[SI-LIST] Re: Need advices
- From: Perry Qu <perry.qu@xxxxxxxxxxx>
- To: yeec@xxxxxxxxxxxxxxxxxx
- Date: Fri, 07 May 2004 10:51:00 -0400
Yee:
You may want to consider broadside coupled diff pair with no reference
plane. e.g., I would propably have the following stackup:
sig1 (difflayer1 - plus)
sig2 (difflayer1 - minus)
sig3 (difflayer2 - plus)
sig4 (difflayer2 - minus)
Try to maximize the horizontal spacing between diff pairs.
Cheers
Perry
Yee Chung wrote:
> 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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--
Perry Qu
Design & Qualification | 600 March Road
Alcatel Canada | Ottawa, ON K2K 2E6, Canada
DID: (613) 7846720 | FAX: (613) 5993642
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