Signal Integrity Simulations for Backplane Bus Connectors

August 20, 2019 Zachariah Peterson

Ethernet ports on a PCB
You’ll need a backplane to stack multiple ports in a rack-mount unit

The days of sub-Gbps data rates are long gone in most systems. Modern systems which interface with a large number of devices in a single enclosure will probably need a backplane, and those that do will likely run above Gbps data rates. Of particular importance is your choice of connectors, and it’s consequential to inform your design and connector choices with simulations.

Backplanes can be quite large, and it’s likely that traces in your bus will act like transmission lines. At a modest data rate of 1 Gbps, traces that are longer than a few inches will act like transmission lines. This makes impedance matching at connectors an important point to consider. Unfortunately, the electrical models for many connectors may not be accurate at every data rate, and they might not match what‘s measured in an actual prototype. Therefore, it is important to accurately simulate the behavior of your backplane as well how backplane bus connectors affects your signal integrity.


Signal Integrity With Backplane Bus Connectors

Backplane bus design is as much about selecting and modeling the right connectors for your particular application as it is about choosing the right routing topology for your system. Signal integrity between successive connectors will primarily be determined by the choice of routing standard and logic family. However, the connectors themselves also affect signal integrity in a backplane bus.

Whether you are designing a backplane or any other device that requires a connector, you should always pay attention to your manufacturer’s specifications on connectors, particularly regarding recommended data rates. Although manufacturers may not state this explicitly, some connectors can be used with higher data rates than specified. This will affect crosstalk coupling in single-ended topologies, as well as creating the potential for signal reflections at a connector.

Improper modeling of backplane bus connectors can create signal integrity problems commonly found on transmission lines. Analog backplanes will have their own set of signal integrity issues, particularly if analog signals are converted to digital directly on the daughterboards. In this case, your digital signals can interfere with sensitive analog signals, producing errors in the converted data. At this point, a grounding and shielding strategy will be critical to ensuring analog signals do not become corrupted by high speed digital data, just as is the case in any mixed-signal PCB.


Simulating Signals on Backplane Bus Connectors

If you purchase connectors from a reputable manufacturer or distributor, they should provide you with IBIS or SPICE models for running simulations with your connectors. However, these models need to be validated within your layout using simulations in order to identify potential signal integrity problems. This is particularly important if you are scaling to a higher data rate, or if you plan to exceed the specifications for your connectors. These problems can include stronger crosstalk at higher data rates and signal reflection due to impedance mismatch.

As much as we would like, only simple passive electrical components have a well-defined impedance spectrum, and this is only really an approximation within a certain frequency range. Anyone who is familiar with self-resonance in a capacitor is familiar with this fact. Therefore, the impedance spectrum of any component will only be flat within a limited range of frequencies.

With analog signals traversing a backplane connector, it is a simple matter to impedance match a given connector at a particular frequency—you only have to run a simulation at a particular frequency and analyze the reflected signal level. You can then determine the return loss at the connector, which then tells you the connector’s impedance at the frequency of interest.

Ideal digital signals contain a spectrum of discrete frequencies with about 75% of the power spectrum being concentrated between the data rate frequency and the knee frequency (approximately one-third of the inverse of the signal rise time). Unfortunately, real digital signals are not ideal, and they contain a continuum of frequencies that are not exactly discrete. Because resistors are the only components with flat impedance spectra, any other component can distort digital signals to varying degrees.

Connectors in backplanes are no different. High speed logic families that are used in backplane topologies have razor thin noise margins, thus minor distortion can lead to a huge increase in bit error rates. This is where simulations become important for optimizing an impedance matching network and analyzing signal distortion along a backplane bus that runs at a high data rate.

Optimizing impedance matching with signal integrity simulations
Optimizing impedance matching between a connector and a differential pair trace

Although intimately related to signal integrity, managing and controlling impedance and parasitics in a backplane is its own topic It also depends on the routing topology, geometry of the backplane, and specific characteristics of connectors. For now, we’ll leave this as a future topic of discussion.


Other Design Aspects That Can Degrade Signal Quality

In addition to signal integrity for digital pulses travelling on a backplane, there are other aspects to consider in your layout which can affect signal integrity. One aspect to consider is the use of active cooling on daughterboards or for the entire system. Placing daughterboards close together on a bus is critical for systems that require a large number of daughterboards, which makes it difficult or impossible to include heatsinks on daughterboards for passive cooling.

Metal heat sinks on a motherboard
Try fitting these heat sinks on active components on your daughterboards

The limited space makes the case for active cooling, as high speed fans mounted directly on an active component can remove more heat than a heat sink with similar size. The problem with a spinning fan is that it can produce radiated and conducted EMI that induces unintended noise in nearby circuits. This means your daughterboards will need to be shielded, or you will want to mount fans to the chassis and power them with a separate shielded unit.

This problem with noise from active components, as well as potential interference between analog and digital signals in mixed-signal backplanes, illustrates the advantage of differential topologies in your backplane. The common mode noise immunity of differential pairs and naturally reduced inductive crosstalk between neighboring pairs goes a long way to ensuring signal integrity in your next backplane.

Backplane design and simulation can be complicated enough, but the full suite of design tools in Altium Designer can help expedite the design process for any backplane. The automated routing tools help you layout your backplane with the right topology, and the simulation tools can help you analyze signal integrity problems on your backplane bus. These tools and many more are all accessible within a single program.

Contact us or download a free trial if you’re interested in learning more about Altium Designer®. You’ll have access to the industry’s best layout, simulation, and data management tools in a single program. Talk to an Altium expert today to learn more.

About the Author

Zachariah Peterson

Zachariah Peterson has an extensive technical background in academia and industry. Prior to working in the PCB industry, he taught at Portland State University. He conducted his Physics M.S. research on chemisorptive gas sensors and his Applied Physics Ph.D. research on random laser theory and stability.

His background in scientific research spans topics in nanoparticle lasers, electronic and optoelectronic semiconductor devices, environmental systems, and financial analytics. His work has been published in several peer-reviewed journals and conference proceedings, and he has written hundreds of technical blogs on PCB design for a number of companies.

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