Hi Tommy,
a sharp dip in S21 comes from a resonance. Check the group delay if there is
the same dip reaching into negative values.
Resonances need two reflection points. These can be mismatched ports,
discontinuities like vias, bends, glass weave, solder-joints... The distance
between the discontinuities sets the resonance frequency, the shorter the
distance, the higher the frequency.
Since you say that 10" and 5" has a dip at the same frequency, I'd suspect that
it isn't port mismatch. You should take a look at the impedance profile of both
traces, maybe you will find two reflections at the same distance on both traces.
BR
Gert
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-----Ursprüngliche Nachricht-----
Von: si-list-bounce@xxxxxxxxxxxxx <si-list-bounce@xxxxxxxxxxxxx> Im Auftrag von
Tommy Huang
Gesendet: Freitag, 28. Mai 2021 12:35
An: si-list@xxxxxxxxxxxxx
Betreff: [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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