Quartz Crystal Stability: How myths and misconceptions mask good value...
Over the years we’ve had many requests for quartz crystals that operate from -40 °C to +85 °C with ±10 ppm stability, despite the fact that nearly all of the applications discussed with our customers didn’t need to operate over this industrial temperature range. It seems that the perceived requirement does not arise from considering the application but from the data sheets of semiconductor manufacturers; the chips are specified to operate from -40 °C to +85 °C and reference designs then suggest that crystals, and other components, should be capable of doing the same.
Now, it’s possible to buy crystals that meet the specification described but the fundamentals of crystal manufacturing make them prohibitively expensive for all but the most demanding applications. Here’s why.
Large stones are cut into small wafers to make crystals and the operating frequency of a crystal is invariably quoted at 25 °C. We vary the temperature characteristics of the crystal by changing the angle at which we cut the wafer.
The graph below shows that to achieve high stability together over a temperature range limits the number of angles at which we can cut the crystal. If you only need a narrow operating temperature range or can tolerate relatively large changes in frequency over temperature, perhaps ±40 ppm, it makes crystal cutting easy. But extending the temperature range or tightening the stability requirement limits the choice of cutting angles.
