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

Mark Randol (
Wed, 17 Mar 1999 13:47:08 -0700

Vinu Arumugham wrote:
> Is a low value of capacitance really important? At 1GHz+, the package inductance dominates the characteristics and so the impedance of a 0.1uF and a 100pF capacitor in an 0603 package will not be very different. As you point out,
> for better performance at higher frequencies one would need a lower inductance 0402.

I'm not as concerned really about the exact number of effective pF as I
am the series reonant frequency (SRF). If the inductance remains the
same, the SRF increases when you decrease the capacitance.


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.

So yes, while I agree a 0.1uF cap will be an inductor @ 1GHz, a 100pF
will still look largely capacitive.

Ray brings up some points that perhaps explain the differences between
what we are seeing. Typically my designs are 2 layer RF boards with
coplanar lines. The "ground" is beside the signal/power line. That can
yield a MUCH higher mounted SRF than if you have a 4 layer board with
the vias to GND/VCC. I still see a reduction between quoted SRF's and
board mounted SRF's, but the difference is not as severe as when vias
are used. The difference in SRF's is only a couple X at most instead of
a couple orders of magnitude. So how much does using an 0402 help when
using > 2 layers since the PCB dominates the inductive parasitic?

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

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