One of the core challenges any RF engineer faces is maintaining frequency stability in crystal oscillators. In an ideal world, an oscillator would transmit at the desired frequency indefinitely and without deviation… but of course, we don’t live in an ideal world, and frequency drift is a real problem in many applications.
There are a number of factors that can cause an oscillator to deviate from the correct frequency, but frequency drift is not an insurmountable issue. There are various approaches engineers can take to improve stability. In this article, we'll discuss the best approaches to maintaining a high degree of stability your in crystal oscillators.
Frequency Stability, Drift, & Influencing Factors
Stability is defined in this context as the ability of an oscillator to maintain a consistent, fixed frequency over a given span of time. The more stable an oscillator is, the more reliable it's considered to be, and in applications like communications and radar, stability is of utmost importance. The loss of stability in crystal oscillators manifests itself as “drift”, a phenomenon which can degrade performance and cause many technical and legal problems, depending on the application.
In RF engineering, the term drift is used to describe changes in the accuracy of a signal over time, which may be caused by environmental factors like humidity and temperature. Heat caused by the circuit operating for a long period of time can also negatively affect stability because it can alter the values of components like inductors, resistors, and capacitors. Other factors include variations in the operating voltage of an oscillator, mechanical vibrations, and more.
Related: 4 Top Factors That Influence Crystal Oscillator Frequency Stability
Challenges Caused by Drift
One of the primary challenges related to achieving lasting stability in crystal oscillators is the fact that in many cases, an oscillator can be in use for years before signs of drift appear, and drift can continue going on for a long time before it's detected.
A simple example of drift that most people have encountered is when the frequency of an FM or AM radio station drifts into an adjacent frequency, causing interference with broadcasts from other stations. This is annoying for listeners and could result in issues with regulatory bodies (like the FCC) for the station that has failed to address their frequency stability issues, especially if it has drifted into a bandwidth in which it is illegal to operate.
In the grand scheme of things, that’s a pretty minor example of the kind of problems that a lack of frequency stability can cause. When you’re dealing with complex communications, radar, and positioning, navigation and timing (PNT) systems, the stakes are much higher. Think about all the hype surrounding self-driving cars; stability is of vital importance for these systems because significant drift in the oscillators used to guide these vehicles could have potentially fatal consequences.
Solutions for Maintaining Stability in Crystal Oscillators
As mentioned above, there is no such thing as a perfect oscillator that remains 100% stable at all times. But you can take steps to improve stability and minimize the damaging effects of drift.
The right approach to take will vary in each case. Making use of swamping capacitors and regulating the power supply to the circuit, for instance, can help mitigate drift and maintain stability. Isolating the circuit from mechanical vibrations is also a simple and effective way to prevent vibrations from throwing off oscillator stability.
But the best way to maintain stability in crystal oscillators and minimize drift is to pick the best design for your application in the first place.
Some form of temperature control or compensation can prevent ambient temperatures (as well as heat generated by the circuit components) from negatively impacting stability. High-quality temperature compensated crystal oscillators (TCXO) and oven controlled crystal oscillators (OCXO) produce much more stable frequencies than crystal oscillators that are exposed to ambient temperatures.
Engineers should also make sure that the cut of the crystal itself is optimized for internal operating temperatures. It's also a good idea to periodically calibrate crystal oscillators — how often this is required will vary depending on the application, but every 6-12 months is a common schedule.
One of the downsides of these more stable oscillators is that they typically consume much more power than conventional oscillators. This has led to the development of a new generation of high-performance, low-power crystal oscillators that maintain strong, stable frequencies with much lower power requirements than previous designs.
Related: Crystal Oscillators: The Beginner's Guide (OCXO, TCXO, VCXO, & Clocks)
Need to Ensure Frequency Stability?
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