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Backplane Differential Channel Microprobe Characterization in Time and Frequency Domains White Paper Table of Contents Differential Channels Will Proliferate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3 The Bottleneck of SMA's . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5 Advantages of Microprobing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 Design for Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7 Physical Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8 Understanding 4-port Mixed Mode Analysis . . . . . . . . . . . . . . . . . . . . . . . . .10 Design Case Study: XAUI Backplane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 Comparing Time Domain and Frequency Domain Data . . . . . . . . . . . . . . . .12 Coupling Pulls Down Differential Impedance . . . . . . . . . . . . . . . . . . . . . . . .13 Eye Diagram Simulation Using 4-port S Parameters . . . . . . . . . . . . . . . . . .15 Non-Ideal Differential Signaling (AKA Mode Conversions) . . . . . . . . . . . . .16 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18 Abstract The chief difficulty with routine characterization of differential channel paths is the current requirement for SMA connectors to interface with the test equipment. With little extra space available on functional backplanes, and the problem of line loading produced by the SMA stubs, it is not practical to use SMA connectors. This means only specially designed test boards can be routinely characterized. This paper introduces a new methodology for testing the passive interconnects associated with a differential channel in a backplane assembly, which can be used for functional, populated backplane assemblies and shows how the pad layout can be optimized for routine probing without impacting the functional system performance. 2 Differential Channels Will Proliferate We are in the beginning of a revolution in input/output formats. All high-speed interface specs are migrating toward the use of differential signals imple- mented with a differential pair of transmission lines. Figure 1 is a partial list of some of these high-speed serial link formats. All Next Generation High Speed Serial Links will use Differential Signaling Serial ATA 1.25 Gbps XZUI 3.125 Gbps Hypertransport 1.6 Gbps PCI Express II 5.0 Gbps AGP8x 2.1 Gbps OC-192 9.953 Gbps Infiniband 2.5 Gbps