Frequency drift

In electrical engineering, and particularly in telecommunications, frequency drift is an unintended and generally arbitrary offset of an oscillator from its nominal frequency. This can be caused by changes in temperature, which can alter the piezoelectric effect in a quartz crystal, or by problems with a voltage regulator which controls the bias voltage to the oscillator.

On a radio transmitter, frequency drift can cause a radio station to drift into an adjacent channel, causing illegal interference. Because of this, there are usually regulations specifying what kind of tolerance such oscillators must have, in order to be in a device which will be type-accepted. A temperature-compensated, voltage-controlled crystal oscillator (TCVCXO) is normally used for FM.

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 invention of the phase-locked loop (PLL) essentially eliminates the drift issue. For transmitters, a numerically-controlled oscillator (NCO) also does not have problems with drift.

It should be noted that this differs from Doppler shift, which is a "perceived" difference in frequency, even though the source is still producing the same wavelength, because the source is moving. It also differs from frequency deviation, which is the inherent and necessary result of modulation in both FM and phase modulation.