INFICON PLO-10i Phase Lock Oscillator - Manual (Page 57)

PLO-10 PHASE LOCK OSCILLATOR
THEORY OF OPERATION 8-9
Figure 18 Crystal Equivalent Circuit
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 19.