[SI-LIST] Re: Decoupling of Oscillator

Zhangkun

I would leave the 10 uF cap in since it is filtering the noise coming from 
the power supply after the Ferrite Bead.   This filtering will minimize any 
indirect modulation of the oscillator through its VCC line.   The other 
caps are also necessary for bypassing the oscillator output current spikes 
and this is where high SRF caps are needed.  All of these caps and ferrite 
should be close to the oscillator VCC to be most effective.  A solid ground 
plane underneath is also needed.

Ed

At 02:43 PM 11/12/02 +0800, Zhangkun wrote:
>Hi all
>
>I meet one question about decoupling of oscillator. In our design, the 
>power supply of OSC is always isolated by one PI filter. In the side near 
>OSC, there are always one tantalum capacitor of 10uF and several ceramic 
>capacitors of 0.1uF or 0.01uF. As we know the resonance frequency of 
>tantalum capacitor is about 3MHz. If the OSC is 50MHz, the spectrum will 
>be speaded at 0, 50MHz, 100MHz, 150MHz, etc. There will be no power in the 
>frequency range between 0 and 25MHz. Therefore, I think I could remove the 
>tantalum capacitor. Is there something wrong?
>
>I think it will have nothing to do with the affection from OSC to outside 
>circuits. I am worrying about the affection from outside circuits to OSC. 
>If there is some noise of 2MHz and my OSC is of 50MHz, there will be 
>modulation between noise of 2MHz and clock signal of 50MHz. The output of 
>OSC and the clock signal will be affected by the noise of 2MHz. The bead 
>will not isolate the noise of 2MHz.
>
>I want to know I could remove the 10uF tantalum capacitor or not. Why?
>
>By the way, is there some people who do not use bead to isolate the power 
>supply of oscillator?
>
>Best Regards
>Zhangkun
>2002.11.12
>
>
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