Causes may include component aging,[1] changes in temperature that alter the piezoelectric effect in a crystal oscillator, or problems with a voltage regulator which controls the bias voltage to the oscillator.
Because of this, Frequency allocation regulations specify the allowed tolerance for such oscillators in a type-accepted device.
A temperature-compensated, voltage-controlled crystal oscillator (TCVCXO) is normally used for frequency modulation.
On the receiver side, frequency drift was mainly a problem in early tuners, particularly for analog dial tuning, and especially on FM, which exhibits a capture effect.
However, the use of a phase-locked loop (PLL) essentially eliminates the drift issue.