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.
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