__Introduction:__

A Voltage controlled crystal oscillator VCXO is an oscillator whose frequency can be changed by applying voltage to the VCXO. A Varactor diode in series with the crystal is used as the means of controlling the oscillator frequency. A change in voltage causes a change in the capacitance of the varactor diode, which changes the CL of the crystal resulting in change of the frequency of the oscillator.

__VCXO Deviation: __

The total frequency change of the VCXO as a result of the change in the control voltage is the VCXO deviation.

__Control Voltage __

The voltage, which is applied to the VCXO for a desired change in the frequency, is the control voltage. For example, a control voltage of 0 to + 5V may result in a frequency deviation of ± 100 ppm.

__Slope __

Slope or transfer function refers to the direction of the frequency change with voltage control. A positive slope denotes that the frequency will increase with increasing positive control voltage. A negative slope denotes that the frequency decreases with increase in positive control voltage.

__Linearity __

Linearity is the measure of the frequency Vs. Control Voltage transfer characteristic as compared to an ideal straight line. If a linearity of ± 10% is specified, the maximum deviation of the actual frequency Vs. control voltage plot cannot exceed by more than ± 10% from the best straight line plot of frequency Vs Control voltage as shown in Figure 1.