Hallo Prahlad,

Erik is absolutely right in telling that ferrites are needed for filtering!

Please have a look at this link:

http://www.murata.com/emc/knowh...8to15e.pdf

It's what I would call 'chapter 1 of filtering bible'. It demonstrates, that a cable somewhere connected to microcontroller board can act as antenna emitting extremely high levels of radiation even if the cable is not carrying any signal!! It all comes from ground noise on PCB and the key word here is 'common mode noise'.

Keep in mind: Only because a cable is labelled to carry 'DC signals only' does not guarantee that no radiation will occur! Just the opposite, everything which is connected to a PCB will suffer from common mode noise and will cause unsane radiation unless measures are taken to prevent this!

First, you must find a quiet place on PCB for the connection point of cables, where common mode noise is low. Such a place is far away from microcontroller and its buses, but next to a chassis plane! Then you must use common mode filtering for each cable leaving the board. The filters Erik proposed can be used, but if signal integrity is degraded by low pass filtering, then a special common mode choke should be used, which filters out common mode noise, but not the signal. Please have a look at this murata site, where the difference of differential signal filtering and common mode filtering is discussed into overwhelming detail! Also the comliance-club link I gave you earlier is a good address to become a master in common mode filtering.

If you have a look at CE compliance testing, you will notice, that an overwhelming number of tests use common mode interference to check the immunity of device under test! So, it's extremely necessary to be weaponed against all sorts of common mode interference, by the help of common mode filtering!

Kai