INFICON RQCM - Quartz Crystal Microbalance Research System - Manual (Page 25)

RQCM – RESEARCH QUARTZ CRYSTAL MICROBALANCE
OPERATION 3-1
3 OPERATION
The heart of the RQCM is the crystal measurement methodology. It is important that the user
understand its operation to ensure proper setup and application.
3.1 GENERAL DESCRIPTION OF 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
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 19. 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 crystal, the in-
phase component (at zero phase error, there is no out of phase component) of the crystal current is
demodulated to a DC voltage. This voltage is amplified and converted into resistance value
which the RQCM outputs to the computer.