Oscillators – Automatic frequency stabilization using a phase or frequency... – Plural a.f.s. for a single oscillator
Patent
1994-11-30
1995-12-05
Grimm, Siegfried H.
Oscillators
Automatic frequency stabilization using a phase or frequency...
Plural a.f.s. for a single oscillator
331 14, 331 17, 331 25, H03L 100, H03L 710
Patent
active
054732840
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to an oscillator unit with an oven stabilized crystal oscillator for a base station or the like in a digital cellular radio network.
BACKGROUND OF THE INVENTION
At a base station of a digital cellular radio system, typically all timing and clock signals are derived from a master clock signal generated locally at the base station. The stability requirement for the master clock signal is 0.05 ppm according to the recommendations of the GSM system, for instance. One approach to meet this stability requirement is to use as master oscillator of the base station a very accurate crystal oscillator, positioned in a space at a stable environmental temperature, a so-called oven. This kind of crystal oscillator is generally called oven stabilized crystal oscillator. A sufficiently accurate oven stabilized crystal oscillator is, however, a very expensive solution, in which long-term instability is inevitable. Consequently, the oven stabilized crystal oscillator must be regularly calibrated to the assigned frequency during operation.
DISCLOSURED OF THE INVENTION
The object of the invention is an oscillator unit with an oven stabilized crystal oscillator, having an improved long-term stability.
This is achieved by means of an oscillator unit, which unit is according to the invention characterized in that the oven stabilized crystal oscillator is adjustable and that the oscillator unit further comprises a non-volatile memory means for storing a control information substantially corresponding to the assigned frequency and defining the frequency of the oven stabilized crystal oscillator at the start of the oscillator unit, a means for deriving a reference frequency from a fixed digital transmission connection between the base station and the remaining cellular radio network, such as a PCM connection, and a frequency control loop for maintaining a long-term stability for the oven stabilized crystal oscillator on the basis of a long-term average of the difference between the frequency of the oven stabilized crystal oscillator and said reference frequency.
In the oscillator unit according to the invention, the oven stabilized crystal oscillator is set at a frequency defined by the stored default control information at the start of the oscillator unit. Thus the oscillator unit reaches a working condition without delay as soon as the oven has warmed up. The oven stabilized crystal oscillator provides a sufficient short-term stability. Long-term stability is obtained by controlling the frequency of the oven stabilized crystal oscillator by means of a reference frequency derived from a digital transmission connection coming to the base station. The frequency of the oscillator is controlled on the basis of a long-term average of the difference between said reference frequency and the frequency of the oven stabilized crystal oscillator. The interval between the adjustments of the oven stabilized crystal oscillator is preferably at least 17.5 minutes to achieve a sufficient capacity of filtering jitter/fluctuation. Thanks to the invention, the accurate cesium norm of a public switched telephone network can be used as a primary long-term frequency norm of the base station. Consequently, the oscillator unit can be provided with an excellent long-term stability, and therefore, the need of calibrating the oscillator unit during operation is reduced or entirely eliminated. Additionally, because only short-term stability is required of the oven stabilized crystal oscillator, it is possible to use a cheaper oven stabilized crystal oscillator than previously. On the other hand, the sufficient shortterm stability of the oven stabilized oscillator guarantees the operation of the oscillator unit, even if the reference frequency derived from the digital transmission connection were temporarily lost. Moreover, the oscillator unit of the invention makes it possible to update the default control information stored for cold starts during operation on the basis of said reference frequency. As resul
REFERENCES:
patent: 4511858 (1985-04-01), Charavit et al.
patent: 4815109 (1989-03-01), Kao
patent: 4847569 (1989-07-01), Dudziak et al.
patent: 4921467 (1990-05-01), Lax
Jantti Arto
Juola Veli
Karki Tapani
Saukkonen Risto
Vaananen Lassi
Grimm Siegfried H.
Nokia Telecommunications Oy
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