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Often, for integrated circuits, a quartz crystal is used to generate the clock signal. However, this only reaches speeds in MHz. What component, or what circuit, generates signals up to 5 GHz like in computer processors?

How is it possible to increase that speed when you overclock a PC (since I don't assume that a crystal speeds up when you put a higher voltage across it, or make it cooler)?

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    \$\begingroup\$ Why would you assume that quartz crystals only reach frequencies up to a few kilohertz ? I have a 27 MHz crystals lying in my drawer. \$\endgroup\$ – Bimpelrekkie Aug 26 '15 at 13:04
  • \$\begingroup\$ You're right @FakeMoustache, but I meant crystals of 1 gigahertz and above. \$\endgroup\$ – Markinson Aug 26 '15 at 13:10
  • \$\begingroup\$ OK, I have seen crystal oscillators up to 150 MHz, in practice up to 50 MHz is used. Frequencies above that are made using a PLL as Wouter mentions. I work on a product where we use a PLL to convert 25 MHz in 60 GHz ! \$\endgroup\$ – Bimpelrekkie Aug 26 '15 at 13:12
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Actually crystal oscillators can easily go up to 10's of MHz. Above that in most cases a PLL (Phase Locked Loop) is used, which is an oscillator that is not very accurate in itself, but can be tuned (its frequency can be adjusted somewhat). The frequency of this high-frequency oscillator is divided by a suitable factor (dividing a signal by a power of 2 is easy and totally accurate), and then compared to a let's say a 10 MHz oscillator. The comparison is used to adjust the high-frequency oscillator. Thus a high frequency is made with (almost) the accuracy of the lower frequency crystal oscillator.

In most cases, the circuitry to do all this is built into the processor chip. This is so it can be configured under software control, and routing such a high-frequency signal between chips is a nightmare.

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    \$\begingroup\$ It was true 5 years ago (and probably still today) that most motherboards have a good old-fashioned 14.318MHz crystal, and a clock generator (PLL) chip that generates other bus frequencies like 33MHz (PCI), 48MHz (USB), and an intermediate "FSB" frequency like 100 or 200 MHz from there. The CPU then takes the FSB frequency and multiplies it up to the GHz range with another PLL on-chip, which avoids the problem of actually transporting the GHz clock any distance or getting it through the CPU pins :) \$\endgroup\$ – hobbs Aug 26 '15 at 17:33
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You don't need a crystal to oscillate, any reactive component, like a capacitor or inductor, with an amplifier can do the job. In fact, a crystal is equivalent to an R, L and C in series, all in parallel with a C. The advantage of a crystal is that the resonant frequency is very precise. To generate higher frequencies, people use other resonant components (e.g. inductors and capacitors inside chips) in their oscillator circuit.

With some oscillator circuits the frequency can be varied with an applied voltage (VCOs). These are used to generate high frequencies accurately, by dividing the output frequency and comparing it to an accurate low frequency source like a crystal then adjusting the control voltage appropriately. A PLL (phase locked loop) is one example, which generates a voltage proportional to the difference in phase between the divided high frequency clock and the reference clock.

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