May 8, 1997

FAST backplane connectors disguise digital transmission lines

Brian Kerridge, Editor

Today's telecommunications and high-end computing designs demand backplane edge connectors that maintain signal integrity and provide high signal density for data rates up to 2.5 Gbps.

In an electronics world, mechanical devices will always have limited scope for advancement. Connectors--particularly backplane edge connectors--are a prime example of metal and plastic parts' having a tough time keeping pace with higher integration and faster signals in today's electronics designs.

Backplane edge connectors find widespread use in telecommunications switching. In broadband ISDN, for example, standard data rates have progressed from 155.52 to 622.08 Mbps, and are now preparing for 2.4883 Gbps. Similar data-rate progressions exist in high-end computing and management of multimedia data streams. Gigabit-per-second data rates move signal transmission well through RF and into microwave territory. Applying microwave design principles to multiple-pin backplane connectors seems incongruous, but, despite the mainly mechanical constraints, connector designers have devised clever methods of adapting their primitive parts to meet these principles.

Whatever the methods, one essential is to treat all electrical aspects of a connector as elements of a transmission line with an inherent characteristic impedance (Z₀), typically 50 ohms. At RF/microwave frequencies, transmission lines offer the only hope of maintaining signal integrity in point-to-point copper high-speed digital communications. If the transmission line itself has a constant Z₀ along its length, and is also terminated at both source and load ends by a purely resistive Z₀, then there are no signal reflections due to line discontinuities--and you have optimal signal transmission.

In practice, of course, there are line discontinuities, and resulting variations in Z₀ give rise to signal distortion, crosstalk, and noise. The levels of degradation partially measure how well connector designers have managed to produce a stable Z₀ through the daughterboard-to-backplane connector interface.

Connector vendors' main design difficulty is in controlling the physical dimensions and separation of signal and return wires, and the intervening dielectric material, because all of these factors govern the value of Z₀. Although controlling the physical dimensions of ICs and pc-board tracks along a signal path is relatively straightforward, having equal control over a moveable connector certainly is not, especially as you insert and withdraw a daughterboard thousands of times during its lifetime.

Vendors offer two basic connector types for controlling the physical backplane connection. The first type makes use of conventional pin-and-socket connectors, such as standard 2-mm five-row-grid type, in which various pin assignments establish multiple signal and ground plane connections. The second type uses surface-contact connections, purposely built for fast signals, that feature low or zero insertion force.

Table 1 details a selection of high-speed backplane connectors ranging from conventional pin-and-socket type to newer surface-contact versions. Specifications shown for all versions provide a single example from a range of performance you can achieve by adopting different signal and ground pin configurations. For example, in principle, all the connectors can support either single-ended or balanced modes of transmission. Vendors adapt their designs to user demand, and performance and characterisation data refer to specific configurations. AMP, for instance, has introduced an eight-row version of its HM connector that promises better performance at the same signal density, although currently the company has not modelled this arrangement.

Of all specifications, crosstalk provides a useful figure of merit, but at the same time it needs full explanation. Simply saying crosstalk is <1% doesn't explain enough. Crosstalk is roughly inversely proportional to the risetime of your input signal, and even that assumes you also define risetime (for example, 10 to 90% of pulse height). Also, crosstalk depends heavily on how many adjacent lines a system is driving.

Pins form transmission lines

Using a standard pin-and-socket connector, you can assign a number of grid pins to form ground path connections for each signal line. By placing ground paths close to, in between, or around signals, you can construct crude transmission lines. Vendors provide various pin layouts that form either single-ended (Z₀=50 ohms) or differential pair lines (Z₀=100 ohms). Using a differential-pair line, as per a stripline, dedicates two pins to each signal. This arrangement improves signal integrity (compared with a single-ended line) by keeping ground currents out of the signal loop. The arrangement also allows you to use other pins in a purely shielding function to reduce crosstalk and noise. In practice, your choice of single-ended or differential transmission depends on whether your logic circuits have single-ended or balanced output drivers.

Using a differential arrangement of two signal pins to one ground pin, Teradyne's HDMplus connectors, for example, achieve adjacent-line cross-talk of 3.0 to 5.5% on a 300-psec risetime with all adjacent lines switching. Clearly, the trade-off with this system is that the more pins you assign to individual signal paths, the lower the overall useable signal density per connector. In the example given, the signal density drops to 10 differential signal pairs/100 mm, whereas the physical pin density is 50 pins/100 mm.

However clever you are in devising connector arrays through a backplane interface, the variable electrical and mechanical tolerances of pc-board plated through holes as well as ageing of the pin and socket ultimately limit the stability and predictability of Z₀. Also, assigning more pins to individual signals means using more pins overall, which, in turn, increases the force you need to insert or withdraw daughterboards. In practice, you'll need to fit board stiffeners to your backplane to prevent flexing, which otherwise adds further mechanical uncertainties.

