In the above equation, we’ve ignored losses (R and G in the transmission line impedance equation), but that’s okay, the point here is to pay attention to spacing. The mutual inductance and capacitance exist to give the two pairs an equivalent total inductance and capacitance, respectively. This means the formula for the typical impedance of a different l pair needs to be decomposed into the odd impedance and the differential impedance, which are defined as follows: Odd-mode and differential-mode impedance formulas. The impedance of a differential pair depends on the self capacitance and self inductance of each trace, and the mutual capacitance and mutual inductance between each trace. The primary parameter in a differential pair that is affected by spacing is the impedance.
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Let’s look at each of the dimensions I mentioned above to see exactly where differential pair spacing plays a role, and how to set the appropriate value. How Differential Pair Spacing Affects Signal Integrity As we’ll see, the focus on tight coupling (whatever that is supposed to mean) has its merits, but it’s often cited as necessary for the wrong reasons. loose coupling in terms of differential pair spacing, as well as how the differential pair spacing affects things like impedance, differential-mode noise, reception of common-mode noise, and termination. In this article, we want to get closer to a realistic description of tight coupling vs. What exactly is “loose coupling” or “tight coupling” in a numerical sense? If you ask 10 different signal integrity gurus, you’ll get 20 different answers! What’s interesting about this design guideline is that it is probably the only PCB design rule of thumb that is most poorly defined. So the question is, how close do the traces in a differential pair need to be to each other, and is the need for “tight coupling” really necessary? Just as important as determining an appropriate trace width for a single-ended trace is determination of an appropriate spacing between two traces in a differential pair. We get a lot of questions about trace impedance and how to calculate the right trace size to hit a specific impedance in a manufacturable PCB.
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