[SI-LIST] : Re: About your SILIST comments

Mark Randol ([email protected])
Fri, 19 Mar 1999 10:46:08 -0700

john lipsius wrote:
> Would you be so kind as to elaborate for me your email...

Sure John. I've already addressed some of it (emails which passed in
the night), but you do bring up some other points.

> "If the SRF is outside my range of interest, then the capacitor will
> still look sufficiently like a capacitor (low Z actually) so that I
> don't have to worry about the resonances effecting the circuit. In
> other words, I'll get the low Z supply and not get the oscillations and
> suckouts in the frequency response that I'd get from a larger valued cap
> with a lower SRF."
> Specifically, I assume you mean you want the SRF always above the
> switching harmonics present, to preserve signal integrity, so the
> impedance is decreasing with greater harmonics. If the SRF is below
> some important switching harmonic then the switched current for that
> sees a higher (inductive) impedance and shows up as V noise on the
> power plane, right?

If the SRF is just below, so that the inductive reactance is still
"low", it's ok, but not desirable. So yes, you are correct. The SRF
occurring *at* the frequency of interest is a good thing since the "cap"
is at it's lowest reactance. I wouldn't want rely on the mounted SRF
being exactly at the frequency of interest over a couple 100MHz for
different boards or component lots. Sometimes that might even be a
little high.

> But why should I be concerned about the cap
> resonance being right at the switching harmonics of interest? What
> "oscillations and suckouts in the frequency response" are you refer-
> ring to?

If I'm looking at the broad band response of an amplifier, mixer, or
oscillator I want to see a smooth amplitude response across the
frequency band. An oscillation in an amplifier isn't desirable, or
unusual if layout and bypassing aren't correct. Neither is a large,
narrow dip in the response, aka a suckout. While both oscillations and
suckouts can occur for reasons other than the supply, the wacky
impedance changes that occur on the supply due to the bypassing being
above resonance are a major cause. Having multiple value caps (hence
SRF's) on the power supply to lower the impedance across the entire
frequency range, not just the harmonics, is generally helpful if they
are placed correctly. So having a cap with an SRF LOWER than the FOI
might be required, but the higher frequencies need to be handled with a
cap with an appropriate SRF.

Sometimes just bypassing isn't enough and sufficient signal gets on the
supply and causes problems. That's when some "blocking impedance" is
needed to get more isolation between stages. Figure if you have ~30dB
reverse isolation and ~35dB forward gain you have pretty good potential
for an oscillator, whether intentional or not! So that's why you need
R's or L's in series sometimes, which is what got us all started on this
:) The local effective supply impedance still needs to be kept low

How's that for a gross simplification? As some others have noted, it
can be highly PCB layout and component design dependent on what has to
be done, as well as the intended function. Sometimes the PCB layout is
so hideous that nothing fixes it <looks at the clouds and whistles a
tune while trying to look innocent>.

Back to work (I'm done proofreading).

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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