[SI-LIST] Re: Return loss
- From: "Qazi Iqbal" <qazi@xxxxxxxxxx>
- To: "'Beal, Weston'" <Weston_Beal@xxxxxxxxxx>, "'bala'" <balaseven@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Fri, 2 Sep 2011 14:03:24 -0700
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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