The dips are often due to contact, ground vias, length matching and/or
meander lines.
If you applied delta-L (i.e., eigenvalue) formula directly, the dips can
appear even when the original data are without dips. (See Pages 36 to 45 of
https://ataitec.com/docs/In-Situ_De-embedding.pdf) In-Situ De-embedding
(ISD) can help reduce or eliminate the dips after de-embedding.
Regards,
Ching-Chao Huang
www.ataitec.com
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx On Behalf Of Tommy Huang
Sent: Friday, May 28, 2021 3:35 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] S21 or Sdd21 with sharp dip
Hi Experts,
When I measure s-parameters of transmission line, I found S21 or Sdd21 cure
with sharp dip at some frequency points, but there is no dip for shorter
trace at same layer.
I read Eric Bogatin's article "How to Read S-Parameters Like a Book", there
is a section is about "sharp dip", but its seem not match my case. In fact
we use standard Delta-L coupon design to do measurement, then implement
de-embedding. When we put Sdd21 curve together, Sdd21 of 10inch trace have
obvious dip, 5inch trace also have but not obvious, 2inch trace no(perhaps
submerged by multi-reflection?),And the dip frequency point of 10inch and
5inch trace are almost same. After de-embedding, the dip is more obvious.
Is there any advice or experience to share? We could do some test and study.
B&R
Tommy
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