OSCILLATORS

Oscillators

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Overview

Crystals

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). See the Oscillator page for information on powered oscillators.

A schematic of a crystal, usually connected to a microcontroller or other digital device that uses a clock. The load capacitance usually varies from 6-25pF per leg (see the crystals datasheet for the correct value).

A schematic of a crystal, usually connected to a microcontroller or other digital device that uses a clock. The load capacitance usually varies from 6-25pF per leg (see the crystals datasheet for the correct value).

Weird name for a crystal, don't you think? Image from http://www.foxonline.com/pdfs/fsrlf.pdf.

Weird name for a crystal, don't you think? Image from http://www.foxonline.com/pdfs/fsrlf.pdf.

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.

The output voltage waveform of a 12MHz crystal being driven by a standard microcontroller.

The output voltage waveform of a 12MHz crystal being 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).

A photo of an N4A series OCXO. Image from http://www.bliley.com/.

A photo of an N4A series OCXO. Image from http://www.bliley.com/.

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.

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).

Oscillators

This site uses the word oscillator to represent a component with an self-contained oscillating feature that has power, ground, and signal out pins. This site uses the word crystal to represent an component which contains a oscillating element (in the form of a crystal), which requires an external oscillation circuit before it useful.

Designators

A common designator prefix to use for oscillators is \(Y\) (e.g. \(Y1\)). I do not recommend using the prefix \(XC\) as this should be reserved for crystal oscillators.

Important Parameters

Phase Noise

Phase noise is a way of describing the stability of the crystal in the frequency domain.

Start-Up Time

Symbol: \(T_{SU}\)

The start-up time for most oscillators is within the range 2-20ms. This start-up time can be important in low-power designs when the start/stop time of the crystal results in wasted energy.

MEMS Oscillators

MEMS oscillators are built using small mechanical structures (less than 0.1mm in any dimension) that vibrate at set frequencies when electrostatic forces are applied. This mechanical vibratory part of a MEMS oscillator is called the MEMS resonator. This is etched into a silicon die, and surrounding electronics contain both the driving, measuring, and compensation circuitry.

They use less power than a crystal-based oscillator, making them suitable for battery-powered devices. They are manufactured using standard IC manufacturing processes, so they are also more durable. They typically have better frequency stability over their operating temperature range, with common values being 10ppm at room temperature and 100pm over their entire operating temperature range.

MEMS oscillators do not like ultrasonic cleaning baths. Ultrasonic baths may permanently damage the oscillator or cause long term reliability issues1.

Packaging

MEMS oscillators have been made in packages which are also commonly used for crystal packages, such as the 2012 SMD package.

A comparison between MEMS and crystal-based oscillators in CSP and larger 2012 SMD packages. Note how the MEMS oscillator sneaks in two extra pins between the standard 2012 pads for power and ground.

A comparison between MEMS and crystal-based oscillators in CSP and larger 2012 SMD packages. Note how the MEMS oscillator sneaks in two extra pins between the standard 2012 pads for power and ground.

Manufacturer Part Numbers

  • SiT1533AI: SiTime standard clock oscillators and MEMS oscillators.
    • SiT1533AI-H4-D14-32.768G: MEMS clock oscillator.

References


  1. https://www.mouser.com/datasheet/2/371/SiT1533_rev1.4_03202018-1324419.pdf, retrieved 2021-01-18. ↩︎


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