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

Wed, 17 Mar 1999 14:03:12 -0800

That's why it's essential to use on-die by-pass caps for fast
drivers.
Apparently, even a smallest value on-board by-pass - say 100 pF - with some
2 nH mounted inductance can only help at 100+ MHz (i.e. bus clock
frequencies). They don't help a 350 ps edge (e.g.), whose bandwidth is ~1
GHz.

-----Original Message-----
From: Mark Randol [mailto:[email protected]]
Sent: Wednesday, March 17, 1999 12:47 PM
To: [email protected]
Subject: Re: [SI-LIST] : A Question About Power Noise.

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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