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WWV's Sharp Tunes Keep World in Step
July 1949 Popular Science

July 1949 Popular Science

July 1949 Popular Science Cover - RF Cafe[Table of Contents]

Wax nostalgic about and learn from the history of early electronics. See articles from Popular Science, published 1872-2021. All copyrights hereby acknowledged.

Back in the 1970s and 80s when I was a regular reader of magazines like Popular Science, Mechanix Illustrated, Popular Mechanics, et al, it never occurred to me that there were so many stories and news tidbits related to electronics and communications. Now, half a century later as I read through many of them, I am amazed to see just how much content there is for posting on RF Cafe. Of course I was nowhere near as familiar with the topics at the time, so the stories did not have the draw they do now. Just as with the contemporary magazines I peruse each month, I typically go through them from cover to cover, reading much of what is there, including the advertisements. This 1949 report is on the National Bureau of Standards' (NBS, now National Institute of Standards and Technology, NIST) frequency and timing standard signals from their original location near Washington, D.C., call sign WWV. Details on the various continually broadcasted signals are covered within, along with some of the equipment used to accomplish the feat. You will need to visit the current WWV website to compare with today's signals.

WWV's Sharp Tunes Keep World in Step

WWV's Sharp Tunes Keep World in Step, July 1949 Popular Science - RF CafeInsulated box guards one of the quartz crystals whose ticks are broadcast to variety of users. They help keep electric clocks on time, hold radio transmitters on the beam, and aid navigators and scientists.

Hidden among the Maryland farms about 20 miles north of Washington, D. C., is a radio station that is on the air 24 hours a day - with some of the world's dullest-sounding programs. Listeners-in hear only whistles, clicks, and a few staccato Morse-code signals, interrupted periodically by a deep voice saying: "This is WWV, radio station of the National Bureau of Standards."

Yet, to a lot of people all over the world, these monotonous sounds are more interesting than Jack Benny - so interesting that the Bureau recently started another station, WWVH, in Hawaii. These odd sounds provide ultra-precise yardsticks for measuring frequency and time. They help hold radio., broadcasts on the beam, tell power companies how to adjust their generators so your electric clock keeps good time, aid the sending of photographs by wire and radio, tune musical instruments, and assist scientists and engineers studying earthquakes, running geological surveys, and making all kinds of measurements.

Radio engineers must keep their transmitters tuned to their assigned frequencies - the FCC permits variations of only 20 cycles, about 0.002 percent. They do this with a frequency meter, which is essentially an extremely accurate radio receiver. But how can they be sure the meter itself is calibrated correctly? They check it against the signals from WWV, which vary less than one part in 50,000,000, or 0.000002 percent.

Some musicians and most of the manufacturers of musical instruments no longer trust their ears to judge the standard note "A." For them, WWV continuously broadcasts the "A" note - a 440-cycle whistle. The companies that make radio sets use one or another of WWV's ultra-precise frequencies to test the instruments with which they adjust new radio and television receivers. And anybody who needs to know what time it is with equal accuracy - and a surprising number of people do - can listen to WWV's second ticks and the time announcement every five minutes in Morse code. The second ticks are actually beeps of 1,000-cycle whistles exactly one second apart and lasting 5/1,000 of a second.

Bureau of Standards scientists keep tabs with automatic recorders on quartz crystal oscillators that set nation's frequencies. Pen wiggles indicate an error - here a few parts in a billion.

Instruments like these help measure power and frequency of microwaves. They are made with extreme mechanical precision, consisting mostly of accurately dimensioned pipes and cylinders.

Interchangeable components permit WWV, Bureau's station, to broadcast on several frequencies. High-powered transmitter covers world night and day with WWV's time and frequency signals.

Quartz crystals may lose their jobs as nation's timekeepers to more reliable ammonia molecules. One proposed ammonia clock (above) uses a microwave "feedback" oscillator controlled by "magic tee," a T-shaped connection of square pipes (at right above). This acts as electric gate, with ammonia as gatekeeper, permitting only correct frequency to get to amplifier and clock.  

Still more accurate clock would send atoms of cesium down long tube between magnets. Microwaves of right frequency change internal magnetism of atoms, making their path through magnetic field shift to take them to collector wire. Atoms hitting collector create signal that holds generator on right frequency. Potential accuracy of the atom clock is one-second error in 300 years.

It takes a lot of very fancy electronic equipment to keep the nation's time (frequency is simply another way of stating time). But the basic gadget that marks seconds and tiny fractions of a second so accurately is nothing but a piece of quartz, chemically the same as sand on a beach.

When quartz is vibrated mechanically, it generates electrical vibrations. Some phonograph pickups, for example, use quartz crystals to change the vibrations of the needle into electricity. WWV has three quartz crystals, shaped , with a jeweler's precision to vibrate exactly 100,000 times a second.

The crystals are protected from drafts, dampness, and cold more cautiously than a week-old baby. First of all, they are mounted inside sealed vacuum tubes. Then each crystal tube is enclosed in a heavily insulated box that has a built-in electric heater and thermostat. And the boxes are kept in an air-conditioned, concrete vault 30 feet below the ground. The vault is locked - no one goes inside except in emergencies, for, despite these precautions, the crystals are so accurate that heat from a human body throws them off!

"Best" Crystal Runs the Show

All three crystals run continuously, but only the one that is "keeping the best time" is selected for broadcasting. This "best" 100,000-cycle wave operates everything at WWV. It is divided, with practically no loss in accuracy, into frequencies as low as 60 cycles, so that it can drive ordinary electric-clock motors used to switch the signals on and off. Other divisions provide the standard 440-cycle and 4,000-cycle whistles. It is multiplied, again with little loss in accuracy, to make the carrier frequencies on which WWV broadcasts. Five transmission frequencies, from 2,500,000 cycles to 35,000,000 cycles, are used to insure world-wide coverage.

