Precise Time-Keeping: The Crystal Oscillator, the Atomic Clock, and the Quantum Clock
Time is really important in electronics, being used in other measurements including the volt. Depending on the application, different ways of time-keeping may be required for electronic devices. Here’s a look at a few ways of time-keeping, and their applications.
Have you ever wondered how time-keeping is achieved on hardware? If you didn’t have a watch would you know how long a second was? Where does the definition of a second even come from? This article will tackle these questions and more.
Time-keeping can be a really interesting topic and a rather peculiar art. We all know that it takes 24 hours for the Earth to complete a full rotation, that each hour is 60 minutes, each minute 60 second… and so a second must be the time it takes for the Earth to make 1/(24*60*60) = 1/86,400th of a rotation. However, it actually takes a bit longer than 24 hours to make a full Earth rotation, which is why we have leap years to occasionally adjust for the time drift. In addition, many other factors cause variation in this exact movement, making the rotation of the Earth itself a poor standard to measure time against.
So how do we measure time? Fortunately, there are more reliable, precise natural occurrences that can be used to standardize time. In 1967, it was decided that one second equates to 9,192,631,770 cycles of radiation of a cesium-133 atom.
Time is really important in electronics, being used in other measurements including voltage. Depending on the application, different ways of time-keeping may be required for electronic devices. Here’s a look at a few methods of time-keeping and their applications.
Quartz Crystal Oscillator
Quartz crystal oscillators (QCOs) are found commonly in microcontrollers, oscilloscopes, watches, and other consumer electronics. Just as the name implies, this component is usually made out of quartz crystal, which is both chemically and mechanically stable. It’s difficult to deform permanently without breaking, and doesn’t experience much in terms of hysteresis—when some force causes it to vibrate, it begins and stops quickly.
A 4MHz quartz crystal oscillator component. Image courtesy of Core Electronics.
In a QCO, when an electric field is applied to the crystal, it will mechanically distort in response, producing a piezoelectric effect (and vice-versa). The crystal will vibrate at a consistent rate when returning to its original shape, and can be used as a reference for time-keeping. Depending on how the quartz is cut, different modes of vibration can be achieved. Depending on which frequency or mode desired, different types of circuits can be paired with the crystal. For example, the overtone mode can help reach higher frequencies, which can be achieved by pairing the crystal oscillator circuit with LC circuits.
Many factors can impact the QCO’s stability: the cut of the crystal, the quality and purity of it, the temperature, humidity, even radiation. Shock can also change the frequency permanently. Over time, the frequency of crystals will also drift due to aging
The cesium atom, which is used to determine the duration of a second, is an example of an atomic clock. The atomic clock is based off the frequency of the energy transition of electrons when an atom at absolute zero. In a chamber, gasses react and oscillate to the change of state in the atom, and so must be tuned to not only detect as many state changes as possible, but also to filter out other noise from things like temperature changes. Eventually, the correct frequency will be reached and then can be used as a time reference.
The clock used to measure one second at the NIST drifts 0.03 nanoseconds per day, with a one second drift in 100 million years. So, fairly stable and precise.
The first atomic clocks were rather large and bulky, but in 2004 a chip-sized atomic clock was introduced. It was low-powered and compact enough to be operable on battery powered devices.
Atomic clocks are used in a lot of time-sensitive application requiring high precision. For example, world time is kept by several atomic clocks across the globe. They occasionally will check for consensus and adjust to match the majority.
Atomic clocks are also used on satellite navigation systems such as GPS. Without precise time-keeping, calculations for the location of a GPS receiver would be off.
This year, the Deep Space Atomic Clock (DSAC) will be launched for testing in low Earth orbit. The DSAC will use mercury-ions, which should not drift more than a second for a billion years. The DSAC will be used for deep space radio navigation, with plans for eventual use in real-time navigation by humans.
Deep space Aaomic clock. Image courtesy of NASA.
Next Generation Atomic Clocks: The Quantum Clock and Optical Lattice Clock
Other clocks have been developed and experimented with, providing more robust time-keeping.
The quantum clock is a type of atomic clock where single ions are cooled with a laser and contained within an electromagnetic trap. These clocks typically use either aluminum or mercury, and use a UV laser to achieve much higher frequencies of vibrations. Quantum clocks are more precise than atomic clocks, and are not affected by temperature or background noise from electric and magnetic fields. Interestingly, it’s not possible to determine if it is more accurate in its measurement of the second, since the standard using cesium-133 can’t be used to measure something more precise than itself.
Optical lattice clock. Image courtesy of the University of Tokyo.
Even more accurate than the quantum clock is the optical lattice clock. This clock would not drift more than 100ms over the lifespan of the universe. The clock is composed of a 3D lattice which hold strontium atoms. A laser is used to induce a vibration in the atoms, which are cooled to 15 nano Kelvins, and will remain coherent for up to 15 seconds.
Time keeping is essential to hardware functionality. What experience do you have with challenges arising from time keeping issues? How about time keeping in device design? Share your thoughts in the comments below.
Several errors: “which is why we have leap years” No, this is why we have leap seconds. Leap years are necessitated by how long it takes the earth to go around its orbit - about 365.25 days. Also, the length of day is rather variable when measured accurately: day-to-day variations are tens of microseconds, longer or shorter.
Cesium clocks do not operate at (or even close to) absolute zero.
“world time is kept by several atomic clocks across the globe” actually by MANY clocks at various national laboratories which are intercompared and averaged. NIST has several 10’s themselves.
The earth rotates once every sidereal day - 23 hours, 56 minutes, 4 seconds approximately. Because it is moving around the sun, it takes another 236 seconds to get the sun in the same position in the sky. Even this is variable as the earth is in an elliptical orbit and moves faster around the sun in January than it does in July. Since the passage of time varies with gravity (or distortion of space) there really is no constant time. Measurements are so precise these days that changing the height of measuring equipment by a meter makes a measurable difference. I worked with a Cesium beam clock in 1967 and the cesium was vapourised to a gas to make the measurements. It required compensation for the Zeeman effect, the atomic transitions affected by magnetic fields. Crude by today’s standard.