INFICON RQCM - Quartz Crystal Microbalance Research System - Manual (Page 67)

RQCM – RESEARCH QUARTZ CRYSTAL MICROBALANCE
THEORY OF OPERATION 5-15
Figure 24 Non-zero Phase Lock
Figure 24 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.
5.7 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 RQCM 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 the crystal’s conductance. This current is monitored by the
RQCM and displayed as crystal resistance. The PLO contains a phase detector that continuously
monitors the phase difference between the crystal’s current and voltage. At frequencies below
CRYSTAL BANDWIDTH