Hi folks, I've enjoyed the discussion on the subject of ferrites and Vcc and feel compelled to throw in another two cents worth.
We have tested numerous products with the ferrite bead in series with the Vcc "island" that feeds power to a chip. In an overwhelming majority of those cases, we were able to trace emissions problems back to this practice. By bonding the "island" back to the main Vcc plane (copper tape and solder wick), we have noted significant improvement in emissions-- when this has been found to be the source of the problem. >From our empirical observations, we have formulated two mechanisms that may explain why this practice can be harmful: Situation #1: RF return impedance Since the Vcc plane forms part of the RF return path, inserting an impedance in series with this path raises the impedance of the circuit return path. As the currents must return to their source (and thus travel through this impedance), a voltage potential is developed between the island structure and the rest of the circuit. This potential elevates the "isolated" circuitry with respect to the rest of the circuit. An RF voltage "hill" develops (that the only way I can describe it). This can be easily detected around such an island using an E-field stub sniffer and a spectrum analyzer. Situation #2: Induced voltage due to bead currents When a gate switches, it pulls current through the bead, regardless of how well-decoupled the Vcc island is. This current generates a voltage across the impedance of the bead. This voltage appears essentially as RF ripple on the Vcc structure (which powers the gates in the "isolated" circuit); thus, this RF energy is coupled to every gate in the isolated circuit. An emissions problem occurs when these gates drive signals OFF of the isolated area (to other areas of the circuit board), and, regardless of the frequency of the intended signal, the ripple is passed along to the rest of the board. This would not be a problem if: - The 0V plane had a zero impedance (Situation #1), and - Decoupling capacitors were perfect (Situation #2) Alas, this is not the case. At RF in general (and in EMC in particular), what may offer a benefit on paper may work in opposition in practice. A logic gate, when switched, requires a tremendous inrush current to support the voltage transition. This charge must be available in the immediate local area of the chip, and a good bit of it is supplied in the capacitance of the Vcc/0V plane (a bit is supplied by the local decoupling caps). As these charges are depleted, current flows from the remainder of the Vcc/0V circuit, decoupling capacitors and power supply to replace and equalize the voltage. If one supplies a large enough contiguous Vcc/0V structure to support this charge demand, the high frequency voltage developed during the transition will be minimum. By creating an island one reduces the available supply of charges. By inserting an impedance, one puts more demands on the local decoupling capacitor AND creates a voltage drop across the impedance. The best source of this current is a fully contiguous, low Z Vcc and 0V structure. Finally, a historical observation: This concept of ferrites and power bus filtering arose in the early 80's when a clock speed of a few MHz was state-of-the art, and the upper frequency of emissions problems was some tens of MHz. I think that the design practice has become ingrained to a certain extent and wherein it didn't matter what you did `in the old days', with 100 MHz fundamental frequencies, these RF problems arise. Mike Violette Washington Labs mi...@wll.com