Oscillators – Plural oscillators – Selectively connected to common output or oscillator...
Patent
1993-06-28
1995-04-11
Mis, David
Oscillators
Plural oscillators
Selectively connected to common output or oscillator...
331 46, 331 74, H03L 700, H03B 2800
Patent
active
054062314
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to an oscillator unit with improved frequency stability.
The specifications of the new European digital mobile radio system GSM (Groupe Special Mobile) define that the nominal frequency of the oscillator unit of the base station, that is, the basic frequency from which the clock and synchronization signals of the base station are derived, must not deviate more than 0.6 Hz from the specified value. Therefore there is a need for an oscillator unit of adequate frequency stability.
SUMMARY OF THE INVENTION
The object of the invention is to provide an oscillator arrangement with reliable frequency stability.
This is achieved by means of an oscillator unit according to the invention, which is characterized in that it comprises: the oscillator unit; for controlling the selecting means to replace the oscillator connected to the output of the oscillator unit with another oscillator if the frequency of the oscillator currently connected to the output deviates from the frequencies of the other oscillators at least by a predetermined frequency difference.
The invention utilizes at least three oscillators of the same nominal frequency. One of the oscillators is connected to generate the output frequency of the oscillator unit, and the oscillator frequencies are compared in pairs with each other. It is thereby possible to identify an individual oscillator the frequency of which starts to drift away from the frequency of the other oscillators. If such an unstable oscillator is currently connected to the output of the oscillator unit, it can be replaced with a more stable stand-by oscillator if the deviation of its output frequency from the output frequency of the other oscillators exceeds a predetermined frequency difference. At the same time, an alarm can be given to indicate the failure of the oscillator. In this way, an oscillator unit of adequate reliability and frequency stability is achieved according to the invention.
BRIEF DESCRIPTION OF THE DRAWING
The invention will now be described in greater detail by means of an illustrating embodiment with reference to the attached drawing, in which: The sole FIGURE shows a block diagram of an oscillator unit according to the invention.
DETAILED DESCRIPTION
The oscillator unit according to the invention comprises at least three oscillators 1, 2 and 3 having the same nominal frequency and comprising outputs 1a, 2a, 3a, respectively, of which one is switched to an output 14a of the entire oscillator unit by a switching unit 14. The switching unit 14 selects the oscillator to be connected to the output 14a in response to a selection signal 13a from a selection unit 13. The selection unit 13, in turn, causes the switching unit 14 to change the oscillator either on receiving information from a special oscillator monitoring circuitry about a change in the frequency of the oscillator connected to the output, or in response to a control signal 15.
To monitor the frequencies of the oscillators 1, 2 and 3, their outputs are connected to a special frequency comparison circuitry which compares the output frequencies of the oscillators in pairs with each other and causes the selection unit 13 to replace the oscillator currently connected to the output 14a of the oscillator arrangement with another redundant oscillator if the frequency of the current oscillator deviates from the frequencies of the redundant oscillators at least by a predetermined frequency difference.
In the preferred embodiment of the invention shown in the figure, the frequency comparison circuitry comprises three circuits 4, 5 and 6 performing the logic XOR operation. The input signals of the XOR circuit 4 include the output signals 1a and 2a of the oscillators 1 and 2; the input signals of the XOR circuit 5 include the output signals 2a and 3a of the oscillators 2 and 3; and the input signals of the XOR circuit 6 include the output signals 1a and 3a of the oscillators 1 and 3. Each XOR circuit 4, 5 and 6 continuously performs the logic XOR operation on the r
REFERENCES:
patent: 4583054 (1986-04-01), Basile
patent: 4629999 (1986-12-01), Hatoh et al.
patent: 5063357 (1991-11-01), Williams et al.
Mis David
Nokia Telecommunications Oy
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