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 Post subject: Ferrite beads
PostPosted: Thu May 08, 2014 2:50 pm 
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Dog
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Joined: Tue Dec 14, 2004 12:26 pm
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Location: Lisbon, Portugal
Here's some good reading on ferrite beads:

http://www.sigcon.com/Pubs/edn/ferritebeads.htm

Worth a read, it is well written and simply explained.
One quote:

Dr. Howard Johnson wrote:
All low-pass filters fall prey to parasitic effects. At extremes of frequency, the inductors all turn into capacitors and the capacitors into inductors, reversing the action of the filter. Beyond some threshold frequency, the filter no longer prevents noise from passing through. The response of such a circuit resembles a band-stop filter more than a low-pass filter.


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 Post subject: Re: Ferrite beads
PostPosted: Thu May 08, 2014 4:26 pm 
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Benjamin
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You missed the most important part:

Quote:
The bead manufacturer should provide you with a curve of impedance versus frequency for your bead. From this curve, you may ascertain the efficacy of a particular ferrite bead at your frequency of interest.


And he leaves out the part about the resistive component of the ferrite bead. Which is what makes ferrites so effective. So, his diagram is not 100%.

Of course, outside of the intended range of the bead, the resistive component diminishes. Never measured one so far outside of its range to see if the resistive component becomes zero. If you are stupid enough to use one that far outside of its range, then you get what you deserve.

My VNA only goes to 200 MHz, so no way I can measure the most common mix, to see what it does at 1 GHz. But, I do have some designed to work down around 5- 10 MHz. Maybe I will measure them, and see what happens at 200 MHz.

Maybe.


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 Post subject: Re: Ferrite beads
PostPosted: Thu May 08, 2014 4:49 pm 
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Dog
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Location: Lisbon, Portugal
Probably the resistive component on the higher end doesn't go down to 0, but the impedance of the cap will rise at some point and then it's not a filter anymore.
I'm curious to see those measurements, Jocko. :grin:

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