Telecommunications – Receiver or analog modulated signal frequency converter – Local control of receiver operation
Reexamination Certificate
1998-11-25
2001-08-21
Bost, Dwayne (Department: 2681)
Telecommunications
Receiver or analog modulated signal frequency converter
Local control of receiver operation
C455S256000, C455S257000, C455S258000, C455S259000, C455S264000, C455S265000, C455S180300, C331S016000, C331S017000
Reexamination Certificate
active
06278867
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to wireless devices and more particularly, to the generation of local frequencies by the wireless devices.
BACKGROUND OF THE INVENTION
In conventional wireless devices such as radiotelephones, it is often necessary to generate a local frequency for use as a fundamental frequency reference for the radiotelephone. One example of the use of local frequencies is seen in a dual mode radiotelephone, such as a radiotelephone which operates in the 800 MHz frequency range for cellular services and in the 1900 MHz range for personal communication services (PCS). In such a radiotelephone, the cellular and PCS bands are divided into a plurality of narrow bandwidth channels, such as 30 KHz wide channels for D-AMPS, 200 KHz channels for GSM or 1.8 MHz channels for CDMA. Such a radiotelephone typically generates a local frequency to be used by transceivers in the radiotelephone to modulate and demodulate signals in these frequency ranges. Accordingly, the frequency accuracy of the radiotelephone should be smaller than the channel spacing. Furthermore, for digital transmission, the frequency accuracy typically must be a small fraction of the information transmission rate, irrespective of the width of the channel. For example, even though an IS95 device may operate with 1.8 MHz channel spacing, a frequency accuracy of 100 Hz may be required.
To obtain these high accuracies, for example, 100 Hz at a carrier frequency of 2 GHz, an accuracy of 5 parts in 10 to the power of 8 may be necessary. Such an accuracy is greater than typically can be achieved by an open loop crystal oscillator. Thus, closed loop synchronization is typically utilized to increase frequency accuracy.
A closed loop system is typically implemented using a voltage controlled oscillator. The local frequency is generated by the voltage controlled oscillator and is synchronized to a frequency transmitted by a base station communicating with the wireless device. Thus, for example, in a conventional radiotelephone, a cellular base station transmits a synchronizing frequency which is received by the radiotelephone and compared to the locally generated frequency. The error between the synchronizing signal and the generated frequency is then typically used to adjust the control voltage to the voltage controlled oscillator to reduce the error between the two signals.
One problem with conventional voltage controlled oscillators is that, typically, the relationship between voltage and frequency may vary with temperature. Furthermore, the variation in frequency may change over time, thereby reducing the likelihood that a single temperature compensation would be suitable for the operational lifetime of the radiotelephone.
A further problem may arise from inaccuracies in the frequencies transmitted by base stations. Older base stations may provide inaccurate frequencies or the radiotelephone may have such a great initial error that a base station signal cannot be detected. Thus, it may not be practical to utilize synchronization with a base station at a given temperature to determine a temperature compensation value for a voltage controlled oscillator because the error in the signal may reflect variance in the synchronizing signal rather than the temperature variation in the voltage controlled oscillator. Conventional radiotelephones might have used averaging of compensation values across multiple base stations so as to reduce the impact of a few inaccurate base stations. The radiotelephone would then learn over time the temperature variations of the voltage controlled oscillator to compensate for such variations. Conventional radiotelephones can also, however, suffer temperature variations in the control sensitivity of their voltage controlled reference oscillators, which were typically not compensated in the prior art.
A further difficulty which may arise in dual band radiotelephones may result from the different operating frequencies of the radiotelephone. The damping factor and gain of the feedback control of the synchronizer circuit of the radiotelephone affect the minimal frequency error or the rate at which the voltage controlled oscillator reaches the synchronized frequency. As a result of the voltage controlled oscillator switching between two frequencies, the gain or damping factor of the feedback in the synchronization circuit of the radiotelephone may change thus, possibly, resulting in less than optimal performance for one or both of the frequencies.
SUMMARY OF THE INVENTION
In view of the above discussion, it is an object of the present invention to provide accurate local reference frequencies.
A further object of the present invention is to provide temperature compensation for voltage controlled oscillators to provide an accurate reference frequency upon initial switch-on before a signal is received.
Still another object of the present invention is to provide consistent performance of an automatic reference frequency adjustment system (AFC) over multiple frequencies and at various operating temperatures.
A further object of the present invention is to provide temperature and loop gain control for a synchronizing circuit which may be utilized with conventional radiotelephones.
These and other objects of the present invention are provided by methods and systems which provide frequency generation suitable for use in wireless devices capable of operating at multiple frequencies. Such systems may change the loop gain of an automatic frequency control loop based on the operating frequency of the wireless device. Furthermore, such a frequency dependent loop gain may be carried out by the selection of subroutines with differing loop gains associated with the subroutines. Furthermore, the loop gain may also be temperature compensated based on the temperature of the wireless device and/or the operating frequency of the device.
In a first embodiment of the present invention, a voltage controlled oscillator which provides a local reference frequency in a wireless device which operates using at least two frequency bands is synchronized to a received reference frequency by determining the frequency band of operation of the wireless device and adjusting a loop gain value based on the determined frequency band of operation of the wireless device. A control voltage is provided to the voltage controlled oscillator based on the determined frequency band of operation of the wireless device so as to provide a frequency adjusted local reference frequency and the control voltage revised based on an error associated with the frequency adjusted local reference frequency, based on the received reference frequency and based on the adjusted loop gain.
In a further embodiment of the present invention, the loop gain is compensated based on an operating temperature of the wireless device. In particular, the operating temperature of the wireless device is determined and a loop gain compensating value obtained from a table of temperature compensation values containing multiplier values selected so as to maintain substantially constant loop gain with variation in temperature. The loop gain is then compensated utilizing the obtained loop gain compensating value.
In another embodiment of the present invention, a change in sign of the error associated with the frequency adjusted local reference frequency is detected and the loop gain value changed if a change in the sign of the error is detected. Preferably, the loop gain decreases by a factor of two. Further, the loop gain may be decreased until a minimum loop gain value is reached. Such a minimum loop gain value is preferably based on the determined operating frequency.
In a particular embodiment of the present invention, the revision of the control voltage is carried out by a subroutine stored in a memory at a location associated with an interrupt vector. When a change in sign of the error associated with the frequency adjusted local reference frequency is detected, the location associated with the interrupt vector to is changed to a memory locati
Dent Paul Wilkinson
Northcutt John W.
Shull Eric Alan
Zien Harvey
Bost Dwayne
Ericsson Inc.
Myers Bigel & Sibley & Sajovec
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