AN2867 datasheet(10/41 Pages) STMICROELECTRONICS

10 / 41 page

Oscillator theory

AN2867

10/41

DocID15287 Rev 9

Figure 4. Block diagram of a typical oscillation loop based on a crystal resonator

Following the small signals theory and when the active branch (oscillator part) is correctly

biased, the latter should have its transconductance equal to the passive branch

conductance in order to maintain a stable oscillation around the oscillator biasing voltage.

However, at startup, the oscillator transconductance should be higher than (multiple of) the

conductance of the passive part of the oscillation loop to maximize the possibility to build up

the oscillation from inherent noise of the oscillation loop. Please note that an excessive

oscillator transconductance compared to the oscillation loop passive branch conductance

may also saturate the oscillation loop and cause a startup failure.

In order to ensure the oscillator ability to startup successfully and maintain stable oscillation,

a ratio between the negative resistance of the oscillation loop and the crystal maximal

equivalent series resistance (ESR) is specified: for STM32 and STM8 microcontrollers, it is

recommended to have a ratio higher than x5 for the HSE oscillators and higher than x3 for

the LSE oscillators.

AN2867 Datasheet(PDF) 10 Page - STMicroelectronics