed above is most appropriate to use.
The guidelines presented here are just that: guidelines. Regardless of the signal speed or length tuning style you work with, it is generally recommended that each side of a differential pair be routed as symmetrically as possible; it is understandable that this is not so simple for wide parallel buses. No matter how you choose to route your differential pairs, you should always verify the behavior of each signal in a differential pair using some simulation tools and, ultimately, using measurements.
It is also difficult to generalize exactly which of these options is objectively "best" for length tuning. Anyone who has seen rules of thumb fail in certain situations knows that you should always check your layout, including length matching for high-speed signals, using post-layout simulation tools. This helps you examine important signal integrity problems like crosstalk, excessive signal reflection at bends, and skew in differential signals or across multiple traces that require precise synchronization.
The powerful interactive routing and post-layout analysis tools in Altium Designer® are built on top of a unified rules-driven design engine, allowing you to implement length matching for high-speed signals and checks for signal integrity. You’ll also have a complete set of tools for building schematics, your layout, and preparing deliverables for your manufacturer.
Now you can download a free trial of Altium Designer and learn more about the industry’s best layout, simulation, and production planning tools. Talk to someone at Altium today to learn more about length matching in PCB design.