Inficon IMM-200, Руководство по эксплуатации

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10 | Measurement Theory INFICON
54 / 67 074-733-P1A IMM-200 Operating Manual
The active oscillator circuit is designed so the crystal is required to produce a phase
shift of 0 degrees, which allows it to operate at the series resonance point. Long and
short-term frequency stabilities are a property of crystal oscillators because very small
frequency changes are needed to sustain the phase shift required for oscillation.
Frequency stability is provided by the quartz crystal even though there are long-term
changes in electrical component values caused by temperature or aging or short-term
noise-induced phase jitter.
As mass is added to a crystal, its electrical characteristics change. A heavily loaded
crystal loses it steep phase slope. 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. 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 an active oscillator monitor will frequently
continue to operate under these conditions; in fact there is no way to tell this has