INFICON RQCM - Quartz Crystal Microbalance Research System - Manual (Page 62)

RQCM – RESEARCH QUARTZ CRYSTAL MICROBALANCE
THEORY OF OPERATION 5-10
Because a crystal’s impedance is minimum at resonance it is convenient to characterize a crystal
in terms of admittance. Admittance is the inverse of impedance, (Y = 1/Z), thus the admittance
reaches a maximum at resonance. While impedance is proportional to the voltage developed
across a device when it is subjected to a current, the admittance is proportional to the current
through the device when it is subjected to a voltage.
At any frequency the admittance of a quartz crystal is a complex value that can be expressed in
terms of magnitude and phase or in terms of a real and imaginary value. The relationship of these
two representations is shown in Figure 17.
Figure 17 Polar Plot of Crystal Admittance
Figure 18 shows the conductance in terms of magnitude and phase, while Figure 19 shows the
same information in terms of the imaginary and real part of the conductance.