What is a VCXO? Why is a VCXO used in Electronics?

What is a VCXO? Why is a VCXO used in Electronics?

A VCXO - Voltage Controlled Oscillator - is a reference timing component that is commonly used in
telecom, consumer and industrial electronics. A VCXO differs from an XO (Oscillator) in that it offers an
capability to "fine-tune" the output frequency within a certain range, after the device is already populated
in a system. This capability is not available on XOs.
Typically, a VCXO is used wherever a clock and data recovery function is performed. Usually, that
involves receivers such as wireless base stations and other telecom equipment as well as consumer
electronic devices such as Set Top Boxes (STB). The VCXO is part of a control loop that involves a PLL -
Phase Locked Loop - that synchronizes the recovered clock on the receiver with the transmitted clock -
to ensure that the two devices are locked and data integrity is maintained.
Pull range and Linearity are two key specs for VCXOs. Typically, crystal oscillator based VCXOs will offer a
pull range of up to 200 PPM of the output frequency. The maximum value of this "fine-tuning" capability
is limited in a crystal-based VCXO because of the type of crystal and circuit that is used. This control is
implemented on the analog oscillator circuit using varactors or switched capacitor arrays, which switch in
and out as needed. In addition, a pullable crystal is also required - which is more expensive and not as
easily available as a standard crystal resonator.
Pulling is achieved by an analog control voltage that is provided as input into the VCXO. The variation of
the output frequency with this control input voltage is measured as linearity - the more linear a VCXO,
the better control and characteristics it exhibits.
A silicon based VCXO offers much better pull range and linearity than a crystal-based VCXO. It does so
because the "pulling" is done using Phase Locked Loops instead of varactors and pullable crystals - thus
achieving a far larger pull range at a far better linearity. VCXOs that are based on Silicon MEMS will offer
pull range of up to 1600 PPM, as well as linearity of less than 1%, at comparable phase noise and jitter
as crystal-based VCXOs. Thus, they are becoming the VCXOs of choice in electronics applications.
Learn more about :- Clock Generator , Silicon Oscillator , Crystal oscillator , Differential Oscillator , TCXO

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