Explanation of Crystal Units
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Since a crystal unit performs mechanical vibration, too much vibration may lead to unstable oscillation frequency, and finally to destruction in the worst case. When designing an oscillation circuit, the level of drive should be examined so as to use an oscillator below the level specified by our company. Fig.3 shows an example method of confirming a level of drive. The method employs a current probe to measure the crystal oscillator current. In this case the level of drive is as follows:
4 Load Capacitance
The load capacitance CL is a factor for determining the "conditions" of a crystal unit when used in the oscillation circuit. In an ordinary oscillation circuit, the crystal unit is used in a range where it functions as an inductive reactance. In such usage, the oscillation circuit operates as a capacitive reactance. In other words, when the oscillation circuit is seen from both terminals of the crystal unit, the oscillation circuit can be expressed as a series circuit of a negative resistance -R and a capacitance CL. At that time this capacitance is called the load capacitance. The relationship between load capacitance and oscillation frequency is not linear. When the load capacitance is small, the amount of frequency variation is large, and when the load capacitance is increased, frequency variation lowers. If the load capacitance is lessened in the oscillation circuit to secure a large allowance for the oscillation frequency, the frequency stability will be greatly influenced even by a small change in the circuit. The load capacitance can be any values, but 10~30pF is better.
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