Telecommunications – Transmitter and receiver at same station – With frequency stabilization
Reexamination Certificate
1998-11-06
2002-06-04
Legree, Tracy (Department: 2681)
Telecommunications
Transmitter and receiver at same station
With frequency stabilization
C455S182200, C375S149000
Reexamination Certificate
active
06400930
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to radio communication devices, and specifically to devices and methods for accurate frequency control of radio transceivers.
BACKGROUND OF THE INVENTION
Radio transceivers used in mobile communication require stable clock sources and frequency synthesizers for use in modulation and demodulation of RF signals. The frequency synthesizers must be capable of tuning and locking onto a base station frequency with high accuracy. In a typical cellular communications system, for example, a radio transceiver must typically be capable of initial tuning, based on dead reckoning to an accuracy of 2 ppm, and then once a base station signal is detected, locking onto the signal frequency to an accuracy of 0.2 ppm. In the 800 MHz cellular band, these requirements translate into 1.6 kHz initial tuning accuracy and 160 Hz locking accuracy. In the 1.5 GHz band, the figures are roughly double.
Crystal oscillators, which are commonly used for frequency control in radio transceivers and other applications, are not in themselves sufficiently stable to provide the levels of accuracy noted above. The inherent frequency of the crystal oscillator is known to vary with temperature and also to change gradually as the crystal ages. A typical crystal oscillator, for example, a PXO-type oscillator, produced by Telequarz of Neckarbischofsheim, Germany, has a frequency accuracy of ±10 ppm. The long-term drift of the frequency of the crystal with age is about 2 ppm per year.
Because of these shortcomings of ordinary crystal oscillators, transceivers known in the art generally use either a temperature-compensated crystal oscillator (TCXO) or a voltage-controlled TCXO (VCTCXO) as a precise frequency reference source. Temperature-compensated oscillators are described, for example, in U.S. Pat. Nos. 3,938,316, 4,015,208, 4,454,483, 5,375,146 and 5,604,468, which are incorporated herein by reference. The TCXO or VCTCXO receives an indication of the ambient temperature and corrects the oscillator output frequency accordingly, so as to compensate for the known characteristic temperature variation of the crystal frequency.
TCXOs and VCTCXOs are larger and heavier than simple crystal oscillators and include elements that are not easily incorporated in monolithic transceiver devices. They also depend on the use of a crystal which has a high motional capacitance to allow for externally-controlled tuning of the oscillator, and is therefore larger than standard crystals used in simple crystal oscillators. Therefore, TCXOs and VCTCXOs add undesirably to the cost and weight of RF communication devices, such as cellular telephones, in which they are used.
SUMMARY OF THE INVENTION
It is an object of some aspects of the present invention to provide methods and devices enabling accurate tuning of a radio transceiver using a low-cost frequency reference source.
It is a further object of some aspects of the present invention to provide an accurate radio frequency source that can readily be incorporated in a monolithic transceiver device.
It is yet another object of some aspects of the present invention to provide an accurate radio frequency source that does not require a TCXO or a VCTCXO.
In preferred embodiments of the present invention, a radio transceiver, which communicates with a base station on a base station carrier frequency, comprises one or more frequency synthesizers, which receive clock signals from a reference oscillator. Preferably, the reference oscillator comprises a simple crystal oscillator without temperature compensation or means for external frequency control. A frequency error of the clock signals relative to a clock frequency required by the radio transceiver is estimated based on the error it causes in the frequency of the transceiver. The estimated error is parsed into a coarse correction estimate and a fine correction estimate. The coarse correction is applied to step one or more of the frequency synthesizers, so that the synthesizers generate frequency signals within a predetermined range of a target frequency. The fine correction is applied by generating a corresponding frequency correction in a correction circuit of the transceiver, preferably in baseband circuitry of the transceiver. By parsing the frequency error and thus applying frequency corrections cooperatively in the synthesizers and the baseband circuitry, the present invention enables the transceiver to tune and lock onto the carrier frequency with high accuracy, while maintaining a fast time response and low noise level, without the need for a temperature-compensated or voltage-controlled oscillator.
In some preferred embodiments of the present invention, additional information is input to the transceiver regarding physical conditions of the crystal oscillator. This information preferably includes the temperature and age of the crystal, as well as predicted or recorded data regarding the response of the crystal oscillator to the physical conditions. The information may also include the oscillator supply voltage and load, as well as any other parameters affecting the performance of the oscillator. The information is used in dead reckoning of the frequency error, which is applied in initial tuning of the transceiver. Thereafter, a closed-loop measurement of the error is preferably made, and the measured error is parsed into a coarse frequency error and a fine residual error and is used in fine-tuning the synthesizers and baseband circuitry.
In some preferred embodiments of the present invention, the frequency correction of the baseband signals is performed by shifting the phase of the signals, most preferably by subtracting the residual frequency error after demodulation of intermediate frequency (IF) signals. Alternatively or additionally, the residual frequency error is corrected by multiplication of the modulated IF signals by a phasor, either in digital or analog form, using a complex multiplier. The phasor applies to the signals a phase shift that varies continuously with time, in a manner calculated to generate the desired frequency correction.
Further alternatively or additionally, the residual frequency error is accounted for during generation of the signals (in transmission) and/or in determination of the values of the signals (in reception), preferably at the stage of modulation and/or demodulation of the signals. For example, in a system utilizing FSK modulation, in which the demodulation is performed by a frequency discriminator, the decision threshold level is adjusted responsive to the residual frequency error. During transmission, the transceiver generates modulating tones at a frequency that differs from the designed frequency by an amount necessary to account for the residual error.
There is therefore provided in accordance with a preferred embodiment of the present invention, a radio communication device, which transmits or receives a signal at a predetermined carrier frequency, including a reference oscillator, which generates a clock frequency having a clock frequency error relative to a specified frequency thereof, a processor, which estimates the clock frequency error and parses the error to determine coarse and fine error correction components, at least one frequency synthesizer, which responsive to the clock frequency and to the coarse error correction component, generates a partially corrected frequency having a residual frequency error, which partially corrected frequency is applied to process the signal, and signal processing circuitry, which applies the fine error correction component to process the signal so as to correct the residual frequency error.
Preferably, the reference oscillator includes a crystal oscillator which is not a temperature-compensated crystal oscillator.
Preferably, the reference oscillator does not receive a control input for adjustment of the clock frequency.
Preferably, the processor estimates the clock frequency error responsive to a known operating characteristic of the reference oscillator.
Preferably, the device includ
DSPC Israel Ltd.
Legree Tracy
Welsh & Katz Ltd.
LandOfFree
Frequency tuning for radio transceivers does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Frequency tuning for radio transceivers, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Frequency tuning for radio transceivers will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2934144