Flyover™ Cables: Inevitable, but Not Easy
Most designs get by just fine with FR4 multilayer boards, but as Samtec showed at DesignCon 2019, bleeding edge interconnects running signals at 28 gigabits per second (Gbps) and beyond require a rethink. Or, in the case of Samtec’s solution, a complete rerouting of the signals using FlyoverTM cables that remove the signals off the backplane entirely, from chip to off-system connector.
The role of Flyover cables is to isolate signals from the limitations of printed circuit boards (pc boards). As signaling speeds increase, the board’s dielectric constant becomes an issue, and traces need to be shaped and routed perfectly to avoid EMI/EMC and signal coupling and crosstalk. Recently, the shift from NRZ to PAM4 modulation encoding for leading-edge server backplanes has made it extremely difficult to meet jitter and noise requirements over any useful length of pc board, despite enormous advances in channel characterization and equalization (Figure 1).
Figure 1: As signaling rates increase to PAM4 (56 Gbits/s) and beyond, pc boards become costly and less reliable, and so are being augmented by Flyover cables. (Image source: Samtec Inc.)
The Flyover concept was conceived by Samtec for its FireFlyTM line. It involves designing cables and connectors specifically to handle high-speed signals, and then routing these cables above and across the board. This saves costs on pc boards by reducing their material and specification requirements, while literally adding flexibility to high-speed interconnects.
At DesignCon, Samtec showcased a 112 Gbps Flyover connection from a Credo Pelican II SERDES ASIC to a front panel connector (Figure 2).
Figure 2: Samtec’s Flyover cable demonstration at DesignCon comprised a 112 Gbps cable (blue) connection from an ASIC to a front panel interface, that went right over the pc boards beneath. (Image source: TechWire International)
At the heart of the demo’s Flyover cable technology is its Twinax low skew, low loss cable design. At 28 gigahertz (GHz) it has ~13 dB lower insertion loss compared to a backplane pc board trace design (Figure 3).
Figure 3: A comparison of Samtec’s low skew, low loss cable to a backplane pc board trace design shows ~13 dB lower insertion loss for the Twinax Flyover cable at 28 GHz. (Image source: Samtec Inc.)
The results of the demo at DesignCon are shown, and they are impressive (Figure 4).
Figure 4: The Flyover demo’s insertion loss plot shows 15 dB of loss at 28 GHz with PAM4 modulation. The eye heights are all greater than 125 millivolts (mV). (Image source: TechWire International)
The Flyover demo’s insertion loss plot shows 15 dB of loss at 28 GHz with PAM4 modulation. The eye heights are all greater than 125 mV, and the bit error ratio of the entire system is 7.05e-09 pre-forward error correction (FEC), which is 4 – 5 orders of magnitude better than the spec.
Flyover cable design
The DesignCon demo used 12 inches of 34 AWG Eye Speed ultra-low skew Twinax cable (Figure 5). However, sometimes how a solution works is just as interesting as what it does.
Figure 5: Samtec’s Twinax cable has many features to ensure signal integrity at up to 112 Gbps, as demonstrated at DesignCon 2019. (Image source: Samtec Inc.)
Speaking with Danny Boesing, product marketing director at Samtec, he laid claim to the Twinax being able to carry faster signals for a given AWG than any cable out there. He may be right, but I was just as curious as to how it worked.
There are a few features that one might expect, such as a high-temp extruded jacket (those servers can get hot), an additional metalized barrier, and underneath that an “advanced” copper alloy shield. Underneath the shield, the two conducting wires are encased in a low dielectric constant (Dk) fluorinated ethylene polypropylene (FEP) or perfluoroalkoxy alkanes (PFA) material. One of the features of these formulations is that they can be melt processed.
However, a large part of the Samtec “secret sauce” lies in the silver-plated copper conductors. I can’t say exactly what it is (as it’s still secret), but the term “co-extruded” is clearly cited in the company’s literature, which points the curious in the right direction.
While it may still be a secret, sometimes innovations simply seem to be a good idea, especially after you find out how it works. Often the secret sauce is simply a matter of executing upon every workable idea toward making the best product possible. In this case it just happens to be high-speed cables.
Expect more FireFly announcements from Samtec with the Optical Fiber in Communications (OFC) coming. In the meantime, here’s a FireFly application design guide available to help you get started with the Flyover concept in the optical domain.