A Bag of Water.

This is a very useful analogy to use when considering an EMC emissions problem, particularly true for radiated emissions in the (often problematic) 30MHz to 1GHz band.

 

Lets get squeezing.

Many of you will have experienced this before. Making a change to an emitting structure inside the equipment by changing the electrical connection between two points results in some emissions going down and some going up.

radiated emissions plot

Then you make another change and this has the opposite effect.

This is like squeezing our bag of water. We can move the water around in the bag much like we can emissions around in the spectrum. The harder we press down in one area, the more it pops up in another.

Emission goes up.

Emission goes down.

 

Reducing the volume

But unless we reduce the amount of water in the bag we will nearly always have a problem. The water is incompressible and it just finds new places to appear.

To achieve this in an EMC context we need to reduce the overall energy in the system.

This could be achieved either by keeping the energy controlled on a PCB away from the radiating structure or by adding lossy components (filters, ferrites, etc) to reduce the amount of energy coupling into the radiating structure.

Changing grounding and bonding within a system without reducing the energy is going to be an exercise in frustration and probably wasted time. Better to address the problem at source where possible.

 

Caveats inbound

There will always be a requirement for us to have to try and achieve the goal of “shaping” our bag of water to fit the radiated emissions limits.

A good example is a manufacturer that has already built a production run of units and needs a quick fix to get them onto the market.

Whilst this is often achievable, there are often significant rework / modification costs involved.

There is also the question of repeatability and consistency. If small changes in bonding of parts can make a large difference to emissions, how can you guarantee that each unit will be compliant? Testing multiple samples can help. As can having good production inspection points during the manufacturing process.

But common mode noise is a slippery customer and these kind of fixes should only ever be considered as temporary pending design changes to address the root cause of the issue.

 

A small plug.

Help is available.

We are really good at this kind of work

We’ve been through the cycle many many times with many many different products.

Using Unit 3 Compliance to help with your emissions problems gets you access to our years of accumulated experience.

Our on site test lab allows us to have a rapid cycle time between analysis of a problem on the bench, developing a fix, and testing in the chamber.

 

Hope this was interesting!

James