On Mar 2, 11:03*am, "N_Cook" wrote:
hr(bob) wrote in message

...
On Mar 2, 5:14 am, "N_Cook" wrote:

Arfa Daily wrote in message

The ferrite cylinders provide a high impedance for common-mode
emissions coming from the source onto the cable so that the cable does
not act as an antenna/wire. *It should have no effect on signals
traveling on the cable that are "balanced". *The cylinder is just an
elongated toroid, and the additional material raises the common-mode
impedance more at the lower frequencies. *A series of smaller toroids
would have the same effect at lower frequencies, and would probably be
even more effective at higher frequencies. *There are a LARGE number
of ferrite compositions, designed to have peak impedance at various
frequencies, and to have a "high Q" or a "low Q", depending on what
frequenciesa are needed to be suppressed. *And as another poster said,
tha manufacturers would not go to the trouble and expense if they did
not have to. *Back in the 1970's, when I first got into EMC, there
were complaints about interference to airplanes in the Baltimore
Washington area, and it was traced to computers. *That led to the
first FCC rules on interference. *Eventually we issued ANSI C63.4 in
conjunction/cooperation with the FCC, setting up rules on how to make
measurements to ensure standards were used to measure equipment to the
FCC emission limits.

*I worked in this area for 40+ years before retiring from Bell Labs 10
years ago. I am still on ANSI C63 and various IEEE EMC Society
committees. In that 50-year time, emissions have increased from the 1
- 100 MHz area up to the 10 MHz - 10 GHz area. *WIth the increase in
frequency, it has become harder to shield noise as the smallest
openings that were not a problem at 10 MHz become slot antennas at 10
GHz. *Ferrites are now being incorporated into the connectors
themselves as putting them on to cables allows for noise to be
radiated from the short stretch of cable between the connector and the
point of application of the ferrite. *Good design will help reduce the
problem, but there aren't enough well-trained EMC designers/engineers
out in the field. *The IEEE EMC Society is constantly working to get
more engineers interested in the field. *It is hard to get people
interested in something they cannot feel, see, hear, smell or touch.
But it's a fun way to be an engineer!

*****

But is that still the case where the wire is coaxial with a thick layer of
metal, shielding the core wire? This type is high current carrying coaxial
wire , not signal level co-axial cable which can have relatively open mesh
shielding.

Yes, the ferrite is effective for any common mode signals that occur
when the outbound signal on the center conductor is not EXACTLY equal
to the return on the outer conductor/shield. Any net difference
between the two signal levels is common mode and that is what the
ferrite will reduce by increasing the common mode impedance.