Dear Hari,

I was just wondering whether your problem kind of lies outside the domain of DC power supplies, ground planes and such stuff on the DC power side of the MCU.

How is your ground setup for the machine ?

In India in most old installations, we have the same conductor going around as both Neutral and Earth! The electrican will call it either depending on who asks. With the result that nowhere in the panel you have a "clean" earth to refer as signal zero.

Just re check to see if your earth line is carrying the return ( neutral ) currents of motors and other such high power gadgets.

In all such installations I have ( and insist ) upon separate Neutral and Ground conductors. The Neutral is run all the way upto the incomer to the building and bound to the Electricity Board neutral. And for earth ( the clean reference ) we use another heavy conductor ( minimum 20sq mm flat ) that serves as safety earth and connected to multiple earth pits all round the factory and FINALLY to the neutral AT THE entry point of building. Actually it is difficult to explain the setup clearly in words. Maybe the following image will help clarify it - I got it through a link given by our Mr Kai Klass vide a series of PDF documents- Thanks.( EMC for systems and Installations by Keith Armstrong ) I am unable to locate link right now. Will try anyway.



If my guess is not wrong, maybe the source of your problem lies there in the Earth. ( And in any case 49MHz is a wee bit on the higher side ;-)

Raghu