Confusion about Quartz tuning fork oscillator  - Page 1
Forum section: electronics / general

author: primegaps
author_rank: Contributor
timestamp: May 07, 2017, 06:13:50 am
content: Hello there! I have a bit of confusion regarding one of these QFT oscillators.I understand that it is possible to model certain mechanical systems with oscillatory circuits, by choosing C, L, and R to be specific values, with certain relations to the original properties of the mechanical system. So you would choose a capacitance that would model your mass, and an inductance to model the restoring forces, and some resistance to model any damping or viscous losses.So you can generate a circuit that will behave fundamentally in the same way the mechanical system you modeled it after would.And of course it goes the other way too, but within limits, I suppose i can't expect a mechanical system to operate at 5 GHz.Okay, so the following is where my confusion stems from.The fork is, of course, a mechanical system, with a resonant frequency dependent on its mechanical properties, such as its mass distribution, restitution, and so on. So of course this system can be modeled with a circuit with L and C such that the resonant frequency is the same, and R such that we can also match the damping or any losses the fork has. And often in papers, the fork is represented as a circuit.But the fork is, apparently, ALSO the circuit. You can just plug it in directly to the breadboard, and it is indeed a circuit with the L, C and R that would match its electrical properties to its mechanical properties. Yes there also is a parasitic capacitance from the shielding and whatever else goes into making the fork.So how is it BOTH the mechanical and the electrical system at the same time?It clearly is an actual mechanical oscillator, but it is also actually the circuit that represents that mechanical system.The tuning fork is quartz, and i have a basic understanding of it's electro-mechanical properties. Of course the electrical and mechanical domains are coupled through the properties of this material, but i don't quite understand how.Why would the capacitance, inductance, and resistance of the fork as a circuit, just so happen to match up to the mechanical properties of the fork in just the right way? Has it just been designed to have a certain geometry such that the parameters approximately line up?Or is it more fundamental, and the fork will have electrical properties that are intrinsically coupled to the mechanical properties, through the quartz?Really, I think of the electrical and mechanical domains (oscillators) of being somewhat distinct. Yes, they can model each other, but are fundamentally different systems based in different physics.So my question is, how can the QFT be both at the same time?I am struggling to put my thoughts into words, i hope i was clear! Let me know if i need to clarify anything, thanks for reading!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 07, 2017, 06:40:07 am
content: Conservation of energy: if the crystal is drawing more power at some frequency,howis it doing that? Simply: the impedance drops. And, since impedance must satisfy certain criteria (continuity, analyticity, conservation of energy, causality), it's not that it simply--plop--there it is, but the impedance changes smoothly across all frequencies. And when the impedance of a two-terminal device changes, it necessarily becomes reactive (rising: inductive, falling: capacitive, leveling off: resistive), so to go from some condition to another, you naturally get an RLC circuit.Tim