Atomic Clocks Aren't

The wonderful "always correct" clocks (and watches) we enjoy today are actually radio wave receivers tuned into station WWV, the standard time reference for the United States. The radio wave broadcasts are based on atomic time standards, but our clocks are not atomic - just radio receivers.

Tune your HF rig or SWL receiver to 5. MHz or 10. MHz and you will hear the WWV broadcast. It is very interesting to hear the beacon and watch your "atomic" clock be in sync with each other.

Canada has the CHU broadcast which just recently changed frequency. For seventy years the Canadian time standard was on 7.335 MHz but as of Jan 1, 2009, the National Research Council of Canada shifted their 40 meter signal to 7.850 MHz. The CHU also broadcasts on 3.330 MHz and 14.670 MHz.

After my original post (above) I started wondering how these clocks had a good enough antenna inside a very small appliance to pick up these signals. Well it turns out there's a whole different broadcast station that is picked up by our clocks ....

Here's a quote from http://tf.nist.gov/stations/radioclocks.htm

In the United States, the signals received by radio controlled clocks originate from NIST Radio Station WWVB, which is located near Fort Collins, Colorado. WWVB broadcasts on a frequency of 60 kHz. Your radio controlled clock actually has a miniature radio receiver inside, which is permanently tuned to receive the 60 kHz signal.

The 60 kHz signal is located in a part of the radio spectrum called LF, which stands for low frequency. This is an appropriate name, because the FM radio and TV broadcasts that we are accustomed to listening to use frequencies thousands of times higher. The lowest frequency received by any of the other radios in your house is probably 530 kHz, the bottom of the AM broadcast band. Even that frequency is nearly 10 times higher than the WWVB signal.

At 60 kHz, there isn’t enough room on the signal (bandwidth) to carry a voice or any type of audio information. Instead, all that is sent is a code, which consists of a series of binary digits, or bits, which have only two possible values (0 or 1). These bits are generated at WWVB by raising and lowering the power of the signal. They are sent at a very slow rate of 1 bit per second, and it takes a full minute to send a complete time code, or a message that tells the clock the current date and time. When you turn a radio controlled clock on, it will probably miss the first time code, so it usually takes more than one minute to set itself (sometimes 5 minutes or longer) depending on the signal quality and the receiver design.

Once your radio controlled clock has decoded the signal from WWVB, it will synchronize its own clock to the message received by radio. Before it does so, it applies a time zone correction, based on the time zone setting that you supplied. The time broadcast by WWVB is Coordinated Universal Time (UTC), or the time kept at the Prime Meridian that passes through Greenwich, England. While a few users like their clocks to display UTC (ham radio operators, for example), most prefer to display local time. This means that the time in your area is corrected by the number of hours shown in the table [ -6 hrs for CST].

Once your radio controlled clock has synchronized, it won’t decode the signal from WWVB again for a while. Some clocks only decode the signal once per day, others do it more often (like every 4 hours or every 6 hours). Those that decode the signal just once per day usually do it at night, since the signal from WWVB is much stronger once the sun goes down. In between synchronizations, the clocks keep time using their quartz crystal oscillators. A typical quartz crystal found in a radio controlled clock can probably keep time to within 1 second for a few days or longer. Therefore, you shouldn’t notice any error when you look at your clock display, since it will appear to be on the right second, even though it has probably gained or lost a fraction of a second since the last synchronization.

These [coverage] maps [on the above website] are based on a field strength of 100 microvolts per meter, which in theory should be a large enough signal for most receivers to work with. In fact, some receivers have much better sensitivity (20 or 30 microvolts per meter). However, simply having a large signal doesn't mean that the receiver will work. What really matters is the signal-to-noise ratio, or the size of the signal compared to the size of the electrical noise near the same frequency. Raising the noise level is just as harmful as reducing the signal level. For example, if the radio controlled clock is near a source of interference (like a computer monitor) the noise level will increase, and the clock might not be able to synchronize. If the radio controlled clock is in a building with a metal roof, much of the signal will be blocked. Therefore, the signal level will be reduced, and the clock might not be able to synchronize.

End Quote

The antenna dilemma becomes more intriguing, tho. But, since everything works, the LF of 60 kHz seems to work very well at low power with the internal antennas in the clocks across North America. Meanwhile, I'm listening to WWV using a 40 meter dipole up about 20 feet! ... AR