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I'm trying to understand how the Pierce oscillator works. Thanks to How does a Crystal work?, I've managed to understand almost all, but I'm still struggling to grasp, what causes the crystal to begin to oscillate at the very beginning? There's still no feedback AC coming into it.

Someone says that it's the "noise" causing the crystal to oscillate, but what noise? Audio noise? Electric noise coming from the circuit?

I would really appreciate if someone could help me to understand this "instant zero" of the Pierce oscillator.

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gabric
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  • It could be any type of noise. An oscillator (before it starts) can be thought of as a pencil balanced on it's point... a little be of noise and away it goes. Sometimes it's a "signal" that get's it started.. look up regenerative receiver and super regen's. – George Herold Feb 14 '15 at 15:51

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"but... what noise? Audio noise? Electric noise coming from the circuit?"

Every circuit (except for superconductors. Maybe) has noise. Every resistor produces a noise voltage, as thermal energy jostles the electrons in it. It's called Johnson noise. This is why, for instance, very-low-noise amplifiers are often characterized by the effective temperature of their inputs.

It's a direct consequence of a) the fact that charge is quantized, and b) thermal energy manifests itself as thermal motion.

There are other sources of noise as well, but that will do for a start.

WhatRoughBeast
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Yes - it is noise if there would be no other source which could disturb the balanced conditions. However, each power switch-on transient is something like a signal step within the circuit that causes a safe start of oscillations. Each simulation can show that the steady-state oscillation conditions are reached in a much shorter time - much shorter than noise could do.

Please note that the principle of oscillations is not explained correctly in the given link. In the PIERCE oscillator the crystal does NOT act as a filter, but as a high-quality inductor. As a consequence, the feedback path consists of a THIRD-order lowpass filter producing a phase shift of -180 deg at one single frequency only.

This lowpass filter consists of two branches: (1) A first-order (r,out-C2) lowpass (r,out: CMOS invert output resistance) and (2) a second order (L-C1) lowpass (L: realized by the crystal). It is a known fact that the properties of such a crystal can be exploited as a series resonant block, a parallel resonant block or as a pure inductor.

LvW
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  • Thanks a lot LvW, very interesting answer. I really want to understand better the oscillators circuits. Do you have any good reference which explains the theory? – gabric Feb 14 '15 at 19:18
  • Theory of the PIERCE oscillator only or something general about oscillators? When the active circuit has inverting gain (as in the present case) the feedback network must produce another -180deg phase shift (at one single frequency). In the other case (non-inv. gain) we need a feedback path with 0 deg (resp. 360deg) phase shift. Only in this case a crystal working as a series resonant block can be used. – LvW Feb 15 '15 at 09:40
  • Thanks, something general about oscillations would be better, maybe something that doesn't assume too much mathematical knowledge – gabric Feb 15 '15 at 09:44