Crystals

Overview

Crystals (also called by the more generic term oscillator, although oscillators are also used to refer to other “oscillating” components) are electrical components which output a periodic waveform that can be used as a clock source for digital logic (which includes microcontrollers, it’s main use).

I have also seen them being called “SMD Tuning Forks”. WTF? I’m putting this down to a bad to-English translation.

Schematic Symbol

Unfortunately, many different designator prefixes are used to represent a crystal. Some of the most common are:

• XC
• XO
• OCXO (oven-controller crystal oscillator)

Important Parameters

Sorted alphabetically.

Accuracy

Like stability, this is measured in ppm.

Combined with the stability, this gives you the total error you can expect from a crystal operating over a particular temperature range.

Frequency

Arguably the most important parameter, this is the nominal frequency of the waveform the crystal generates.

Typical values: 20kHz-48MHz

Lead Capacitance

Typical Values: 8pF-20pF

Operating Mode

Most crystals operate in the “fundamental” operating mode, that is, they are running at their fundamental frequency.

Operating Temperature

The operating temperature is the temperature range in which the crystal is guaranteed to operate within it’s specified tolerances (e.g. stability and accuracy).

Typical operating temperatures are -40 to +85°C.

Power Consumption

The power consumption of a crystal is a measure of how much power the crystal consumes during normal operation. This can be of concern for extremely low power designs.

Typical power consumption for a TH/SMD crystal is around 50uW.

Note that this is the power consumption of the crystal itself, and does not take into account the power consumption of the associated drive circuitry (which can be much greater!).

Stability

Stability is a measure of how repetitive, or stable, the clocks frequency is over time. It is measured in parts-per-million (ppm).

Typical stability is around 30-50ppm.

What The Crystal Output Looks Like…

The following waveform is the voltage on one of a 12MHz SMD crystals pins, when driven by a standard microcontroller.

Oven-Controlled Crystal Oscillators (OCXOs)

High-performance crystal oscillators are kept with temperature-controlled environments to increase the stability of the oscillator. They are called oven-controlled crystal oscillators (OCXOs).

The crystals are designed to have a turning-point, a point of greatest stability, close to the oven temperature. OCXOs, rather than having a temperature stability in the ppm (parts-per-million) range like normal crystals, have a stability in the ppb (parts-per-billion) range (20ppb would be a viable stability).

Peltier devices can be used as the “oven” to keep the crystal’s temperature constant.

Popular Crystal Packages

The HC-49/U package is a popular choice for older through-hole crystals.

Newer crystals come in small, custom SMD packages, with typically either 2 or 4 pins (with the 4-pin packages usually have two GND pins).