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

Mark Randol (ryvw50@email.sps.mot.com)
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.

SRF~1/[6.28*(Leff*C)^0.5]

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?

-- 
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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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