INFICON PLO-10i Phase Lock Oscillator - Manual (Page 27)

PLO-10 PHASE LOCK OSCILLATOR
ADJUSTING THE CAPACITANCE CANCELLATION 4-1
4 ADJUSTING THE CAPACITANCE CANCELLATION
Proper adjustment of the Capacitance Cancellation is critical in obtaining accurate results
with high resistance crystals. See Section 1.2.3 FREQUENCY ERROR DUE TO
IMPERFECT CAPACITANCE CANCELLATION. 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 compensation clockwise until it begins to
flash then back up until it just stops. If it is flashing, turn the fine adjustment counter
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.
4.1 ADJUSTING CAPACITANCE CANCELLATION TRIMMER CAPACITORS
Setting up the capacitance cancellation is fairly straightforward. The thing to remember
is that there are two variable capacitors, a course and a fine with the total compensation
capacitance being the sum of the two. These trim capacitors have no stops so it’s not
obvious when they are at their minimum or their maximum.
The capacitors have 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 capacitors are rotated
clockwise they go through a full cycle from maximum to minimum and back to
maximum. Or, depending on where you start they may go first toward a minimum, then
to a maximum and then back toward a minimum. To avoid confusion, we always want to
be turning clockwise as we approach the desired capacitance and we want the capacitance
to be decreasing.