What is a VCXO and How do they Work?

Voltage Controlled Crystal Oscillators more commonly known as VCXOs are used for generating controllable and stable frequency reference signals in a whole host of applications from broadcast to telecommunication technologies, most often being used in the design of clock generators, providing a timing signal to help maintain the synchronicity of the circuit’s operation. Throughout the rest of this guide, we’ll investigate exactly what is a VCXO, what makes it different from other oscillators, how they work and why you should look to invest in VCXOs for your applications.
But if you already know that a Voltage Controlled Crystal Oscillator is what your application requires then why not browse the wide range of VCXOs that industry leading manufacturer and supplier of quartz crystals and oscillators, EuroQuartz provide!
What is a VCXO?
As we mentioned in the introduction a VCXO or Voltage Controlled Crystal Oscillator is a device that is used with certain applications where a stable frequency source required, while allowing for the capability for the fine tuning of the signal via an electronic voltage. The basic circuit for most VCXOs comprises of a standard crystal oscillator but with the means of applying an electronic voltage to tune or “pull” the frequency.
How do VCXOs Work?
Like all forms of crystal oscillators, VCXOs utilise vibrations from the quartz crystal to generate a signal, but what makes VCXOs stand out and provide their unique functionality is the use of electronic diodes to fine tune and “pull” the frequency of the signal, which is measured in part per million (ppm).
The “pulling” of the frequency is usually accomplished by placing varicap or varactor diodes on to either side of the crystal, here the diodes then serve as a variable capacitor, applying a voltage across the diodes, and therefore “pulling” the signal as desired. This process is most commonly referred to as controlled voltage, with the change in frequency being referred to as deviation. Mapped against each other the control voltage and deviation can be graphically recorded and interpreted in the form of Transfer Function or Slope Polarity.
But it is not just the amount of controlled voltage that can influence the degree by which the VCXO frequency can be altered and tuned, as other factors such as the type of crystal used and the VCXO circuit design can all play a part. The higher the “pulling” of the frequency the higher the tuning range is, and this isn’t always a positive property, as the process of “pulling”causes the frequency to become unstable which may increase phase noise, meaning that for the most stable frequency it’s essential to limit the level of pullability applied via the controlled voltage.
Another concern that should be taken into consideration when using VCXOs for your applications is the risk of temperature drift, here due to the drift over the varying levels of controlled voltage it can’t be optimised for all the levels, therefore, compromising the final design frequency and subsequent readings. This, however, can be overcome by incorporating temperature compensation aspects into the application to offset the possible drift.
So there you have it, a comprehensive guide to just what is a VCXO and how and why you may require them for your broadcast or telecommunication applications, if you’d like to find out more about VCXOs why not browse EuroQuartz full range or speak to one of their experts about your application and let them help you find the perfect Crystal Oscillator for your requirements.

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