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
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