4–64
Altera Corporation
Stratix GX Device Handbook, Volume 3
February 2005
Transmission Lines
For differential traces, it is important to match the length of the positive
and negative signals. A mismatch in lengths causes duty cycle distortion,
which can decrease the timing margin. Altera recommends that the
lengths be matched to ± 50 mils. If the signal goes through a turn and the
outer trace is longer than the inner trace, you can use a second turn in the
opposite direction as an offset.
Figure 4–66 shows an example of high-
speed transceiver routing to achieve length matching on the Stratix GX
development board.
Figure 4–66. Transceiver Signal Routing for Length Matching
Other Transmission Line Issues
This section describes transmission line issues other than routing.
Stackup
The stackup design is an iterative process in which the relative
permittivity (
εr), trace width, layer count, and transmission line
structures are set by engineers to achieve the best system performance at
the minimum cost. These parameters are selected based on the
P and N Lengths of this
Pair are Matched
P and N Lengths of this
Pair are Matched