[SI-LIST] Re: Return loss
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: Qazi Iqbal <qazi@xxxxxxxxxx>
- Date: Fri, 02 Sep 2011 18:53:48 -0400
Qazi
S11 is a frequency domain parameter. It has nothing to do with time
domain signal rise time. S11 is measured as the ratio of the forward
power to the reverse power at the port. The physical size of the
discontinuities does matter.
Scott
Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax
http://www.teraspeed.com
Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC
On 9/2/2011 6:45 PM, Qazi Iqbal wrote:
> Thanks Scott, great explanation!
>
> Anything on the rise time of the signal comparing to the flight time of the
> signal throughout the whole interconnect on this regard?
>
> -----Original Message-----
> From: Scott McMorrow [mailto:scott@xxxxxxxxxxxxx]
> Sent: Friday, September 02, 2011 3:27 PM
> To: Ravinder.Ajmani@xxxxxxxxxxxxxx
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Return loss
>
> Ravi
> Generally yes!
>
> Loss is your friend. It attenuates reflections in the channel. But remember
> that what we measure as S11 is superposition of multiple reflections. As
> the channel becomes longer, resonant frequency of multiple reflective
> structures becomes lower, pushing peaking down to lower and lower
> frequencies where there is less loss. Depending on the size of the
> discontinuities, there is a crossover point where additional channel length
> causes reduced return loss.
>
> Adding raw resistance will always reduce return loss. Narrow traces and
> resistors work quite nicely in this regard.
>
> Scott
>
> Scott McMorrow
> Teraspeed Consulting Group LLC
> 121 North River Drive
> Narragansett, RI 02882
> (401) 284-1827 Business
> (401) 284-1840 Fax
>
> http://www.teraspeed.com
>
> TeraspeedR is the registered service mark of Teraspeed Consulting Group LLC
>
>
> On 9/2/2011 6:05 PM, Ravinder.Ajmani@xxxxxxxxxxxxxx wrote:
>> Hi Scott,
>>
>> Thanks for the explanation. Doesn't this mean that for the same
>> Channel architecture, a longer interconnect will exhibit improved
>> Return Loss due to higher signal loss in the interconnect.
>>
>> Regards,
>> Ravinder Ajmani
>> Server PCB Development
>> Hitachi Global Storage Technologies
>>
>>
>> Email: Ravinder.Ajmani@xxxxxxxxxxxxxx
>>
>>
>> *Scott McMorrow<scott@xxxxxxxxxxxxx>* Sent by:
>> si-list-bounce@xxxxxxxxxxxxx
>>
>> 09/02/2011 02:36 PM
>>
>>
>> To
>> si-list@xxxxxxxxxxxxx
>> cc
>>
>> Subject
>> [SI-LIST] Re: Return loss
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> Qazi
>>
>> Return loss represents the sum total of power reflected back to the
>> measured port. This includes power reflected within the length of the
>> interconnect and from the far end port. When reflections occur away
>> from the near end port being measured, that reflection is subject to
>> attenuation to the reflection and from the reflection. Thus, the
>> reflection is attenuated. In RF this fact is used to improve return
>> loss by using attenuation pads to improve return loss.
>>
>> best regards,
>>
>> Scott
>>
>> Scott McMorrow
>> Teraspeed Consulting Group LLC
>> 121 North River Drive
>> Narragansett, RI 02882
>> (401) 284-1827 Business
>> (401) 284-1840 Fax
>>
>> http://www.teraspeed.com
>>
>> TeraspeedR is the registered service mark of Teraspeed Consulting
>> Group LLC
>>
>>
>> On 9/2/2011 5:03 PM, Qazi Iqbal wrote:
>>> Weston,
>>>
>>> I am a bit confused when we co-relate reflection and the length of
>>> interconnect. Isn't impedance an instantaneous concept when
>> calculating the
>>> reflection co-efficient at the termination? There should be
>> attenuation for
>>> the added length (be it dielectric or resistive) but why would there
>> be any
>>> effect of length on the reflection?
>>>
>>> Rgds,
>>> Qazi
>>>
>>>
>>>
>>> -----Original Message-----
>>> From: Beal, Weston [mailto:Weston_Beal@xxxxxxxxxx]
>>> Sent: Friday, September 02, 2011 1:44 PM
>>> To: bala; si-list@xxxxxxxxxxxxx
>>> Subject: [SI-LIST] Re: Return loss
>>>
>>> Bala,
>>>
>>> The word loss in Return Loss often confuses people this way. Return
>> loss is
>>> not a loss in the way we usually think of where an electrical signal
>>> is partially dissipated as heat. Return loss is really a measure of
>>> signal reflection caused by impedance discontinuity.
>>>
>>> Whether a short channel or a long channel has less return loss
>> depends on
>>> the lossless impedance of the interconnect. If the lossless
>>> impedance is already very near the normalizing impedance (usually
>>> 100 Ohms
>> differential)
>>> then the addition of dielectric and resistive loss will increase the
>> trace
>>> impedance and thereby increase the impedance discontinuity and the
>>> reflection. On the other hand, if the lossless impedance is lower
>> than the
>>> normalizing impedance, the additional loss from the longer channel
>> moves the
>>> impedance closer to the normalizing impedance, reduced the impedance
>>> difference, and reducing the reflection.
>>>
>>> Regards,
>>> Weston
>>>
>>>
>>> -----Original Message-----
>>> From: si-list-bounce@xxxxxxxxxxxxx
>> [mailto:si-list-bounce@xxxxxxxxxxxxx] On
>>> Behalf Of bala
>>> Sent: Friday, September 02, 2011 11:40 AM
>>> To: si-list@xxxxxxxxxxxxx
>>> Subject: [SI-LIST] Return loss
>>>
>>> Hi experts,
>>> I have generated sdd11 waveform for 7.5gbits serdes channel..and i
>> got good
>>> return loss results for longest length and bad results(-6db) for
>> shortest
>>> length..my dk-3.9 df-.008.generally longest path should have high
>> impedence
>>> discontinuties and bad return loss.am i right.
>>> -bala
>>>
>>>
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