Comment by kurthr

Building an oscillator is "just" putting some gain around a large enough phase delay (>90deg). The challenge is in making the oscillator predictable and STABLE.

You want a frequency generator that oscillates with a constant period/frequency. Even an unbalanced oscillator can just be divided by two to provide uniformity. However, it turns out that building something that is not sensitive to any outside inputs (temperature, strain, voltage, time, etc) is really hard to do over a very wide frequency range (from ~DC to many MHz), but from that you can build a stable clock.

Look up Allan Variance, if you're looking for bit of a rabbit hole on clocks and oscillators and other sensors.