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It’s about time

By Chuck Newcombe

I awoke this morning at about 5:00 a.m. (give or take daylight savings time). I’ve done that for years, and without the aid of an alarm clock.

As I understand it, our bodies are tuned to a natural circadian rhythm, just a little longer than 24 hours. It is generally believed that we adjust our internal clocks to the slightly shorter 24 hour cycle each day based on the daylight to night cycle of the earth as it spins on its axis in the solar system. I regularly stressed this system as my internal clock was confused repeatedly by my travels around the globe during my years at Fluke, but given enough time, it somehow always readjusted to the environment. It’s truly an amazing system.

Humans have long used the 24 hour solar cycle as the basis for our clock-based time measurements, dividing the day into hours, minutes, and seconds. And, technology has advanced to the point that today, even inexpensive modern wristwatches can easily maintain an accuracy of ±1 minute/month. That equates to an accuracy of about ±0.002%.

This accuracy is based on the mechanical vibration of a piezoelectric crystal in an electronic oscillator circuit. That circuit produces a frequency of 32,768 Hz, and that frequency is divided by 2, the result divided by 2, etc., sixteen times to produce the “ticks” that represent seconds in the watch.

The world’s most accurate clock, maintained by the National Institute of Standards and Technology (NIST), is so finely tuned it demonstrates that your head is older than your feet.

Using time to navigate the world

John Harrison, a Yorkshire carpenter and experimenter, invented in 1761 a mechanical clock, called a marine chronometer, that overcame gravitational and centrifugal force effects of a ship rolling at sea to provide the time accuracy necessary to precisely navigate in longitude using celestial measurements. Precise, in those days, meant something like several tens of miles. »

We’ve come a long way since then. Today, there are 31 satellites in orbit about 12,500 miles above the earth’s surface, operating as the space segment of the Global Positioning System (GPS). A United States Air Force group maintains the control segment of this system. Among their duties is the precise coordination of time settings for the satellites to within a few nanoseconds of each other, allowing users to potentially achieve theoretical accuracies in position down to a foot or so.

Since the receivers in a GPS system don’t have atomic clocks in them, clock error correction techniques ensure that their time information will be correct to within at least a microsecond. This level of accuracy can determine position to better than 1000 feet based on arrival times of signals from 6 to 12 of the satellites (depending on a user’s position and that of the satellites). As a result, besides knowing their positions, users also have access to precise (to the second) local time information as a secondary benefit.

Managing networks, such as cell phone systems, worldwide banking computer network transactions, and smart grid power transmission switching, requires precise time information, approaching the resolution and accuracy of GPS. Toward this end, the Fluke 1760 Power Quality Monitor can be equipped with a GPS receiver to synchronize recorded three-phase power events.

Timing in Fluke test tools

The technology used today in a wristwatch is at the heart of the internal timing of the computing processes in a Fluke digital multimeter (DMM). Besides providing the timing for the microcomputer, it is the basis of a measurement mode that allows you to measure frequency using the model 289 - from a 100 Hz range with 0.001 Hz resolution, to 1 MHz with 10 kHz resolution. Related uses of timing information allow measurement of waveform duty cycles and pulse widths.

The timing of industrial and commercial processes can be critical to accurate, efficient and safe operations.

Take, for example, the switching of ac power from line to UPS standby sources. If you are to maintain proper operation of computing and timing systems, it’s a good idea to make the switch to a synchronized source in less than one-fourth of a 60 Hz cycle - about four milliseconds. You can easily analyze the process by using any two-channel Fluke ScopeMeter test tool. This example uses relative time measurements and doesn’t require absolute (world clock) time information. Similar, more challenging timing measurements may be needed when troubleshooting the digital circuitry in a modern automobile.

Finally, the top-of-the-line Fluke 190 Series ScopeMeter tools are up to many of the most stringent relative timing measurement tasks using the fastest sweep range of five nanoseconds per division. With a sampling rate of 2.5 billion samples per second, a Fluke 199c digital oscilloscope can resolve a 1 nanosecond difference between signals in a single sweep.

I think back to the complex processes I had to implement in the 1960s to maintain oven stabilized crystal oscillator lab frequency standards in multiple labs a few miles apart using the oscilloscope technology of the day. A Fluke 190 Series ’scope would have reduced hours of complex test procedures to a few minutes at the most.

Maybe the good old days weren’t so good after all.