Tripling AdvancedTCA backplane performance while reducing costs
Justin and Tim discuss recent efforts to improve signal performance by removing the longest data signal paths from the main backplane.
AdvancedTCA continues to grow in telecommunications applications, including the Central Office and access edge. The architecture is also winning designs in some benign-environment military programs as well as other markets. With the emergence of IPTV, volumes of videos online, growth of social networking, and new software applications, bandwidth demand is exploding. AdvancedTCA is evolving with the performance demands, and efforts to achieve 40 Gb per channel (10 Gb per signal pair x 4) are underway. This would provide a scalable bandwidth of up to 10 terabits per second. This performance pressure will further strain signal performance across the backplane.
Signal integrity versus cost
Today’s AdvancedTCA backplanes at 5 Gb per signal pair often require some techniques to maintain channel compliance, which can increase the backplane costs. Methods for achieving signal performance across the backplane include:
n Backdrilling of vias
n Expensive laminates/materials
n More/thicker layers
n Signal conditioning technologies
The vias (plated through holes) are used for connector terminations, connecting the various layers of the Printed Circuit Board (PCB). The “unused” portion of a via hole becomes a circuit “stub,” which can cause reflections and resonances, harming the signal continuity. Backdrilling the vias creates a larger opening at the ends of the hole, thereby reducing the negative effects of the stubs. This approach does not completely solve these problems; it only reduces the effects on signal quality.
High-grade laminate materials such as Nelco 4000-13, Rogers 4350, Megtron 6, and similar, can also improve the backplane performance. With lower dielectric losses than standard FR-4 PCB material, the channel compliance is improved. However, costs typically rise in proportion to higher-grade material use.
Adding more layers to the backplane doesn’t really solve the problem. Increasing the number of backplane layers brings an array of potential problems, including manufacturability, signal integrity, longer stubs, potential issues press-fitting connectors, and the associated costs. In fact, all the measures noted earlier add costs to the AdvancedTCA backplane.
Ideally, backplane manufacturers would use FR-4 and somehow reduce backplane layers and thus costs. Moreover, we’d prefer to eliminate backdrilling and avoid expensive laminates. To do this, we need to think not in two dimensions, but three.
Thinking three dimensionally
Imagine, if we could take the longest data signal paths (which typically have the worst signal integrity results) and remove them from the main backplane, leaving only the control signals and shorter data paths. This would take a typical Dual Star AdvancedTCA backplane from 18 layers to 6-8 layers and vastly improve the signal performance of the backplane with fewer layers (meaning shorter, if any, stubs) and shorter/cleaner signals. That leaves a concern about long traces with high-speed signals. This is where the third dimension comes in.
For the high-speed signals a separate FR-4 press-fit board plugs into the rear of the backplane, carrying the high-speed data signals directly in this “Z coordinate plane or Z dimension.” (See Figure 1.) These boards carry the high-speed signals from slot to slot. An adapter with guide pins can be used to firmly secure the rear plane PCB in place and relieve strain. This adapter has a short, impedance-matched connection between the rear “Z dimension” PCB (or Link) and the backplane connector. They also have staggered arrays, so they can be stacked adjacent to one another, and they come in press-fit pin or compliant pin termination, depending on the backplane thickness.
How does the board fit?
Interestingly, and perhaps surprisingly, in point-to-point serial systems the PCB Links run at a consistent angle across the backplane. Therefore, a linear connection system as described can be routed. The physical hardware defines the angle at which the Link traverses the backplane, and the connector slot pitch and connector row spacing determine the angle. Each level of the Link is terminated with an adapter, which directly interconnects to the appropriate pins of the backplane connector on the rear of the backplane. Various versions of adapters can address special needs.
The PCB Link itself can be manufactured in various sizes, lengths, and routing configurations (see Figure 2). The Link boards can be designed as single-, double-, or four-layer PCBs with one, two, or four channels per board. The two-layer Link board can be used with the Dual Star configuration to connect two slots on either end. The four-layer Link board is designed for a hybrid full Mesh system.
How reliable is this?
Using standard and long-accepted PCB interconnection practices, this Z-Plane, Inc., interconnect scheme is highly reliable. The solder joints between the rear PCB plane and the adapter utilize standard Surface Mount Technology (SMT) soldering practices. The press-fit and compliant pins on the adapters also use tried-and-true practices. Therefore, the parts are easily handled and do not require special tooling or manufacturing technology. For extra protection, a protective plate (see Figure 3) can be incorporated to prevent damage during handling or for applications requiring higher levels of ruggedization.
Rear Transition Module (RTM) implementation is another consideration. One might ask whether these Links would prevent rear I/O. The Links for Dual Star versions are short enough not to interfere with the plugging of rear modules. Initial proof-of-concept Mesh version Links (with more channels) initially were too high, but the latest versions have been reduced in height and are expected to provide enough clearance. It should also be noted that many AdvancedTCA applications don’t require RTMs, so this is probably not a significant issue.
Does it work?
Using an Elma Bustronic Dual Star ATCA backplane and Z-Plane, Inc.’s, rear Link PCBs and adapters, we set out to answer exactly this question. In this 14-slot design, the hub slots are located on the left side of the backplane. The standard backplane has 18 layers with a thickness of 0.132" and works well within specifications at speeds under 5 Gbps.
To test this new “3D” design concept, the AdvancedTCA backplane was redesigned with eight layers, keeping the control lines and shorter channels within the backplane. The six longest channels were reserved for the rear PCB Link configuration. The rear PCB Links with the adapters easily plugged into the rear of the AdvancedTCA backplane with no manufacturing issues or concerns. The characterization results were very positive. For 10 Gbps speeds, TDR, crosstalk, and attenuation measurements show that this system would meet if not exceed the IEEE 802.3ap requirements (see Figure 4). Comparing the standard backplane and the backplane with the rear PCB Links, we can see the results in Table 1.
The PCB price for the eight-layer backplane was less than one-half the cost of the 18 layer, and the backplane assembly costs were a little less. Adding in the manufacturing and material costs of the rear PCB Links, the entire assembly was still approximately 20-30 percent less than the standard backplane. The performance of the eight-layer backplane with the rear PCB was two-three times higher.
This was a first test across a new technology. As the design is optimized in final design implementation, the results will be even more impressive. Further, using the high-speed laminates across the rear PCB (as opposed to the entire backplane), it is expected this design technique could perform beyond 20 Gbps without seriously impacting the costs.
It should be pointed out that although a Dual Star AdvancedTCA backplane was used in this study, a Full Mesh or custom design implementation is also easy to accommodate. Patents for this new technology are currently pending.
As performance demands strain the limits of AdvancedTCA backplane signals, new solutions are needed. Without going down the path of ever-higher backplane costs, we can triple the bandwidth of AdvancedTCA backplanes. Rear-mounting PCB Links in the Z-axis of backplanes can be incorporated using conventional materials, processes, and connector designs. Whether it’s AdvancedTCA or other high-speed architectures, this design concept can accommodate legacy systems and is scalable well into the future.
Justin Moll is Director of Marketing at Elma Bustronic.
Tim Lemke is Chief Technology Officer at Z-Plane, Inc.