[SI-LIST] Re: Return loss
- From: "Beal, Weston" <Weston_Beal@xxxxxxxxxx>
- To: bala <balaseven@xxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Fri, 2 Sep 2011 20:43:48 +0000
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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