Active channels replace passive interconnects to meet high-speed demands

Active channels replace passive interconnects to meet high-speed demands
In the past, backplane component vendors focused on the performance of their components in an isolated nonsystem-like environment. But as the high-speed serial links approach multigigabit data rates, manufacturers have found that this approach insufficient, and they have focused on understanding the entire system interconnect and the synergistic roles of the transmitter, receiver and channel.

Designers face the task of developing architectures where every system interconnect must be able to pass data at the given rate of operation for all distances within the given architecture and in a reliable manner. In a typical design, designers chose the various components of the channel based on their individual performance capabilities, such as the dielectric constants and loss tangents of the transmission media. Designers looked at impedance, potential crosstalk and density characteristics when choosing connectors.

The active devices were chosen for their ability to provide enough signal power to drive a given channel, while also having input and output impedances that matched the impedance of the channel to minimize reflections. This approach, however, reflects a limited understanding of a system's interconnect and its synergistic response to the drivers and receivers the designers chose. Consequently, the limited speed at which the system interconnect actually runs would appear to be a result of the backplane and connectors, rather than the result of device-to-interconnect pairing.

System interconnects using devices that do not compensate for the behavior of the channel between them can be considered to be passive. These systems typically run at 2.5 Gbits/second and lower. At these speeds, signal losses in the system are predictable and often can be improved through the selection of higher-grade transmission media. Developments under way in the semiconductor industry, however, are causing the system interconnect to evolve from a passive to an active component-with sophisticated interactions. Designers are now able to fashion devices that compensate for the predictable losses of the channel between the transmitter and receiver, which means systems can run faster and over greater lengths than traditionally thought. In addition, designers will encounter fewer instances where they must use specialized high-performance transmission media.

The need for increased bandwidth has driven the frequency of operation upward, into a region of performance where the influences among the various components of the channel no longer can be ignored. This type of very high-frequency interaction produces a channel whose losses are unpredictable. It is this unpredictable nature that limits the effectiveness of high-performance materials-or semiconductor devices meant to compensate for the otherwise predictable losses between the driver and receiver. Thus, it is the underlying passive channel that ultimately limits the speed of operation and length for which it may run.

Ordinarily, two characteristics of a passive channel determine its data rate capacity: the ability to pass the signal and the inherent noise performance. To try to better understand how the connector, the passive channel and the active interconnect influence each other, Tyco Electronics' engineers have experimented with systems using the company's Z-Pack HM-Zd connector, one of the lowest-noise connectors available. With the noise characteristic of channel performance minimized through the use of the Z-Pack HM-Zd connector, it's possible to measure the signal throughput of the channel. Signal losses associated with dielectric loss of printed circuit board material and trace skin effects are well known and documented, and are considered predictable in nature. Thus, they are correctable by means of better material selection, wider traces or through semiconductor techniques.

Incalculable outcome
However, upredictable losses, are not so easily compensated for. These losses are associated with the impedance discontinuities of the pc board vias in the daughter card and backplane into which the connector is inserted.

Further, the specific layers into which the signal connects on these boards will determine the nature of the unpredictable losses. A frequency domain plot will illustrate the impact of the layer connection on the performance of the channel. With a top-layer connection, there is a significant roll-off in throughput compared with the bottom layer-especially as the signal approaches a 10-GHz frequency. On an eye-pattern measurement, the roll-off will translate to a reduced eye-opening and increased jitter. Therefore, one can expect the variation in performance to change as the thickness of the connector/board interface grows.

Some manufacturers will compensate for the performance of the underlying passive channel by employing a transmit "de-emphasis" at 3.125 Gbits/s. In essence, this transmit de-emphasis adjusts the transmitter output so that the amplitude of lower-frequency components is reduced, compensating for the channel's compression of high-frequency components. This approach produces a better-balanced mix of low- and high-frequency content in the signal that reaches the output of the channel, resulting in lower jitter and intersymbol interference. Voltage constraints within semiconductor devices, however, eventually will limit the effectiveness of de-emphasis at higher data rates.

For serial data transmission at 10 Gbits/s, equalization at the receiver has proved to be an effective solution. Tyco engineers, working with engineers at Gennum Corp., have demonstrated a wide-open eye diagram at 10.7 Gbits/s. Using a channel based on bottom-layer signal connections, the signal traversed the backplane through 2 HM-Zd connectors for a total length of 22 inches using standard FR-4. The Gennum GN2001 receiver was easily able to recover the eye.Testing showed a bit error rate (BER) of better than 10-15 when tested over a period of two days — no errors were found in 1.8x1015 bits.

Tyco's engineers have characterized the pairing of high-speed interconnects with specially compensating ICs as an active interconnect. Tyco's relationships with semiconductor companies, through its involvement in XAUI, HSBI, XFP and 10-Gbit/s serial efforts, have given it insight into the performance requirements for connectors in the active interconnect. This knowledge has been employed in other backplane product offerings, such as the company's MultiGig family.

The active interconnect concept is not limited to backplane applications. Mezzanine and cabling may be used in topologies employing these chip techniques.