# Generation of clock signals

Inside a computer, a crystal oscillator sends signals to a microcontroller or microprocessors. I want to know if the crystal oscillator is increasing the frequency or generating pulses. My questions are:

1) Clock rates of computers are high which means crystal oscillator's frequency is high too. Is the crystal oscillator fed by high frequency voltage or does it generate the high frequency voltage?

2) We all use a 230 V or 110 V AC power supply with a 50 Hz frequency. What are the main steps to obtain clock signals just after power supply?

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1. I don't know what you mean when you say "Clock rates of computers are too high". Too high for what? They're high, yes, often as high as technology will allow (GHz), often as low as the application allows, for instance a 32kHz watch crystal, if only little processing power is required, or power has to be limited (power consumption is a direct function of clock frequency). Clock frequency is determined by the resonance frequency of a quartz crystal, and you can cut a crystal to about any frequency you want (see edit below). Sometimes processors run of a different type of oscillator, without a crystal, like RC, if the precision a crystal offers isn't required.

2. You don't use the 50Hz frequency to obtain a clock from, though it's not impossible to do so. It's much easier to use a crystal, like I explained above. In many cases, like for PCs, the 50Hz is only used for the power transport (transformers, overhead power lines). Once inside the PC it's converted to several DC voltages, and the 50Hz is no longer relevant. A PC can also run from a battery, and then there is no AC at all.

edit
Like Cybergibbons says there's an upper limit for crystals' frequencies. He mentions 300MHz, Digikey's highest listed crystal frequency is 155MHz. Same Digikey lists crystal ocsillators up to 1.35GHz, but those are overtone oscillators. Other high frequency oscillators may use SAW (Surface Acoustic Wave) technology.
But even a 100MHz crystal can clock a 3.2GHz PC. This uses a PLL, or Phase Locked Loop. It has an internal variable frequency oscillator, and a frequency divider which divded by 32. The frequency this gets us is compared with the 100MHz from the crystal, and the difference (technically the phase difference) is used to adjust the internal oscillator to the required 3.2GHz. Since it has to cintinuously adjust, the 3.2GHz shows slight deviations, known as jitter. So a PLL clock is never as stable as its input signal, but for clocking a PC the deviations are negligible.

another edit

"Is the crystal oscillator fed by high frequency voltage"

When you switch a crystal oscillator on it's just an amplifier, you don't get the desired frequency yet. The only thing that's there is a low-level noise over a wide bandwidth. The oscillator will amplify that noise and pass it through the crystal, upon which it enters the oscillator again which amplifies it again and so on. Shouldn't that get you just very much noise? No, the crystal's properties are such that it will pass only a very small amount of the noise, around its resonance frequency. All the rest will be attenuated. So in the end it's only that resonance frequency which is left, and then we're oscillating.

You can compare it with a trampoline. Imagine a bunch of kids jumping on it randomly. The trampoline doesn't move much and the kids have to make a lot of effort to jump just 20cm up. But after some time they will start to synchronize and the trampoline will follow the jumping. The kids will jump higher and higher with less effort. The trampoline will oscillate at its resonance frequency (about 1Hz) and it will be hard to jump faster or slower. That's the frequencies that will be filtered out.

For most crystal this startup goes fast enough not to be a problem. But 32.768kHz watch crystals can only be driven with very low power, typically 1$\mu$W vs. 500$\mu$W for a 12MHz crystal. As a result a 32kHz crystal oscillator takes much more time to stabilize, often a few seconds.

