Electronics notes/frequency generation
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Simpler things that go tick can be anything that intentionally resonates - oscillators, crystals, ceramic resonators, flip-flop constructions, a 555, or whatnot.
- clock source (for digital calculation and communication)
- signal generation
- reference clock
- often based on crystals, because those can be made with smaller error.
In electronics you see a bunch of ceramic resonators (cheap, can be used when the rate is more imporant that it is for long-term precision, simpler quartz crystal oscillators (more accuracy than resonators), and corrected quartz crystal oscillators (more accuracy than basic quartz, necessary for some applications).
Frequency tolerance refers to how close the real frequency will be to the spec.
This is about production quality, what a specific crystal ends up doing exactly, and some specific effects like that of temperature on quartz.
Any given crystal is likely to be relatively consistent. A crystal that is ±50ppm will probably not be +30 one day and -40 the next. This means that you can often calibrate away the 'average' error for a specific crystal once you measure it, and have noticeably smaller error left.
Frequency tolerance of ±250ppm or more is considered relatively relaxed, ±20 ppm is pretty good, lower than that is fairly strict.
You can spend ten bucks on a TCXO that is 2ppm (approx. 1 second per ~5 days, approx. 1 minute per year). For regularity this is good stuff.
...but for accurate timekeeping it's still only so-so. It's relevant that there are various ways of receiving time wirelessly, and if you can do that frequently, then larger error in your crystal only matters on the scale of, say, minutes, so basically not at all.
This includes GPS (which requires accurate time to even work), and most of the western world has radio-wave synchronization (which is the cheaper option unless your device had GPS anyway).
Digital watches typically have quartz crystals with few-dozen ppm accuracy. And yes, that means they're often off by a few minutes per year. Fancy digital watches are typically radio synchronized - it's easier and cheaper for long term accuracy.
Frequency generation, loosely:
- harmonic/linear, oscillator (waveform like a sine wave) 
- relaxation oscillator (waveform like a sawtooth) 
- Low-Frequency Oscillator (LFO) - designs that generates a waveform below ~20 Hz. Used in synthesizers and such.
- Surface acoustic wave (SAW) oscillators - quartz crystals that achieve higher frequencies though a standing-wave construction. More expensive, but necessary for devices that
- frequency tolerance: on the order of 0.0001
- ceramic resonators are less accurate than crystals, but good enough whenever timing need not be that accurate
- frequency tolerance: on the order of 0.5% (5000ppm) (7.2min/day)
- crystal oscillators - piezoelectric quartz.
- frequency tolerance: on the order order of ~0.001% (~10ppm) (~0.86sec per day),
- Tunable within a small range of frequencies
- Fairly cheap
- not accurate enough for certain some applications (such as some radio transmission, long-term timekeeping, stability under varying temperatures)
- TCXO (temperature-compensated crystal oscillator)
- less than 5 PPM, usually 2 or 3(verify), down to 0.1 PPM is theoretically possible. A little more complex than basic crystals, and still fairly affordable
- MCXO (microcontroller-compensated crystal oscillator)
- refers to designs that are more stable and avoid more noise and drift (useful when supporting less predictable things like uCs) (verify)
- Down to 0.1PPM