SCD21-SDD21 is first order approximation with some strong assumptions. It's
still really useful. However, a better approximation requires taking into
account more terms e.g. re-reflections. It's very analogous to VNA
calibration theory. Specific inputs to a better approximation of SCD21
output would be common-mode reflection properties, common-mode transmission
properties, differential reflection properties, differential transmission
properties, and the asymmetries of the channel. You can mathematically
model this with certain assumptions. I've derived an approximation before
and can pass on my notes if I can find them. After that, it just becomes
unpragmatic and more.
So think about reflections. Most cable channels have a very high SCC11/22.
Also, shorter and less lossy channels generally have more reflection energy
whether diff or common.
Regarding your question on old vs newer cables... One hypothesis would be
to think about how different wrap styles affect common signals e.g.
helical/spiral vs cig/longitudinal. Helical has far greater common-mode
attenuation than cig wrap (at least this has always been my experience).
Might also be insightful to plot time domain and do some gating techniques
to gain some confidence/intuition.
In the end, MC is complicated and even the best approximations can't always
explain the underlying cause of SCD21 levels.
-Ryan
On Thu, Dec 19, 2019, 9:44 AM <talented.libran@xxxxxxxxx> wrote:
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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