[SI-LIST] Re: Differential pairs commom mode impedance

  • From: steve weir <weirsi@xxxxxxxxxx>
  • To: Vinu Arumugham <vinu@xxxxxxxxx>
  • Date: Tue, 09 Nov 2010 13:09:24 -0800

If the Z matrix has essentially zero line to line coupling at the 
transmitters and receivers, then I am at a loss as to how a channel that 
does not match that will not suffer some amount of mode conversion.  The 
reality is that a perfect match is unattainable.  But the further one 
deviates, the more degradation will result. 

Steve.
Vinu Arumugham wrote:
> I don't think the cited section of the spec. below can be interpreted as 
> requiring loosely coupled 50 ohm single ended lines. 
> "XFI differential reference impedance for driver, *transmission line*, and
> receivers are defined as 100 Ohms. XFI common mode reference impedance for
> driver, *transmission lines*, and receivers are specified as 25 Ohms."
>
> The reference impedance is the port reference impedance used to check 
> compliance of the driver, receiver or transmission line parameters.
>
> I don't think the spec. places a direct limit on the actual common mode 
> impedance of the interconnect. Only differential insertion and return 
> loss requirements are specified. So, a 90 ohm differential pair with 30 
> ohm common mode impedance could be compliant.
>
> Thanks,
> Vinu
>
> steve weir wrote:
>   
>> It's basically telling you to use loosely coupled pairs where each line 
>> has a single ended impedance of 50 Ohms.  For zero line to line 
>> coupling: Zcm = Zse /2, and Zdiff = Zse*2.
>>
>> Steve.
>>
>>
>> Alvaro alkschbirs wrote:
>>   
>>     
>>> Hi gurus,
>>> I have a question about differential pairs routing.
>>>
>>> The XFP MSA Specifications says:
>>>
>>> "XFI differential reference impedance for driver, *transmission line*, and
>>> receivers are defined as 100 Ohms. XFI common mode reference impedance for
>>> driver, *transmission lines*, and receivers are specified as 25 Ohms."
>>>
>>> I think that it says that my signal need to have both 25ohms commom mode
>>> impedance and 100ohms differential impedace. In pratical terms, I need to
>>> make this traces thinner and more spaced. Thinner traces are more critical
>>> and add  too.
>>>
>>> Is this requirement true for any type of 100ohms differential impedance
>>> pair?
>>>
>>> If yes, this requeriment will add some problems to make BGA fanouts without
>>> route the pairs separately by other pins fanout vias.
>>>
>>> Why I need control common mode impedance?
>>> How critical is this requirement?
>>> Is 30 ohms commom mode impedance acceptable?
>>>
>>> I will appreciate your help!
>>> Rgds,
>>> Álvaro
>>>
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>
>
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