Telecommunications – Receiver or analog modulated signal frequency converter – Local control of receiver operation
Reexamination Certificate
1997-12-12
2001-04-10
Nguyen, Lee (Department: 2746)
Telecommunications
Receiver or analog modulated signal frequency converter
Local control of receiver operation
C455S257000, C455S265000, C331S176000
Reexamination Certificate
active
06215990
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to the minimization of frequency errors during power up of a transceiver unit, and more particularly, to a method for minimizing frequency errors due to the effects of temperature differences on a voltage controlled, temperature compensated crystal oscillator.
2. Description of Related Art
The frequency at which a transceiver unit, such as a cellular telephone initially operates during power up is different from the final frequency that the unit operates at once the unit is locked onto a control channel frequency. The further the initial operating frequency of a cellular telephone is from the final frequency of the control channel, the greater the likelihood that the phone may never lock onto the control frequency or the greater amount of time is required to accomplish this goal. As the initial frequency error increases, the cellular telephone's ability to acquire synchronization with the digital control channel (DCCH) decreases.
The factor most greatly affecting the initial frequency error is the initial frequency signal produced by a voltage controlled, temperature compensated crystal oscillator (VCTCXO) within the cellular telephone. Most radio products, such as a cellular telephone, use a modular VCTCXO in which an analog control network compensates for frequency errors caused by variances in the temperature of the crystal oscillator. The analog circuit keeps the VCTCXO frequency within a specified ppm error over a variety of temperatures. The control voltage of the VCTCXO is used to calibrate the device at room temperature and for automatic frequency control (AFC). The initial control voltage value is stored and applied to the VCTCXO at system power up. An AFC algorithm based upon received frequency error reports is used to lock onto the digital control channel.
Due to the high cost of modular VCTCXO products, manufacturers are increasingly producing radios including discrete VCTCXOs with temperature compensation tables. For discrete circuits, the maximum expected frequency error relies upon the specifications of the crystal. The operating characteristics of the crystal can be characterized over various temperature ranges, and multiple temperature tables generated to improve the frequency performance at each particular temperature range. However, the use of a plurality of temperature tables greatly increases the cost of the VCTCXO. The discrete VCTCXO works similarly to the modular VCTCXO. The difference arises in the method of temperature compensation. A discrete VCTCXO stores a hex temperature compensation value that is added or subtracted from a factory calibrated, hex control voltage value. For each temperature value detected by an associated thermistor network, there is an associated hex temperature compensation value stored in the temperature table. Thus, for a given temperature, the microprocessor generates the initial control voltage indicated by the factory calibration value plus a temperature compensation value.
Thus, the need has arisen for a means for improving compensations for frequency drift caused by temperature changes within a cellular telephone that is less complex and more efficient than existing systems.
SUMMARY OF THE INVENTION
The present invention overcomes the foregoing problems with an improved method and apparatus for minimizing initial frequency errors in a voltage controlled temperature compensated crystal oscillator (VCTCXO). Coupled with the crystal oscillator is a digital-to-analoged controller generating a voltage signal to the crystal oscillator in response to an input voltage control signal received from a processing unit. A thermistor network detects the temperature of the crystal oscillator and provides a temperature signal to the processing unit.
The processing unit in response to the temperature signal and input from a default table containing default voltage control values for each of a plurality of temperatures and a compensation table including a plurality of updatable compensation values generates an input voltage control value.
In response to the detected temperature signal and the associated values from the default and compensation tables, the processor calculates an error in the presently applied control voltage signal. From the calculated error a compensation value necessary to overcome the error in the applied controlled voltage value is determined. Once the compensation value is determined a number of tests are performed to assure that the value is not too far from a set of predetermined limits in an attempt to assure the compensation value is not changed due to a non-standard base station. Upon passing the tests the compensation value within the compensation table at the detected temperature may be updated using the determined compensation value.
REFERENCES:
patent: 4921467 (1990-05-01), Lax
patent: 5392005 (1995-02-01), Bortolini et al.
Ericsson Inc.
Jenkens & Gilchrist P.C.
Nguyen Lee
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