Although pin-and-socket connectors have some limitations in handling fast signals, their overriding advantage is price. Currently, this type of connector is readily available and easy to second source.

Increasing signal density

To better utilise the area connector pins occupy, and to reduce insertion force, Cinch, Packard Hughes, and Siemens have devised backplane connectors that adopt surface contact rather than pin-and-socket connections. These designs share an important feature in controlling the mechanical tolerances in the signal path from daughterboard to backplane.

Cinch's "Cin::Edge" design achieves this control by using a flexible multilayer printed circuit mounted on the edge of the daughterboard to carry signal tracks and a ground plane (Figure 1a). Early examples of this design provide 50 ohms single-ended transmission lines, although the basic principle can adapt to other values of Z₀ for balanced or single-ended lines. The design can also incorporate different levels of shielding by using different track/spacing dimensions and more layers in the flexible circuit.

Siemens' Speedpac design introduces a separate clamp mechanism that embodies pairs of conductors that connect to gold-plated pads on the daughterboard and backplane (Figure 1b). The metal clamp also houses multiple grounding contacts that act as crosstalk shields around every pair of signal conductors. The ends of the signal pairs are spring-loaded and contact the daughterboard and backplane with a wiping action. As well as cleaning the contacts, this action accommodates pc-board manufacturing tolerances and enables the signal pairs to maintain a constant separation and, in turn, a stable Z₀. Speedpac connectors can accommodate daughterboard thicknesses in the range of 1.6 to 3.6 mm, ±0.2 mm.

Cin::Edge, EZpac, and Speedpac connectors benefit from a more stable and predictable impedance at the connection surface than is possible with an overlapping pin-and-contact joint. These designs also require low or zero insertion force.

The Speedpac design provides four rows of contacts--with 2.5-mm spacing for signal pairs--resulting in a signal density of 140 pairs/100 mm. An alternative Speedpac configuration using 1.75-mm spacing on single-ended lines increases that density to 196 lines/100 mm.

Both Cinch and Siemens announced fast connectors at the Electronica show in 1996 and are currently shipping low quantities. Volume production will be available by the end of this year. Regarding price, Siemens' objective is to be absolutely competitive in "cost per signal line" with earlier generations of high-speed connectors. Speedpac looks likely to achieve this aim because of its simplicity of design. Consisting of only one part, Speedpac does away with the conventional male/female connector sections on both the daughterboard and backplane. Speedpac only requires mating pads on both the daughterboard and backplane, and thus reduces cost by simplifying pc-board assembly.

Simulation is essential

If you think connector designers have a hard time meeting their design objectives, don't underestimate the problems you'll face when designing-in their ready-made parts (Reference 1). Connector vendors are quick to point out that running Gigabit signals on FR4 pc board is more of a limiting factor to a design than using their connectors. Without doubt, you'll need to observe strict pc-board-layout design rules. You'll also need to set up a simulation of the whole transmission path from source to load, including backplane connectors.

For this reason, vendors provide some form of Spice model for analogue simulation. As with any simulation, the validity of the models limits the value of simulation results. Simple RLC models lose their validity as switching transitions exceed the 1-nsec region mainly because dielectric constants in pc-board and connector materials cease to be constant. Establishing simulation models beyond this level involves multiple and meticulous measurements using time-domain reflectometry and switching risetimes in the region of 50 psec. With these difficulties in mind, it's no surprise that connector vendors pass along this characterisation work to the care of specialist companies. Siemens, for example, uses US-based North East Systems Associates (NESA) (see box, "For more information...").

NESA's president Ed Sayre says characterisation work carried by the company covers both pin-and-socket and surface-contact type connectors. Sayre also notes that, in pin-and-socket type connector characterisations, the grounding pattern has a great deal to do with the crosstalk response of the connector. Although high-speed transmission is possible, one of the reasons designers often don't consider using the speed capability is because the number of pins you have to engage becomes a physically difficult task. Sayre cites an example of a system using 1000 pin-and-socket connections where the daughterboard engagement force exceeds 40 kg. That force puts a lot of structural overhead on cardcage enclosure design, which becomes necessary to avoid EMI-compliance problems.

In regard to work on Siemens' surface-contact connector system (Reference 2), Sayre says the crosstalk response is what really sets the design apart from any other connector system he's seen. The system has substantially negligible crosstalk--even at highest speeds up to 2.5 Gbps. Even so, Sayre regards his assessment as rather conservative. In fact, Sayre's test essentially ran out of performance in the generator he used at around 3 Gbps. Sayre believes the system is good enough for wireless communications--for example, to carry 900-MHz cellular RF.

References

1. Travis, Bill, "High-speed-connector systems," EDN Europe, March 1996, pgs 26 to 33.

2. Sayre and Baxter, "Specification and Characterisation of a Multi-GigaHertz Differential Backplane Connector," North East Systems Associates, Stow, MA, USA.

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