Back-to-Basics - Specifying quartz crystals: additional parameters to consider for your application
Posted: 27th May 2021
In the last blog, we looked into the main parameters to consider when designing-in a quartz crystal in your application. This time, we will focus on other parameters to extend the understanding of quartz crystals and support designing electronic solutions.
Equivalent Series Resistance (ESR) is a measurement of the resistance of the crystal at series resonance and it is measured in Ohms.
A quartz crystal can be represented by an equivalent circuit comprising of shunt capacitance (C0) in parallel with a series arm consisting of motional capacitance (C1), motional inductance (L1) and motional resistance (R1). This is shown in Figure 1. The value of R1 represents the internal ‘losses’ in the quartz crystal.
Figure 1: equivalent circuit diagram for a quartz crystal
For general use, it is normal to use ESR and R1 values interchangeably, but they do have a mathematical relationship when looked at in detail.
Engineers must note that as the size of the quartz crystal gets smaller, the maximum ESR value generally increases. Therefore, the ESR value of a quartz crystal is an important value to consider during the design stage. If for example the ESR value is too high, the oscillator circuit may not have enough gain to overcome the crystal losses and may fail to start.
Note also that ESR values are usually higher in strip-crystal designs compared with traditional round quartz designs, and so even if using the exact electrical specification and frequency in two different package styles, this difference has to be taken into account during the design phase.
The maximum ESR is stated on datasheets. In particular, this information is listed under the Electrical Parameters section of IQD datasheets.
For more information on ESR, read our blog: Does size really matter? – Design effect of crystal downsizing.
Ageing is the change in crystal frequency over time. This effect should always be taken into consideration whenever a crystal is used for an application. There are two main causes of ageing in quartz crystals; mass-transfer, due to outgassing from the materials used to mount the crystal and seal the device; and the other due to stresses from the manufacturing process.
Ageing is an important aspect to take into consideration when selecting a device for a product. The design engineer needs to be mindful of the ageing characteristics of the selected device and ensure that the overall frequency does not stray beyond desired operational parameters during its operation.
Failure to consider ageing may result in a shorter than expected life cycle and could result in product failure in the field.
For more information on quartz crystal ageing, read our application note here
3. Drive level
The crystal drive level is the power that is dissipated by the crystal and is usually specified in micro-watts or milli-watts. However, exceeding the maximum recommended drive level can cause degradation in performance and shorten the crystal's life span. The maximum drive level is stated on the crystal datasheet.
The size of the quartz crystal also determines how much power can be dissipated, and the general rule is that smaller quartz devices have lower maximum drive levels. It is important to remember that if an oscillator circuit re-design is required and the new crystal chosen is smaller than the original device, the drive level applied to the crystal may need to be adjusted.
For more information on key parameters to consider when designing an application, please contact our Applications Support team.