Electronics Forum / Electrical Engineering / July 2008

I am having some issues with radiated emissions and RF immunity. I
wanted to see if anyone had any suggestions.

My system is separated into 3 sections.
Section 1 - battery
Section 2 - microcontroller , power, memory
Section 3 - sensors

The different sections are connected together with unshielded wires.
The plan was to use some Murata EMI filters for all the cables coming
out from the device and the individual wires from section to
section.   To try and contain the noise two  plastic SLA's were
created to serve as a can on the top and  bottom part of the main
board(section 2)  This was coated with EMI spray and holes were
designed in for the wire connections to the other sections. The EMI
filters were installed  inside our cans.

From my observations and from the staff at our testing house it seems
the filters are not installed correctly and noise is coupling on the
point where the signals should be getting filtered.  For example, we
have some filters that were positioned directly in the middle of the
board.  In this case there is no way to isolate the noisy from the
filtered signal.  When I used near field probes the cables were
extremely hot at the frequency 672.70 MHz that was pushing us over the
class B limit.

Next the plastic shields that where created didn't fit correctly and
would melt while trying to solder to the ground ring.  We ended up
having to glue this in place instead of soldering.

The results from the test house showed the highest peak was detected
when their antenna was setup in the  horizontal position.  This
matches up with how the wire harnesses between sections were oriented
in this same horizontal position.  When I used the near field probes
the wire connections between the boards outside of can were  extremely
hot.

Also when the system was radiated with RF our measured signals would
die out.

What I wanted to do was to use a legit metal can that can be properly
soldered to our PCB.  I also wanted to have the filters properly
installed.  From my conversations with our test house,  they suggested
that I put the metal can around the center part of the filter.  This
way we have the input of the filter inside of the can, the can
surrounding the filter acting as a barrier and the output of the
filter out to the cables.

We also tried conductive mental spray but with this we still have the
cabling problems, and there is a huge cutout for our radio module.

It seems challenging to fit a metal can over filter components on the
board.  How do people do this to make sure the filters are installed
correctly?

Generally speaking how would an EMI filter (a low pass filter from
Murata) compare with a ferrite clamp on the cable itself? Equivalent?
Trade-offs?

Thanks!

Jim