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Rule of thumb: How long a stub is too long?

19 Jan 2015  | Eric Bogatin

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Spoiler summary: The maximum acceptable stub length is roughly Len[in] < 0.3/BR[Gbps], or, Len[cm] < 0.75/BR[Gbps].

In the last rule of thumb, we identified the frequency of the dip in the insertion loss from the quarter wave stub resonance. The effect of a stub routing topology, from whatever source, can suck out a significant fraction of the signal energy at and near the quarter wave stub resonance frequency. We identified that the resonant frequency depends on the length of the stub.

If we know the bandwidth of the signal, we can estimate the longest stub length before the quarter wave stub resonance begins to affect the signal quality.

As a rough rule of thumb, for the presence of the stub resonance to have little impact on the signal, we would like to engineer the stub length so it is short enough to push the resonant frequency well above the bandwidth of the signal.

We translate "well above" into a resonant frequency at least twice the signal bandwidth.

This condition for the stub length having little impact is:

In the case of a high speed serial link, in the best case, the bandwidth of the signal is about 2.5 × Bit Rate (BR). This is Rule of thumb: Bandwidth of high speed signal.

The quarter wave stub resonant frequency = 1.5/Len[inches], Rule of Thumb #17. Combining these, we have:

After a little algebra, we arrive at:

If the data rate is 1Gbit/s, the longest stub length, in FR4, before the stub begins to affect the signal quality is 0.3/1 = 0.3 inches, or 0.75 cm. This says that trying to engineer an interconnect channel operating at 1Gbit/s or higher is very tough to do with any appreciable routing or test line stubs.

In the gigabit regime, only point-to-point routing architectures will work. This is one reason engineering a dual bank DDR4 design at 2.3Gbit/s is such a challenge. Point-to-point, with all routing stubs shorter than 0.15 inch, makes the design possible.

Now you try it:

1. What is the longest via stub before problems may arise with a PCIe gen II design?

2. In a 28Gbit/s design, what is the longest routing stub before problems may arise?

About the author
Eric Bogatin is Signal Integrity Evangelist at Teledyne LeCroy.

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