Voltage Controlled Crystal Oscillators

Voltage Controlled Crystal Oscillators


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 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 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.