To see the trace-only return loss, you need to de-embed the launches.
For causal de-embedding, see http://ataitec.com/products/isd/
Ching-Chao Huang
On Mon, Dec 12, 2016 at 1:42 AM, Havermann, Gert
<Gert.Havermann@xxxxxxxxxxx> wrote:
Hi Sunil,------------------------------------------------------------------
Return Loss is not only a function of Impedance. The Return Loss is just
showing you the reflections over the complete signal path, and reflections
can be caused by impedance mismatch and even stronger by impedance
discontinuities. Bad launch into the test cupon, signal vias in the test
cupon, missing return vias, Return vias at bad positions, Plane voids,
via-antipads, etching roughness and copper roughness can and will all impact
your Return loss way more then the trace impedance itself.
Look at Insertion Loss and Return loss in one diagram to find out whats going
on. If you see opposing peaks in both curves at the same frequency, then
there is a resonance, and those are always caused by at least two
discuntinuities. The frequency of appearance indicates the distance between
the discuntinuities.
In Time Domain (TDR) those discuntinuities are easier to find and to quantify.
BR
Gert
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On ;
Behalf Of sunil bharadwaz
Sent: Sunday, December 11, 2016 5:18 PM
To: si-list <si-list@xxxxxxxxxxxxx>
Subject: [SI-LIST] 50 ohm - RF Impedance Traces
Hello Group ,
We seem to be running into issues while getting PCB's fabricated with 50 ohm
lines.Our Freq band of interest is 2 Ghz to 6 Ghz.Tried checking these traces
on a VNA.The return loss looks decent @ 2 Ghz , but seldom looks good at 5Ghz
& above .
The fabricator does send test coupons & impedance reports @ TDR .This does
notcover the freq response .
This is not issue with any single fab . This seems to be a general problem ,
where return loss isnot flat across freq band .
Wanted to check if there are ways to tackle this issue .
Thanks much.
RegardsSunil.B
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