The mode-lock oscillator

INFICON has developed a new technology for overcoming these constraints on the active oscillator. The new system constantly analyzes Ihe response of the crystal fo an applied frequency not only fo determine the (series) resonance frequency, but also fo ensure that the quartz oscillates in the desired mode. The new system is insensitive te mode hopping and the resultant inaccuracy. It is fast and precise. The crystal frequency is determined 10 times a second w/ith an accuracy to less than 0.0005 Hz. 

The crystal impedance is capacitive at frequencies below the fundamental wave and inductive at frequencies above the resonance. This information is useful if the resonance frequency of a crystal is unknown. A brief frequency sweep is carried out until the phase comparator changes over and thus marks the resonance. For AT quartzes we know that the lowest usable frequency is the fundamental wave. The anharmonics are slightly above that. This information is not only important for the beginning, but also in the rare case that the instrument loses track of the fundamental wave. Once the frequency spectrum of the crystal is determined, the instrument must track the shift in resonance frequency, constantly carry out frequency measurements and then convert them into thickness. 

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In 1989 A. Wajid invented the mode-lock oscillator. He presumed that a connection existed between the fundamental wave and one of the anharmonics, similar to that ascertained by Benes between the fundamental oscillation and the third quasi-harmonic oscillation. The frequencies of the fundamental and the enharmonic oscillations are very similar and they solve the problem of the capacity of long cables. He found the necessary considerations for establishing this connection in works by Wilson (1954) as well as Tiersten and Smythe (1979). 

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