[<< wikibooks] Practical Electronics/Oscillators
Crystal Oscillator provides an Oscillator

f = nfoUsing LS tuned tank

f

o

=

1

2
π

1

L
C

{\displaystyle f_{o}={\frac {1}{2\pi }}{\sqrt {\frac {1}{LC}}}}
The "negative resistance test" measures the oscillation allowance of a crystal oscillator circuit.
Most manufacturers recommend that the oscillation allowance of the circuit be at least five times the maximum resistance value of the quartz crystal unit used in the circuit.