A trio of articles explain how QSFP-DD and MicroQSFP connectors high-speed connectors handle excess heat and what engineers must do the minimize EMI.
Datacenters never always need higher data rates, which forces engineers to develop new forms of connectivity. The latest iterations of the popular SFP connectors — QSFP-DD and microQSFP — bring 100 Gbps speeds to the faceplates of switches, routers, and servers while managing to keep their cool.
A trio of articles on EE Times' companion site EDN recently featured two articles that provide a look at the latest connectors and how their designs manage heat. A third looked at the EMI aspects of high-speed connectors.
SFP optical modules: Legacy compatibility vs. improved performance by Nathan Tracy of TE Connectivity looks at two new connector styles — QSFP-DD and microQSPF — and the place each holds in the overall connector ecosystem. QSFP-DD (Quad Small Form Factor Pluggable Double Density) adds a second row of connections in the QSFP connector, effectively doubling the density of electrical or optical connections in the same space. Combined with PAM4 modulation, that quadruples overall data rate.
microQSFP form factor modules can insert into single-height or double-height cages. Source: TE Connectivity.
The downside if QSFP-DD is "while the receptacles are backward-compatible, SFP-DD modules can't be used in old equipment; the design requires an entirely new PCB layout." That said, you can use QSFP-DD receptacles with QSFP modules, you just won't get the speed advantages. If, however, you don't need backward compatibility but require a path to future generations, then microQSFP is an option.
Tracy looks at how module designs handles the heating issue, covering heat sinks and airflow. The problem with QSFP-DD is the double row of connectors, which limits airflow.
Joe Dambach from Molex also looked at QSFP-DD connectors and their thermal issues. In, QSFP-DD pluggable modules boost data density
he noted that QSFP-DD modules require heat sinks when used with optics, but have operated without heat sinks when operating as passive electrical cables. But, that's changing as date rates increase. "An I/O module with fewer than four lanes can typically be cooled using airflow intended to cool other components. With four signal lanes, the QSFP form factor requires a combination of airflow and heat sinks." Data rates of 100 Gbps require four lanes.
With a heat sink (left), A QSFP-DD module can support optics. Without the heat sink (right), a module can support passive cables. Source: Molex
Because the the double density of QSFP-DD modules, what used to be a 3.5-W module becomes a 7-W module. Dambach goes on to demonstrate how engineers test QSFP-DD modules for cooling capacity.
Besides having thermal issue, high-speed connectors also have EMI issues. As reported in Connectors: Not just schematic symbols
Colin Brench, EMI engineer at Amphenol explained that, unfortunately, EMI doesn’t top the list of design considerations when it comes to these modules. At frequencies over 5 GHz, backplane connectors can be one wavelength in size, turning them into potential radiators. Under those conditions, EMI can be a significant issue, but, as Brench pointed out, EMI lags behind signal integrity, manufacturability, and cost on the list of important design considerations.
Connectors can produce EMI at high data rates.
If you think that today's data rates won't increase in the future, think again. 400 Gbps Ethernet is on the cusp of deployment. What that's optical, you can be sure that an electrical version will occur.
covers test and measurement for EE Times
. Contact him at martin.rowe@AspenCore.com