INFICON PLO-10i Phase Lock Oscillator - Manual (Page 11)

PLO-10 PHASE LOCK OSCILLATOR
GENERAL DESCRIPTION 1-1
1 GENERAL DESCRIPTION
The INFICON Phase Lock Oscillator 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 provides a dc voltage that is proportional to the crystal’s
conductance (1/resistance). This provides additional information in the study of
lossy films and 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 converted to a voltage, demodulated and amplified to create a dc
voltage proportional to crystal conductance.
The PLO contains a phase detector that continuously monitors the phase
difference between the crystal’s current and voltage. At frequencies below the
crystal’s resonant frequency the current leads the voltage and the phase goes to 90
degrees as the frequency separation continues to increase, see Figure 20. Above
the resonant point the current lags the voltage and the phase go to minus 90
degrees. As the frequency increases through the resonant frequency, the phase
goes from plus 90 through 0 to minus 90. It is interesting to note that the phase
angle is 45 degrees when the VCO frequency is one half of the crystal’s
bandwidth above or below the crystal’s resonant frequency.
The output of the phase detector is fed into an integrator. The integrator
accumulates the phase error such that any positive phase error causes the
integrator output to climb; a negative phase causes the integrator output to fall.
With zero phase error the Integrator output holds steady.
The integrator output is connected to the VCO. Thus, if the VCO frequency is
initially below the crystal resonant frequency, the phase will be positive,
producing a positive output at the phase detector. This causes the Integrator
output to climb, which causes the VCO frequency to increase. When the VCO
frequency matches the resonant frequency of the crystal the phase will decrease to
zero, the phase detector output will go to zero, the Integrator output will hold
steady and the VCO frequency will be “locked” to the crystal’s resonant
frequency.
If the crystal’s resonant frequency moves up or down, a phase difference between
the crystal voltage and current will develop, producing a phase detector output.
The non-zero phase detector output will drive the Integrator output up or down
until the phase is zero once again, thus keeping the VCO frequency locked to the
crystal’s resonant frequency.
Once the frequency of the VCO is locked to the series resonant frequency of the