During start-up, the inverting amplifier amplifies noise. The crystal will act as a bandpass filter and feed back only the crystal resonance frequency component, which is then amplified. Before achieving steady-state oscillation, the loop gain of the crystal/inverting amplifier loop is greater than 1 and the signal amplitude will increase. At steady-state oscillation, the loop gain will fulfill the Barkhausen criteria with a loop gain of 1, and constant amplitude.
Factors affecting the start-up time:
- High-ESR crystals will start more slowly than low-ESR crystals
- High Q-factor crystals will start more slowly than low Q-factor crystals
- High load capacitance will increase start-up time
- Oscillator amplifier drive capabilities (see more details on oscillator allowance in Section 3.2, Negative Resistance Test and Safety Factor)
In addition, crystal frequency will affect the start-up time (faster crystals will start faster), but this parameter is fixed for 32.768 kHz crystals.
Figure 1. Start-Up of a Crystal
Oscillator
