What this technique is used for
The use of EMC techniques in printed circuit board (PCB) design, placement and layout is one of the most powerful and cost-effective ways to reduce emissions and improve immunity.
For low-frequency analogue devices and circuits which do not have appreciable emissions, these techniques are used to improve immunity.
Good PCB design for EMC can reduce the costs of filters and shielding, reduce the number of development iterations, improve circuit functional performance (improve signal integrity and signal-to-noise ratios), improve time-to-market, and reduce project financial risks.
How this technique is used
From the very first – PCBs should be designed and laid out using a number of well-proven EMC principles.
It is not anything like as cost-effective (or time-saving) if the PCB EMC techniques are applied later on, when EMC problems have been found and solutions are being sought.
Key issues in employing this technique
Circuit segregation or ‘zoning’
 Firstly the ‘outside world’ and ‘inside worlds’ are identified. The ‘inside world’ is the area or volume where the designer has control over all EMC phenomena (e.g. by shielding and/or filtering), and the outside world is everywhere else.
Secondly the ‘inside world’ is further segregated into subsidiary zones. The traditional segregation between analogue, digital, and switch-mode power conversion circuits is an example of this internal ‘zoning’.
All of the devices and conductors in each zone are rigorously separated from all of the devices and conductors in the other zones, to reduce the electromagnetic coupling between them as much as is possible in the space available.
It may be necessary (for EMC cost-saving or signal integrity reasons) to fit shielding over individual zones. Five-sided metal boxes soldered to the PCB’s 0V plane at numerous points can create a fully enclosed shielded box around a zone.
Only the essential interconnections between zones are permitted to cross the boundaries between zones, and any/all of them may need filtering or otherwise suppressing.
|
