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32 The PCB Design Magazine • October 2017 culation" tab at the bottom of the Si9000e main input window. When the pop-up window ap- pears, choose the appropriate transmission line geometry from the pallet along the left-hand side, and then enter the specific design param- eters in the boxes. Change the various param- eters until the desired impedance is reached 2. The next step is to select the "Frequency Dependent Calculation" tab at the bottom of the input panel and enter line length, conduc- tivity and frequencies in the appropriate boxes in the main window. 3. Under the "Extended Substrate Data" sec- tion, choose "Causally Extrapolate Er / TanD" radio button and click "Edit" to enter the D keff parameters in a pop-up window as shown. Click "Calculate" to view causal D keff over frequency. Click "Close" to return to main window. 4. Under the "Surface Roughness Compen- sation" section from the main window, select Huray radio button and click "Edit" to enter the appropriate roughness parameters in the pop-up window. Enter r eff and A flat in the boxes shown. Enter 1.00 for "Ratio of Areas" and 14 for the "Number of Balls in Area" boxes. Click "Apply" to return to main window. 5. In the main window, hit calculate. Once the simulation has run, then export the respec- tive touchstone file under the "File" menu. After repeating this for each transmission line segment of the topology, there should be three touchstone files generated. One file is for the single-ended traces on the daughter card. Another one is for the differential pair on the daughter card and the last one is for the differ- ential pair on the backplane. Keysight ADS is used to model and simulate the entire backplane channel, as shown in Fig- ure 7 . The two schematics in Figure 7A use the S-parameter pallet to model and compare the channel in the frequency domain. The two sche- matics in Figure 7B use the "ChannelSim" pallet for transient simulation and eye diagram analy- sis. All the Polar generated s-parameter files are concatenated together, as shown, including the Examax connector s-parameter files. Via and co- ax connector models are not included, because I want that "OK answer now." You can always model and add them later to get that "good an- swer later" if need be. The results of the simulation are plotted in Figure 8. As you can see there is excellent corre- lation for both differential SDD21 insertion loss on the left and differential time-domain reflec- tometry (TDR) TDD11on the right. Figure 9 shows plots of transmit eyes on top and receive eyes on bottom at 28GB/s. The simulated channel is on the left and the mea- sured channel is on the right. Even though the measured transmit eye on the top right shows slightly more noise and jitter, the received eyes are virtually the same, suggesting the "OK PRACTICAL MODELING OF HIGH-SPEED BACKPLANE CHANNELS Figure 5: Summary to determine effective sphere radius (r eff ) and flat tile area (A flat ) parameters for input into a Polar Si900s Huray model.