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XTC/3 Operating Manual
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 frequencies slightly higher than resonance it is inductive in nature. This
information is useful if the resonance frequency of a crystal is unknown. A quick
sweep of frequencies can be undertaken until the output of the phase comparator
changes, marking the resonance event. For AT crystals we know that the lowest
frequency event encountered is the fundamental. The events slightly higher in
frequency are anharmonics. This information is useful not only for initialization, but
also for the rare case when the instrument loses track of the fundamental. Once
the frequency spectrum of the crystal is determined, the instrument’s task is to
follow the changing resonance frequency and to periodically provide a
measurement of the frequency for subsequent conversion to thickness.
Figure 8-6 Heavily loaded crystal
The use of the “intelligent” measurement system has a series of very apparent
advantages when compared to the previous generation of active oscillators,
namely immunity from mode hopping, speed of measurement and precision of
measurement.
Phase (degrees)
-90
Log |Z| (ohms)
Frequency (MHz)
4.541500
4.512500
4.543600
4.544500
4.544500
1000
100
Phase
Impedance
Series resonance
+90
1000
100
0