Breaking up a pair

摘要

The two traces comprising a differential pair, when routed close together, share a certain amount of cross-coupling. This coupling lowers the differential impedance between the traces. For example, when two 50Ω traces are well-separated, the differential impedance between them should be precisely 2Z_0= 100Ω. When you jam these traces close together, as you might do to improve routing density, the differential impedance will be a lower value, perhaps something like 82Ω. If the coupling effect lowers the differential impedance too much for your taste, you can fix it. Just reduce the trace widths, thus raising their impedance. The traces remain coupled, but you can push the differential impedance back up to 100Ω.