Back-to-Basics - Specifying quartz crystals
Posted: 5th May 2021
Today the usage of timing devices increases with each new technological breakthrough. Gradually, more engineers become responsible for specifying crystal parameters for their electronic designs. This drives the need to better understand crystals. In this blog, we will cover the key crystal parameters to consider when designing an application.
The frequency of a quartz crystal is a measure of the number of oscillations per second the piece of quartz will resonate at, i.e. 10 MHz means the quartz is vibrating at 10,000,000 times a second! This frequency changes with the thickness of the quartz blank. The thinner the blank, the higher the frequency will be.
The frequency required is generally application dependant and is considered in conjunction with the IC that the device is used with, e.g. GPS applications typically work with 26 MHz.
The correct operating frequency must be chosen, particularly when systems need to exchange data. If the incorrect frequency is selected, then systems may be unable to ‘communicate’ to each other or lose data.
Historically, quartz-based devices were usually made using through-hole metal packages. However, with the arrival of surface mount technology, devices reduced in size and construction changed to ceramic-based packages.
Usually, the crystal is housed in an industry standard package size. These range from 7.0 x 5.0 mm packages down to 1.2 x 1.0 mm packages. Nowadays, there are even crystals that are housed in a 1.0 x 0.8 mm package.
It is also worth considering that the frequency range will be reduced when using smaller packaged timing devices. For more information, read our blog: Finding the perfect size – considerations when choosing the size of your timing device.
3. Frequency tolerance
The frequency tolerance defines the accuracy at room temperature (25 °C) and is set during the manufacturing of the crystal.
It is important not to over-specify very low values of frequency tolerance as this affects the unit cost and the accuracy achieved in the application circuit when in conjunction with low values of load capacitance.
4. Frequency stability
Frequency stability is a measure of how the frequency changes with temperature, and the performance obtained varies with the cut angle of the quartz used.
In Figure 1, the frequency behaviour over temperature for different cutting angles is shown.
Figure 1: Frequency behaviour over temperature graph
Frequency stability is one of the most important and application dependant parameters aside from nominal frequency. The performance of an electronic system will depend upon the overall stability value and can range from wide values of ±100 ppm down to ±10 ppm.
If you require tighter stability, one of our other timing devices such as temperature compensated crystal oscillator (TCXO) or oven controlled crystal oscillator (OCXO) could be a better choice for you.
5. Operating temperature range
The operating temperature range is specified in conjunction with the frequency stability as the two values are related. The temperature range is also application dependant ranging from commercial use, at for example -20 to 70 °C, through to automotive use at -40 to 125 °C.
It is essential not to over-specify the temperature range or stability as this can have cost implications.
A quartz crystal specified at -20 to 70 °C will still operate at wider temperature ranges but will not achieve the same stability performance.
6. Load capacitance
The load capacitance is the capacitance value that the quartz crystal needs to see in the oscillator circuit to achieve optimum frequency accuracy at room temperature.
If the wrong load capacitance value is presented to the crystal, it will pull the frequency away from its nominal value and, at worst, cause the frequency to be out of tolerance.
It is also important to note that specifying low load capacitance values makes the crystal more pullable, meaning the frequency will deviate more. This makes it more challenging to achieve nominal frequency accuracy.
For more information about load capacitance, pullability and choosing the right capacitor values, read our blog posts: Myths around load capacitance – how to choose the right capacitors and How a few picofarads can ruin your timing accuracy.
To reach design goals, other parameters may need to be considered in addition to the standard parameters; these include equivalent series resistance (ESR), ageing and drive level.
For more information on these additional values for an application, please contact our Applications Support team.