INFICON PLO-10i Phase Lock Oscillator - Manual (Page 63)

PLO-10 PHASE LOCK OSCILLATOR
THEORY OF OPERATION 8-15
Figure 26 Non-zero Phase Lock
Figure 26 shows the result of a non-zero phase lock. Note that the frequency difference
between the top of the conductance circle and the bottom is equal to the bandwidth of the
crystal. For a high Q, (high conductance, low resistance) crystal, the bandwidth is very
narrow and small errors in phase lock angle are insignificant. For a low Q crystal the
bandwidth can be quite large and small phase errors can result in significant frequency
errors. See the equations in the error discussion section.
8.7 CHARACTERIZING THE CRYSTAL MEASUREMENT
The INFICON Phase Lock Oscillator (used on the Crystal Measurement Card) was
developed specifically to support the use of the quartz crystal microbalance in the
measurement of lossy films and in liquid applications. In addition to accurately tracking
the frequency of heavily damped crystals, the PLO-10 also tracks the crystal’s resistance.
This provides additional information in the study of lossy films and/or viscous solutions.
The PLO utilizes an internal oscillator referred to as a Voltage Controlled Oscillator
(VCO) to drive the crystal. The crystal current is monitored and the frequency of the
oscillator is adjusted until there is zero phase between the crystal voltage and current.
Assuming that the crystal’s electrode capacitance has been effectively cancelled, this
point of zero phase between the crystal current and voltage is the exact series resonant
point of the crystal. The magnitude of the current at this point is directly proportional to
CRYSTAL BANDWIDTH