Thanks for the input. However we need very short cables in some cases less
than 0.5m mainly to have an extremely low loss channel. Increasing the
gauge is an option but I could go with longer wire for this to improve
SCD21. I see one of the pairs in a 4 pair or 8 pair wire to show SCD21
issue, and you're right I'll check the skew intra pair skew. I'll let cable
vendor worry about the quality control but do I need to follow any bundling
procedures to prevent any mishandling of the cables which might induce more
skew?
On Thu, Dec 19, 2019, 2:20 PM Mick Felton <
Mickey.Felton@xxxxxxxxxxxxxxxxxxxxx> wrote:
You have a lot of questions here, I'll give you a couple of tidbits that
will help you along the way.
1. The insertion loss is smaller for a short cable and the return loss is
sky high with a short cable especially with older generation connectors, as
such, impedance issues at the near connector and at the far connector start
to both cause issues with not only reflections but skew and likely
crosstalk as well. It's a cornucopia of issues with a short cable, this is
why its better to change the gauge to increase the insertion loss to handle
some of the reflections/skew etc in a short cable.. So most cables may use
a 30gauge for a 0.5meter, try using a 33gauge for the short cable.. This
will significantly impact your SCD and improve it as SCD is impacted by
IL.. As another point, your SCD could be caused by bad skew on the traces
going to the connector/cable in the first place, take a look with a scope
and check the phase of + and -.
2. The SAS2.1 cables have an enormous suck out at higher frequencies, so
it affectively wipes out your signal at higher frequencies, so if your
trying to compare a 20Ghz signal on a SAS2.1 vs a SAS4 cable there will be
almost no signal at 20Ghz present on SAS2.1.. So that may be why your
second sentence is happening.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx <si-list-bounce@xxxxxxxxxxxxx> On
Behalf Of talented.libran@xxxxxxxxx
Sent: Thursday, December 19, 2019 10:43 AM
To: SI-LIST <si-list@xxxxxxxxxxxxx>
Subject: [SI-LIST] mode conversion cable length
Hello Si-List'ers,
I see from cable vendors, the shorter cables 0.5m and 1m exhibit higher
SCD21 and SCD21-SDD21 data than 2m and 3m high speed cables. I know the
length dependency would affect mode conversion insertion loss, longer
cables would show lower SCD21 and the relevant spec is loss independent
mode conversion.
Can I ignore the SCD21 failures for shorter cables 0.5m and 1m cases and
consider pass? I wonder why do I see high bandwidth 24Gbps cables exhibit
mode conversion issues than the lower bandwidth for example SAS 2.1 cables.
Any insight would be appreciated.
Thanks
Robert
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List forum is accessible at:
http://tech.groups.yahoo.com/group/si-list
List archives are viewable at:
//www.freelists.org/archives/si-list
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
