[SI-LIST] Re: Return current at board transitions

The center cavity is already horribly thick as far as power delivery 
goes, so why not use reasonable thicknesses for the microstrip 
dielectrics and keep the SI in line?  Path continuity is your friend.  
You are only going to get away with  cheating if the region is small 
compared to your rise times.  What kind of rise times do you have? 

Keeping the Cu to the outside of a thick board assy should not be either 
a technical or cost issue.

Steve


Joel Brown wrote:
> One board has 28 mil dielectric which I think of as really thick.
> I could try to reduce it but since the board is only 4 layers and 93 mils
> thick that could be difficult. The diff pair traces are only 400 mils long
> and only 125 mils run too close to another pair so hoping crosstalk will not
> be a problem.
>
> Joel
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
> Behalf Of steve weir
> Sent: Monday, December 29, 2008 4:45 PM
> To: Joel Brown
> Cc: SI-List
> Subject: [SI-LIST] Re: Return current at board transitions
>
> Mode conversion happens.  But not nearly as much as you would think.  
> Unless the board with the large dielectric spacing has really thick 
> dielectric, most of the energy is still confined between each trace and 
> the nearest plane.  That's a good thing too, otherwise your pair to pair 
> crosstalk would be terrible.
>
> Steve.
> Joel Brown wrote:
>   
>> Two boards are mated together with impedance controlled high speed
>> connectors.
>> One board has diff pairs with a large dielectric spacing so a large
>> percentage of the return currents flow in the opposite trace of the diff
>> pair.
>>
>> The other board has a small dielectric height so a large percentage of the
>> return currents flow in the adjacent ground planes.
>>
>> What happens at the transition? How does the return current change from
>> flowing in the opposite signal trace to a ground plane?
>>
>> Is this a discontinuity even though the impedances are the same? Does it
>>     
> not
>   
>> matter since the differential currents cancel?
>>
>>  
>>
>> Thanks - Joel
>>
>>  
>>
>>  
>>
>>
>>
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>>
>>   
>>     
>
>
>   


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