Back-to-Basics-Specifying Quartz Crystal

30 Dec.,2024

 

Back-to-Basics-Specifying Quartz Crystal

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.

 

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  • Frequency

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.

  • Package

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.

  • 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.

  • 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.

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.

  • 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.

For more information, please visit quartz crystal resonator.

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.

  • 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: 

At what frequency would a quartz crystal shatter?

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That would be difficult, and so violent if achieved that other things would break too.

Just about all electronic watches use 32,768 Hz crystals. A special shape is used for these crystals. A chunk of quarts suitably small to put in a watch would resonate in the MHz range if it were just a block. The breakthru that allowed for accurate electronic watches was to shape the quarts like a tuning fork. That allows for much lower resonance frequency from a small amount of quartz.

Imagine trying to get a ordinary tuning fork vibrating at high amplitude just by shaking the container it is mounted in. That wouldn't work very well because the oscillations of the tuning fork are differential mode whereas your shaking can only be common mode. Everything won't be perfectly balanced, so some fraction of your common mode signal will be differential, which will cause the tuning fork to resonate. However, the package will have to be subjected to quite strong shaking to get a strong response from the tuning fork. Quartz crystals are more fragile than steel tuning forks, but the same principle of common mode versus differential mode still applies.

Another problem for your scenario is that crystals are often deliberately mounted with a little flexibility. This helps reduce frequency errors from ordinary bumps and vibrations.

By the time you get the crystal resonating at 32,768 Hz hard enough to break it, you have probably already obliterated the watch it was mounted in.

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