INFICON PLO-10i Phase Lock Oscillator - Manual (Page 13)

PLO-10 PHASE LOCK OSCILLATOR
GENERAL DESCRIPTION 1-3
1.1.6 CRYSTAL FACE ISOLATION (PLO-10i Models only)
The PLO-10i Models provide transformer isolation of the crystal front face
electrode. This feature allows user to connect the crystal face to an
electrochemical instrument such as a potentiostat.
1.2 CHARACTERIZING THE PLO
1.2.1 FREQUENCY ERRORS
The first thing we want to know regarding the performance of the PLO, is “What
is the magnitude of the frequency error we can expect from 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 temperature 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.
1.2.2 FREQUENCY ERROR DUE TO PHASE ERROR
Given some finite zero phase error, the resulting frequency error depends on the
sensing crystal’s Q, the higher the Q, the lower the error. For phase errors below
10 degrees the frequency error is 0.087 PPM per degree for crystals with a Q of
100,000. Thus a one degree phase error in the PLO results in a 0.44 Hz frequency
error for a 5MHz crystal with a Q of 100,000. For a 5 MHz crystal with a Q of
10,000, the error is 10 time greater or 4.4 Hz per degree.
Frequency Error/degree = df/f = PI/(360*Q)
1.2.3 FREQUENCY ERROR DUE TO IMPERFECT CAPACITANCE
CANCELLATION
The effect of imperfect electrode capacitance cancellation can also be viewed as
an equivalent phase error. This error is directly proportional to crystal resistance.
The equivalent phase error due to a non-zero shunt capacitance equal to 1 pfd is
one degree for a crystal with a series resistance of 556 Ω. Since the equivalent
phase error is proportional to the crystal resistance, a 1-pfd residual capacitance
error will result in a 10-degree equivalent error for a sensing crystal with a
resistance of 5.56 KΩ.