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Use of an LCD back panel as an image plane to reduce radiated emissions

EMC Radiated Emissions Fault Finding Case Study

I’m really happy to have one of my blog articles featured on Interference Technology.

Problem solving and fault finding EMC problems, especially radiated emissions, is one of my specialities and oddly enough is one of the facets of my job that I enjoy the most. After a successful exercise in helping a customer out with their product, getting the chance to write about it and share it with you is a real bonus.

Fixing radiated emissions is at it’s most challenging when the scope for modification to the unit are limited by the fact there are significant stock of PCBs or components that would require scrapping and redesign. Finding a way to use the existing stock was key in this example as the customer had significant time and money invested into the project. Thankfully I was able to help them out.

Head on over to Interference Technology and have a read through – I even put pictures in! Hopefully it will give you an idea of how I work and the sort of EMC issues that I can help you solve.

Case Study: Poor PC Board Layout Causes Radiated Emissions

 

Unit 3 Compliance anechoic chamber

Recent Work: Medical Laboratory Equipment EMC Testing and more

It has been an good couple of weeks here at Unit 3 Compliance with some very interesting products to work on.

Firstly, I received confirmation from a previous customer that their product has passed the required EMC certification testing at their accredited laboratory with the modifications that we incorporated during our problem resolution work. This is excellent news for all concerned!

Then things started off with some radiated emissions EMC testing and fault finding on quite a complex and clever piece of medical laboratory equipment with multiple interconnected boards, display, motors and servos. A few problems were identified and feedback given to the customer about potential improvements.

Noisy DC motors were again the theme in some more radiated emissions testing, requiring additional suppression on the motor terminals, made all the more challenging by the tight mechanical constraints of the product. Differential mode suppression on the terminals using ferrite beads to reduce the brush noise is the most effective solution but without a well defined RF return path to the brushes any noise reduction will ultimately be a compromise. More testing is being performed with some small filter PCBs mounted right on the motor terminals.

Lastly, our screened room test facility is almost completed and is being used for some mains and DC port conducted emissions testing with buck converter switching noise providing a challenge.

“Problems worthy of attack prove their worth by fighting back!” 

Unit 3 Compliance anechoic chamber

Radiated emissions fault finding and pre-compliance in the Unit 3 Compliance fully anechoic chamber

 

radiated emissions plot showing improvements made to customers product

Brushed DC Motor – EMC Radiated Emissions Problems and Improvements

I’ve been working with a customer whose product had significant broadband radiated emissions throughout the spectrum, helping them with fault finding and improving their EMC performance. Partial information from their certifying test lab appeared to show a regular harmonic series suggesting noise from a digital clock of some kind. However, near field probing with a spectrum analyser on the bench disproved this theory and identified the brushed DC motor as the cause. It is believed that a combination of the certifying test lab only performing one frequency sweep, the long cycle time of the EUT and the random arcing noise from the DC motor brushes caused a series of regular peaks to appear distributed throughout the spectrum.

In reality, both spectrum analyser measurements and the radiated emissions plot shown below show a wide range of broadband noise once the graph had multiple sweeps through the spectrum to enable it to become fully populated.

Working closely with the customer, we selected an alternate motor from the same manufacturer part with suppression capacitors built in to decouple the brush noise at the source; always the most efficient way of solving an EMC problem is to knock it on the head where is is being generated. Putting the EUT in the anechoic chamber showed that this provided a significant improvement, especially at higher frequencies but there remained an hump at around 150MHz to 300MHz.

We followed up with testing various ferrite cores around the DC motor power leads to further reduce the noise being conducted down the lead, leading to the selection of a small ferrite core with two turns around the motor DC supply leads.

Below can be seen the scan data showing before any modifications (blue) and after changing the motor and adding the ferrite core (green). Testing also revealed what appeared to be a satisfied customer 😉

radiated emissions plot showing improvements made to customers product