The impact of tuning circuit configuration on the start-up time of crystal oscillators

It is understood that a crystal oscillator does not generate a periodic signal at the instant power is applied to the circuit. Start-up time is affected by many factors as explained in various literature. In this paper, it is shown that start-up time can be a function of the arrangement of frequency determining components within the oscillator loop. The reason for the resulting change in loop gain is not necessarily obvious. Frequency tuning elements are known to influence the effective series resistance within the loop, so it is necessary that they be included in open loop gain and negative resistance analyses. Empirical results and circuit simulations provide insight into the observed behavior. Variations of the Colpitts oscillator as well as other oscillator circuit topologies are examined for this phenomenon.