Explanation of Crystal Units
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5 Equivalent Circuit of Crystal Oscillation Circuit
When a crystal unit is actuated as an inductive reactance in an oscillation circuit, the relationship between crystal unit and oscillation circuit is shown in Fig.4. To improve the starting conditions of the oscillation circuit, it is preferable to increase the value of negative resistance -R which parameter of the oscillation circuit. The starting conditions will become worse if a circuit without much allowance in negative resistance (less negative resistance) is combined with a crystal unit having a larger resonance resistance. The oscillation circuit should be designed to a goal such that the value of negative resistance is 5 to 10 times the resonance resistance. It is also necessary that the center value of load capacitance (to determine the absolute value of oscillation frequency) and the variable range (fine adjustment range of oscillation frequency) are maintained at the optimum values in the oscillation circuit.
6 Oscillation circuit
A typical oscillation circuit composed of a crystal unit is introduced below. Element constants used are for example.
(1) Crystal units are designed to have lower limitable level of drive, of 100µW and below. Prior to use, the crystal current should be examined in an actually mounting circuit. (See Fig.3.)
(2) The negative resistance of a circuit must be checked. Confirmation of negative resistance is possible according to Fig.6. A goal of negative resistance is designed to 5 or more times of the resonance resistance.
(3) The Rd in the circuit diagram is indispensable when used in a C-MOS oscillation circuit. (See Fig.5.) If this Rd is attached, the level of drive is kept within the specified value and stable oscillation frequency can be obtained.
(4) Cg and Cd should be used within the range of 10 ? 30 pF. If Cg and Cd are used below 10 pF or above 30 pF, oscillation may be easily affected by circuit performance, level of drive may increase, or negative resistance may decrease, thus failing in maintaining stable oscillation.
(5) The layout for crystal oscillation circuits should be arranged as short as possible. The stray capacitance between circuits and ground patterns should be reduced. Crossing of crystal oscillation circuits patterns over other circuit patterns should be avoided.
(6) If the circuits used, IC types, and IC manufacturers are different, frequency, level of drive, and negative resistance should be confirmed.
* Overtone oscillation circuits need additional consultation
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