RadioClock-Homepage - Detail information about the time

Contens:

Realization and dissemination of the legal time of the Federal Republic of Germany
Atomic second and atomic clocks
Time signal and standard frequency transmitter DCF77
Pseudo-random carrier phase shift keying
PTB Telephone Time Service
Extract from the Time Act of 1978 and from a decree of 1994 of the Federal Government

Realization and dissemination of the legal time of the Federal Republic of Germany

The CS3 and CS4 atomic clocks of the PTB

Atomic clocks CS3 and CS4 (PTB)

The Time Act of 1978 has entrusted the Physikalisch-Technische Bundesanstalt (PTB) with the task of giving the time which determines public life in the Federal Republic of Germany. For this purpose, the PTB designed and constructed tour primary caesium clocks, CS1 to CS4, which am among the most accurate clocks in the world. Within one year, the readings of these clocks differ from one another by less than one millionth of a second.

Everybody receiving the long-wave transmitter DCF77 can use the PTB`s time. This transmitter is located near Frankfurt and disseminates the PTB time in continuous operation. The Time Act refers to these PTB tasks as the "realization and dissemination of Legal Time".

Throughout Germany, the time signals emitted by DCF77 can be used to keep radio clocks in compliance with Legal Time with an accuracy better than one millisecond. The time information given by the broadcasting and television stations, the clocks of the Federal Railways Administration and of the speaking-clock announcement of the Deutsche Telekom AG as well as a great number of tariff counters of power industry, traffic control devices and traffic light equipment are also controlled by DCF77. In industry and science, the time signals emitted by the PTB serve to control and supervise complicated processes. Furthermore, everybody can buy radio-controlled clocks for private use.

The CS1 and CS2 atomic clocks of the PTB

The unit of time, the second, of the International System of Units (UI) which is based on the oscillation of the caesium-133 atom was defined in 1967. This consistently led to an international atomic time scale related to the second at sea level and to the zero meridian. This scale superseded the "universal time" - also known as "Greenwich Mean Time" obtained from astronomic observations. The time scale used now is called "Universal Time Coordinated" (UTC). Leap seconds intercalated into the UTC time scale about once a year ensure that UTC never deviates from the Universal Time determined by die Position of the sun by more than 0,9 second.

The Legal Time of die Federal Republic of Germany is either die Central European Time CET(D) or the Central European Summer Time CEST(D). Whether or not CEST(D) is adopted is stipulated in advance by a decree issued by die German government.

The following relations are valid between UTC and CET and between UTC and CEST:
CET(D) = UTC(PTB) + 1 h,
CEST(D) = UTC(PTB) +2 h.

Atomic second and atomic clocks

Atomic second

Pursuant to international agreements, the second as the unit of time is defined as follows:

The second is the duration of 9.192.631.770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.

According to this definition, the unit of time is realised with caesium atomic clocks which are manufactured by industry or constructed and operated by research laboratories to meet highest accuracy demands. Only about ten specimen's of the latter exist world-wide.
Principle underlying an atomic clock
Atomic clocks work on the following principle: Atoms occur in different energy states one of which is identified by the symbol (+) and one by the symbol (-). The transition of an atom from the (+) to the (-) state can be stimulated and is connected with the emission of electromagnetic radiation of a characteristic frequency. In the ease of the caesium atom, this frequency, fcs has a value of 9.192.631.770 Hz, corresponding to an oscillation period of (1 / 9.192.631.770) seconds. According to the laws of atomic physics, fcs is equal to the energy difference between the (+) and (-) states divided by the Planck constant h. For the caesium atom in particular, the constancy in time of fcs much better than that of, for example, the oscillation period of a pendulum, the oscillation frequency of a quartz or the rotation period of the earth.

Caesium atoms are evaporated in the vacuum chamber of an atomic clock. The magnet arranged behind the oven separates the atoms such that only atoms in the (+) state enter the cavity resonator. Here exposure to a microwave field stimulates the atoms to pass into the (-) state. The second magnet then directs to the detector only these atoms. The number of atoms at the collector reaches a maximum when the frequency of the irradiation field has the value fcs. A feedback circuit ensures that the microwave oscillator Q is kept at the frequency fcs. By the counting of 9.192.631.770 periods, the time interval of one second is obtained from the oscillator signal.

Diagram of an atomic clock:
O	atomic bem oven	Q	microwave oscillator
M	sorting magnet	A	detector
H	cavity resonator	R	servo control circuit

Horizontal section through the CS2 primary time and frequency standard of the PTB

Horizontal section through the CS2 primary time and frequency standard of the PTB
O	caesium beam oven	S	sorting magnets
V	vacuum recipient	M	mu-metal shields
H	cavity resonator	W	beam reversal manipulator
C	coil to generate a homogeneous magnatic field (C-field)	A	detector

Time signal and standard frequency transmitter DCF77

	Coding schema:
	M	minute marker (0,1 s)
	R	second marker No. 15 has a duration of 0,2 s when the standby antenna is used
	A1	announcement of a forthcoming change from CET to CEST or vice versa
	Z1, Z2	zone time bits
	A2	announcement of a leap second
	S	starting bit of the coded time information (0,2 s)
	P1-P3	parity check bits