[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
>>>>>
>>>>> ------------------------------------------------------------------
>>>>> To unsubscribe from si-list:
>>>>> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>>>>>
>>>>> or to administer your membership from a web page, go to:
>>>>> //www.freelists.org/webpage/si-list
>>>>>
>>>>> For help:
>>>>> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>>>>>
>>>>>
>>>>> List technical documents are available at:
>>>>> http://www.si-list.net
>>>>>
>>>>> List archives are viewable at:
>>>>> //www.freelists.org/archives/si-list
>>>>>
>>>>> Old (prior to June 6, 2001) list archives are viewable at:
>>>>> http://www.qsl.net/wb6tpu
>>>>>
>>>>>
>>>>>
>>>>>
>>>>
>>>>
>>>
>>> ------------------------------------------------------------------
>>> To unsubscribe from si-list:
>>> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>>>
>>> or to administer your membership from a web page, go to:
>>> //www.freelists.org/webpage/si-list
>>>
>>> For help:
>>> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>>>
>>>
>>> List technical documents are available at:
>>> http://www.si-list.net
>>>
>>> List archives are viewable at:
>>> //www.freelists.org/archives/si-list
>>>
>>> Old (prior to June 6, 2001) list archives are viewable at:
>>> http://www.qsl.net/wb6tpu
>>>
>>>
>>>
>>>
>>
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List technical documents are available at:
> http://www.si-list.net
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
>
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
>
>
--
Steve Weir
IPBLOX, LLC
150 N. Center St. #211
Reno, NV 89501
www.ipblox.com
(775) 299-4236 Business
(866) 675-4630 Toll-free
(707) 780-1951 Fax
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
