Pulse or digital communications – Receivers – Automatic frequency control
Reexamination Certificate
1998-06-18
2001-05-15
Pham, Chi (Department: 2631)
Pulse or digital communications
Receivers
Automatic frequency control
C455S264000
Reexamination Certificate
active
06233292
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a digital communication device comprising a receiver having a mixer which is coupled to a local frequency generating arrangement, a demodulator which is coupled to the receiver, a microcontroller which is coupled to the demodulator being arranged for providing a frequency offset value being a difference between a frequency value representative of an output frequency of the local frequency generating arrangement and a frequency value representative of a desired tuning frequency, and frequency adjustment means for adjusting the frequency of the local frequency generating arrangement in dependence of the frequency offset value. Such a digits communication device can be a pager, or any other suitable digital communication device such as a cellular or cordless telephone, or the like. The receiver can be a super heterodyne receiver, a direct conversion quadrature receiver, or any other suitable receiver.
BACKGROUND OF THE INVENTION
A digital communication device of this kind is known from the European Patent Application EP 0 735 675 A2. The known device comprises an automatic frequency control (AFC) loop for adjusting the frequency of a local oscillator in accordance with a frequency offset between a frequency of the local oscillator and a desired tuning frequency. In order to avoid erroneous tuning to a strong adjacent channel, in such communication devices frequency offset compensation should only be done in a limited range. The known communication device has a temperature sensor so as take into account temperature effects the AFC control loop. Such a temperature sensor renders the communication device complicated. Besides, no measures are described for taking into account long term effects such as aging. Herewith, in the long run, the communication device can run out of its specifications.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a more robust digital communication device in which both the effects of long term and short term frequency drift effects are taken into account.
To this end the communication device according to the present invention is characterised in that the microcontroller is arranged for processing the frequency offset value such that long term frequency drift is separated from short term frequency drift, whereby the frequency adjustment means comprises a frequency adjustment reference value which is updated in accordance with the separated long term frequency drift. Herewith, under all circumstances maximum freedom is achieved for compensating short frequency drift such as temperature drift or drift caused by co-channel interference. Alternatively, cost reduction can be achieved by allowing greater temperature and aging drifts. The present invention is based upon the insight to adapt the frequency adjustment reference value over a relatively long period such that it follows the long term frequency drift such as drift caused by aging effects in components of the communication device, such as a reference crystal in the local frequency generating arrangement. Also incidents causing a similar effect as aging are compensated for. Such an incident can be a sudden change in parameter values of a crystal oscillator when the receiver is subjected to a shock, e.g., by being dropped. Such a quasi short term drift will initially add to the short term offset which may be partly compensated, but will eventually be fully compensated for because in the long run it is considered by the drift compensation mechanism as a long term drift effect.
Embodiments of a digital communication device according to the present invention are given in the dependent claims. By initially aligning the frequency adjustment reference value such that the frequency affect becomes zero, it is achieved that in the factory the digital communication device is optimally aligned before being sold. By determining the short term driff and the long term drift by respective averaging of the frequency offset value over averaging periods of defferent lengths, it is achieved that a good separation is made between short term and long term drift, whereby short term averaging can be done over minutes and long term averaging can be done over months, for instance. By relating a frequency adjustment control value and the frequency adjustment reference value to each other in accordance with a predetermined function, it is achieved that a non-linear relationship or any other predetermined relationship between reference and control value is taken into account, whereas storing the predetermined function as a look-up table in the microcontroller gives an advantageous embodiment thereof. When the digital communication device is a pager, whereby the frequency offset value is determined at a synchrosinsation word received by the pager, proper measurement of the frequency offset values in a pager is achieved. Generally, the demodulator is a digital demodulator. Such demodulators have the capability to provide frequency offset values.
REFERENCES:
patent: 4921467 (1990-05-01), Lax
patent: 5113416 (1992-05-01), Lindell
patent: 5172075 (1992-12-01), Yerbury et al.
patent: 0735675A2 (1996-02-01), None
patent: WO962498 A1 (1996-08-01), None
Van Bezooijen Adrianus
Van Veldhuizen Evert D.
Pham Chi
Phu Phuong
U.S. Philips Corporation
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