[SI-LIST] Re: 60 MHz sinusoidal clock Decoupling
- From: Ray Anderson <Raymond.Anderson@xxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 13 Apr 2004 09:20:54 -0700
Chris Chalmers wrote:
> SI,
>
> This is theoretical. Is there any point in
> putting standard 0603 0.1uF capacitors (SRF ~ 12MHz) on to
> decouple a clock oscillator that is producing a pure
> 60MHz Sine wave. To my mind there would be no point
> because 0.1uF caps won't have the low impedance we
> want at that frequency. Is this correct?
>
> Thanks in advance
>
> Chris
>
Chris-
Depending on the actual circuit used to implement your 60 MHz
oscillator, if it is not sufficiently decoupled over a broad range of
frequencies (at low frequencies and at higher frequencies) it _may_ be
prone to spurious oscillation at undeisered frequencies. In adequate
decoupling of oscillators has also been know to induce oscillator
startup problems in some circuits.
Also note that having good decoupling at frequencies lower than the
operating frequency of the oscillator will help to keep lower frequency
noise present on the power planes out of the oscillator and can assist
in maintaining a clean spectral output from the oscillator.
Also note that your presumed SRF of 12 MHz for the .1uF decap presumes a
total mounted loop inductance of about 1.7nH. This is indeed possible,
however much higher mounted inductances are also possible depending on
the mounting geometries and via escapes in which case the SRF could be
somewhat lower than 12 MHz.
-Ray Anderson
Sun Microsystems Inc.
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