In late April Samtec kicked off its series of gEEk® spEEk webinars. gEEk spEEk allows engineers to interact with Samtec’s engineering leaders and SI experts on a variety of selected SI-related topics.
Samtec SI Engineer Steve Krooswyk presented one of the first webinars entitled “PCI Express: Is 85 Ohms Really Needed?” This webinar was well-attended and incredibly well-received.
I spoke to Steve about the presentation and why he selected this topic, about the PCIe 85 ohm requirement, and about PCIe 85 ohm over cabling, to name a few topics. The following is a summary of our conversation.
DANNY: Steve, can you give us a brief overview of this webinar? Why did you select this topic?
STEVE: High speed systems today face many simultaneous impedance requirements that must be achieved in the same stack-up – differential impedances at 100 ohms for ethernet, USB at 90 ohms, PCIe at 85 ohms, and a myriad of single-ended DDR requirements. We want to follow the specification, selecting components and routing traces at the correct impedance. However, sometimes there are hurdles – the preferred component is not available in 85 ohms, or the upstream package is yet another impedance. We want to follow this mantra of 85 ohms for PCIe, but it may not be so easy. What can we do, and what does the specification say?
DANNY: Is an 85-ohm connector required for PCIe?
STEVE: Lets go directly to the specification. About the PCIe 85 ohm requirement, the specification says “This requirement does not apply to vias, connectors, packages, cables, and other similar structures” PCI Express Card EletroMechanical Specification Revision 4.0 version 1.0, section 4.7.8.
Connectors outside of 85 ohms are acceptable, and in fact this is common. For short durations, the contacts are exposed to air (like CPU sockets), which raises the impedance. These short excursions will be higher than 85 ohm, but play no detrimental role in the return loss. In fact, it is quite common to observe 110 ohms on compliant Gen3 and Gen4 connectors.
DANNY: On the PCB, is 85 ohm routing required?
STEVE: It is required for the case of interoperable systems and cards that follow the CEM (Card EletroMechanical) specification – ensuring a matched impedance for any mated device.
All other systems — with any number of connectors or cables — may use any PCB impedance. Many packages and connectors are designed to support multiple I/O and will have an impedance of 100 ohms, or even 93 ohms as a balanced compromise. I recommend to review and determine what impedance may be optimal for your specific design.
It is becoming more common to see 93 ohm routing, but it is not unreasonable to continue with 85 ohm routing alongside higher impedance packages and connectors. Lower PCB impedance does tend to have some advantages including lower losses, better matching to dense BGAs, and more tolerance to impedance variation to name a few.
DANNY: Would PCIe over cabling require 85 ohm twinax?
STEVE: It is tempting to select cable impedance to match what is used on the PCB. However there are a couple of anecdotal considerations. First, higher frequency reflections are a greater function of the mating connector impedance than the PCB itself. If an incoming signal experiences an increase from 85 on PCB up to 93 ohms for a given connector, it is best to remain at 93 ohm and not create new reflections by following it with a decrease to 85 ohm cable.
Secondly, the relationship between impedance and loss is inverse of the PCB relationship. That is, higher impedance cable has less insertion loss whereas higher impedance PCB would otherwise be more loss. The loss difference between 85 and 100 ohms is as much as 14%, and is explained in the actual geometry of the twinax. This is the experience for Samtec twinax eyespeed cable and may not be the experience for all cable suppliers.
Ultimately, cable impedance choice depends on the priorities for your system. If insertion loss is the greatest constraint, then 93 or 100 ohm cable is the best choice.
DANNY: Excellent! Any closing comments?
STEVE: Hopefully our readers are enabled to review their specific design, select the optimal impedance, and not be constrained to 85 ohms. Remember, the specification only requires 85 ohms for the PCB routing when using the interoperable CEM slot. When it comes to connectors, cables, packages, and PCB routing to other connectors, the specification permits any impedance that is optimal for your design.
This summary only scratches the surface of everything covered in the webinar. If you want to check out the entire presentation and discussion, here’s a link to the “PCI Express: Is 85 Ohms Really Needed?” gEEk spEEk webinar.
Here’s a list of upcoming gEEk spEEk webinars as well as links to the previous webinars if you want to listen to them.
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