On 7/14/2012 11:14 AM, pfizenmayer wrote:
> One thing that has always bugged me is very very few of these discussions
> ever seem to consider what impedance the choke is working between. Seem to
> me if you dont know that it really does not matter what the choke impedance
> is at some frequency.
Those are excellent questions, but they are applications questions, and
they can be VERY complex to analyze in virtually any world situation.
Think about it -- we're using chokes over a wide frequency range,
inserting them on cables whose ELECTRICAL length and terminations
determines both the common mode voltage and the common mode impedance,
and that electrical length varies greatly with frequency.
> Which leads me to ask - what is wrong with measuring the attenuation with a
> two port VNA with the choke simply in series between port 1 and port 2 ?(of
> course calibrating out any cables if used) This is basically a "VI"
> measurement and seems to me to give valid measure of the atteuation if the
> load impedance presented to the choke is 50 ohms and the choke is driven
> from a 50 ohm source.
If you read what I've written earlier in this thread, and what is in
greater detail in my RFI tutorial, that's close to what I've done in my
lab to obtain the impedance data that I've published for coax chokes. I
took data point by point, put it in a spreadsheet and did the math, and
plotted it. It's "scalar" data -- that is, magnitude only, no phase
data. But because a parallel resonant circuit is a good first
approximation of these chokes, R is equal to Z at the resonant peak, L
can be computed from the linear part of the curve well below resonance,
and C can be computed from L and the resonant frequency.
73, Jim K9YC
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