1.2 WHAT ARE TIME AND FREQUENCY?

We should pause a moment to consider what is meant by the word "time" as we commonly use it. Time of day or date is the most often used meaning, and even that is usually presented in a brief form of hours, minutes, and seconds, whereas a complete statement of the time of day would also include the day of the week, month, and year. It could also extend to units of time smaller than the second going down through milliseconds, microseconds, nanoseconds, and picoseconds.

TIME OF DAY:

9:00 A.M.

5 MINUTES TIME INTERVAL

We also use the word time when we mean the length of time between two events, called time intervals. The word time almost always needs additional terms to clarify its meaning; for instance, time of day or time interval.

Today time is based on the definition of a second. A second is a time interval and it is defined in terms of the cesium atom. This is explained in some detail in the chapter on atomic frequency sources (chapter 11). Let us say here that a second consists of counting 9,192,631,770 periods of the radiation associated with the cesium-133 atom.

The definition of frequency is also based on this definition. The term used to describe frequency is the hertz which is defined as one cycle per second.

What does all this mean to users of time and frequency? Where does the laboratory scientist, industrial engineer, or for that matter, the man on the street go when he needs information about frequency and time measurements or about performing those measurements himself? That's what this book is about. It has been written for the person with a casual interest who wants to set his watch and for those with a specific need for frequency and time services to adjust oscillators or perform related scientific measurements.

This book has been deliberately written at a level which will satisfy all of these

You can get time of day by listening to high frequency radio broadcasts, or you can call the time-of-day telephone services. You can decode time codes broadcast on various high, low, and very low frequency radio stations. You can measure frequency by accessing certain signals on radio and television broadcasts. You can obtain literature explaining the various services. And if your needs are critical, you can carry a portable clock to NBS or the USNO for comparison. This book describes many of these services and explains how to use them for time and frequency calibrations.

1.3 WHAT IS A STANDARD?

Of course, before you can have a standard frequency and time service, you have to have a standard, but what is it? The definition of a legal standard is, according to Webster, "something set up and established by authority, custom, or general consent as a model or example." A standard is the ultimate unit used for comparison. In the United States, the National Bureau of Standards (NBS) is legally responsible for maintaining and disseminating all of the standards of physical measurement.

There are four independent standards or base units of measurement. These are length, mass, time, and temperature. By calling them independent, we mean that all other measurements can be derived from them. It can be shown mathematically that voltage and pressure measurements can be obtained from measurements of these four base units. It is also true

that frequency or its inverse, time interval, can be controlled and measured with the smallest percentage error of any physical quantity. Since a clock is simply a machine that counts frequency or time intervals, then time is kept with equal accuracy.

1.3.1 CAN TIME REALLY BE A STANDARD?

Time is not a "standard" in the same sense as the meter stick or a standard set of weights. The real quantity involved here is that of time interval (the length of time between two events). You can make a time interval calibration by using the ticks or tones on WWV, for example, to obtain second, minute, or hour information, but you usually need one more piece of information to make that effort worthwhile. The information you need is the time of day. All national laboratories, the National Bureau of Standards among them, do keep the time of day; and even though it is not a "standard" in the usual sense, extreme care is exercised in the maintenance of the Bureau's clocks so that they will always agree to within a few microseconds with the clocks in other national laboratories and those of the U. S. Naval Observatory. Also, many manufacturing companies, universities, and independent laboratories find it convenient to keep accurate time at their facilities.

1.3.2 THE NBS STANDARDS OF TIME AND FREQUENCY

As explained later in this chapter and elsewhere in this book, the United States standards of frequency and time are part of a coordinated worldwide system. Almost the entire world uses the second as a standard unit of time, and any variation in time of day from country to country is extremely small.

But unlike the other standards, time is always changing; so can you really have a time standard? We often hear the term standard time used in conjunction with time zones. But is there a standard time kept by the National Bureau of Standards? Yes there is, but because of its changing nature, it doesn't have the same properties as the other physical standards, such as length and mass. Although NBS does operate a source of time, it is adjusted periodically to agree with clocks in other countries. In the next chapter we will attempt to explain the basis for making such changes and how they are managed and organized throughout the world.

So even though NBS does not have a glassenclosed clock that is untouched or unadjusted, we do have a standard for frequency and time interval. This standard is located in Boulder, Colorado. It carries the designation NBS-6 because it is the sixth in a series of

SATELLITE

atomic oscillators built and maintained by NBS to provide the official reference for frequency and time interval in the United States. NBS-6 is referred to as the "master" or "primary" clock. It is used to calibrate other oscillators ("secondary" clocks) which are used to operate the time scale. (A time scale is a system of counting pendulum swings.) The organization of the NBS frequency and time standard is illustrated in figure 1.1. Shown is the primary atomic standard NBS-6, which is used to calibrate an ensemble of other atomic oscillators which operate the time scale, which is in turn compared and adjusted to similar time scales in other countries. Periodic adjustments are made to keep them all in agreement.

The useful output of the NBS standard and its associated time scale are the services provided to users. These services can take many forms and use several methods to get the actual needed calibration information to the user.

