Ferrite beads question

[kalel]

A very simple question about these and similar ferrite beads:



Are they supposed to be used as is, can you make a coil around them? Do they already have a coil inside?

If they are used as is, what are they used for, filtering?

[cdev]

They act as an lossy inductance and RF choke, and no there is no coil, they are used to dampen RFI and prevent oscillation etc.

[FlyingHacker]

I have not seen ferrite beads like that with the wire already through them, but typically they are used because they provide impedance at particular frequencies. As you suggest their effectiveness increases with multiple turns through the core (goes up with the square of the number of turns, where two turns through is the same as four beads in series, three turn is nine beads, etc.).

There are various "mix" ferrite materials that have different properties. You really want to consult the data sheet to find the impedance at various frequencies. You could also test it well enough for most uses with a scope, or better a spectrum analyzer with a tracking generator.

[kalel]

There are various "mix" ferrite materials that have different properties. You really want to consult the data sheet to find the impedance at various frequencies. You could also test it well enough for most uses with a scope, or better a spectrum analyzer with a tracking generator.

I would probably have to stick with the datasheets when available or avoid the cases where none is available.

[FlyingHacker]

That is the wise choice.

[floobydust]

There are single-turn and multi-turn ferrite beads.
I gave Fair-Rite Products as an example, but many other companies make them.
Sometimes the same core is used for a 1, 2, 3 etc. turn part.

You can also get just the bead and slide it on a component, like a resistor or jumper.

[w2aew]

You might enjoy my video on the basics of ferrite beads:

[robinb]

When looking at catalogue entries for ferrite cores I see "Maximum Frequency" figures given ... Dumb question: is that the maximum frequency you should try to pass through it, or the maximum frequency that it can usefully suppress?


Cheers,
       Robin.

[cdev]

 I have uploaded a small PDF which gives the basic outline on ferrites for suppressing EMI.

I find using a signal generator and scope is really helpful in seeing what a given ferrite does.

[robinb]

Thanks for that, very nice intro.
My actual issue is that I want to suppress a 1.3GHz signal but every ferrite I can find seems to have a "maximum frequency" value of between 300 - 500MHz. Impedance graphs all seem to be heading upwards at the point where the plots stop (usually a few hundred MHz). Is it likely that the ferrites will be more or less useless at higher frequencies? 

Cheers,
      Robin.

[cdev]

It seems likely that a ferrite or even better a combination bypass capacitor ferrite- the 2 dimensional equivalent of a 3 dimensional feed-through cap and choke-which likely is marketed for exactly such a purpose, would present an almost total block to signals at that high a frequency. But you should check their specs. Coils have internal capacitance that sometimes passes high frequencies. You should look for specific parts that act as LC filters- if you can wait until an order reaches you its highly likely a specific part is available which does exactly what you want.

I have a few SMD feed through filters., Which go in the same place as my more traditional feed through (axial) filters (combination of inductor and cap for filtering RF off of a DC line) . they are three terminal devices which look like a standard inductor or bypass cap but with a pair (or one) terminals in the middle which go to ground. They are supposed to be connected to ground in the middle. Look under EMI filters.

They may also be framed as "3 terminal capacitors" some which incorporate ferrites or inductors. they are made for filtering RF off of DC lines.

See below.. an example of what I mean.