Technical note 6

Power planes, not so good

To distribute DC power to various electronic circuits, often ground planes and power planes are applied, as the circuits are thus easily accessed. Active circuits, however, behave as current sources that inject noise currents into these planes, which cause emission. The active circuits must therefore be 'isolated' from the power plane by means of high-frequency filters. In digital equipment, this 'isolation' is usually omitted and only decoupling capacitors are connected. Resonances now occur between the power plane and the ground plane and even a thin isolation of high-permittivity material between the two planes is no guarantee of low emissions. Splitting-up the supply plane in short DC lines with dedicated L-C filtering decreases emissions more than 10 dB. The cost of the extra components is more than compensated by the fewer EMC measures that are required.