Crystal Oscillators
2009
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Crystal Oscillators
Advantages and Disadvantages of Oven Controlled Crystal Oscillators-OCXOs
The main advantage of an OCXOS is its stability, which is unparallel by other crystal oscillator types. The frequency against temperature stability of an OCXOS depends on the static and dynamic F vs. T characteristics of the resonator, the design temperature range of the OCXOS, the stability of the oven and of the components in the sustaining circuitry, and the accuracy with which the oven is set to the turnover temperature of the resonator. Typical fractional stability can range from ± 20 ppb (±20E-9) to ±100 ppb. This stability can be valid for a temperature range of –40 degree C to + 85 degree C. Improved stability can be obtained over narrow temperature ranges.
The main drawback of an OCXOS is power eating, unit size, warm-up time and cost. The amount of oven power required is determined mainly by the quality of insulation used and the temperature differential between the oven and the external environment. Increasing amount of insulation to reduce heat loss requires an increase in size, resulting in a tradeoff between power and size. Warm-up time is the time required for the oven to reach operating temperature and for the frequency to stabilize. It is largely dependent on available power, the thermal mass of the oven, the quality of insulation, and ambient temperature. Typical warm-up times are from 15 seconds to 5 minutes.
The OCXOS operating temperature is required several degrees higher than the highest ambient temperature in which OCXO has to operate by which the oven may maintain good control. There are disadvantages associated with high oven temperature operation. First, the crystal's frequency vs. temperature characteristic is sharper with the higher turnover crystal. Second one is more important, crystal aging degrade with an increasing temperature. That's why it is important while designing an OCXO. It should be low as practicable, but it must be high to drive good control at the maximum ambient operating temperature.
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Why crystal oscillators are used rathe than other oscillators in microprocessors?
microprocessor 8085
They're an economical way of producing a stable accurate clock generator. Ceramic resonators are almost as good but are more susceptible to temperature and worst of all is an LC network.
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