VCXO Voltage-Controlled Crystal Oscillators

A voltage-controlled crystal oscillator (VCXO) is a type of crystal oscillator that relies on a voltage control input to adjust the oscillation frequency that is generated by the oscillator’s crystal. It allows for the adjustment of the frequency of the electronic device across small ranges. The principle of VCXO frequency offset is also described as pulling.

The VCXO circuit design is very similar to that of a traditional crystal oscillator except that a VCXO includes a varactor diode to which the voltage current is applied at a specified voltage. The purpose of this is to alter the frequency of the oscillator’s crystal. The oscillator’s diodes are typically arranged back to back and function as a variable capacitor. The tuning range of the voltage applied to the oscillator’s diode can go from -150ppm up to +150ppm depends on the oscillator crystal’s high Q factor that enables pulling over a frequency range. The oscillator’s capacitance level dictates the maximum amount of pulling that can be applied to the crystal to alter the oscillation frequency level. When an electronic device requires that the VCXO be pulled over a wide range, an inductor is typically utilized within the oscillator’s circuit.

A VCXO is commonly found in devices used in the telecommunications and broadcast industries. They are frequently used in temperature-controlled crystal oscillators (TCXOs) to minimize drift by applying the temperature-stabilizing voltage to the VCXO’s control terminal. In addition, a VCXO can be used in narrow-band phase-locked loops (PLLs) that need a small amount of frequency alteration and maintain appropriate phase noise levels in their closed loops.

Although a VCXO can efficiently achieve a frequency alteration of more than  a hundred ppm, this may result in a decline in the performance of the oscillator’s circuit. Also, temperature controls may be required to reduce the amount of temperature drift resulting from the pulling of voltage across the circuit. A major advantage of including VCXOs as part of the circuitry of an electronic device is that they are much more affordable than other oven-controlled crystal oscillators.

Often, a VCXO is procured from a specialty supplier of these modules. Without the proper specifications for the VCXO, it may not work as intended for the desired purpose. Be sure to refer to the specific information sheets for the VCXOs that you are ordering.

The frequency of the specific VCXO will be identified in terms of MHz when it is at 1.0 MHz or above. When the frequency is below that level, it will be designated in kHz. Be sure that you are also matching up the correct decimal places when examining the frequency specifications for a VCXO.

The VCXO’s output requirements must be defined and matched to ensure the proper functioning of the VCXO for its individual application. The application of VCXOs can vary from analog to driving various forms of logics like CMOS, Clipped sinewave, Sinewave, LVPECL, and LVDS

The VCXO’s frequency stability is designated in ppm in standard intervals of 25 ppm, including 25 ppm, 50 ppm and 100 ppm. This range is generally specified at both room temperature and when the oscillator is at its operational level.

The supply voltage is crucial to maintain so that the proper amount of pulling can be achieved across the crystal. A high-quality VCXO will function slightly outside of this range to ensure efficiency.

There are generally three main operating temperature ranges for VCXOs. First, the commercial temperature operating range for oscillators in laboratory conditions is from -20 degrees Celsius to 70 degrees Celsius. The industrial temperature range for a VCXO is from -40 degrees Celsius to 85 degrees Celsius. Lastly, the VCXO operating temperature range for use in military equipment is from -40 degrees Celsius to 125 degrees Celsius.

For a VCXO, the pullability refers to the oscillation frequency range that the oscillator can be pulled across to alter the control voltage. With a higher pullability range, the VCXO will have a larger tuning range for using voltage to control the oscillator’s frequency. The trade-off for a lower pullability range in a VCXO is an improvement in its frequency stability and a reduction in the oscillator’s phase noise.

For a VCXO, the pullability refers to the oscillation frequency range that the oscillator can be pulled across to alter the control voltage. With a higher pullability range, the VCXO will have a larger tuning range for using voltage to control the oscillator’s frequency. The trade-off for a lower pullability range in a VCXO is an improvement in its frequency stability and a reduction in the oscillator’s phase noise.

The package for the VCXO should be specified according to its application. There are options for either dip type or surface mount VCXOs. In addition, when manufacturing VCXOs in mass quantities, it is also possible to specify tape and reel package requirements for surface mounted type.

Linearity for a VCXO is identified as a percentage in terms of a deviation from the tuning curve along a straight line fit. Given that the slope over the curve can vary considerably at either end, the linearity for a VCXO may be expressed as a range of percentages.

While the DC voltage input controls the oscillation frequency of a VCO, a current-controlled device depends on the amount of the current to determine the oscillation frequency. The difference in these types of oscillators is the types of inputs that are used to alter or maintain the devices’ frequencies.

Primarily, a VCO is used for the modulation of frequency or phase and is also the main component of a PLL. The application of a DC voltage to the control input of the VCO allows for this modulation.

Unlike a VCO, a fixed frequency oscillator does not rely on input controls in the form of voltage or current to alter the oscillator’s frequency. Its frequency output signals are not subject to alteration or modulation. In comparison to a VCO, a fixed frequency oscillator typically has a higher Q factor and less phase noise and jitter.

Given that a PLL relies on an input signal to alter and adjust its oscillation frequency, it is useful in stabilizing or recovering a signal from a communications channel that has suffered from an interruption in data. PLLs enable wireless communications and digital data transmission.