To suppress high frequency noise by using ferrite bead

[onemilimeter]

I've a switched-mode AC/DC adaptor. Let's say it's used to provide a 5VDC to Circuit-A. At the end of the DC output wire, there is a ferrite bead as shown in the image below. Is the ferrite bead used to suppress high frequency noise from the switch-mode AC/DC adaptor to the Circuit-A, or to suppress high frequency noise from the Circuit-A to switch-mode AC/DC adaptor?

[onemilimeter]

The GND of Board-A and Board-B are connected together.
An isolated DC/DC converter is used to supply Board-C.
If a ferrite bead is used to connect the GND of Board-B and the GND of Board-A, can the ferrite bead prevent the high frequency noise (from DC/DC converter) to flow into Board-A?

Thanks.

[Zero999]

The ferrite is there to suppress the noise from the adaptor and any RF picked up by the cable itself.

[the_raptor]

Ferrite chokes like that are for suppressing RF. Remember that anything that oscilates and is connected to an "antenna" (ie wiring, circuit traces, ungrounded chassis) is an RF transmitter*.

These days any cable that doesn't come with RF chokes should be banned as unfit for purpose.

* I can obscure a 50 watt FM carrier from a repeater a few km's away by simply moving my handheld to within about a metre of my computers monitor.

[onemilimeter]

I wander how the wires are wound on the ferrite bead to suppress RF noise. Are both (+) and (rtn) wires wound in the same direction?

[Zero999]

Both wires just go through the middle of the bead.

[Neilm]

A ferrite works by going around a wire. The ferrite will then act as an impedence of a few ohms at the frequencies required to attenuate the signal - usually in the order of 100-500MHz or so. For greater affect the wire can be wrapped around the ferrite so it passes through multiple times. The effect of the ferrite is then increased  by the number of times though the ferrite.

Ferrites like this are most often used to eliminate the noise of high frequency switching - not just the signal itself but the harmonics in the switch edge. This might not be from the PSU iself, it may be from whatever it was designed to be plugged into. I have had to use these on mains leads to suppress noise generated in the box under test. Ferrites are also used to prevent noise entering a unit via the cable.

These days any cable that doesn't come with RF chokes should be banned as unfit for purpose.

 I must say I disagree - the design should be done well enough that noise is either not generated or the unit is immune to it. (Tell your boss that by making a minor mod to a PCB will mean you don't have to fit £2.50 worth of ferrite to a unit you make 3k a year and what his reaction.)

Yours

Neil