Release date:2020-07-20
The oscillating circuit with quartz resonator is unstable.
Oscillating circuits are usually designed with a negative resistance value, which is usually related to the feedback coefficient of the circuit and the amplification factor of the active element. The quartz resonator as a frequency selecting element has an equivalent resonant resistance. The resonant resistance depends on the package size and frequency of the resonator. The lower the frequency, the greater the resonance resistance. Oscillations become unstable when the negative resistance of the circuit is less than twice the resistance of the quartz resonator. In particular, when the working voltage of the oscillating circuit fluctuates, the deviation of the active element and the feedback element value will cause the negative resistance to deviate from the design value. When the actual working environment temperature changes, the resonant resistance of the quartz resonator also changes, thus destroying the normal working conditions of the oscillating circuit.
The solution is to adjust the feedback circuit to increase the negative resistance and reduce the resonant resistance of the quartz resonator. In general, the first approach is easier to implement. Reducing the resistance of quartz resonators is sometimes difficult and will greatly increase the manufacturing cost. In order to ensure the normal operation of the circuit, it is recommended that the negative resistance in the circuit design should be about 3-10 times the resistance of the quartz resonator. When replacing a large-size resonator with a small-size quartz resonator (such as hC-49S instead of HC-49U), extra care should be taken to adjust circuit parameters.