Quartz crystals are the best standards of time and frequency we now have, but they do possess some disadvantages. They drift steadily. A good crystal clock may gain several seconds a year, even though its accuracy over 10-minute periods is better than one part in a billion. They operate at comparatively low frequencies, so that measurements up in the high microwave region can be made only by frequency multiplication, which involves much equipment and some loss in accuracy.

Good frequency standards are badly needed at the very high frequencies. The microwave relays that form intercity television networks, for example, operate around 3 billion cycles. An error of 0.002 percent seems small, but at those frequencies it amounts to 60,000 cycles. You can fit six standard radio stations in 60,000 cycles! Thus, by reducing the amount of error, it might be possible to squeeze more stations into the overcrowded radio space.

In television, too, better frequency control may become vital if many networks adopt RCA's new system of synchronizing broadcasts from two stations using the same channel but in different cities. Extremely accurate synchronization would permit use of the same channel by stations much closer together than is now possible. Without it, televiewers in between the two stations would see "venetian-blind" interference on their screens.

The very high frequencies are finding many other applications, too. Radar works up there, for example. So do the new navigation systems, weather-reporting networks, blind-bombing techniques, control systems for guided missiles, atom smashers, industrial heating devices, and some medical instruments.

One of the new uses of very high frequencies is microwave spectroscopy - the study of the way atoms and molecules absorb microwave radio beams. From this study are coming promising methods of controlling frequency and telling time.

All atoms and molecules can be made to change their internal arrangements if fed energy of the correct frequency. They will soak up energy of just that frequency and no other. This means they can be used as standards of frequency or time. They are more accurate than quartz crystals and would never drift. They would even be more accurate than the present ultimate standard of time, the rotation of the earth - which is slowing down enough to make a difference of around half a minute every century, besides changing for no good reason every once in a while.

For some substances, like ammonia, one of these absorption frequencies occurs in the microwave range. The Bureau of Standards has already made a combination frequency standard and clock that is controlled by the microwave absorption of ammonia (PS, Mar. '49, p. 143). It has a quartz crystal and frequency multipliers to generate the 24-billion-cycle signal absorbed by ammonia. The signal is fed into a tube containing ammonia, which will absorb most of it as long as it is at exactly the right frequency, creating a dip in the output from the other end of the ammonia container. Variations in the dip automatically adjust the crystal to bring it back to the right frequency.

But the ammonia clock still requires clumsy and expensive equipment - quartz crystals, frequency multipliers, and servo-mechanisms. Dr. Harold Lyons, who built the first model, is now trying to eliminate these by working down from the high atomic frequencies instead of up from the low crystal frequencies.

"Magic-Tee" Controls Atom Clock

One possible method will use a "magic tee," a T-shaped arrangement of square copper pipes (waveguides). The magic tee acts as an electric gate. When the electrical loads on the two arms are balanced, no current (very, very high frequency current, that is) gets through the straight part. But when the loads are unbalanced, current will flow. In Dr. Lyons' proposed setup, the load on one arm will be a cell filled with ammonia gas, which will balance the load on the other arm so long as it does not absorb any current. The ammonia will absorb current only at its 24-billion-cycle absorption frequency, so the loads can be unbalanced only at this frequency, and only current of this frequency can get through the magic-tee gate. This precisely regulated frequency can then be divided into any desired value. The chief difficulty here is building the 24-billion-cycle amplifier needed to go with the device.

An ammonia clock should be accurate to about one part in 100,000,000 - twice as good as a quartz-crystal clock. It might be even more accurate except for a characteristic of all atoms and molecules. We have been telling you that atoms and molecules absorb radio waves of exactly one frequency. Actually, however, this is not quite true. They really absorb waves over narrow ranges of frequencies, which thus limits their accuracy. The width of these ranges depends on the particular atom or molecule, the speed at which it moves, and how much it is bounced around by bumps against the walls of the container and other atoms or molecules.

In the search for still higher accuracy, a somewhat different atomic clock is being designed for the Bureau by Prof. Polykarp Kusch, of Columbia University's atomic-beam laboratory. Dr. Kusch's clock will use atoms of cesium, a soft, silvery metal used in photoelectric cells. They will travel down a long vacuum tube, past several magnets, to a hot tungsten collector wire. The magnets will be arranged to make the cesium atoms move in a curved path that ordinarily misses the collector wire. However, when radio waves of just the right frequency (about 10 billion cycles) are applied to the cesium atoms near the center of their path, the internal magnetism of the atoms will be altered, changing their path so that they do hit the collector wire. Almost as soon as the cesium atoms strike the hot tungsten, they boil off, but leave behind their electrons. The electrons added to the collector make a signal current that is amplified to control the 10-billion-cycle radio-wave generator, the signal appearing only as long as the generator is at the right frequency. The signal's absence automatically corrects the generator.

The beam clock, still in the planning stage, is expected to be accurate to about one part in 10 billion. It will be much better than the ammonia clock, mainly because its thin stream of cesium atoms will not suffer as much from collisions. One difficulty, however, will be getting rid of the cesium metal that will build up on parts of the apparatus, eventually clogging the narrow slits that keep the beam sharp.

These new devices - and others that are still vague ideas in the brains of Bureau of Standards physicists - will make time and frequency measurements more fantastically precise than ever before. They will make WWV's broadcasts more interesting to scientists, engineers, and military men. But to anybody else, the whistles, clicks, and code signals will still sound as dull as ever.

 

 

Posted December 22, 2023

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