INFICON SQM-160 Thin Film Deposition Monitor - Manual (Page 108)

8 - 6
IP
N 07
4-
51
1-
P1
C
SQM-160 Operating Manual
for a number of materials, and has been found to be valid for frequency shifts
equivalent to F
f
= 0.4F
q
. Keep in mind that equation [2] was valid to only 0.02F
q
and equation [3] was valid only to ~0.05F
q
.
8.1.4 Active Oscillator
The SQM-160 relies on the use of an active oscillator circuit, Specifically the type
schematically shown in
Figure 8-4 . This circuit actively keeps the crystal in
resonance, so that any type of period or frequency measurement may be made. In
this type of circuit, oscillation is sustained as long as the gain provided by the
amplifiers is sufficient to offset losses in the crystal and circuit and the crystal can
provide the required phase shift. The basic crystal oscillator’s stability is derived
from the rapid change of phase for a small change in the crystal’s frequency near
the series resonance point, as shown in
Figure 8-6 on page 8-7 .
Figure 8-4 Active Oscillator Circuit
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. Figure 8-5 on
page 8-7 is the same plot as Figure 8-6 overlaid with the response of a heavily
loaded crystal.