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I know that in quartz watches a quartz crystal is used to keep time by counting it's vibrations, caused by the voltage applied to it.

I don't understand if this voltage is DC or AC:

If DC is used, why does the quartz oscillate instead of changing position permanently?

If AC is used, doesn't he quartz oscillator have to oscillate at the same frequency as the AC? If so, then there's no need for the mechanical transducer.

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  • \$\begingroup\$ it is both AC & DC, negative feedback DC to self-bias inverting amplifier for max gain somewhere near~ Vdd/2 but also Negative FeedBack,NFB for AC since these Xtals shift 180'deg to satisfy AC PFB requirement to continue oscillating with AC drive at self resonant (parallel tank) frequency. \$\endgroup\$ – Sunnyskyguy EE75 Apr 21 at 20:50
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It's AC, BUT:

It's not really that an AC voltage is applied to the quartz:

The quartz has the nice property of that if you invert the voltage across it, it takes some time for the quartz to "follow":

quartz inverter circuit

For a start, assume the left hand side of the quartz X has just become negative, the right hand side positive. That will lead to the quartz crystal expanding (or contracting) in a specific way, which will overshoot, and a little later invert that voltage. Since that will revert the voltage at the input of the first inverter, that will invert its output – and the cycle starts again.

The specific delay at which this happens due to the mechanical properties of the quartz leads to a resonance frequency that the quartz defines.

So, no, no AC voltage is applied, but with a bit of negative feedback, you can make a quartz oscillate at a stable freqeuncy.

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  • \$\begingroup\$ reconsider "No AC is applied" because there is. Since it expects a 180 deg phase shift on Xtal output, it initially applies an opposite DC voltage until the IC input= Xtal output, reaches Vdd/2 then if flips again and repeats at the AC output frequency of the Xtal. It cannot sustain these oscillations without an output AC signal to drive the Xtal. \$\endgroup\$ – Sunnyskyguy EE75 Apr 21 at 20:10
  • \$\begingroup\$ yeah, but that AC input really only needs to be the initial phase inversion – after that, the negative feedback self-sustains the oscillation. \$\endgroup\$ – Marcus Müller Apr 21 at 21:01
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    \$\begingroup\$ I describe it as DC negative feedback to bias AC gain which becomes AC driven positive feedback for oscillation after 180 phase shift in the parallel resonant crystal. \$\endgroup\$ – Sunnyskyguy EE75 Apr 21 at 22:20
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A quartz crystal is used in an oscillator circuit. The oscillator will be powered by DC, but generates an AC signal. The frequency of the generated signal will be determined by the characteristics of the crystal.

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Here is progressively more details on Quartz oscillator behavior

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ best answer so far \$\endgroup\$ – Sunnyskyguy EE75 Apr 21 at 22:23
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There are simpler answers, but incomplete, so challenge yourself to understand and ask better questions. This is just a supplement to @analogsystemsrf fine answer.

There must be a DC bias for the inverter and it must be self-biased with this negative feedback to allow it to operate with max. the gain in the linear mode.

  • Gain can be 10 to 100 typ. per stage in the inverter and frequency limited by output rise time BW=0.35/Tr.

    • Every CMOS inverter has a gain-bandwidth factor ( often selected as unbuffered) but has excess gain to convert the sine-wave input biased at Vdd/2 into a full-swing square-wave output at, ( or near) a 50% duty cycle.

Since this question suggests a design using the inverting parallel resonant crystals, (Xtal) the CMOS inverter creates the gain-phase condition required to sustain oscillation by AC positive feedback.

doesn't the quartz oscillator have to oscillate at the same frequency as the AC? enter image description here

It does not mean you get 5kV out of the crystal with 1V input because the gain is limited by the damping of the ESD input clamp diodes. Due to XTAL power limits of 10uW, a driver Rs of 1k is often mandatory for best MTBF.

Crystals > 20 MHz tend to operate as harmonic oscillators 3rd 5th or 7th harmonics called "overtones".

Anecdotal: I used this fact to design an injection locked Xtal Oscillator that required only 10 clk cycles then no data transitions for 1024 NRZ data bits at 4Mbps to self-clock synchronous-data with no need for bi-phase or a separate clock signal to be transmitted. This was broadcast on TV in the early '80s using blanking interval lines to download games etc to home computers such as ][+, Commodore and TRS-80. Sadly ahead of it's time for slow decision makers in Cable TV to buy our network design.

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