Editor’s note: This question was asked in response to Interference Technology’s recent webinar by Keith Armstrong. To view the webinar, click here.
Question: Is there a simple method to choose a filter for a given purpose?
Answer: No. Sorry!
But we can use appropriate design techniques based on the CM and DM impedances of both the source and load circuits, and the CM and DM attenuations we want our filters to achieve, to get into at least the right ballpark.
(This is another question that is not related to my webinar, but I still have some time to fill.)
The problem is that the CM impedances and amounts of CM noise to be filtered are all caused by various kinds of tiny imbalances, and we generally don’t know what they are with sufficient detail and/or accuracy to design our filters accurately. “Ball-park” (i.e. in the right order of magnitude) performance is actually pretty good!
We can use full-blown 3D Field Solvers to analyze all the imbalances in the various conductor structures in our designs (including inside the components) to accurately predict CM and DM levels, spectra and impedances and get filtering right first time – at significant cost per seat.
Or we can aim to get in the right ballpark and leave ourselves with room to maneuver in our filters’ PCB pad patterns or the panel space we provide for them – then do pre-compliance EMC testing at the earliest opportunity to determine what filters we actually need.
Most manufacturing companies are missing an important trick by not investing in costly 3D Field Solvers and training in how to use them. As a business investment, they can easily pay back their cost within one year, on the first project they are used on, by shortening the overall project timescale by the time that would otherwise have been spent in iterating the design to pass its functional and EMC specifications. (But of course, if design iteration isn’t on the Critical Path because something else delays market introduction, then a field solver won’t improve competitiveness by as much as it could.)
Note: A survey by a major international accountancy firm found that, since 2000, the greatest impact on the profitability of a new electronic product was its time-to-market.
Not its BOM (Bill Of Materials) cost, which was in second place!
-Keith Armstrong
