Design engineers in many industries rely on a variety of oscillators in their work. From simple RC circuits to temperature-controlled crystal oscillators, they’re often used in electronics to provide a base frequency reference and/or synchronise activity.
Commonly found in clocks and radios, crystal oscillators control communication and speed. These are made with crystal quartz due to its mechanical and chemical stability, and its natural ability to regulate frequencies.
The specific frequency that quartz crystals can generate is obviously important, but the accuracy they afford in oscillators is vital for exact timekeeping and communications – especially in portable battery-powered equipment.
Accuracy and stability are usually measured in parts per million, or ppm. To find out how accurate crystal oscillators are, we’ll need to discuss how they work, how their stability can affect their accuracy, and which factors can cause frequencies to age and shift.
How do crystal oscillators work accurately?
Natural mineral quartz is a crystalline form of silicon dioxide, which is often synthetically produced to keep up with demand. Though the material doesn’t conduct electricity itself, crystal quartz is Piezoelectric. This means that applying a voltage across the crystal face will cause the crystal structure to vibrate, reversing the voltage.
Pressure and an AC voltage will mechanically distort the crystal, causing it to vibrate at a consistent rate as it returns to its original shape. This sustained and stable resonation allows for accurate timekeeping when used as a reference in a wider circuit.
When different frequencies or vibration modes are required, quartz crystals can be optimised by cutting them to specific shapes and sizes. Once you cut the crystal to the desired dimensions, it will become a stable source for the appropriate frequency. They generally span a range of 10kHz to 10 MHz, though crystals with higher frequencies are available for higher prices.
Crystal quartz is a passive component, so it must be combined with any of the various types of oscillator circuits to reach higher and more accurate frequencies. Its mechanical properties mean that even standard quartz crystals are accurate and stable enough to maintain their frequency for a long time with little variation.
When higher degrees of accuracy are essential for a design, high-performance crystal-based oscillators are used to control and adjust the frequency to the necessary level, even if it varies. These include temperature compensated crystal oscillators (TCXOs), voltage controlledcrystal oscillators (VCXOs), oven controlled crystal oscillators (OCXOs), and phase-locked-loop systems (PLLs).
Though engineers would love it if there was a crystal oscillator that continued to transmit the designated frequency without deviating for its entire lifespan, this unfortunately will never exist. Factors such as changing temperatures will inevitably cause its accuracy and stability to degrade.
What can affect the accuracy of a crystal oscillator?
Oscillator modules are far more accurate than basic circuits, but many variables can cause the crystal to age and the frequency to drift. This includes the quality and purity of the quartz crystals, humidity and temperature, radiation, and shock from overloading or alternating the voltage.
Packaging and housing with an airtight seal can fend off environmental humidity and pressure, but temperature changes are still a slight issue. Extreme temperature shifts can cause frequencies to drift by dozens of ppm. Some devices include cooling fans to avoid this, but this isn’t always practical or possible in many applications.
Using an oscillator system specially designed to control temperature can minimise this risk. For example, TCXOs and OCXOs use sensors to automatically adjust and maintain the frequency. It’s common for crystal oscillators to achieve an accuracy within 50ppm and a stability of around 50ppm/°C (per Centigrade). When the temperature and/or voltage is regulated, this can promise variations as small as 1-2ppm per day, which can easily be corrected.
It’s important to remember that while accuracy and stability can affect each other, and both be affected by the same external factors, they aren’t the same thing. An oscillator can produce a stable but inaccurate frequency, or inconsistently produce an accurate frequency. You’ll need to know both of these figures to understand how much variation you can expect.
Variations in crystal oscillator performance can be measured using a frequency counter or timing interval analysing equipment. This allows you to record the standard deviation over a specific output period, and analyse how near-term variations will affect long-term performance accuracy. Short-term variations are known as jitters or phase noise.
Information on these is crucial to understand how to correct the frequency and slow down the ageing and shifting of the crystal (if it can’t be completely reversed). Too much unchecked variation could prevent the crystal from returning to its original frequency. Even if the frequency only drifted by 1ppm each day, after a week or so you would have a very inaccurate device without adjusting the oscillator.
Can you rely on crystal oscillator accuracy?
As we’ve explained above, crystal quartz is already a very stable material, able to produce and sustain a frequency with a high degree of accuracy. It’s the particular applications of the crystal in oscillator form and its ambient conditions when in use that can cause this natural accuracy to gradually decrease over time.
However, there are ways to keep frequency shifts so small that they’re barely noticeable, as with TCXOs, OCXOs, and VCXOs. The majority of potential applications for crystal oscillators are tolerant of minor variations, where an accuracy within up to 5-10% of the nominal frequency is acceptable in most cases. Just make sure that when choosing and operating oscillatorslike this, their specifications and possible variations are in line with the intended use and results.