Jitter
Measure of the modulation in phase or frequency of the oscillator output.
Stand by Function
A function built in the IC that temporary turns off the oscillator to save
power.
3. Voltage Controlled Crystal Oscillators
A VCXO is a quartz crystal oscillator that includes a varactor diode and
associated circuitry allowing the frequency to be changed by application of a
voltage across that diode. This can
be accomplished in a simple logic clock or sine wave crystal oscillator, or a
TCXO (resulting in a TC/VCXO-temperature compensated voltage controlled crystal
oscillator), or an oven controlled type (resulting in an OC/VCXO-oven controlled
voltage crystal oscillator). In addition to those characteristics that define the fixed
frequency crystal oscillators (XOs), there are several characteristics peculiar
to VCXOs. Primary among the
specifications that are peculiar to VCXOs are the following:
Control Voltage
This is the varying voltage, which is applied to the VCXO input terminal
causing a change in frequency. It is sometimes referred to as Modulation Voltage, especially
if the input is an AC signal.
Deviation/Pulling Range
This is the amount of frequency change that results from changes in control
voltage. For example, a change of
control voltage from 0.5 to 4.5 volt might result in a deviation of 100 ppm.
This parameter can also define so-called average slope which is the total
frequency deviation divided by the total control voltage swing.
Slope Polarity/Transfer Function - This denotes the direction of frequency
changes with respect to the control voltage.
A positive transfer function denotes an increase in frequency for an
increasing positive control voltage. Conversely,
if the frequency decreases with a more positive (or less negative) control
voltage, the transfer function is negative.
Linearity
The generally accepted definition of linearity is that specified in
MIL-0-55310. It is the ratio
between frequency error and total deviation, expressed in percent, where
frequency error is the maximum frequency excursion from the so-called “Best
Straight Line” drawn through a plot of output frequency vs control voltage.
If the specification for an oscillator requires a linearity of ±10% and
the actual deviation is 10 kHz total as an example, the curve of output
frequency vs control voltage input could vary as much as ±1 kHz (10 kHz ±10%)
from the Best Straight Line. On the
other hand, if the maximum deviation from the Best Straight Line is 20 ppm and
the total deviation is 200 ppm, the linearity is ± 20 ppm/200 ppm = ± 10%,
which is the linearity value for a typical VCXO.
Good VCXO design dictates that the voltage vs frequency curve be smooth
(no discontinuities) and monotonic.
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