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Measurement of patch cable / terminating impedance - RF Cafe Forums

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Post subject: Measurement of patch cable / terminating impedance Posted: Tue Jun 20, 2006 5:18 am


Joined: Mon Jun 19, 2006 10:27 am
Posts: 7

I've to measure the properties (return loss, cross talk) of patchcables,
which are assembled with typical RJ45 connectors as well as with other kind of 'differential' connectors, such as Firewire, USB or industrial round connectors.
RJ45 isn't a problem, but there are no instruments available with other
connectors. So I thought about constructing such an adapter on my own. Starting from a network analyzer I would connect a SMA-connector to a balun going further to the desired connector. The following problem occurs:

- Because I've got different types of connectors I'll need several
adapters. To provide an easy operability it would be convenient
if all adapters are located on just one circuit board with one
sma-connector. With the help of a switch I want to choose which
connector is currently connected with the sma-connector. So I need
something like a power-divider, 1-to-8 (8 as an example).
But I don't believe that this is sufficient, because the signal will first
be divided and the parts of the signal which travel to the branches which
currently are not connected with no connector will be reflected at the branch ends. So the branches have to be terminated with the line impedance.
How do I achieve, that a branch can be connected with a connector or
alternatively with the impedance ?

(if you don't understand my description, please have a look at ... leindx.htm,
they offer such system, it's just too expensive )


Post subject: Measuring patch cablesPosted: Thu Jun 22, 2006 12:28 am


Joined: Wed Jun 21, 2006 8:33 pm
Posts: 21
Location: Queen Creek, Arizona
First off: I could not get your link to work..missing page

I am not quite sure how to answer your question but I would offer an alternative.
If all you are trying to do is measure impedance and s-param's of different cables, I would simply design a two layer board (easy for building accurate strip-lines), and SMB connectors (They can be easy to use unless you do not mind threading on and off SMA's). Then design several traces from the SMB's to the various cableing connectors (RJ45, USB, 1394), at the same time you could construct each pair of traces that allow for terminating with shorts, opens (if needed thur's) for easy calibration and deembedding of the boards and your adapters themseles. I personally would avoid the switch, not because it is impossible to do what you are doing, but because it would still be a deembedding risk and stray parasitics could be problematic). That would be the beauty of SMB's, as easy to use as a switch unless your automating the system.

So my adpater would be a two layer board, with 2 smb's and 2 connectors for each type of cable, two traces for each cable type (port 1 and port 2) obviously connecting the smb's to the cable jacks, with the possibility of jumpering for a short calibration.

I apologize if I misunderstand what you are attempting to do and this repsonse is of no use.

CMOS RF and Analog ESD Specialist!


Post subject: Posted: Thu Jun 22, 2006 4:27 am


Joined: Mon Jun 19, 2006 10:27 am
Posts: 7
Thanks Stephen for your answer !

What you described matches quite well my intented assembly. Unfortunately I don't know how do insert an image in this forum, that would simplify of my project.

But I realized that I don't need a power-divider, something like a switch (doesn't need to be automatic) would be sufficient.
Pin-diode or a GaAs-switch are too exaggerated, so I believe a HF-Relay would be the right thing ? What kind of 'mechanical' switches are available ?



Post subject: Posted: Thu Jun 22, 2006 9:03 am


Joined: Wed Jun 21, 2006 8:33 pm
Posts: 21
Location: Queen Creek, Arizona
In some of the other forums, this manufacturer was recommended:

There was also a pretty detailed response rgerading impedanced matched switches.

CMOS RF and Analog ESD Specialist!

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