How to Specify a VCXO

One thing that design engineers, buyers and supply chain specialists all share in common is the challenge of working within fast paced and deadline driven environments that demand projects, programs, and orders move forward. Understanding how to specify parts may appear straightforward to the casual observer, but for loyal customers and PRX Professionals that couldn't be further from the truth.

Unlike an ordinary voltage controlled oscillator (VCO), the crystal-controlled VCXO possesses the ability to accurately maintain a reference frequency, even in the event of a loss of the control signal and despite changes in the environment such as temperature or supply voltage. This characteristic is especially important in wireless applications where drop-outs can occur and is also of equal value for high-speed wire-line links such as ISDN, ATM and xDSL, where VCXOs maintain the essential frequency with minimal noise or jitter.

There are two most commonly used methods for specifying VCXO's. First, is the APR Method which specifies all performance operating conditions for the device. The second method is known as the Separate Method. The Separate Method specifies device performance under specific operating conditions and calls for defining separate operating parameters such as the frequency's deviation, stability, and tolerance over operating temperature range, output load, and supply voltage. 

The APR Method uses the absolute pull range which allows the customer to simply specify one parameter. The Separate Method considers variations from nominal as the design engineer calculates the absolute pull range by starting with the total pull range of the VCXO, then subtracts the sum of all variations and tolerances such as calibration, temperature, power supply and load.

In conclusion, the APR Method is preferred because customers, buyers, and suppliers can easily calculate the requirements to the bottom line without concern toward individual factors.  

Oscillators and Crystals (Light)

In an industry that can be overwhelming to the new recruit, assistant buyer, or casual observer and in an era where parts are always getting smaller, smarter, and faster, there remains fundamental principles that have withstood the test of time. It is in this realm that we delve in an effort to give our readers a better understanding of the science behind crystals and oscillators.

An oscillator is a mechanical or electronic device that works on the principles of oscillation: a periodic fluctuation between two things based on changes in energy. Computers, clocks, watches, radios, and metal detectors are among the many devices that use oscillators.

A clock pendulum is a simple type of mechanical oscillator. The most accurate timepiece in the world, the atomic clock, keeps time according to the oscillation within atoms. Electronic oscillators are used to generate signals in computers, wireless receivers and transmitters, and audio-frequency equipment, particularly music synthesizers. There are many types of electronic oscillators, but they all operate according to the same basic principle: an oscillator always employs a sensitive amplifier whose output is fed back to the input in phase. Thus, the signal regenerates and sustains itself. This is known as positive feedback. It is the same process that sometimes causes unwanted "howling" in public-address systems.

The frequency at which an oscillator works is usually determined by a quartz crystal. When a direct current is applied to such a crystal, it vibrates at a frequency that depends on its thickness, and on the manner in which it is cut from the original mineral rock. Some oscillators employ combinations of inductors, resistors, and capacitors to determine the frequency however, the best stability is obtained in oscillators that use quartz crystals.