[SI-LIST] Re: Decoupling of Oscillator
- From: Ed Miguel <emiguel@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 13 Nov 2002 09:00:50 -0600
Mike,
I think the main reason for the PI filter that the other circuits you
mentioned do not have is an indirect modulation coefficient that can couple
power supply noise into the output frequency thus causing phase noise
issues or jitter at frequencies that switching power supplies run at.
Ed
>Anand,
>
>It is relatively common for the oscillator to be treated as just another
>load, and omit everything in the filter except the HF capacitor.
>
>I think the main purpose of the dedicated pi filter for the oscillator
>is protection of the oscillator, because the other loads are relatively
>immune to LF/MF variation in their supply voltages. That doesn't say
>that the filter doesn't provide other functions as well.
>
>The pi network mentioned will certainly keep the oscillator current
>spikes out of the supply. Is that its main purpose? Every load driven
>by the oscillator is also drawing current at the oscillator frequency,
>or 2x the oscillator frequency. The other loads don't use a dedicated
>overt pi filter, but rely on HF decoupling. Why not the oscillator?
> Its current spikes aren't that much different than the other loads.
>
>I think the question here is "Who's being protected from what?",
>followed closely by "What's the most cost-effective way to provide the
>protection?"
>
>Regards,
>Mike
>
>Kuriakose, Anand wrote:
>
> >Mike,
> >
> >You said "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."
> >
> >I an addition to isolating the power rail noise from the VCC of the OSC, the
> >current spikes generated on the VCC rail due to the OSC switching is being
> >decoupled by the decoupling scheme mentioned below.
> >
> >That is to say that the filter isolates noise originating from either sides
> >of the filter, which is precisely what one should do to make all devices
> >connected to the common power rail operate without causing too much of
> >noise.
> >
> >Please correct me if i am wrong.
> >
> >Regards,
> >Anand.
> >
> >
> >
> >-----Original Message-----
> >From: Mike Brown [mailto:bmgman@xxxxxxxxxx]
> >Sent: Tuesday, November 12, 2002 7:42 PM
> >To: zhang_kun@xxxxxxxxxx
> >Cc: si-list
> >Subject: [SI-LIST] Re: 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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> >
> >
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