High-speed signals jump over PCB traces
Cable assemblies used to bypass PCB traces go by names such as Sliver, Firefly, and BiPass. In this article, I'll refer to the class of products as "jump-over" cable assemblies. Jump-over cables are made using straight and right-angle connectors connected by a series of differential-pair cables commonly called "twinax." Figure 4 shows Samtec's approach to their Eyespeed high performance twinax cable.
Figure 4. This twinax cable consists of two wire pairs, a dielectric, two metal shields, and a jacket. Source: Samtec.
Note that the wires are in parallel as opposed to being a twisted pair. Twisted-pair wires are typically used in long-reach cables such as telephone lines. "Twisted pairs are good to about 5 GBits/s," said Walz. The twisting reduces susceptibility to interference, but at speeds of 28 Gbits/s NRZ or 56 Gbits/s PAM4 where clock frequencies start at 14 GHz, you need a parallel shielded pair because of crosstalk problems with twisted pairs. "Twinax cables attenuate cross talk by 70 dB to 80 dB," added Walz.
There are differences in the construction of these cables. Figure 5 shows a cross section of the cable in a Molex BiPass cable assembly. In addition to shielding, Molex adds a drain wire that connects to a reference plane, which minimizes excess charge inside the cable. Figure 6 shows the cross-section of a twinax cable from TE Connectivity.
Figure 5. This cross section shows the twinax pair of wires, shields, and a drain wire. Source: Molex.
Figure 6. Twinax cable from TE connectivity doesn’t need the drain wire, nor does it need a second layer of shielding, according to the company. Source: TE Connectivity.
Because of the high frequencies they carry, each wire in the differential pair must be the same length. "We extrude both conductors at the same time under tight process controls as opposed to extruding each wire separately and fusing them together" said Guetig. "Doing so achieves consistency in the dielectric constant of the insulating materials." That consistency minimizes skew, a condition that occurs when signals traveling along the pair reach their destination at different times. At 14 GHz and higher, skew is so much of a problem that it's affected by the glass weave of a PCB as a trace passes over or between weaves. So, unless both pairs of a differential trace pass over the same points, skew will appear in the signal. Skew is less of a problem at, say 10 Gbits/s (5 GHz). Jump-over cables minimize skew because both sides or the differential pair pass through the same materials as opposed to possible differences that can occur in PCB traces.