[SI-LIST] Need advices
- From: "Yee Chung" <yeec@xxxxxxxxxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Fri, 7 May 2004 10:00:40 -0400
Hi SI experts,
I am in the process of designing a connector and would like to get some
idea from the experts.
I need to put a PCB (4 layers sig1- ref1 - ref2 - sig2) in a connector
to minimize the crosstalk between differential pairs. The routing will
be microstrip edge coupled and length of the PCB is only about 500mils
or less. (There are still lead frame between the PCB) The rise time of
the signal is about 700ps.
Here is what I am thinking
1) If I use 155ps/in for microstip line propagation delay, 1/10 of rise
time is about 450mils. So the PCB will look like a lump element
instead of transmission line. Do I really need to worry about the PCB
impedance control?
2) Let say if I do need impedance control in the PCB for 100 ohm
differential. I calculated the impedance requirement for the regular PCB
then I need two 10mils traces, pitch to pitch 20 mils and the reference
plane height about 8 mils. The connector is connected to unshield
twisted pairs cable. Since each cable pair is tight coupling to each
other the return path will be on the other cable.
Here is what I am getting confuse, the reference plane in connector PCB
is floating (Not connect to any ground). And the edge couple trace only
have coupling effect about 14% to each other because the 10 mils
spacing. If I look at the PCB itself, the impedance should be 100 ohm.
However, the connector PCB have connected with twisted pair cable though
the lead frame, it seems to be the return path won't be on the PCB
reference plane much but mainly on the other differential trace. If this
is the case will the differential impedance on the PCB still is 100 ohm?
Do I expect to see impedance mismatch on the differential signal. I
assume the reference plane still good for crosstalk isolation if I route
edge couple trace on both top and bottom layer right.
BTW Can any one recommend any simulation tool that is good for
connector?
I would appreciate any feedback.
Thanks,
Yee
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