[SI-LIST] Re: Differential measurement with normal VNA
- From: Peter.Pupalaikis@xxxxxxxxxx
- To: Matthias Bergmann <MBergmann@xxxxxxxxxxxxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Mon, 6 Apr 2009 10:16:13 -0400
Hi Matthias:
I don't know anything about the rules of thumb you asked about in 1)
regarding 2), I think you will be unable to make the connector lines
completely
coupled, so I would try to make each side of the differential pair the
same. You will
probably need to make room for your connectors, and then come together
where you
have your actual line that you want to measure.
When the lines are coupled, the lines will have an odd mode impedance that
is typically
targeted to 50 Ohms, but the even mode impedance will be different (meaning
you keep
a good differential termination, but sacrifice the common mode termination
a bit).
I would handle this measurement by calibrating to your cable plane and then
de-embedding
your fixture (the connectors to your transmission line). You probably
cannot easily measure
your fixture, but you can use some impedance peeling techniques. You can
use impedance
profiles to measure the differential mode and common mode impedance profile
looking
into your connectors. Using this method, you will see the discontinuity
(in time) clearly from the
connectors and where the impedances change as the lines come together.
Then you
can cut the fixture from your measurement and de-embed it. If your fixture
is really short, you
can usually neglect loss, but otherwise, you can measure and incorporate
the loss (or
simulate it).
I have a paper that generally deals with a situation like this. It
contains an extreme case of
bad connectors and fixturing, but hopefully is helpful:
http://www.lecroy.com/tm/Library/WhitePapers/PDF/DesignCon_2008_Measurement_Based_Models.pdf
Sincerely,
Pete Pupalaikis, LeCroy Corporation
Matthias Bergmann
<MBergmann@phoeni
xcontact.com> To
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Subject
[SI-LIST] Differential measurement
04/06/2009 09:32 with normal VNA
AM
Hello SI-LIST,
I would like to measure a differential structure with a network analyzer
and have got the
following questions:
First I will measure single S-parameters which are afterwards converted
mathematically to
differential S-parameters as described by Bockelmann and Eisenstadt.
1) My VNA measurement system is connected to the DUT by cables and SMA
connectors.
I want to calibrate the network analyzer to the end of the cables using a
standard SMA
calibration kit, but I am not sure if this will be sufficient.
Is there anything like a rule of thumb until which frequency it is
tolerable to neglet the effect of
the connectors and when I have to use a custom-designed calibration ?
2) Shall I design the traces between the differential DUT and the
connectors as tightly-
coupled transmission lines with a differential impedance of 100 Ohms or as
two loosely-
coupled traces with a single impedance of 50 Ohms each one ?
In their books Bogatin and Johnson write that two loosely-coupled traces
with 50 Ohms
indeed have got a differential impedance of 100 Ohms and that tightly
coupled traces are just
used because of limited space on the pcb ? Furthermore I don`t really
understand the
appendix about coupled and uncoupled lines in the original article from
Bockelmann and
Eisenstadt.
3) Assuming that I will use coupled lines:
Probably the differential impedance would reach the aimed 100 Ohms, but the
single impedances would be somewhat higher than 50 Ohms. If I terminate the
unused terminals with 50 Ohms SMA-termination plugs, can I neglet the
imperfect termination or how should the terminatin look like ?
How does the broadening / drift of the traces in the region of the sma
connectors influence the measurement ? Can I neglet this effect ?
Thank you, Matthias
____________________________________
Business Unit Geräteanschlusstechnik
Technologielabor
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