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alex_shrab
Post subject: De-embedding lossy transmission line Posted: Wed Jul 25, 2007 4:40 pm

Lieutenant


Joined: Wed Jul 25, 2007 4:23 pm
Posts: 1
Hi all experts ,

I'm doing the following experiment:

I have a system which consists of a pair of SMA cables, backplane with microstrip differential pair and another pair of cables which S-parameters I'm trying to find.

Unfortunately, I can't just measure S-parameters of the second pair of cables by using a VNA. But I have S-parameters of the whole system: [SMA cables - backplane - cables], and S-parameters of [SMA cables] and [backplane] separately. The backplane is relatively ‘lossy' at high frequencies (>10GHz).

I'm converting S-parameters to T-parameters, then taking T[backplane]^-1*T[SMA-cables]^-1*T[whole system]. Then, I'm converting back to S expecting to get "second" cables S matrix. Beyond some frequency (~12GHz), I'm getting very high gain and obviously wrong behavior. I used this technique in the past to de-embed NOT very ‘lossy' components...
What could be wrong?
Thanks a lot in advance,
Alex.


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nubbage
Post subject: Posted: Thu Jul 26, 2007 3:39 am

General


Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
Hi Alex
One time I encountered something similar, where a VNA plot turned to rat doo-dooz above a certain frequency, it turned out to be an unexpected resonance. This produced a rapid variation in S parameters around the resonant frequency, and at harmonics of it.
Is there any chance your set-up could exhibit a resonance?





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