Measuring Ferrite Beads

[burningman]

I was wondering if anyone know what kinds of things are measured / considered when using ferrite beads in audio circuits as EMI supressors?
Would you measure with a multimeter or would use of a scope and signal generator be necessary?
Thanks.

[brett]

Hi
I'm no expert, but ferrite beads offer inductance.  They are often used to filter high-frequency transients or signals from lower frequency lines. I am guessing that they offer a few microhenries (or nanohenries?) of inductance.  There are formulae available on the web to calculate the inductances for standard materials and geometries.
Sometime ago I submitted a thread about a cheap inductance meter (but you need a signal generator and preferably a scope).
cheers

[R.G.]

Brett's right, they add inductance. However, their primary use is in blocking and/or attenuating radio frequency signals. While the manufacturers do specify inductance with a wire through the middle, I don't know anyone who's ever actually measured one, including in a fully equipped, fully functioning RF testing facility back where I used to work. I'm sure someone has measured one sometime, but it takes good high frequency equipment to do it, well out of the scope of what us audio hacks need.

The most interesting thing to RF-blocking for beads is that they are not only inductive, they are dissipative - they actually convert some of the high frequencies to heat, not just making it bounce around or tuning it. That's the primary use - either reflecting or absorbing incoming RF.

NB - there are a number of DIY inductance meters I've run into in looking at microcontroller projects. I've never bothered to make one because in general, with the exceptions of a wah inductor and maybe some pickup sims or graphic eq inductors, if you have to measure an inductor, you're approaching the problem from a direction that will make it difficult to analyze and realize.

[burningman]

I am trying to learn more about circuits that use ferrites for RFI blocking. Is there any way of identifying beads (cylinders) that do not have any markings?
Are there standard specs when it comes to RFI blocking from audio signals?

[amptramp]

You would probably use the ones R.G. was talking about that are somewhat conductive.  These operate in the low frequency range and can be distinguished by the fact that they have a relatively low electrical resistance between any two points on the bead.  I believe some manufacturers call this a "B" ferrite, which is actually powdered iron held together by a resin binder.  If the ferrite is non-conductive, it is designed for higher frequencies and will probably not dissipate RF power the way R.G. has suggested.

[burningman]

Thank you for the further clarification.
Are there any existing circuits or examples of how people implement RFI suppression with use of ferrites?

[brett]

Hi again
look for old schematics of discrete hi-fi amps.  The phono pickups used to pick up plenty of RFI.
You might therefore find a BC107/108/109 with a ferrite bead on the base in the pre-amp stage. 
Electronics Australia or Electronics Today International would be worth a look (1970 to 1980?).

In pedals we usually try to achieve a similar thing with a small cap to ground at the input (e.g. 0.001uF). 
(Maybe phono cartidges couldn't drive capacitive loads?  Someone should know why they used inductors)
cheers

[bancika]

Thank you for the further clarification.
Are there any existing circuits or examples of how people implement RFI suppression with use of ferrites?

some tube amps have them on the input stage grid, few Mesas and probably other brands...