Dan, It is a lossy material and most commercially available ferrite beads are 'tuned' to peak around the 100 MHz (VHF band II ?). I suppose there must be lots of exception but I have not use any outside this region. Most of them have different Q to describe their individual characteristics. Obviously, there are lots of RFI from many FM radio stations making it a necessity for many design. The material's 'lossy' property are subjected to saturation etc...
Here is how I suppose it works :-) or fail to work often..., the presence
of a ferrite bead in the ground path increases the inductance of the return
path at some frequencies, it also introduces a form of RF losses that can
be modelled as an ac resistance. Hence, it is a differential mode filter
of sorts, it will not work against CM interference if the bead is only in
the path of the return lead (signal ground). There is always other grounds
that RF breakthrough can come around.
However, so much being said, pay careful attention to the layout
(installation and placement) because in general reference designs often do
not provide reference 'photographs' for best layout practices. Some do
provide PCB layout but very often in the interest to avoid verbosity, the
all important 'notes' are absent. Inter-conductor capacitance can ruin
the performance of the ferrite bead.
Just my 2 ยข
cheerio...
Tim Foo
Dan Pierce
<dpie...@openglobe.net To:
emc-p...@majordomo.ieee.org
> cc: (bcc: Wan Juang
Foo/ece/staff/npnet)
Sent by: Subject: Ferrites for GND
owner-emc-pstc@majordo
mo.ieee.org
10/12/02 03:01 AM
Please respond to Dan
Pierce
I have always been reluctant to place ferrite beads in the ground path, but
I see them frequently in reference designs for USB and Analog Audio.
What kind material should this be and what characteristics would this type
of ferrite have. I am assuming this ferrite would not have 600 Ohm
impedance @ 100MHz
Thanks in advance,
Daniel J. Pierce
Sr. Design Engineer
OpenGlobe, Inc.
> (An Escient Technologies Affiliate)
6325 Digital Way
Indianapolis, IN 46278
mailto:dpie...@openglobe.net
P: (317) 616.6587
F: (317) 616.6587
(See attached file: Dan Pierce.vcf)
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