INFICON RQCM - Quartz Crystal Microbalance Research System - Manual (Page 31)

RQCM – RESEARCH QUARTZ CRYSTAL MICROBALANCE
OPERATION 3-7
films.
♦ Use a shortest cable possible for connection from the Crystal Holder to the RQCM. Cables’
capacitance changes with temperature and position, the longer the cable the larger the
capacitance; hence the larger change in capacitance.
♦ Capacitance cancellation should be checked and readjusted every time the environment of
the crystal is changed, for example, transition from air to a liquid phase.
♦ The capacitance cancellation adjustment must be performed with the Crystal Holder and
crystal in the actual measurement environment.
3.6 WORKING WITH VERY LOW Q CRYSTALS
Very low Q crystals require very close adjustment of the compensating capacitance to insure a
successful lock. To adjust the compensation capacitance one pushes the Reset button and adjusts
the capacitance to the point where the Sweep LED just ceases to flash. With very low Q crystals,
the PLO may not lock upon release of the Reset button. The Unlock LED will be on and the
Sweep LED will be flashing. This is normal. Even so it may be possible to lock on the crystal by
slowly adjusting the fine capacitance clockwise until the Sweep LED again ceases to flash. Lock
is evidenced by the Lock LED turning on or by a crystal resistance of less than 10 KΩ.
Once lock is achieved the true series resonant point can be found by adjusting the capacitance for
minimum resistance value. The limits of the crystal bandwidth can be determined by adjusting
the capacitance and reading the maximum frequency and the minimum frequency just before the
RQCM loses lock.
3.7 NORMAL OPERATION
The RQCM comes set up for operation with a INFICON cable and crystal holder. If a INFICON cable
and crystal holder is being used, then no initial adjustments should be needed.
During normal operation with a crystal installed and connected to the oscillator, the green Lock
LED will be on and the frequency output will reflect the crystal resonance. The red Unlock LED
will be off.
If the Unlock LED is on, the Sweep LED should slowly flash. Continuous sweeping of the
frequency range indicates that the crystal’s resonant frequency is outside of the PLO’s frequency
range or the crystal’s conductance is below the conductance threshold.
No flashing of the Sweep LED when the Unlock LED is on can mean one of two things. First, if
the VCO frequency is sitting at its low limit, it means the electrode capacitance is over
compensated. Second, in some cases, even though the crystal conductance has fallen below the
threshold necessary to indicate lock, the internal signals are still sufficient to keep the VCO
locked to the crystal. In that case, the PLO really is locked and the VCO frequency will be sitting
at the crystal frequency somewhere between its minimum and maximum frequencies.
If the VCO frequency is sitting at its low limit, press and hold the Reset switch and adjust the fine
capacitance trimmer a few degrees counterclockwise (not more than ten) until the Sweep LED
begins to flash.
3.8 HOOKUP FOR ELECTROCHEMICAL EXPERIMENTS
Figure 4 shows a typical connection diagram for an electrochemical QCM using the INFICON
RQCM. Note that the crystal’s front electrode becomes the “working electrode” when the Crystal