RE: [802.3ae_Serial] 12 dB return loss for transmitter?
Tom,
I too think it means the return loss looking back into the transmitter. I
had hoped that for an optical transmitter it could be measured with the
transmitter turned off. It was introduced to control the reflection noise
with a receiver that reflects similarly; reflection noise is (nearly) the
same thing as the ripple which Ron analyses. I think Ron's analysis is in
electrical dB while table 52-12 would be in optical dB (aside to editor: is
that obvious? does it need spelling out somewhere?).
Piers
-----Original Message-----
From: Ron Miller [mailto:rmiller@xxxxxxxxxxx]
Sent: 06 February 2002 06:40
To: 'Lindsay, Tom'; Serial reflector 802.3ae (E-mail)
Subject: RE: [802.3ae_Serial] 12 dB return loss for transmitter?
Tom
This is the return loss looking back into the transmitter. Typically it is
measured with the
drive circuit powered. It is an AC measurement, and may vary according to
the signal that
the transmitter is sending out.
The value of this measurement is in calculating or simulating the third pass
signal, which is the
combination of the driving signal together with the reflection from the
destination, coming back to the
source and reflecting again. The total reflection from the source
termination and the load termination
should be at least 20 db total for +/-10 percent ripple in the rails of the
eye pattern. So, 12 db at
each end would be 24 db or about +/ -6 percent ripple. So, you normally
want to do better. 20 db is
a very safe number at both ends resulting in 40 db or only about +/-1%
ripple in the rails.
Ron miller
-----Original Message-----
From: Lindsay, Tom [mailto:tlindsay@xxxxxxxxxxxxxxxxxxxx]
Sent: Tuesday, February 05, 2002 9:24 PM
To: Serial reflector 802.3ae (E-mail)
Subject: [802.3ae_Serial] 12 dB return loss for transmitter?
Dave Kabal, Dubravko and I were discussing the 12 dB return loss spec in
Table 52-12.
What is it? Is this return loss looking back into the transmitter port from
outside, or is it tolerance to return loss of the cable plant and receiver
combination?
If the former, what is the technical reasoning? If the latter, it should at
least be renamed or more preferably removed.
Thanks, Tom
Stratos NW
425/672-8035 x105