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Below are all of the forum threads, including all
the responses to the original posts.
Post subject: Cascaded RL Calculations?
Wed Aug 11, 2004 5:13 pm
Aug 11, 2004 5:11 pm
I’m trying to characterize the
return loss of a cascaded path. The path elements are very simple… cables,
attenuators and switches. I have SWR and insertion loss data for each
element, but am unsure how to calculate the return loss or SWR of the
entire path. Do you know of a tool or method that may be useful?
Thank-you very much, any help would be greatly appreciated.
postPosted: Thu Aug 12, 2004 12:49 pm
Do you have (complex)
s-parameter data for the components? If so, the s-parameters of the
cascade can be computed by converting the s-parameters of each of the
elements to their ABCD matrices, then computing the ABCD matrix of the
system by multiplying the individual ABCD matrices, then converting
back to s-parameters.
A math package can be very helpful for
this type of calculation. There is also free MW software that will compute
this easily, though I've never used it.
Post subject: Itay
Unread postPosted: Thu Aug 12, 2004 2:06 pm
If your elements are cascaded then you should
multiply the SWR of each element and you will get the cascaded SWR.
From this it is easy to obtain the relflection coefficient and RL, as
SWR=1+rho/1-rho (rho- Reflection Coefficient)
RL=20 log |rho|
Hope this helps,
Unread postPosted: Thu Aug 12,
2004 2:14 pm
Thanks for your replies!
I have already
calculated the cascaded VSWR and RL using the RSS method (very similar
result to simply multiplying each components VSWR as Itay mentioned),
but the problem is that the result is not even close to the measured
value of return loss. The calculated value is MUCH worse. I am using
the specified max VSWR for each device which would of course cause the
resulting calculated VSWR to be higher than would typically be measured.
My paths are also up to 12 devices long which is compounding the errors
due to the assumptions made in the RSS method.
I think part of
the problem, if I understand this method correctly, is that it assumes
no device losses and only looks at the 1st order reflections. In this
model the reflected waves do not experience any attenuation because
device losses have not been considered. This results in much larger
SWRs and much smaller return loss values than would actually occur.
Of course the RSS method calculates the absolute max possible VSWR
only. I will use the information on http://www.rfcafe.com/references/electr
... smatch.htm to calculate the minimum...hopefully my measurements
are somewhere between the two ;-) Otherwise I suppose I’ll need to get
a hold of the complex s-parameters for each device from their manufacturers
and try Vuk Borich's suggested method.
Unread postPosted: Thu Aug 12, 2004 4:09 pm
Why make your life complicated? Use a VNA to measure exactly the
Return Loss/VSWR at the wanted frequency range for each element in your
cascade (don't meaure it outside the frequency band you are using).
Then apply the method you named, which is similar to the one I noted
before. If you start to convert ABCD matrices to S-parameters you will
end the calculation next year ... long and winded calculations are prone
to errors. :!:
Also check that all your elements are in good
shape, a cable that is faulty might give you a short circuit or an open
circuit and then of course you will end with a wrong measurements. Check
that the insertion losses of the cables are in the range and that the
cables are matched to 50-ohm (Use the Smith Chart for that).
Unread postPosted: Thu Aug 12, 2004 6:54 pm
Joined: Wed Aug 11, 2004 5:11 pm
I'm actually trying
to determine the limits that a measured result should be between. I'm
more or less satified with my current results. The RSS method is providing
a maximum SWR (minimum RL) and the method of deviding the larger VSWR
by the smaller VSWR thought the cascade (used here http://www.rfcafe.com/references/electr
... smatch.htm) is providing the min SWR (max RL).
any details of the theory behind this min VSWR calculation?