[SI-LIST] Re: Return loss
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Fri, 02 Sep 2011 17:36:39 -0400
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
Teraspeed® 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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