Quartz Crystal Stability: How myths and misconceptions mask good value...
Over the years we’ve had many requests for quartz crystals that operate at -40 °C to +85 °C with a stability of ±10 ppm. When discussing the intended use for such crystals with our customers, nearly all of their applications didn’t actually need a crystal that would meet this specification, but confusion was being caused by the semiconductor datasheets and chipset reference designs, which suggested that crystals and other components used should be capable of meeting a stability of ±10 ppm at -40 °C to +85 °C.
It is possible to buy crystals at this specification, but the fundamentals of crystal manufacturing make them prohibitively expensive for all but the most demanding applications.
Large stones are cut into small wafers to make crystals. The temperature characteristics of crystals are varied by changing the angle at which the wafer is cut. The wider the temperature range and the tighter the stability requirements, the more limited the choices become for the cutting angle.
The graph below shows these limitations to the cutting angle:
Even when using the most appropriate cutting angles, the yield of crystals across the -40 °C to +85 °C temperature range that will stay within the ±10 ppm stability specification is small.
Consequently, the cost of these crystals is relatively high – prohibitively so for many commercial, consumer, and IoT (Internet-of-Things) applications.
It is possible to source cheap crystals in distributor catalogues that claim ±10 ppm stability across -40 °C to +85 °C, but we’ve bought a selection of these components, tested them exhaustively, and are yet to find a single low-cost crystal that fully meets the specification; some get close, but they were always outside the ±10 ppm specification at the lower end of the temperature range and, in some instances, there were occasional failures at the higher end, too.
It’s important to bear in mind that some manufacturers simply don’t have the facilities to test crystals accurately and rely on quoting approximations.
To reiterate, you can buy crystals with ±10 ppm stability at-40 °C to +85 °C, but to guarantee purchasing a crystal that actually meets that specification, it’s going to be a relatively expensive component, and rarely one that’s optimal for your application.
Our advice is always to think about your individual application first, rather than relying on the reference designs of chipset manufacturers, which are by their very nature generic, and often inaccurate.
Talk to our Engineering and Application Support Team, who will always point you towards the most economical crystal or oscillator for your design.
We’ve been in the crystal business for over 50 years and will only ever provide accurate, honest specifications that are proven in exhaustive test procedures.
