[SI-LIST] Re: Decoupling of Oscillator
- From: "EVANS,JEFF (HP-Cupertino,ex3)" <jeff_evans@xxxxxx>
- To: si-list <si-list@xxxxxxxxxxxxx>
- Date: Tue, 12 Nov 2002 16:47:39 -0800
I know from experience on many systems over the last 20 years that the
pi-filter on the crystal oscillator power pin reduces emissions (EMI). Most
EMC failures are related to repetitive clock harmonics. Contamination of
the power system with clock noise couples to other circuits (power supply
feed through) such as I/O causing system EMC failures.
Additionally, there was a good paper recently (last year) co-authored by
Howard Johnson and Bruce Archembeault (IBM) advocating the output series
termination of clock circuits to reduce EMI. Basically the source series
resistor reduces the current driven down the circuit line and thus radiation
and coupling from the repetitive clock signal and it's harmonics.
Cheers!
-Jeff Evans
-----Original Message-----
From: Zhangkun [mailto:zhang_kun@xxxxxxxxxx]
Sent: Tuesday, November 12, 2002 4:38 PM
To: Mike Brown
Cc: si-list
Subject: [SI-LIST] Re: Decoupling of Oscillator
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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>
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