On Fri, 19 Apr 1996, Mike Violette wrote:
> 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 > [email protected] > So the conclusion is that inserting a ferrite bead only does not create a filter, before we never heard of L-Filters, only LC-filters. THerefore there must be an decoupling capacitor across the VCC of the IC and the ground plane. THe decoupling capacitor can either be a discrete component or an VCC island under or very near to the IC. The VCC island is then connected to the main DC Island by above mentioned ferrite bead coil. I remember before flat capacitors were available that could be placed right under the DIP package of an IC with connections near the VCC and GND of the IC rgds Rene
