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

Mark Randol (ryvw50@email.sps.mot.com)
Fri, 05 Mar 1999 13:54:29 -0700

OK, some comments from the low power RF/Microwave perspective...

Ray Anderson wrote:
> 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.
>
> 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 agree somewhat with both Michael and Ray. The idea is to keep the
currents as electrically local as possible. When nearing 1GHz, the way
to do that is with several surface mount capacitors. A few
strategically placed 0603 100pF caps can work wonders. Power planes
can't be counted on for this function due to the inductance in the via,
resonances in the larger structure, and the propagation delay of the
charge in the planes. The last two are related. You can't count on
the capacitance you have on the power planes once they are electrically
far away from your supply point. Instead, you'll effectively have a
series inductor referenced back to the via, due to the "transmission
line" length. If that length happens to correspond to a frequency in
your part's band of activity (not necessarily the desired band of
operation), nastiness of various forms can occur.

I may be preaching to the choir now, but...

The problem is, at 1GHz you are approaching resonance on many of the
surface mount parts if you aren't careful. Larger value components of
the same package size will have lower SRF's. That makes the component
selections based not only on impedance, but what the resonant
frequencies of those components are. Again, lots of little capacitors
can be much better than the one big one. If you put them in the right
places.

If the quiescent and dynamic current requirements are sufficiently low,
even a series resistor/shunt capacitor configuration can be used in the
supply. The "DC" current is supplied through the resistor while the
smaller currents are supplied by the electrically local capacitor. This
way the RF signals can be contained close to the part while still
maintaining a sufficiently low impedance for the "DC" current.

For higher currents, inductors provide some of the series impedance,
while resistors provide a smaller portion. For even higher currents,
only series inductors might be used, but with more capacitors.

> 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.

Again, it depends on whether the signals have square or rounded edges.
For most of my circuits, SI means a VCO with a minimal amount of phase
noise, or an amplifier WITHOUT excessive harmonic content.

Different tools for different jobs. The same ideas still apply, it just
doesn't seem like it.

Regards,

-- 
---------------------------------------------------------------
Mark Randol, RF Systems Engineer        | Motorola SPS, Inc.
(602)413-8052 Voice                     | M/S EL379
(602)413-4150 FAX                       | 2100 E. Elliot Road
ryvw50@email.sps.mot.com                | Tempe, AZ 85284
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