Role of a resonator in oscillator

[muheeb16]

Is the resonator a frequency sensitive device or is it a signal source?I believe its the former but why does a transistor needs resonator to operate as an oscillator(A resonator is just selecting a frequency,its not producing oscillation by itself)?A parallel oscillator just has the highest impedance at its resonant frequency.How can it make the transistor oscillate at the same frequency (where is the signal coming from in an otherwise "DC" circuit)?

 

[dick_freebird]

Same as a crystal (which is a specific sort of resonator).
It passes a particular frequency and in a feedback loop
with an active amplifier, creates oscillation.

There is no such thing as pure DC. Always some noise to
start the process, unless you're at absolute zero.