Column: Design with frequency components


systems, wearables and IoT nodes, meters, loggers, clock and industrial control, smartphones and tablets, and automotive electronics, among many others. A single crystal is usually used when


the MCU has an internal RTC oscillator, time accuracy is not so critical (e.g., in toys), the device does not enter the sleep mode, and high accuracy time stamping is not required.


Component selection Here we will determine what must be considered when selecting a tuning fork crystal. Besides the regular parameters that


are important for a good design with crystals, which we addressed in the previous articles in this series, there is one particular aspect to be considered for the tuning fork crystal – temperature stability. While -40°C to +85°C is a standard


today, requirements for +105°C and even +125°C are no longer uncommon. Although these temperatures are not a problem for a normal quartz crystal, special attention is required when designing with 32.768kHz crystals. Figure 1 shows that the thermal


curve of a tuning fork crystal is totally different to the curve of a regular AT-cut crystal. The temperature response (frequency


downwards with a vertex at T0


deviation over temperature) of a tuning fork quartz is described by the formula: Deviation [ppm] = –PC [ppm/°C2 , which is a parabola that opens = +25°C


] × (T-T0)2


±5°C (reference temperature), where PC is the parabolic coefficient; see Figure 2. From Figure 2 it can be seen that


the quartz always lags behind, as soon as the operating temperature deviates from the reference temperature, and the further away from +25°C, the greater the deviation. At +125°C, this is typically –350ppm in this case; when all tolerances (e.g., load capacities) are considered, it can be significantly higher. This may not matter in applications


where a clock crystal is only keeping a controller alive in standby. However, if


The choice of a crystal with greater accuracy does not generally bring the desired success, since the specification of the basic accuracy refers only to +25°C


the application relies on a precise time base or RTC, even the smallest time errors can add up enormously over the year – several tens of minutes are not uncommon! The choice of a crystal with greater


accuracy does not generally bring the desired success, since the specification of the basic accuracy refers only to +25°C. The basic curve therefore does not change. A solution by hardware – e.g., an


adjustment of the circuit by means of adapted load capacitances – is only promising in a narrow temperature range; for example, a wristwatch. So, what is important after all for


obtaining a good design? The answer is: the choice of the component according to the checklist (Table 1) and a good design verification mechanism.


Design verification for tuning fork crystals For applications that depend on an accurate time base or require precise timing over a longer period of time, e.g., metering/consumption recording, and so on, it is essential to compensate for the typical temperature curve using software (e.g., look-up table) and, at the same time, to synchronise the time at regular intervals with a master of some kind (data collector, radio clock, human). Only with these measures can the correct and reliable functioning of the application be guaranteed. To determine the advantages of lower


load capacitance crystals, two clock crystals of 12.5pF and 7pF have been compared; see Table 2.


However, one disadvantage


should not be ignored: Due to the low load capacitance, the oscillator circuit with the 7pF crystal is more sensitive to component tolerances of the circuitry. So, if power is a concern, use a quartz with a low load capacitance. For applications with higher precision use a quartz with a higher load capacitance.


Tuning fork crystal alternatives For higher temperature stability requirements, one can use temperature compensated 32kHz oscillators as an alternative, either in the 3.2 × 1.5 mm footprint or as a regular HCMOS oscillator, such as 1.6 × 1.2 mm, for example. Table 3 shows that an alternative


crystal oscillator with a current consumption of only 1µA has a much lower frequency deviation over the temperature and operating voltage range (Figure 3) than a clock crystal and may therefore be the better choice for some applications.


Not a trivial matter Design with tuning fork crystals is not trivial and takes more than just a few approaches presented here. However, with the right choice of a tuning fork crystal and, if necessary, the correct temperature compensation and master clock synchronisation, along with good engineering support, you will see the crystals “tick” reliably even in the extended temperature range.


www.electronicsworld.co.uk December 2025/January 2026 22


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