[SI-LIST] Re: Decoupling of Oscillator

  • From: Mike Brown <bmgman@xxxxxxxxxx>
  • To: Zhangkun <zhang_kun@xxxxxxxxxx>
  • Date: Tue, 12 Nov 2002 21:19:16 -0600


The system was for production.  The two oscillators provided were for 
normal operation and for frequency margining (Normal frequency +10%, if 
I remember correctly).  Both oscillators ran continuously, and a 
glitchless switch was performed between the two clock sources. Switching 
could be accomplished on the fly.



Zhangkun wrote:

>Hi Mike
>You said you have built a system with two osillators. Is it simulation or 
>Best Regards
>----- 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
>>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.
>>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
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