Buying crystals: what to watch out for

Many applications call for a watch crystal that resonates at 32.768kHz but why this frequency and what do buyers need to consider when looking at these parts? AEL Crystals provides some answer.

Quartz resonators operating at 32.768kHz were originally designed for use in quartz wrist watches and are used for real time clock applications where a stamp is needed in a product. The specific frequency of 32.768kHz is used as it is easy to divide using standard frequency dividers or binary counters in order to give a signal with a one second resolution.

Quartz crystals of this type commonly use a ‘tuning fork’ quartz blank which looks like a tuning fork. Historically these parts were offered in through hole packages measuring 3.0 by x 8.0mm and whilst these are still readily available, watch crystals are also produced in many surface mount packages ranging from 7.0 by 1.5mm to 1.2 by 1.0mm.

In order to supply parts for a customer’s application there are several parameters that AEL Crystals needs to be aware of in order to offer the correct part.

Know your specs
As mentioned above, parts can be supplied in numerous package styles and sizes, including both through hole and surface mount. You’ll also need to cons

ider the initial stability, which is the tolerance of the crystal’s frequency at 25°C defined in parts per million. The operating temperature range, meanwhile, is the temperature range over which the crystal is expected to be able to operate.

Another important factor is load capacitance. This relates to the capacitance that the crystal is subject to in the circuit. This value is often specified by a chipset manufacturer as they will use the crystal’s ability to change the frequency when this value is varied. Specifying the wrong value here can have a significant effect on the crystal’s frequency in circuit.

Equivalent series resistance is the impedance or effective resistance that a part will present in the circuit. This parameter has become more important with requirements for low power circuits, such as those that are battery powered. If the ESR of the crystal is too high, this can lead to poor start-up in the circuit. Note that the standard ESR value will usually increase as the physical package size reduces. AEL Crystals can offer parts with lower ESR in smaller packages, however, if required.

Is stability an issue?
You may be surprised to see that stability over temperature, which is usually discussed with crystals, is not in the list above. This is due to the specific quartz blank used by watch crystals. This blank has a parabolic temperature stability curve, which is fixed by the physical design of the parts. Frequency will always reduce once temperature is moved from 25°C and can lead to significant frequency changes of over -120ppm for wider temperature ranges.

AEL Crystals increasingly receives requests for 32.768kHz parts that offer a much tighter stability over temperature than that available with standard watch crystals. Fortunately, there are numerous options available to combat this issue.

Quartz based temperature compensated oscillators at 32.678kHz are available with stabilities of ±5ppm over a -40 to 85°C temperature range in 3.2 by 2.5mm packages and more recently MEMS silicon based oscillators have become available offering ±3ppm stability over the same temperature range but in a chip scale 1.5 by 0.8mm package.

All these parts are available from AEL Crystals, along with in-depth engineering experience to help customers achieve a solution that meets both technical and commercial requirements and will work reliably right from the start of the project.