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

Mark Randol (ryvw50@email.sps.mot.com)
Wed, 17 Mar 1999 10:01:20 -0700

Zhang, Michael T wrote:
>
> So it is desirable for the series
> inductive element to have high impedance at the problem frequency, but low
> impedance at the signal (fundamental) frequency. This is why ferrite bead, a
> frequency selective device, is widely used in this technique. Practically,
> signal edge rate will be slowed down a bit. Then it is a trade-off design
> between meeting SI and EMI requirements.

Agreed. As 1GHz is approached, I've had problems finding beads that are
rated at that high a frequency. The chip beads that I have found work
pretty well. Mostly they've been used to kill supply path oscillations
on boards where there is no other option. They aren't cheap.

> As far as power delivery is concerned, ferrite beads have low impedance at
> DC and low frequency. Charging up decoupling caps at a slow speed is not
> degraded. High-frequency surge current will be blocked by the ferrite beads,
> but will be provided by the high-frequency decoupling caps placed between
> the beads and the VCC pins of the chip. This is exactly what the design goal
> is - don't want to see high-frequency current on VCC/GND plane.

Another way to look at it is that you are providing a definate, low Z
path for the AC return.

> When EMI problems are at close to 1GHz, caps are effectively inductors.
> Putting more caps down will lower the effective decoupling impedance, but
> limited by cost and space (if you can't get caps close to pins, it is a
> waste of money).

SMT 0603 ceramic caps are still pretty good at 1GHz+ as long as you keep
the value under a couple 100pF. 0402's go up through a couple GHz.

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