Xilinx Virtex-4 RocketIO User Manual page 259

R
A dielectric constant of 4.4 has been used in these via models. All unused pads have been
removed.
Since the vias have very little resistive and dielectric losses, the insertion loss (IL) in linear
magnitude can be computed as:
Where the RL has been converted from dB to linear magnitude with:
Since a single-pole response of +6dB/octave has been assumed for the RL with increasing
frequency, then RL at frequencies higher than 1 GHz can be computed with:
In
linear IL magnitude at higher frequencies.
the IL in dB. This has been done for frequencies of 2, 3, 4, and 5 GHz; the results are
displayed in
Since a single-pole response has been used to extrapolate RL and IL from 1 GHz to higher
frequencies, the accuracy of the results decreases for increasing IL. For example, for an
extrapolated IL of 0.5 dB, the actual loss is about 0.45 dB. As the IL increases to a few tenths
of a dB, it is recommended that a detailed circuit simulation of the channel be relied upon
for high-level loss budgeting, rather than using this table.
Not only does IL inaccuracy become an issue, but more importantly, the RL typically
creates ripple in the frequency response. This ripple correlates to significant inter-symbol
interference (ISI), which closes the RX eye. Specifically, the ISI is caused by multiple
reflections from high-capacitance transitions in the channel, as well as pulse spreading due
to RC charging of the capacitance. In addition, the lumped capacitance approximation
begins to lose accuracy for larger vias and other transitions, and the accuracy of the loss
increases as the data rate increases.
Table 12-1: Model-Derived Loss for Differential Vias of Various Dimensions
Board Stub Via
Thickness Length Diameter
(mils) (mils) (mils)
25
0
20
25
200 60
20
25
140
20
Virtex-4 RocketIO MGT User Guide
UG076 (v4.1) November 2, 2008
IL
Mag
RL
RL = 20
,
dB f
Equation 12-3, f is the desired frequency. Now
Table 12-1. For other frequencies, simply use the equations above.
Via Excess
RL @1GHz
Pitch Capac.
(mils) (fF)
60 415
80 173
60 92
80 430
60 622
80 107
60 301
80 131
60 848
80 379
60 594
80 201
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Channel Budgeting Considerations
2
( )
= 1 – RL
Mag
( ⁄ )
RL 20
dB
10
=
Mag
( ⁄ ( ) )
log f 1GHz + RL
,
dB 1GHz
Equation 12-1
Equation 12-2
IL @1GHz IL @2GHz
(dB) (dB) (dB)
-23.7 -0.019 -0.074
-31.3 -0.003 -0.013
-36.8 -0.001 -0.004
-23.4 -0.020 -0.080
-20.2 -0.042 -0.168
-35.5 -0.001 -0.005
-26.5 -0.010 -0.039
-33.7 -0.002 -0.007
-17.5 -0.078 -0.319
-24.5 -0.015 -0.062
-20.6 -0.038 -0.153
-30.0 -0.004 -0.017
Equation 12-1
Equation 12-2
Equation 12-3
can be used to compute the
can be used for the IL to compute
IL @3GHz IL @4GHz IL @5GHz
(dB) (dB)
-0.170 -0.304
-0.029 -0.051
-0.008 -0.014
-0.182 -0.327
-0.390 -0.713
-0.011 -0.019
-0.088 -0.157
-0.017 -0.029
-0.757 -1.438
-0.141 -0.251
-0.354 -0.645
-0.039 -0.069
(dB)
-0.490
-0.081
-0.023
-0.527
-1.185
-0.031
-0.250
-0.047
-2.554
-0.403
-1.067
-0.110
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