INFICON XTC/C Thin Film Deposition Controller - Manual (Page 143)

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XTC/C - XTC/2 Operating Manual
suddenly apparently thinner by an amount equivalent to the frequency
difference between the fundamental and the anharmonic that is sustaining the
oscillation.
Figure 5-6 Heavily Loaded Crystal
5.5.6 ModeLock Oscillator
INFICON has created a new technology
7
that eliminates the active oscillator
and its limitations. This new system constantly tests the crystal’s response to
an applied frequency in order to not only determine the resonant frequency, but
also to verify that the crystal is oscillating in the desired mode. This new system
is essentially immune to mode hopping and the resulting inaccuracies. It is fast
and accurate, determining the crystal’s frequency to less than .05 Hz at a rate
of 4 times per second. Because of the system’s ability to identify and then
measure particular crystal modes, it is now possible to offer new features that
take advantage of the additional informational content of these modes.
This “intelligent” measurement system uses the phase/frequency properties of
the quartz crystal to determine the resonant frequency. It operates by applying
a synthesized sine wave of specific frequency to the crystal and measuring the
phase difference between the applied signal’s voltage and the current passing
through the crystal. At series resonance, this phase difference is exactly 0
degrees; that is, the crystal behaves like a pure resistance. By separating the
applied voltage and the current returned from the crystal and monitoring the
output of a phase comparator it is possible to establish whether the applied
frequency is higher or lower than the crystal’s resonance point. At frequencies
well below the fundamental, the crystal’s impedance is capacitive and at
7.US Patent 5,117,192 (May 27 1992)