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

  • From: steve weir <weirsi@xxxxxxxxxx>
  • To: Vinu Arumugham <vinu@xxxxxxxxx>
  • Date: Tue, 09 Nov 2010 13:27:43 -0800

Vinu, perhaps I am just tired this afternoon, but Zdiff = 4*Zcm pretty 
much stipulates zero line to line coupling.  Otherwise the ratio would 
be less than 4.

Steve.
Vinu Arumugham wrote:
> True, but the spec. does not require zero line to line coupling at the 
> transmitters or receivers either.
>
> Thanks,
> Vinu
>
> steve weir wrote:
>   
>> 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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