DesignCon 2002

Figure 19: Post-layout results for the extracted Switch-Backplane-Blade channel extracted from
SIwave
using VerifEye analysis with 8 taps of FFE and 3 taps of DFE at the receiver.
Measured Analysis
To determine if the simulated solution space was representative of the actual design an
end-to-end frequency domain channel measurement was made and the measured
Touchstone file was used to determine the channel BER using the statistical simulation
tool. The measurement methodology used Gore 3.5 mm (EN0CB0CA036) cables along
with a 3.5 mm to SMP adapter cable (Rosenberger 101076-4604) that allowed mating to
the female SMP connectors on the Switch and Blade. A SOLT calibration was performed
on an Agilent E8361A PNA with an N4421BH67 test switch up to 20 GHz to the end of
the Gore cables and therefore did not calibrate out the 3.5 mm to SMP adapter cable. This
adapter cable has not been taken into consideration in any of the simulations. The
measurement was taken from 10 MHz to 10 GHz with a frequency step of 10 MHz. The
authors realize this measurement methodology will not provide the most accurate data.
However, given the physical size and constraints of the entire system which included:
three PWBs along (Switch, Backplane, and Blade), a right angle connector, and a left
angle connector the measurement setup was considered sufficient. The awkwardness of
the physical setup precluded the use of RF probes with which extremely accurate
measurements could have been made. Due to limitations in the measurement setup the
authors limited the upper frequency of the measurement to 10 GHz and believe error
introduction started to become a factor beyond 6 GHz.
The measured .s4p file was imported into the circuit simulator tool and then simulated
using VerifEye with 8 taps of FFE and 3 taps of DFE at the receiver. Comparing the pre-
and post-layout simulations to the measured channel .s4p simulation good correlation was
obtained. For a BER of 1x10 -12 the measured s-parameters yielded an eye opening of 43
ps where the pre- and post-layout simulations yielded eye opening of 49 ps and 51ps
respectively. The authors believe even better correlation can be obtained by incorporating
the actual VHDM connector model and obtaining better measurement results up to 15
GHz for the simulation. However, despite these anomalies good correlation (8 ps for a
BER of 1x10 -12 ) was achieved. These results have been used in defining an operating
BladeServer KR channel (5 GHz or 10 Gb/s) which includes over 26 inches of FR4 with
very large via stubs (up to 125 mils) within the channel and an additional four 50 mil via
stubs to support the measurement methodology.