Pulse or digital communications – Spread spectrum – Direct sequence
Patent
1983-11-08
1986-01-07
Griffin, Robert L.
Pulse or digital communications
Spread spectrum
Direct sequence
375108, 375120, 331 11, H04J 306
Patent
active
045637679
DESCRIPTION:
BRIEF SUMMARY
FIELD OF INVENTION
The invention relates to a method of and apparatus for phase-synchronising a transit exchange in a digital telecommunication network, including a signal-controlled oscillator, the control signal of which is influenced by the collectively weighted phase difference between the local transit exchange clock and that of the other exchanges.
BACKGROUND
A synchronisation of the type mentioned involves each station in the network forming its frequency as a mean value of the frequencies of the incoming links. This is done by measuring the phase differences between incoming link clock signals and the local clock, subsequent to which the phase positions are added or weighted together to provide a control value acting on the clock in the respective exchange via a regulator. The phase detector may be analog or digital, although only a digital phase detector is suitable for use in a time multiplex system.
A phase detector represents a periodic function, i.e. the variation of the faulty signal between zero and the highest deviation corresponding to the phase difference 2.pi. is repeated for each 2.pi. phase difference. This means that large phase jumps occur just when the phase position exceeds or falls below its measuring range. Even if the sum of the phase positions is kept constant in a network, the individual phase positions can move apart and cause phase jumps. This can result in difficulties when the clock frequency rapidly changes, thus possibly causing new phase jumps. Since the entire network is synchronised, such a phase jump can cause the whole network to hunt.
SUMMARY OF INVENTION
An object of the invention is to eliminate this drawback and to provide a method by which phase jumps in the phase detection are avoided. This is accomplished in that the change of phase difference in respect of each of the clock signals of the incoming links is monitored, and if this change exceeds a given limit indicating that a phase jump has taken place, the signal is corrected before it is used for regulation of the oscillator.
More particularly to achieve the above and other objects of the invention, there is provided a method of synchronizing the phase of clocks from links from different stations in a transit exchange in a digital telecommunication network including a signal controlled oscillator, the output of which is dependent on collectively weighted phase differences between a local exchange clock and the clocks of the other stations, the method being characterized in that for avoiding a phase jump a signal corresponding to the phase difference is sampled for each of the incoming links thereby producing sampled values. These sampled values are differentiated thereby producing differentiated values. The differentiated values are compared with a limiting value for indicating a phase jump when the limiting value is exceeded. When the limiting value is exceeded, a compensation signal having a control signal's highest value but with opposite sign is added to the differentiated value to form a compensated differentiated value. After possible weighting, the compensated differentiated value is added to the compensated differentiated values corresponding to the other links producing a sum value and the sum value is integrated for obtaining the control signal.
In further accordance with the invention apparatus is provided for synchronizing the phase of clocks from links from different stations in a transit exchange in a digital communication network including a signal controlled oscillator, the output of which is dependent on collectively weighted phase differences between a local exchange clock and the clocks of the other stations. The apparatus includes in accordance with a preferred embodiment phase detectors for sensing the phase difference between the local exchange clock from the signal controlled oscillator and the clocks of each of the incoming links for generating a signal proportional to the phase difference. Differentiating circuits are provided for differentiating the phase difference-proporti
REFERENCES:
patent: 3555194 (1971-01-01), Goto
patent: 3597552 (1971-08-01), Goto
patent: 3862365 (1975-01-01), Kobayashi et al.
patent: 4075248 (1978-02-01), Ghisler et al.
patent: 4339817 (1982-07-01), Hata et al.
Glenny Raymond C.
Griffin Robert L.
Telefonaktiebolaget LM Ericsson
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