Inficon SQM-160, Operating Manual

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SQM-160 Operating Manual
Figure 8-5 Heavily Loaded Crystal
The crystal has lost the steep slope displayed in Figure 8-6 . Because the phase
slope is less steep, any noise in the oscillator circuit translates into a greater
frequency shift than that which would be produced with a new crystal. In the
extreme, the basic phase/frequency shape is not preserved and the crystal is not
able to provide a full 90 degrees of phase shift.
Figure 8-6 Crystal Frequency Near Series Resonance Point
The impedance, |Z|, is also noted to rise to an extremely high value. When this
happens it is often more favorable for the oscillator to resonate at one of the
anharmonic frequencies. This condition is sometimes short lived, with the oscillator
switching between the fundamental and anharmonic modes, or it may continue to
oscillate at the anharmonic. This condition is known as mode hopping and in
addition to annoying rate noise can also lead to false termination of the film
because of the apparent frequency change. It is important to note that the
SQM-160 will frequently continue to operate under these conditions; in fact there
is no way to tell this has happened except that the film’s thickness is suddenly
apparently thinner by an amount equivalent to the frequency difference between
the fundamental and the anharmonic that is sustaining the oscillation.