EMI: Back to the Drawing Board - Again!
Friday, May 29, 2009 in Uncategorized
By John Whalen
All too often during product design, well intentioned product managers, rightfully scared from prior EMI encounters, especially during the beginning of a new project, schedule large amounts of time for EMI evaluation and prevention. Program managers add in time for the illusively fiendish problems that always seem to delay projects at the very end. As schedules inevitably are delayed, the big gaps are minimized to small gaps, or eliminated. The simulations and Printed Circuit Board (PCB) evaluations and trace routing guidelines are placed in the hands of the PCB Autorouter.
Often, the EMI engineer at the end of a project will carry the new consumer product back from his/her EMI chamber and trusty spectrum analyzer, with a fist full of graphs with large peaks and valleys. “It’s got problems”, he/she says, “but we can fix it. You just need to eliminate this spike from the system clock that is generating harmonics large enough to turn on a light bulb.” Rerouting the clock requires a new Printed Circuit Board layout. That can, in itself, create new problems associated with parasitics. The next PCB revision takes a full eight weeks to design, populate, test and evaluate.
This time, the clock traces have been placed too close to the image sensor system. The result is that when the camera is turned on with this new handset, the video screen has vertical bars from left to right corrupting the screen. Again, the device is put through EMI testing. This time the handset passes with flying colors…. however, due to the close proximity of the image sensor system, the PCB must be redesigned yet again. What has been your experience in solving these challenges?
About the author:
Business Development Manager, Signal Path Analog
Tags: EMI, Fairchild Semiconductor, PCB, Printed Circuit Board
June 3rd, 2009 at 12:07 pm
EMI fixes are generally easy and free to fix during the initial circuit andf PWB design (keep loop areas small…keep high speed digital signals away from sensitive analog circuits…don’t run that clock over a slot in the ground plane, ha!) but maddening and expensive after the PWB and mechanical design is locked down. It’s an area where the Art of Electronics is in play. If you intuitively and instinctively “enjoy” minimizing current loops and keeping loud and noisy signals away from the areas that should be quiet…you’re way ahead of the game.
June 8th, 2009 at 2:55 pm
Any onlookers who have experience in edge rate control for EMI mitigation? I’m currently on a class-D amplifier project where output edge rate control is a required option to be competitive. Interesting design challenge.
June 8th, 2009 at 6:19 pm
My experience with EMI and class-d audio circuits has left me with some memorable encounters. An EMI complaint from a customer designing a 100W subwoofer using a switching amp for the first time prompted a trip to taiwan. When I saw that the design used a wooden box for shielding I knew I had my work cut out for me. After explaining the concept of a Faraday shield and port decoupling to this engineer I left him to spin his design. This solution was the brute force method and would have worked. One year later I returned to see that the customer was still trying to pass FCC with a wooden box………
Years later EMI is now a concern in every consumer application. Well described application notes and reference designs with layout guidelines go a long way if the customer is willing to listen to your suggestions.
When I hear auto-route I think wooden box.