I've been working on a Intel based PC design, where I think
ringing/reflection on the
66 MHz clock to AGP (Video port) is going to be an EMC & Signal Integrity
issue due to long track lengths.
We have used a 6 layer standard design layout of
Signal/GND/Signal/Signal/VCC/Signal, with all clock lines buried
interlayer. The fab house designs the geometry to control the stackup to
make the 65 Ohm on these critical lines. As you may already know, this 66
MHz clock line splits into two line which one loops back to the internal
PLL on the Intel chip and the other feeds directly to AGP port. Each
branch has its own damping resistor. According to design specification
both lines have equal length (Approx. 8 inch) but the loop back line is
cerpentined. The cerpentined feed back segment Placed interlayer next to
GND plane and placed right under the Intel chip. The clock line that
feeds AGP connector only switches once from inter layer plane A to B.
According to the Emission data gathered with AGP video card in place, we
are still marginally above limit. We have already tried different values
of damping resistor at source and load side. Also, tried small value of
Caps on both ends to match the impedance of the line with source and
load, but no cleaner signal or better emission performance observed.
Also, enabling the Spread Spectrum on clock generator has no effect on
the 66 MHz clock that comes out of the PLL section of the Intel chip.
Does anyone know how to best resolve this Signal Integrity and EMC issue?
What is the best termination configuration?
What is the best layout design to cure this problem?