Influence oscillating circuit three important indicators

There are three important indexes that affect the oscillation circuit: frequency error, negative impedance and excitation level

(1) The frequency error is too large, resulting in the actual frequency offset nominal frequency resulting in crystal.

Solution: Choose a product with the right PPM value.

(2) The negative impedance is too large and too small will cause the crystal to fail to oscillate.

Solution: The negative impedance is too large, you can increase the value of the external capacitor Cd and Cg to reduce the negative impedance; If the negative impedance is too small, the values of external capacitors Cd and Cg can be reduced to increase the negative impedance. In general, the negative impedance value should be no less than 3-5 times the nominal maximum impedance of the crystal.

(3) The excitation level is too large or too small will also cause the crystal not to oscillate

Solution: By adjusting the size of Rd in the circuit to adjust the oscillator circuit to the crystal output excitation level. In general, the smaller the excitation level, the better, in addition to low processing power consumption, but also related to the stability of the oscillation circuit and the service life of the crystal.