I am currently working on a mid-speed (<50MHz) digital circuit pack and =
would like to proper terminate some long lines(up to 15 inches). It =
consists of CPU, DSP, SRAM, Flash etc. I am approaching the problem by =
using Daisy-Chained tracks and AC terminations. There are some findings =
that I would like your comments:
1. When using uncontrolled-impedance FR4 PCB, based on my calculation on =
a 6 layer board, the micro-strip (8 mil) Z0 is about 75 Ohm and the =
micro-strip Z0 is about 45 Ohm. That means a mismatch every time I =
switch layers. Does anyone has experience on this? Does it matter for 5 =
ns rise time? How about 1ns rise time?
2. My EDA simulation package showed that after termination, my signals =
do not look much cleaner. In fact, it look worse and seems to be loaded =
down (Vpeak is about 3.5-4V instead of 5V). Does this make sense?
3. At want point could I use T instead of Daisy-Chain so that the stub =
look like capacitance, no transmission lines? A lot of time daisy chain =
line is longer than a treed line.
All comments on this matter are welcome.
Hardware Design Engineer
Nortel, Wireless Development Center, Calgary