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. 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. This method is often found in applications involving a parallel bus of many single-ended signals the typical example is DDR. The layout shown below uses multple trace extensions of different distances. 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. ![]() Sawtooth length matching for high-speed signals: the “3W” rule.Īccordion tuning is also often referred to as serpentine length tuning. These dimensions are used to minimize any impedance discontinuities along the length of the 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. 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 trace should be precisely spaced, as shown below. In the sawtooth tuning example below, there are no smooth bends along the trace. ![]() 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. ![]() The most popular example of length tuning is sawtooth tuning, sometimes also called serpentine tuning. Below I've presented three common length tuning options found in high-speed PCB layouts. Length tuning structures will always create three problems: input odd-mode impedance mismatch, NEXT, and mode conversion in differential pairs. When selecting a length tuning option, we have to consider two important points: 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. At low speeds, the difference between the different length-matching styles is superficial due to the longer rise time of those signals. ![]() 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. Length Matching for High-Speed Signals Options 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. 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. The remaining choice for a designer is deciding which length-matching scheme to use: trombone, accordion, or sawtooth routing. 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. 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.
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