INFICON PLO-10i Phase Lock Oscillator - Manual (Page 64)

PLO-10 PHASE LOCK OSCILLATOR
THEORY OF OPERATION 8-16
the crystal’s conductance. This current is monitored by the PLO-10 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 21. 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.
8.7.1 FREQUENCY ERRORS
The first thing we want to know regarding the performance of the crystal measurement is
“What is the magnitude of the frequency error we can expect from the crystal
measurement portion of the PLO-10?”
In any oscillator and sensing crystal system, the error in the frequency measurement, is a
function of both the oscillator and the sensing crystal. The same is true for phase locked
loops. Any phase error will introduce a frequency error and this frequency error will be
inversely proportional to the sensing crystal’s Q. These errors are over and above any
change in crystal frequency due to stress, temperature, adsorption, and humidity changes.
There are four important parameters that determine the frequency error of the PLO and
sensing crystal system or indeed, any oscillator and sensing crystal system. The first two,
the zero phase error and the electrode capacitance cancellation errors, are characteristics
of the PLO. The second two are characteristics of the crystal, the Q of the crystal and the
conductance (1/resistance) of the crystal.