RE: [SI-LIST] : A Question About Power Noise.

Ray Anderson (raymonda@radium.eng.sun.com)
Fri, 5 Mar 1999 09:45:11 -0800 (PST)

Michael Zhang wrote:

>
> I agree with Han's statement about series inductance and decoupling caps.
> However, a series inductor is still needed even when decoupling caps are
> properly placed.
>
> High frequency currents take the least inductive path. At the presence of
> decoupling capacitors across VCC/GND, there are two possible paths: through
> decoupling caps or through VCC/GND planes. Caps have series inductance
> (ESL), which makes them not as effective above their resonant frequencies.
> Therefore, to prevent high-f current to be sourced from VCC/GND planes
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> causing EMI problems, one fix is to have a series inductor - typically a
> ferrite bead - to increase the VCC/GND loop inductance with respect to the
> decoupling cap ESL. This technique is generally applied to VCC, but could be
> extended to GND as well. The app note, "CK100 Clock Buffer Preliminary EMI
> Layout Guideline," available at http://developer.intel.com/ial/sdt/, gives
> such an example.
>
> -Michael T. Zhang
> Platform Architecture Lab (PAL), Intel Corp
> (503) 264-2301

The underlined statement above is correct with respect to the fact
a series inductance will prevent high freq. current from being drawn from
the power planes, but I draw a different conclusion.

A power distribution system (PDS) is usually composed of
at least 4 parts: Voltage Regulator, bulk capacitance, ceramic decaps, and
power planes. Each part is effective over a certain bandwidth. The typical
VR can provide current in response to a delta-I of the load up to a perhaps
10KHz, above that the bulk caps provide a current source that can respond up
to maybe 100KHz or so. Then the ceramic decoupling caps come into play and
are effective up to perhaps a couple hundred MHz. Then the system relies on
the energy stored in the interplane capacitance to respond at higher
frequencies.

By intentionally placing inductance in series with the power planes
and the load you are limiting the ability of the PDS to act as a low impedance
source of power for the load. Granted, the conducted EMI radiation might be
reduced, but then again if the system doesn't perform properly because you've
introduced a high impedance in the PDS at a critical frequency what have you
gained? Also, I'm not convinced the inductor is a panacea for radiated EMI
interference.

I think it is more effective to provide proper decoupling capacitors
on the plane properly placed to achieve a low broadband impedance rather
than introducing inductance to try to mask the EMI effects.

Proper bypassing can provide a good SI environment as well as
a good EMI environment in a system, whereas the addition of series
inductance may be OK from a EMI perspective but is definitely counter-
productive from a SI perspective.

Anyway, that is my humble personal perspective on the topic.


Ray Anderson

Sun Microsystems Inc.

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