Invention of the radio clock

The Inventor of the Radio Clock
“With the sound of the last tone it’ll be twelve o’clock: Beep, Beep, Beep, Beep, Beep.”.

This was state-of-the-art on synchronization of clocks to standard Middle European Time in private homes by radio broadcast stations, when Wolfgang Hilberg in the year 1967 submitted his patent “Method for continuously transmitting clock time” to the German Patent Office in Munich. Until then there were mechanical clocks in use, e.g., mounted to church towers, showing in best case a time signal derived from the astronomic time in turn derived from earth rotation. More recently but until then, the reference time was brought by portable high precision clocks, or was transmitted via telecommunications networks by means of switching impulses to church towers, broadcast stations, railway stations and the like.

The idea of transmitting clock time via radio was first published in the year 1899 by Sir Howard Grubb in the journal “Scientific Proceedings for the Royal Dublin Society” titled “Proposal for the Utilization of the ´Marconi´ System of Wireless Telegraphy for the Control of Public and Other Clocks”. This paper does not give any teaching of hint on technical actions to perform to achieve this goal. The technical realization of this idea was progressed over a long time span in many small steps, which were fertilized by both, an increasing demand on the precision and actuality of the clock time, and progresses in radio transmitters’ and receivers’ design and signal processing techniques. The first couple of German patents addressing technical systems for clock time transmission are dated from the first decade of the past century. From the original idea (in 1899) to the realization of a robust technical solution for continuous radio transmission with a sufficient precision of clock time, date of the day, day in the week, etc. it took, however, 75 years.

Hilberg managed to achieve a break-through in clock time transmission, which crowned a 75 years long development process and gained a one forever final technical solution: All terrestrial radio clock transmitters known make use of his invention. This is the reason why the Board of the Eduard Rhein Foundation, based on a proposal by its Kuratorium, today grants to Wolfgang Hilberg the Technology Prize 2011

“For inventing digitally coded reference clock signals continuously transmitted over radio and for contributions on realizing radio clock receivers”.

Since 1899 until Hilberg came with his proposal, other inventors had developed methods for transmitting clock time via radio, which have been unsatisfactory and, accordingly, without success. The reasons were that the clock time was available only at specific time instants during a day or that the clock time was updated stepwise by transmitted time impulses, which must be detected without gaps by the receiver to progress the clock mechanism correctly, thereby gaining the timing information displayed. Since atmospheric and technical disturbances impact reception of the time pulses transmitted via radio, these clocks tend to fall after the actual time with an unknown error. Exactly this weakness was overcome through the invention by Hilberg, in that the complete information transmitted within short subsequent time intervals, say a minute, providing to the receiver in a time message the actual valid parameters: year, month, day of week, hour, and minute by means of a binary impulse code. The seconds signal is extracted either from the carrier frequency of the radio clock transmitter or may be extracted from the bit tact of the signal that carries one signal per second.

Hilberg, expressively, notes that no clock must be contained in the radio clock receiver, which only contains technical measures for decoding and displaying the actual decoded clock time signals. The receiver, after switched on, has to wait until the complete message is received before displaying it. Under bad receive condition this may last for a long time duration, or may even not result in any time information displayed. To avoid a long wait of the user, the receiver displays the receive signal quality to motivate the user to move the position of the radio clock to get an optimal signal reception quality. Today’s radio clocks contain an autonomous quartz stabilized clock synchronized by sample signals from the radio clock signal received. Thereby, a battery life-time of the radio clock receiver of many years is achieved.

There are several radio clock transmitters operating with up to 50 kW transmit power on long wave frequencies (100-300 kHz) with a range of coverage of up to 2.000 km that came-up after the German radio clock transmitter DCF 77 was established, e.g., in Great Britain, Russia, USA, Japan, China. And there are many radio clock transmitters operating on medium wave frequencies (0.3-2 MHz) with, typically, much less power and range. DCF 77 operates since the year 1972 using the time and date coding proposed by Hilberg in his 1967 patent, all the other radio clock transmitters use a very similar binary signal encoding.

The German law “Gesetz über die Zeitbestimmung (Zeitgesetz)” was very helpful to introduce the radio clock according to Hilberg’s patent in Germany. The law issued in the year 1978 requested the Physikalisch-Technische Bundesanstalt (PTB) to broadcast Middle European Time (MEZ) throughout Germany and beyond. Hilberg did his best to motivate industry to develop and market radio clocks, and himself developed a number of radio clock prototypes. However, only after his patent expired, instustry started mass production of radio clocks. His invention was too far ahead of its time. The number of pieces manufactured since then is estimated to exceed a 100 million radio clocks.

Besides radio clocks, since the 70-ies, satellite based radio systems have been developed for high precision localization of mobile devices. This equipment also provides high precision time information as known, e.g., from the Global Positioning System (GPS) and similar systems like Glonass and Galileo. The satellite receivers must have line-of-sight connectivity to a number of, say at least four satellite transmitters to achieve the geo-location precision desired. Therefore, synchronization of GPS (and related) receivers to their serving satellites is successful only, outdoors. A broadband transmission system is used resulting in complex signal processing circuitry, compared to the very narrow frequency band and simple signal encoding used with radio clock transmitters. Accordingly, battery life-time of satellite based time reference systems tends to be much shorter than with terrestrial radio clock systems. This is the reason why radio clocks based on satellite systems, in general, can be found only in terminals that have a powerful electric accumulator battery as energy resource, as usual with navigator terminals.

Atomic clocks are known able to keep worldwide time references that differ only by a few microseconds. Distribution of this normal time value via satellite systems and radio clock transmitters is common practice, today. Besides the desire to measure and date time dependent processes in industrial and scientific research with a high precision, there is a wide interest to receive atomic clock signals with simple radio equipment to operate radio clocks. The precision required with commonly used clocks is about 0.1 seconds. With a little more technical effort, precision can be driven up to 0.1 microseconds, e.g., based on the pseudo-random phase shift of the amplitude modulated time signal transmitted by the German radio clock transmitter DCF 77. This transmitter was in experimental operation since 1972, and is in continuously operating since 1977 to transmit at 50 kW transmit power in a bandwidth of 1.7 kHz at the carrier frequency of 77.5 kHz, once per minute, a complete time message modulated in amplitude and pulse width with a data rate of 45 bit/s.

Prof. Dr.-Ing. Bernhard Walke
RWTH Aachen