INFICON RQCM - Quartz Crystal Microbalance Research System - Manual (Page 29)

RQCM – RESEARCH QUARTZ CRYSTAL MICROBALANCE
OPERATION 3-5
3.4 ADJUSTING THE CAPACITANCE CANCELLATION
Proper adjustment of the Capacitance Cancellation is critical in obtaining accurate results with
high resistance (heavily damped) crystals. See Section 5.8. The cancellation adjustment should
be performed with the crystal holder and crystal in the measurement environment. For instance, if
liquid measurements are to be made, insert the crystal and its holder into the liquid where the
measurement will be made.
With the crystal and holder in the measurement environment, press and hold the Reset switch.
Pressing and holding the Reset switch forces the VCO to its minimum frequency, turns on the
Lock LED, and turns off the quadrature current injector. Forcing the VCO to its minimum
frequency insures that the crystal is being driven at a frequency far from its resonant frequency
where its impedance is essentially due only to the shunt electrode capacitance. With the
quadrature current injector turned off, the measured current is due only to the net shunt
capacitance. The measured net shunt capacitance is the capacitance of the cable, holder and
crystal electrodes minus the compensation capacitance. If the capacitance is under compensated,
the phase of the measured current leads the voltage, (a phase angle of plus 90 degrees). If the
capacitance is over compensated, it lags the voltage, (a phase angle of minus 90 degrees).
The Yellow Sweep LED is used to determine whether the crystal capacitance is over
compensated or under compensated. The Sweep LED flashes whenever the crystal capacitance in
under compensated.
If the Sweep LED is not flashing, turn the fine adjustment counterclockwise until it begins to
flash then back up until it just stops. If it is flashing, turn the fine adjustment clockwise until it
just stops flashing. This is a very fine adjustment. Go back and forth until you are sure you are
right on the edge. The sensitivity of the fine adjustment is approximately 0.05 pfd per degree. In
situations where the crystal resistance is very high (over 1 KΩ) a net capacitance of over 0.5 pfd
can result in a significant frequency error so try to get this adjustment to within a couple of
degrees. Remember to keep the Reset switch depressed while making this adjustment.
3.5 ADJUSTING CAPACITANCE CANCELLATION TRIMMER & SWITCH
Setting up the capacitance cancellation is fairly straightforward. The thing to remember is that
there are two adjustments, a course (rotary switch) and a fine (capacitor trimmer) with the total
compensation capacitance being the sum of the two. The trim capacitor has no stops so it’s not
obvious when it is at its minimum or its maximum.
The fine adjustment capacitor has circular, rotor plates that mesh into fixed stator plates. The
capacitance is at a maximum when the plates are fully meshed and a minimum when rotor plates
are above the stator plates and not meshed. As the capacitor is rotated clockwise it goes through
a full cycle from maximum to minimum and back to maximum. Or, depending on where you
start it may go first toward a minimum, then to a maximum and then back toward a minimum. To
avoid confusion, always turn the fine adjustment clockwise as we approach the desired
capacitance and we want the capacitance to be decreasing.
The coarse adjustment is a rotary switch. Like the fine adjustment, it goes from its minimum to
its maximum, then back to its minimum capacitance value in a full rotation. The difference is
that it has 16 positive stops. Observe the “V” notch on the switch (Figure 3). The coarse
adjustment is at its minimum capacitance when the “V” notch is pointing straight upward (zero
position). The capacitance is increased with each stop as the switch is rotated clockwise. It
reaches maximum capacitance at the 15 th stop – one stop before returning to the zero position.
The RQCM is setup for a standard INFICON crystal holder and cable from the factory so you
should not have to change the course adjustment. Simply connect the cable and crystal holder to