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• Voltage Controlled Crystal Oscillator Terms and Definitions
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A VCXO is aquartz crystal oscillator that includes a varactor diode and associatedcircuitry allowing the frequency to be changed by application of a voltageacross that diode. By varying the control voltage, the capacitance of thevaractor changes accordingly, thus forces the output frequency changed. Thiscan be accomplished in a simple logic clock or sine wave crystal oscillator. Inaddition to those terms that define the fixed frequency crystal oscillator,there are several characteristics peculiar to VCXOs.

Control voltage (Vc):

An externalvoltage applied to the input of the VCXO. By changing the voltage, thefrequency varies accordingly. Typical Vc is 0V to5V; 0.5V to 4.5V; -0.5V to4.5V.

Deviation or Frequency pullability:

The minimumchange in the output frequency with respect to the change in control voltage.Unit is measured in ppm. Standard pullability is ±50ppm minimum, ±100ppmminimum.

Linearity:

The deviationfrom the best straight-line slope of the frequency vs. control voltage curve.If the specification for an oscillator requires a linearity of ±10% and theactual deviation is 10 kHz total as an example, the curve of output frequencyvs control voltage input could vary as much as ±1 kHz (10 kHz ±10%) from theBest Straight Line. On the other hand, if maximum deviation from the BestStraight 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.

Input impedance:

A measure ofisolation between the input port of the VCXO network and the voltage controlsource. Typical input impedance is > 50 Ohms @ 10 Khz.

Center frequency or nominal frequency: