INFICON PLO-10i Phase Lock Oscillator - Manual (Page 12)

PLO-10 PHASE LOCK OSCILLATOR
GENERAL DESCRIPTION 1-2
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. The amplified
output of the demodulator is provided at the Conductance output.
1.1 FEATURES
1.1.1 VERY WIDE FREQUENCY RANGE
The PLO-10 supports a wide frequency range from 3.8 to over 6 MHz. It will
support both 5 and 6 MHz crystals; and with a low limit of 3.8 MHz it will
support 1.2 MHz of frequency shift on a 5MHz crystal. Also, available is the
PLO-10-2 to support higher frequency crystals. Its frequency range is 5.1 to over
10 MHz.
1.1.2 SUPPORT FOR VERY LOW Q, HIGHLY DAMPED, CRYSTALS
The PLO-10 will reliably lock to crystals with resistance of 5 KΩ or less. In most
cases it will maintain lock up to a resistance of 10 KΩ. It will support crystal
oscillation in highly viscous solutions of more that 88% glycol in water.
1.1.3 DIRECT REAL TIME MEASUREMENT OF CRYSTAL RESISTANCE
The PLO-10 provides a dc voltage output that is proportional to the crystal’s
conductance. Conductance is the inverse of resistance. Based on the measured
conductance output voltage, the crystal resistance is easily calculated.
1.1.4 ELECTRODE CAPACITANCE CANCELLATION
The total quartz crystal impedance includes a shunt capacitance (due to the
capacitance of the crystal electrodes and holder) in parallel with the series
resonant arm. The total current through the crystal is the sum of the current
through the shunt capacitance plus the current through the series resonant arm.
The physical motion of the crystal is reflected in the values of the L, R and C in
the series arm of the crystal only, and therefore we want to subtract out or
otherwise cancel the current through the shunt electrode capacitance. The
INFICON
PLO includes a method of canceling the electrode capacitance insuring
that the measured crystal current does not include the current through the
electrode capacitance and therefore is essentially the current through the series
resonant arm of the crystal only.
1.1.5 “AUTOLOCK”
When the PLO-10 loses lock, the VCO is ramped up to the maximum frequency
at which time it is automatically reset to the minimum frequency and a new scan
is initiated.
To insure that the VCO ramps up in frequency, a small amount of quadrature
current is injected into the current to voltage buffer whenever the PLO is
unlocked. This current is equivalent to a shunt capacitance of about 1.5 pfd. As
soon as lock is detected, the quadrature current is turned off.