[SI-LIST] Re: Decoupling of Oscillator

  • From: Zhangkun <zhang_kun@xxxxxxxxxx>
  • To: Mike Brown <bmgman@xxxxxxxxxx>
  • Date: Wed, 13 Nov 2002 08:38:22 +0800

Hi Mike

You said you have built a system with two osillators. Is it simulation or 
measurement?

Best Regards
Zhangkun
2002.11.13
----- Original Message ----- 
From: Mike Brown <bmgman@xxxxxxxxxx>
To: <zhang_kun@xxxxxxxxxx>
Cc: si-list <si-list@xxxxxxxxxxxxx>
Sent: Tuesday, November 12, 2002 10:12 PM
Subject: Re: [SI-LIST] Decoupling of Oscillator


> Zhangkun,
> 
> You are right - that filter network is not to keep the oscillator 
> spectrum out of the power supply, but to keep power supply noise from 
> modulating the oscillator.  Jitter will be introduced into the system 
> timing if this modulation occurs.  Some jitter will unavoidably occur 
> but the filter, including the tantalum cap, will minimize the amplitude. 
>  The noise frequency will be determined by the timing of the loops in 
> the software, which change the power loading periodically.  Any load 
> variation above the regulator cutoff frequency is a possible noise source.
> 
> I prefer to isolate the power to the oscillator.  Others don't, and they 
> get away with it if their system is not jitter sensitive.  I once built 
> a system with two oscillators and no isolation and found the resulting 
> jitter due to the asynchronous noise to be intolerable.  Isolation 
> solved the problem.
> 
> Regards,
> Mike
> 
> 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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