Indeed such is the case, and it is quite pronounced with high Q inductors.
You may wish, however, to model and/or measure two series chokes when
both are wound on lossy ferrite material. The response looks quite
different. The resistance in parallel with the inductors radically
modifies the response when the material causes the inductor Q to be < 1.
I'll E-mail you some early measurements and a quick simulation plots.
Jack K8ZOA
Alan Bloom wrote:
When you wire two chokes of different value in series, you almost always
get a series-resonant "hole" in the attenuation somewhere between the
parallel-resonant frequencies of the two chokes. It's not hard to see
why that's true if you model each choke as an ideal inductor in parallel
with a capacitor. So you need to design the combination so that the
series-resonant frequency falls somewhere unimportant.
Al N1AL
On Thu, 2008-07-31 at 09:14, Jack Smith wrote:
Jim:
Thank you for the reference.
Indeed, the self-resonant frequency of the typical small (FT50 size)
chokes I've wound are in the 5-10 MHz range, but at 100 MHz some
(depending upon the core material) still show enough Z to be useful.
For truly wideband 10 KHz - 100 MHz choke action, it's necessary to
series two wound with different core material and turns, e.g., 35 turns
on Steward 40 material for the 2.5 mH, followed by, e.g., 10 turns on
FairRite 43 material for > 50 MHz.
You have to pay particular attention to the u' and u'' values of the
ferrite material and how they change with frequency as this is a case
where core loss at high frequencies can be good.
Jack K8ZOA
Jim Brown wrote:
On Wed, 30 Jul 2008 20:33:53 -0400, Jack Smith wrote:
Yes, I have several ferrite core 2.5 mH chokes here, including the
Hammond one you mention. There's a significant difference in high
frequency performance of the pi wound on ceramic form versus the smaller
pi-wound over ferrite and that's one of the things I'm hoping to
illustrate in the article.
The RFI and Ferrite tutorial on my website includes clear explanations of
the nature of ferrite materials from a circuit point of view. You may find
it helpful in explaining why your very correct in your analysis. You are
welcome to cite it as a reference.
http://audiosystemsgroup.com/RFI-Ham.pdf
However -- ANY 2.5 mH choke has a good chance of looking capacitive at 50
MHz.
73,
Jim Brown K9YC
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