Appropriately cut quartz crystals can be used as high-quality electromechanical resonators. Their piezoelectric properties (voltage across the crystal deforms it; deforming the crystal generates a voltage) allow them to be the frequency-determining element in electronic circuits. Crystals are widely used in oscillators, time bases, and frequency synthesizers for their high quality factor (QF); excellent frequency stability; tight production tolerances; and relatively low cost. All crystal oscillator circuits have a start-up time. Start-up time is an important consideration in battery-powered applications where the device is often switched on and off. Shorter oscillator start-up times reduce wasted energy in low-power systems.
The definition for the start-up time of a crystal oscillator can vary, depending on the type of system. For a microprocessor system, the start-up time is often the time from initial power application to the time a stable clock signal is available. The start-up time for a phase-locked loop (PLL) is often the time from initial power application to when a stable reference signal is available, often settled to within an acceptable frequency offset from the final steady state oscillation frequency. The start-up time of a crystal oscillator may be determined by the noise or transient conditions at turn-on; small-signal envelope expansion due to negative resistance; and large-signal amplitude limiting.
Crystal oscillators are known to have a long start-up time. Once there is a small build-up of signal which itself takes a long time, they may take additional 2,000 to 8,000 cycles to settle based on their motion arm inductance. For a 32 KHz oscillator start-up time can be over 3 to 4 seconds, and for 24-32 MHz, start-up time may extend up to 5-20 milliseconds. These long start-up times of the crystal oscillators have been reported as a problem by many users. Kick starting of the crystal oscillator with a mono-stable one shot oscillator has been employed at equipment turn-on but has not always been successful since the energy frequency spectrum of a single pulse from a mono-stable one shot oscillator tends to fall far outside the crystal oscillation operating frequency and thus cannot add enough energy to the crystal-inductor element for quick and reliable start-up of the crystal oscillator circuit.
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