Voltage Controlled Oscillators- Fine Tuning The Communication Systems

Voltage Controlled Oscillators- Fine Tuning The Communication Systems

The technological world, which helps communication across geographical boundaries easier and faster, is
not as simple as it sounds. Understanding the intricacies of the network and the components, which go
into making communication lines with clarity of voice and speech, is itself a Herculean task.
A Voltage Controlled Oscillator (VCXO) is a component for reference timing used in virtually all
wireless systems. A VCXO has the capability to fine-tune the output frequency within a certain range,
even after the device is already populated in a system. With the varactor diode as the principal variable or
tuning element, the VCO is tuned across its band by a dc voltage applied to the diode. This fine-tuning of
a clock frequency within an electronic system is used to synchronize the receiving system with a
transmitting system, which is not at the same location. Synchronous communication standards like
SONET and GiGe require extremely tight timing control. These standards are found everywhere from
long-haul, wide-area network (WAN) global communication to the metro-area network (MAN) found in
cities and neighborhoods. An error in the timing between the receiver and transmitter translates to bit
errors, dropped transactions and systems coming to a grinding halt. An example is a phone call. When
you talk on your mobile phone while driving, the wireless infrastructure is moving your call from one
base station or repeater to another to maintain contact. If any of the base stations or repeaters were out of
synch, the call would be dropped or larger pieces of the conversation would be lost.

Voltage controlled oscillators (VCXO) are generally made of quartz crystals, but recent technological
developments have led to the usage of silicon oscillators with MEMS technology. Generally used for
performing a clock and data recovery function, a VCXO is used with receivers or wireless base stations
and with consumer electronic devices and set top boxes. Two main specifications with a VCXO are pull
range and linearity. The crystal oscillator offers an output frequency pull range up to 200PPM, but
typically only 10-15PPM for higher precision VCXOs. An all-silicon VCXO offers significantly greater
pull range up to 1600PPM. This wide pull range will enable new applications as more system designs
realize the availability of a VCXO with such a large pull range. However, a wide pull range is not very
appealing if the linearity is poor. Fortunately, this is not the case with MEMS VCXOs. Linearity is
typically 1%, resulting in a monotonic pull range over the entire 1600 PPM. This represents a 10-to-20-
times improvement compared to a quartz-crystal VCXO with a 200PPM pull range. MEMS Oscillators
are flexible and facilitate innovation of in telecom clock synchronization, wireless networking, FPGA-
based systems and various other applications.
An all-silicon VCXO showcases a number of other unique key features such as configurability, frequency
and supply-voltage flexibility, plastic packaging, and drop-in, pin-to-pin compatibility to existing quartz
crystal VCXOs.
Learn more about :- Clock Oscillator & TCXO

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