Notice that figure 1.1 looks a lot like any timepiece. We have a source of frequency, a means of counting that frequency and keeping time of day in a time scale, and as mentioned, a way of getting a useful output. This is exactly what is contained in almost every clock or watch in use today. The frequency source can be a 60 Hz power line, a balance wheel, tuning fork, or quartz crytal. The counter for totaling the cycles of frequency into seconds, minutes, and hours can be gears or electronics. The readout can be a face clock or digits.

In early times, the sun was the only timekeeping device available. It had certain disadvantages--cloudy days, for one. And also, you could not measure the sun's angle very accurately. People started developing clocks so they could have time indoors and at night. Surely, though, the sun was their "standard" instead of a clock. Outdoors, the sundial was their counter and readout. It is reasonable to suppose that if you had a large hour glass, you could hold it next to the sundial and write the hour on it, turn it over, and then transport it indoors to keep the time fairly accurately for the next hour. A similar situation exists today in that secondary clocks are brought to the master or primary clock for accurate setting and then used to keep time.

1.4 HOW TIME AND FREQUENCY STANDARDS

ARE DISTRIBUTED

In addition to generating and distributing standard frequency and time interval, the National Bureau of Standards also broadcasts

In private industry and other government agencies where standards labs are maintained, a considerable amount of time and frequency work is performed, depending on the end product of the company. As you might guess, electronics manufacturers who deal with counters, oscillators, and signal generators are very interested in having an available source of accurate frequency. This will very often take the form of an atomic oscillator kept in the company's standards lab and calibrated against NBS.

Although company practices differ, a typical arrangement might be for the company standards lab to distribute a stable frequency signal to the areas where engineers can use the signal for calibration. Alternatively, test equipment can periodically be routed to the standards lab for checking and adjustment.

As mentioned elsewhere in this book, the level of accuracy that can be achieved by a standards lab when making a frequency calibration ranges from a few parts per thousand to one part in a thousand billion, or 1 part in 1012. Precise calibrations do involve more careful attention to details, but are not in fact that difficult to achieve.

1.5 THE NBS ROLE IN INSURING ACCURATE TIME AND FREQUENCY

The National Bureau of Standards is responsible for generating, maintaining, and

distributing the standards of time and frequency. It is not a regulatory agency so it does not enforce any legislation. That is, you cannot get a citation from NBS for having the incorrect time or frequency.

Other government agencies, however, do issue citations. Most notable among these is the Federal Communications Commission. This agency regulates all radio and television broadcasts, and has the authority to issue citations and/or fines to those stations that do not stay within their allocated frequencies. The FCC uses NBS time and frequency services to calibrate their instruments which, in turn, are used to check broadcast transmitters.

The Bureau's role is simply to provide access to the standards of frequency and time interval to users and enforcement agencies alike. When radio and television stations make calibrations referenced to NBS, they can be confident that they are on frequency and are not in violation of the broadcasting regulations.

1.6 WHEN DOES A MEASUREMENT BECOME A CALIBRATION?

Using an ordinary watch as an example, you can either move the hands to set the time of day, or you can change the rate or adjust the frequency at which it runs. Is this measurement a calibration? It depends on what reference you use to set the watch.

If the source you use for comparison is traceable at a suitable level of accuracy back to the National Bureau of Standards, (or the U.S. Naval Observatory in the case of DOD users), then you can say you have performed a calibration. It is very important to keep in mind that every calibration carries with it a measure of the accuracy with which the calibration was performed.

The idea of traceability is sometimes. difficult to explain, but here is an example. Suppose you set your watch from a time signal in your laboratory that comes to you via a company-operated distribution system. If you

trace the signal backward toward its source, you will find that it perhaps goes through a distribution amplifier system to a frequency source on the manufacturing plant grounds. Let's say this source is an oscillator. Some means will have been provided to calibrate its output by using one of several methods--let's say NBS radio signals. With suitable records, taken and maintained at regular intervals, the oscillator (frequency source) can claim an accuracy of a certain level compared to NBS. This accuracy is transferred to your laboratory at perhaps a slightly reduced accuracy. Taking all these factors into account, the signal in your laboratory (and therefore your watch) is traceable to NBS at a certain level of accuracy.

What kind of accuracy is obtainable? As we said before, frequency and time can both be measured to very high accuracies with very great measurement resolution. As this book explains, there are many techniques available to perform calibrations. Your accuracy depends on which technique you choose and what errors you make in your measurements. Typically, frequency calibration accuracies range from parts per million by high frequency radio signals to parts per hundred billion for television or Loran-C methods. A great deal depends on how much effort you are willing to expend to get a good, accurate measurement.

1.7 TERMS USED

1.7.1 MEGA, MILLI, PARTS PER... AND PERCENTS

Throughout this book, we refer to such things as kilohertz and Megahertz, milliseconds and microseconds. We further talk about accuracies of parts in 109 or 0.5%. What do all of these terms mean? The following tables explain the meanings and should serve as a convenient reference for the reader.

Table 1.2 gives the meaning of 1 x 10-6 but what is 3 x 10-6? You can convert in the same way. Three parts per million or 3 x 10-6 is .0003%. Manufacturers often quote percentage accuracies in their literature rather than "parts per. . . .