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 which can cause an oscillator to deviate from the correct frequency, but it is not an insurmountable issue, and there are various approaches engineers can take to improve stability. In this article, we will discuss approaches to maintaining a high degree of stability in crystal oscillators.
Stability, Drift, and Why They Matter
“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 is 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 even 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 such as humidity and temperature. Heat caused by the circuit operating for a long period of time can also negatively affect stability since 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. One of the primary challenges related to achieving lasting stability in crystal oscillators is the fact that in many cases, an oscillator may have been in use for years before signs of drift begin to appear, and it could continue going on for a long time before the drift is detected.
One 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 other 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 systems, however, the stakes are much higher. Think about all the hype surrounding self-driving cars; stability will be 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, this is not an ideal world; there is no such thing as a perfect oscillator that remains 100% stable at all times. But steps can be taken 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 (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.
Periodic calibrations of the crystal oscillator are a good idea— 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 types of oscillators is that they typically consume much more power than conventional oscillators. This has led to the development of a new generation of high-performing, low-power crystal oscillators that maintain strong, stable frequencies with much lower power requirements than previous designs.
Bliley Technologies is proud to play a leading role in maintaining stability in crystal oscillators. No matter what your project is, our designs are a perfect fit for many applications in markets ranging from defense and telecommunications to aerospace applications, and much more. Check out our series of world-class oscillators and clocks and let us provide real solutions to your frequency stability issues.