"Bead", what is that?

[klitgt]

I have found a schematic diagram for a separate PSU for a CDP clock that includes a "bead" (appears as a coil).

What is this?
I understand from searching here that it is a device that kills RF etc. But such a thing may come in several forms?
What should I ask for at the parts store?

[pinkmouse]

I would imagine you mean a ferrite bead. They come in all sizes, but I suspect the one you need is like a small hollow cylinder, about 4mm diameter and 3mm or so long, you can slip it over a component lead. You can also buy them with wire already attached. What you need specifically depends on you application.

[mrjam]

Try this link:

http://www.murata.com/emc/index.html

Murata offers a wide production of good emi filters in both surface mount and through-hole packages.

Ferrite bead in a digital supply track (close the Vdd pin) doesn't allow the spikes produced by saturating digital ICs to pollute the other power lines, due to his high reactance in comparison to MHz frequencies used in digital audio circuits.

It gives its best when followed by a small decoupling chip cap (100nF) soldered underneath the IC.

Bye

Roberto

[klitgt]

Originally posted by pinkmouse
I would imagine you mean a ferrite bead. They come in all sizes, but I suspect the one you need is like a small hollow cylinder, about 4mm diameter and 3mm or so long, you can slip it over a component lead. You can also buy them with wire already attached. What you need specifically depends on you application.

Like this

[mrjam]

It's the supply for a TentLabs XO
The bead attenuates the noise produced by the oscillator forming a filter (L-C) with the cap. I suggest you to add a small smd cap in parallel with the one on the schematic to improve the decoupling at high frequencies.

Bye

Roberto

[mrjam]

Sorry, I've confused with a similar circuit.
Anyway the purpose of the bead is the same!!

Bye

Roberto

[klitgt]

Originally posted by mrjam
Sorry, I've confused with a similar circuit.
Anyway the purpose of the bead is the same!!

Bye

Roberto

Thank you, I did not know that it is a tentlab circuit. The tentlab website does show the circuit but not the kind of bead to use.

Can anyone help me so that I purchase the right component?

[Jocko Homo]

It does not act like an L-C filter!

Beads work by acting as a resistor at a wide range of frequencies, without having any actual resistance. The frequencies that they are effective at is determined by the "mix" it is composed of. Most beads you guys will find floating around work in the 20-100 MHz range, varying some as there is more than one popular "mix" in that range, depending on who made it.

Jocko

[mrjam]

Hi Jocko!



I studied that even a simple wire has an inductive reactance, if you surround a conductor with ferrite you increase the mutual induction of the wire.

The resistance of a resistor is theoreticaly indipendent from the frequency since it follow ohm's law, while an inductor (air wound coil, ferrite core coil, high-mu core coil, our ferrite bead and a pcb track in several hundreds MHz circuits) has a variable resistance related to the frequency of the ac current that flows across it (digital circuits are full of spurs and harmonics due to square waves, saturations, etc).

Am I wrong considering it an L? Is the behaviour of a resistor different?

Of course ferrite bead has to be selected to work best at the frequencies of interest (for example: around 12MHz in a 44.1KHz OS DAC)
So klitgt you should observe the Resistivity Vs. Frequency curve on the data sheet, it shows you at what range of frequencies it is most effective.

Bye
Roberto

[mrjam]

Hi klitgt! Eureka!!

Maybe this attachment could help you, it's an abstract from Digikey catalogue which shows the appearance and the curves of Panasonic Excel series ferrite beads.

Bye

Roberto