Ferrite bead as low pass filter search

[aryenudelman]

Hi

i'm looking for a ferrite bead that will block 1-20 Mhz
frequancies and pass all lower ones

is it possible ?

can someone to advice me for one specific?


Arye

 

[Phrak]

That's not really what bead will do. What's the application?

 

[aryenudelman]

really

if i need to block a frequancy around 10MHZ it is not possible using ferrite?

but that's the advice a got...

strange

Thanks

 

[Phrak]

really

if i need to block a frequancy around 10MHZ it is not possible using ferrite?

but that's the advice a got...

strange

Thanks

You know, I read your question backwards? I'd though you wanted to block the lows. My mistake. As I understand it, it's an engineer's prerogative to get things bass ackwards, so I'm good.

You should look up the frequency response of various ferrite materials to see if you get the attenuation you want at the frequencies you require.

 

[aryenudelman]

thanks

i thought if maybe someone knows a specific bead

thanks anyway
Arye

 

[vk6kro]

Ferrite acts to increase the inductance of a coil. That means it has more inductance for the number of turns than it would have if it was wound on wood or plastic for example.

It is not able to act as a filter by itself.

For this, it needs to be used as part of a filter with capacitors and possibly other inductors.

Even then, a filter is seldom perfect and it will allow some of the unwanted signal to pass through it and it may also reject some of the wanted signal.

This is a high pass filter, just to give you an idea:

The coil may be wound on ferrite, but the other components are necessary.
A practical filter would usually be a lot more complex than this.

 

[aryenudelman]

thanks

but I'm not sure you are right
ferrite beads are freq dependent impedance and can be very high in some desired freq therefore blocking the signal

thanks
Arye

 

[vk6kro]

No, that just isn't true.

Ferrites do have permeability (meaning they increase the inductance of a coil wound on them) that depends on frequency, but this just starts off high at low frequencies and drops rapidly as frequency increases.

They do not show big peaks or dips in permeability or impedance at specific frequencies.

They can only be used to enhance the inductance of coils and these can then be used with capacitors (and/or integrated circuits) to produce filters.

Coils wound on ferrite can be placed in cables to reduce interference. You will find such coils in the video leads of computer monitors. Their effect has more to do with the actual value of inductance produced than the type of ferrite in use. They have very broad response and have to be carefully designed to avoid degrading the video signals.

 

[aryenudelman]

ok thanks a lot for the description but what does mean the graph on page 131
isnt it showing a 1200 ohm at 10^7 hz?
http://www.fair-rite.com/newfair/pdf/CUP%20Paper.pdf
Arye

 

[sophiecentaur]

Actually, it is common to use beads as HF blockers because the material is chosen to be very lossy at high frequencies. The bead actually behaves like a series Resistor above a certain frequency. It's a very elegant way of doing the job of a whole RLC filter with one small wound component. The cleverness is in the particular choice of ferrite.

 

[Phrak]

Actually, it is common to use beads as HF blockers because the material is chosen to be very lossy at high frequencies. The bead actually behaves like a series Resistor above a certain frequency. It's a very elegant way of doing the job of a whole RLC filter with one small wound component. The cleverness is in the particular choice of ferrite.

Yeah, what's going on with that?

Naively, one would think that the series resistance would be proportional to the number of turns around the BH curve per second times the amplitude of the curve, or

R = kfI

But this isn't the case on examining figure 2 within the web page provided by aryenudelman.

 

[Phrak]

Hi

i'm looking for a ferrite bead that will block 1-20 Mhz
frequancies and pass all lower ones

Arye, It doesn't look like ferrites will attenuate well at 1 MHz. You might look into a low frequency iron powered toroid made of material such as #3 in the following web page. However these materials seem to be selected for good Q rather than bad, so you should really try a broader search than I have.

http://www.bytemark.com/products/ipmaterl.htm"

 

[aryenudelman]

thank you all for your attention
the freq i want to block is 10 Mhz and not 1 Mhz
does it make any difference?

 

[aryenudelman]

and i need it to be a low profile means low then 1mm height
for a PCB board

 

[Phrak]

thank you all for your attention
the freq i want to block is 10 Mhz and not 1 Mhz
does it make any difference?

Originally you said 10 MHz. But it seems to make a difference on material selection. You're going to have to do the research your self on ferrites and iron powder, unless you can convince someone else to help.

and i need it to be a low profile means low then 1mm height
for a PCB board

40 mils is a tall order. Can you go 40 mils on one side or both? Can you put a hole in the board to accommodate large components? How thick is the board? And what frequencies do you want to pass? Do you have a signal with respect to ground? Is the signal differential mode. If so, is it differental voltage or current? Is it shielded? Is the shield ground? If you have a ground shield, which end is it tied to--transmitter, receiver or both? Or do you have a twisted pair? Do you want to block common mode or differential mode noise? Is it really noise you are trying to block or a signal source?

I'm willing to bet that only one or two of my questions have anything to do with your application. With a restraint of 40 mils you have to be very specific about what you are trying doing.

What are you trying to do?