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

Mark Randol ([email protected])
Thu, 18 Mar 1999 09:21:09 -0700

Stephen Peters wrote:
> A question. I assume that in RF design the purpose of power plane
> 'decoupling' is to keep RF energy out of a devices power leads -- i.e.,
> preserve a quiet DC reference. This is done by providing a low impedance
> path between power and ground at the frequency of interest. Therefore,
> it seems to me that you would want a decoupling capacitor to be
> operating right near its SRF, but if I read your comments correctly, you
> imply you do not want it to operate it at that point. What am I missing?

Since mounting it on the board actually yields a lower SRF than is
quoted in the data sheets, having it somewhat higher turns out just
about right. For impedance matching, you don't want to be anywhere near
the SRF. Since those values are typically much less than
decoupling/bypassing values, it generally isn't a problem if you use the
same sizes (0603 ~1GHz, 0402 ~2GHz+)

> (Note: In the digital world, the rule is to use as big a decoupling
> capacitor as possible (for a given package inductance). The reason being, of
> course, is that in digital design the purpose of a 'decoupling capacitor' is
> to supply transient current to a devices when it switches. In other words,
> the decoupling capacitor is acting as a charge storage device, and
> the bigger the capacitor the better. It's interesting that while we both
> refer to those capacitors as "decoupling capacitors", they are used for
> very different reasons.)

I remember there being a distinction drawn between "bypassing" and
"decoupling" functions, but they seem almost to be the same thing.
Really we are accomplishing the same thing. While a decoupling cap
supplies the surge current it is lowering the supply impedance (V/I).
When/if it(they) runs out the available current drops, increasing the

Seems many times digital and analog people are talking about the same
thing, but the terminologies get in the way. Examples:

D/A Convertor :)
rise time~=bandwidth, f.max
impedance matching~=attenuator
filter~=impedance matching

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

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