[SI-LIST] Re: Need advices
- From: steve weir <weirsp@xxxxxxxxxx>
- To: yeec@xxxxxxxxxxxxxxxxxx, <si-list@xxxxxxxxxxxxx>
- Date: Fri, 07 May 2004 07:15:16 -0700
Yee, it depends on what that PCB is connecting between. If it were the
only thing between a driver and receiver, then being electrically short
would be OK. However, if there enough trace that when combined with that
connector you do have a transmission line, then you would be well advised
to pay attention to what that connector looks like to the rest of the
transmission line.
If you are not running a bunch of ground shields in that cable, then you
have mode conversion to worry about.
The confusion that you have is from the energy headed out of that assembly
in the form of EMI.
Steve.
At 10:00 AM 5/7/2004 -0400, 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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