The coordinate problem between negative resistances of electro circuit and the resistance of resonat

Problem and phenomenon: The oscillation of circuit upon which quartz crystal resonator installed is not stable, some fail to oscillate.

Cause analysis: There is negative resistance value on the design of oscillatory circuit, which is related with the feedback coefficient and the amplification factor of active component. As the frequency selection component, quartz crystal resonator has equivalent resistance, which is related with encapsulation size, frequency. In general, the smaller the resonator is, the higher the resonant resistance is; the lower the frequency is, the higher the resistance is. The oscillation will become unstable when the negative resistance of the electro circuit is less than twice of resonator resistance. Especially when the working voltage fluctuates, the value of active components and feedback elements will make negative resistance deviate the previous design.

The solution to the problem: one method is to make adjustments to the feedback of electro circuit, increase negative resistance value. When the operating temperature changes, the resonant resistance will also change, which will destroy its normal working conditions. Another method is to decrease resonant resistance. The first method is easy to realize, however, it is hard to reduce the resonant resistance, which may increase manufacturing cost. In order to ensure electro circuit can work well, it is advisable that negative resistance should be three to ten times larger than that of quartz crystal resonator. When small size resonators are used to replace big size resonators (e.g.HC-49S resonators replace HC-49U resonators), adjustments of electro circuit parameters should be paid special attention.