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Might be worth adding that the practical limit of crystal oscillators is currently around 300MHz, and anything higher than this is generated by a form of clock multiplication, like a PLL. – Cybergibbons Jun 1 '12 at 18:06
@cybergibbons - Added, thanks. Given the upvotes for your comment it seems highly relevant for others. – stevenvh Jun 2 '12 at 10:58
@stevenvh, it is something that people often do not know, I dont want some poor users looking for a 10GHz crystal. – Kortuk Jun 4 '12 at 12:00
@Kortuk - If they need 10GHz, I would expect they have some years of experience in electronics. If he doesn't know by then, you're giving the assignment to the wrong engineer IMO. – stevenvh Jun 4 '12 at 12:09
@stevenvh, Ha. I was just explaining and using an extreme case, It is a useful. Using a PLL to step a slow oscillator is a pretty easy step. – Kortuk Jun 4 '12 at 12:12
1. Oscillators, as the name suggest, oscillate, thus generating the clock from scratch - which means, you don't need to give them an oscillation, just a supply.
2. They are feeded by a DC voltage, the same that supplies the microcontroller; the AC mains voltage has to be rectified and lowered by the PSU (Power Supply Unit) that you have in the PC or embedded system.
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thanks, but what component is increasing that high frequencies from 5O Hz.? Is that a high pas filter or something? If it is not oscillator so what is oscillator used for? i mean crystal oscillator or rlc components creating the high frequency? sorry ima bit confused – cmd1024 Apr 14 '12 at 16:48
The oscillator generates a high frequency from a DC (0 Hz) power supply. There is no 50 Hz involved. – The Photon Apr 14 '12 at 16:51
A power supply generates a DC voltage from the 50 Hz mains. There is no "high frequency" involved. – The Photon Apr 14 '12 at 16:51
@cmd1024 - nothing "increases .. from 50 Hz", b/c oscillators start from DC, which is actually 0 Hz. An oscillator makes up a time varying signal right from DC. – JustJeff Apr 14 '12 at 16:52
@cmd1024 In a properly working AC power system, there are no harmonics. – David Kessner Apr 14 '12 at 17:07

Most electronic circuits do not use 50Hz or 60Hz clock to generate clock frequencies. Instead they use crystals to provide a very stable clock. A PLL can then be used to multiply the frequency up to the 2.4GHz etc that the CPUs need. Every microprocessor has a PLL to get the desired frequency

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This is nice causality question.

For the theorist with 99% knowledge of complete theory of electricity and missing few pieces the method of obtaining a reliable high frequencies source will be a mistery. If such theorist will create a Spice software, the models will be perfectly working except models of oscillators.

1) It is self caused

The crystal oscillators are impossible to run without help of thermal noise. Crystals are just linear filters with very narrow band. Amplified signal in MHz range is not the AC, but thermal noise of all components in oscillator. Which is very broadband noise. The exception is quantum based generators: atomic clocks. (but not quantum amplifiers: lasers). Fortunately AC signal from power generator helps to make start of oscillaton somewhat deterministic. (For simplicity consider that power supply is not a switching kind)

2) Use chaos theory coupling

Second question is how is it possible to exploit the existing source of oscillation when creating your own causality root. Or how is it affecting it, and perhaps how to isolate effects or control degree of effect.

This class of problems boils to coupling of oscillators. If they are build in same universe, then they are coupled. There is no symmetry in coupling. AC of 50-60 Hz is affecting local oscillator more than oscillator affects generator of the power station. The effects backwards involve enormously small numbers, but they exist.

For all practical means power generator is not affected by oscillator. For simplicity: It is only one way. The effect causes phases to be coupled with some strength. The strength is rarely considered advantageous, because designers want their devices to be deterministic. However, no matter how hard one will try to exclude the coupling, it will always exist.

Consequently near 100% of crystal oscillators powered on particular continent are participating in one very broad and loosely coupled causality tree rooted in NIST.

When branches of the tree are tangled, the effect is stronger, especially on close frequencies. If observer will measure phase differences and resulting frequency walk between neighbouring oscillators the 2D picture over long time will show more than one cluster of preferred values with abrupt transition between clusters. This trajectory is nicknamed strange attractor.

Problems of indeterministic behaviour of deterministic systems is studied by chaos theory

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 Re: -1. I understand that my example of atomic clocks requre some elaboration. It will be my Saturday study, if they are truly immune to the entropy of physical implementation or if they rely on it – Rocket Surgeon Jun 2 '12 at 12:02 I'm -2 :) It's not clear how this relates to the question, but if it did: why would you need chaos theory to trigger an oscillator? You can also build startup circuits, if you don't want to rely on noise (which is always present, being thermal, mains, RF or whatever) – clabacchio♦ Jun 4 '12 at 11:11 Is it really true that "near 100% of crystal oscillators powered on particular continent are participating in one very broad and loosely coupled causality tree rooted in NIST."? How is that measured, and where can I find out more? – Justin Jun 4 '12 at 17:14