Choosing Murata Ferrite Bead Impedance for EMI Suppression Above 30MHz on VCC Line

Hi,

I am using some Murata ferrite beads for EMI suppression above
30MHz. These Murata ferrite beads in any particular model are available
in various impedances @100MHz. For example 10 - 1000 ohms. I don't
understand why anything other than the maximum impedance type would be
chosen. Why are there so many impedance types?

There are so many models available that I am really not sure what to
get. The attached picture shows six graphs for six devices. The DC
resistance and current capability is about the same for all of them, and
will suffice for my application.There
are three different physical sizes, 15, 18 & 21, and any of those
size are OK for me. I need one bead on the VCC line of the 64MHz MCU,
and perhaps also on the 4MHz SPI line, for the clock, data in and also
data out, I guess.

It seems to me that if I want to limit radiated emissions at 30MHz+ then
the best choice is BLM21RK102N1, because its resistive area starts
lower than any of the others, at about 7MHz (where X crosses R). But if
it is that simple then I don't know why there are so many choices and
why anyone would ever choose anything else.

It seems that a device with a resistive curve starting above 30MHz is no
good and so none of the devices on the right are satisfactory. But
again with everything commercial needing EMI certification for radiating
emissions above 30MHz, I don't know why there are so many options above 30MHz.

appreciate any information. thanks.

Firstly, radiated EMI suppression is more about layout and ground planes of your PCB, as well as component choices. Minimizing conducted radiation from signals going off board, for example USB and power supplies is where filtering is most likely to be needed. You definitely don't want to put heavy filtering on the SPI bus, you could potentially degrade the rise and fall times, since you will be removing the high frequency components of your clock and data lines.Good power supply decoupling is all you should need on the Vcc lines, at the optimal locations.There is a lot of information about on EMI suppression, HP have a very good application book on the subject, as do ADI.With respect to PCB layout, Freescale have a good article called Effective Printed Circuit Board Design. There are a lot of devices above 30MHz because radiated nose is measured up well beyond 200Mhz, as opposed to conducted noise. Often you are only looking for 10-20dB of attenuation.Cheers,Richard