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 > > > > > >------------------------------------------------------------------ > >To unsubscribe from si-list: > >si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field > > > >or to administer your membership from a web page, go to: > >//www.freelists.org/webpage/si-list > > > >For help: > >si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > > > >List archives are viewable at: > > //www.freelists.org/archives/si-list > >or at our remote archives: > > http://groups.yahoo.com/group/si-list/messages > >Old (prior to June 6, 2001) list archives are viewable at: > > http://www.qsl.net/wb6tpu > > > > > > > > > > > > > ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu