MXPA97003412A - Method and apparatus for temperature compensation of a reference oscillator in a communication device - Google Patents

Abstract

The present invention relates to a reference oscillator (118) in a communication device (100) for example a radiotelephone is controlled by compensating the temperature of the reference oscillator (118). At the time of manufacture, the characterization data is stored in the non-volatile memory (128) in the communication device (100). When the communication device 100 is turned on, the characterization data is read and used to provide an initial correction 206 at the reference frequency of the reference oscillator 118. Subsequently, an automatic frequency control operation is performed using the radiofrequency signal received from a remote transmitter as a reference signal. A frequency correction (214) is determined in the form of step size and frequency step direction. The output frequency of the reference oscillator of repeated steps until the frequency error is minimized

Description

METHOD AND APPARATUS FOR TEMPERATURE COMPENSATION OF A REFERENCE OSCILLATOR IN A COMMUNICATIONS DEVICE Field of the Invention This invention relates to communication devices in general. The invention relates more particularly to the compensation of the temperature of a reference oscillator in a communications device.

Background of the Invention Communication devices such as radiotelephones generally include a radio receiver and / or a radio transmitter. To control the frequency of reception or transmission, the radio also includes a local oscillator or reference oscillator. The reference oscillator provides an oscillating signal at a known frequency. Commonly, a crystal is used as the source of the oscillating signal. The receiver and the transmitter are tuned to the appropriate frequencies using this oscillating signal as a reference.

To achieve reliable operation, the signal produced by the reference oscillator must maintain its frequency without shifting. Two sources of shifting in the output frequency are the variation in the operating temperature of the reference oscillator and the aging of the crystal in the operating time of the communications device.The compensation is used to produce an oscillating signal that is largely immune to variations in temperature and due to aging.

A known method of compensation in the communication device is automatic frequency control using the frequency of a received signal as a stable frequency reference. For ex. a mobile station in a communication system can adjust its reference oscillator by comparing the frequency of the oscillating signal produced by the reference oscillator with the frequency of the signals received from the base station. In response to a binary error signal produced in response to the comparison, the mobile station adjusts the frequency of the reference oscillator, reducing the frequency if the error signal is "Hl" and increasing the frequency if the error signal is "LO" " By repeating the comparison periodically, the frequency of the reference oscillator remains within acceptable limits.

The effectiveness of this method is limited by the uncertainty regarding the magnitude and the sign of the frequency error after activation. The automatic frequency control may have to spend a substantial amount of time searching for the correct correction, creating a time delay in activating the radio connection. This delay may be inconvenient or unacceptable to the user.

Another known method of compensation in a communication device is to characterize the frequency variation of the reference oscillator with the temperature at the time of manufacture. The data that relate to the variation of frequency with temperature are stored in a memory to call them later. Then, during the operation of the radio, the temperature measurements are made repeatedly using a thermometer. A decoder and controller respond to an analog voltage or temperature signal from the thermometer to read an appropriate correction factor from the memory and apply a control voltage to the local oscillator. After having applied the control voltage, the controller measures the frequency error between the local oscillator and a transmitted signal received by the radio. If the error is within acceptable limits, the temperature is read again and the process is repeated. If the error is not within acceptable limits, the correction is made by detecting the degree of frequency shift and generating a control voltage until the frequency shift is compensated. New data indicating the appropriate control voltage for that temperature is stored in memory.

The need to measure continuously the temperature limits the effectiveness of this conventional method. Temperature measurements by a thermometer are subject to error. Another error occurs when the analog voltage corresponding to the temperature is quantified as a digital number. The calculation to determine the control voltage for the particular temperature produces one more error. Even small errors can produce substantial errors in the determined output frequency of the reference oscillator. For ex. a frequency error of + l, 5ppm corresponds to +1.35 KHz at 900 MHz of carrier frequency. This is a substantial error for radios used in a communications system in accordance with the Advanced Mobile Telephone Service (AMPS) standards, where the bandwidth of the channel is 30 KHz or for radios in accordance with the standards of the Advanced Mobile Telephone Service. Narrowband (NAMPS), where the bandwidth of the channel is only 10 KHz.

Accordingly, it is necessary in the art to find an improved method and apparatus for compensation of the temperature of a reference oscillator in a communications device.

Brief Description of the Drawings The features of this invention, which are considered new, are particularly described in the appended claims. The invention, together with other objects and advantages, may be better understood by referring to the following description, taken together with the accompanying drawings, in which different figures the same reference numbers identify the same elements, and where: FIG. is a block diagram of a communication device according to the invention. Fig. 2 is "a flow diagram illustrating a method of operation of the communication device of Fig. 1 according to this invention.

Fig. 3 is a flow diagram illustrating also a method of operation of the communication device of Fig. 1 according to this invention.

Detailed description of the invention Referring now to FIG. 1, a communications device 100 includes an antenna 101, a receiver 102, a frequency error detector 103, a demodulator 104, a sound processor 106 and a controller. The communications device 100 also includes a modulator 110, a transmitter 112 and a power amplifier 114. The communications device 100 also includes a digital-to-analog converter or DAC 116, a reference oscillator 118, a frequency synthesizer 120 and a voltage controlled oscillator or VCO 122. To control the operation of the communication device 100 by a user, the communication device 100 also includes a subscriber interconnect 124. To read the temperature, the communication device 100 also includes a temperature sensor and for storing data, the communication device 100 also includes a memory 128. The communication device 100 may be a radiotelephone, e.g. a cellular or cordless handset, or the communications device 100 may be a mobile radio.

For the reception of voice or data by the communications device 100, the antenna 101 is configured to receive electromagnetic energy transmitted by a remote transceiver and produce electrical signals in radio frequencies. The receiver 102 is coupled to the antenna 101 and is configured to receive a radio frequency (RF) signal. In general, the radio frequency signal is transmitted by a remote transceiver according to a predefined communication protocol. Examples of these protocols include the Advanced Mobile Telephone Service (AMPS) or Advanced Mobile Narrowband Telephone Service (NAMPS). In these protocols, the communication channels are predefined at specified frequencies for the transmission and reception of radio frequency signals between the communications device 100 and the remote transceiver. The radiofrequency signals are suitably modulated to carry voice or data information. The demodulator 104 receives the modulated radiofrequency signals from the receiver 102 and demodulates them to extract voice or data. The demodulator 104 provides the voice or data to the radio processor 106 for conversion into sound signals. An indication of the sound signals is provided to the controller 108 and the user interconnect 124. The user interconnect 124 generally includes a speaker, a microphone, a keyboard and a screen.

For the transmission of voice or data from the communications device 100, the controller 108 and the user interconnect provide an indication of sound signals to the sound processor 106. The sound processor 106 conducts the voice or data to the modulator 110, which provides sound signals to the transmitter 112. The transmitter 112 modulates a radiofrequency carrier signal in accordance with the radio signals received from the modulator 110 to produce radio frequency signals. The amplifier 114 amplifies the radiofrequency signals and provides them to the antenna for transmission to the remote transceiver.

To provide cnaal selectivity at the receiver 102 and the transmitter 112, the VCO 122 provides a well-regulated oscillating signal to the receiver 102 and the transmitter 112 under the control of the synthesizer 120. The synthesizer 120 provides an oscillating signal to the VCO 122 at a frequency of reception or transmission, or several of them. In response to the control signals received from the controller 108, the synthesizer varies the reception or transmission frequency to tune the receiver 102 or the transmitter 112, respectively with a particular channel or frequency. Some lines and other connections are omitted from the block diagram of Fig. 1 so as not to unduly complicate the drawing. The oscillating signals provided to the VCO 122 by the synthesizer 120 are produced in response to an oscillating signal provided by the oscillator 118.

The reference oscillator 118 includes an oscillator circuit 130, a crystal 132 and a varactor 134. The reference oscillator 118 has an output 136 for providing the oscillating signal and an input 138 for receiving a control voltage. The reference oscillator 118 is configured to provide an oscillating signal to the output 136 at an output frequency. The output frequency varies with the temperature according to a predetermined ratio. The output frequency is variable in response to the control voltage. The control voltage is an analog voltage. To allow the controller 108 to vary the control voltage, the 'DAC 116 has an input 140 for receiving the digital control signal from the controller 108. In response to the digital control signal, the DAC 116 produces an analog voltage at an output 142 corresponding to the digital control signal. Preferably, the DAC 116 is a digital to analog bit converter, but other configurations can also be used.

The crystal 132 has a terminal 144 and a terminal 146. The crystal 132 provides a stable frequency impedance for the reference oscillator 118. The characteristic frequency of the crystal 132 has a frequency variation with temperature defined according to the following ratio: where ? F is the frequency variation in parts per million (ppm), A and C are coefficients and Ti is the inflection temperature. According to the invention, the glass 132 printed directly on the packaging a crystal code that defines the ratio between the frequency and the temperature of the crystal. Specifically, the crystal code includes letters or numbers that define A, C, and T.

The oscillator circuit 130 is a conventional circuit that receives the oscillating signal from the crystal 132 and leads it to the synthesizer 120. The oscillator circuit 130 can provide regulation, impedance matching, level displacement and other functions to ensure accurate reception of the signal. oscillating signal on synthesizer 120.

The temperature sensor 126 has an output 148. The temperature sensor 126 provides at the output 148 an analog signal proportional to the measured temperature. Alternatively, the temperature sensor 126 can be formed using a thermistor or a diode. Preferably, the temperature sensor 126 is located in the communication device 100 as close to the reference oscillator 118 as possible to ensure that the temperature sensor 126 measures the temperature of the reference oscillator 118, in general, and the crystal 132 in particular. This positioning minimizes errors due to the temperature gradients in the communications device 100. Similarly, the varactor 134, which may also have a temperature dependence, should be placed close to the glass 132 and the temperature sensor 126.

The memory 128 is preferably a non-volatile read-write memory, e.g. an electrically erasable programmable read-only memory, or EPROM. The memory is steered by the controller 108 to store and read data. Non-volatility is an important attribute of the memory 128 so that the data stored in the memory 128 during manufacture and testing of the communication device 100 is retained in the memory 128 for use during the subsequent operation of the communication device.

The frequency error detector 103 has a first input 154 coupled to the receiver, a second input 156 coupled to the VCO 122 and an output 158 coupled to the controller 108. On the first output 154, the frequency error detector 103 receives a signal of reference from the receiver. The reference signal is a radiofrequency signal transmitted by the transceiver and received by the receiver 102. The frequency at which this reference signal is received is set to be accurate for the channel on which the communications device 100 is transmitting. As will be described below, the frequency of the reference signal is used to provide automatic frequency control by the communications device 100. In the second output 156, the frequency error detector 103 receives the output signal of the reference oscillator. 118 The frequency error detector 103 produces a frequency error indication between the output frequency and the reference oscillator 118 and a frequency 1 received, the frequency of the reference signal. In accordance with this invention, the output of the frequency error detector 103 is in the form of a frequency correction step size and a frequency correction step direction. The frequency correction step size is preferably a binary value indicating a degree of increase or decrease of the output frequency of the reference oscillator to reduce the frequency error indication, and the frequency correction step direction indicates whether the output frequency must be increased or decreased. The frequency error detector 103 is then coupled to the receiver 102 to determine a frequency error indication between the reference frequency and the output frequency of the reference oscillator 118.

The controller 108 is preferably a microcontroller. The controller 103 operates in response to a predetermined instruction program and data stored in memory, e.g. the memory 128. Among these instructions is a group of instructions - that form - the automatic frequency control, or AFC routine 152. The controller 108 may itself include memory for storing instructions or data, or additional storage devices such as a random access memory (RAM) or a read-only memory (ROM) can be coupled to the controller 108.

The controller 108 includes an analog to digital converter or ADC 150 coupled to the temperature sensor 126. The ADC 150 receives the analog signal produced by the temperature sensor 126 and produces a digital value that is used by the controller 108 as a temperature indication . The ADC 150 is preferably an analog to digital converter of 8 to 10 bits. Alternatively, the ADC 150 may be a separate element of the communications device 100. However, it is preferred to use a controller eg. the controller 108 that incorporates an ADC eg. the ADC 150 in the controller, since the design goals for the communications device include minimizing the number and physical size of the communications device parts 100 in order to minimize the manufacturing cost.

According to this invention, at the time of manufacture, the glass code is automatically read from the glass 132 using the machine vision incorporated in the manufacturing equipment. These values are conducted from the manufacturing equipment to the controller 108, processed by the controller 108 and stored in the memory 128 as data representative of a relationship between a temperature indication and a corresponding output frequency of the reference oscillator 118 and the crystal 132. Subsequently, during the operation of the communication device 100 at the time the communication device 100 is turned on, the controller 108 determines a temperature indication of the reference oscillator or crystal 132. In response to the temperature indication, the output frequency of the reference oscillator is corrected to a nominal value for that temperature. The output frequency is curved according to the following equation: (2) curvature = B + A (TAD-Ti) + C (TA / D-Ti) 3 where A, B and C are coefficients read from memory 128 and TA / D is the temperature indication provided to controller 108 by temperature sensor 126 and ADC 150. Controller 108 curves or corrects the output frequency of reference oscillator 118 by providing a control signal to DAC 116 in response to the indication of temperature and data of the memory 128. In response, the DAC 116 provides a control voltage to the reference oscillator to adjust the output frequency of the reference oscillator 118. After turning on the communications device 100, during continuous operation of control of communication device 100, no temperature measurements are made for the purpose of correcting frequency errors.

After correction or initial temperature compensation of the output frequency of the reference oscillator using the stored data, the correction of the frequency error is provided to the automatic frequency control under the direction of the AFC 152 routine. The error detector of frequency 103 determines via indication of frequency error between the output frequency of the frequency oscillator and the received frequency of the reference signal, received from the remote transceiver. The frequency error may be due to the variation in the temperature of the communication device 100, particularly the variation in the temperature of the crystal 132, or it may be due to the aging of the crystal 132. In response to the size of the frequency step correction and the direction of frequency step correction, the controller 108 provides a control signal to the DAC 116 which, in response, provides a control voltage to the reference oscillator 118. The process of detecting frequency errors, determining step size and direction Step and correction is repeated until the frequency error indication is less than a predetermined maximum. Therefore, after the first correction of the output frequency, the controller 108 provides the control voltage to the reference oscillator 118 in response to the frequency error indication.

Because the automatic frequency control function is applied using only discrete frequency steps at a specified step size and frequency step direction, limits are needed to ensure that the frequency step is an exact choice and it is not possible to move the communication device more out of frequency setting with the received signal. In accordance with this invention, one or more limit values are calculated at the time of manufacture in response to the data stored in the memory 128 and the temperature indication provided by the temperature sensor. Typical relationships to initially determine limit compensation values are indicated below: limit = 0.82 Ks - 2 2 - . 2 - Wi.

Here, i and W2 are digital to analog converter step values and F2 and Fi are frequency values. Other limit calculations may also be adequate.

The limit compensation value is preferably the same for each curvature value independent of temperature. However, when the temperature changes, the curvature value changes as defined by the equation (2) previous. Consequently, the limit value is corrected periodically according to the new temperature. Therefore, after waiting for a predetermined period, the communication device according to this invention determines an indication of the temperature of the oscillator and, in response to the indication of the temperature, updates the limit value. The limit value is updated by determining the curvature value, then adding to the curvature value, the limit compensation determined initially according to equations (3) - (6).

With reference to the. Fig.2, it shows a flow diagram illustrating the operation of the communications device. 100 of FIG. 1 according to the invention. The method begins at step 202, after the characteristic data of the predetermined relation between the output frequency of the reference oscillator and the temperature are stored in the memory 128. In step 202, the communications device is turned on. In step 204, the method includes determining an indication of the temperature of the reference oscillator 118. That is, the temperature sensor 126 provides an analog signal indicating the temperature, whose signal is converted into digital data by the ADC 150.

In step 206, in response to the temperature indication, the method proceeds to correct the output frequency of the reference oscillator 118. The controller 108 reads the data from the memory 128 which defines the coefficients A, B and C corresponding to the temperature indication and calculates a curvature value according to equation (2) above. The controller 108 provides a control signal to the DAC 116 to bend the reference oscillator 118, the control signal is converted into a control voltage by the DAC 116.

The method continues in step 208 where the communications device 100 sweeps in search of a valid channel. The controller-. 108. controls the synthesizer 120 to tune the receiver 102 to the frequencies corresponding to the channels defined by the communication protocol, e.g. AMPS If a valid channel is not located, in step 210, the control returns to step 204 to repeat the temperature measurement. If a valid channel is located, control continues in step 212.

In step 212, the frequency error is measured. The frequency error detector 103 compares the output frequency of the reference oscillator 118 and the frequency received, the frequency of the reference signal. The frequency error detector 103 determines a frequency correction in the form of frequency correction size and frequency correction step direction, provided to the controller 108. For example, the frequency correction step direction may indicate the change of the output frequency of the reference oscillator by a factor of two and the frequency correction step direction may indicate increase, rather than decrease, of the frequency. Alternatively, the frequency correction step size can be predefined as a default value selected by the controller 108.

In step 216, the controller 108 determines whether the frequency correction step size is smaller than a limit value. When the frequency correction is less than the limit value, in step 218 the method takes the frequency step, corrects the output frequency of the reference oscillator 118 according to the frequency correction. The frequency error is measured again in step 220 and compared with a predetermined maximum error. In step 222, if the frequency error is not less than the predetermined maximum, the control returns to step 214 for a new frequency correction. If, in step 222, the frequency error indication is less than the predetermined maximum, the method ends in step 224. Alternatively, the method may terminate when the frequency correction step address changes sign or its binary value , indicating that the difference between the output frequency of the reference oscillator 118 and the received frequency has a changed sign. This corresponds to a zero error coordinate.

If, in step 216, the frequency correction step size was not less than the limit value, the control returns to step 204. to measure the temperature again. It should be noted that this is an unlikely condition and indicates the failure of a component.

To improve the accuracy of the automatic frequency control operation, steps 214,216,218,220 and 222 can be repeated using the smaller step sizes successively. For example, in a "quick fix" or coarse setting operation, the frequency step size may be relatively large and the reference oscillator output frequency step given up to the frequency correction step direction changes sign. Therefore, this method according to the invention includes the step of adjusting the output frequency by a coarse pitch size until the difference or error satisfies a first condition. At that point, the step size can be reduced, eg. , to one-half or one-third of the size of the quick fixing step for carrying out a "slow fixing" operation. When the frequency correction step direction again changes sign, the step size may be reduced one final time, again by a factor of one-half or one-third, for example. With this small step size, the automatic frequency control routine performs a "tracing" operation, keeping the frequency of the reference oscillator output closely fixed to the received frequency. Therefore, the method according to this invention would include the additional step of adjusting the output frequency in a fine pitch size until the difference or error satisfies a second condition. The first condition and the second condition correspond to the change of sign or direction of the frequency error correction. However, other conditions, po. ex. Obtaining a frequency error less than a predetermined degree can also be applied.

Fig. 3 is a flow diagram illustrating a method according to this invention for updating the limit value of the pitch size. This routine is preferably carried out periodically, for example, four times per minute while the communications device is operating. The method begins in step 302. In step 304, it is determined whether a predetermined time eg. 15 seconds (named in Fig.3 as term), has elapsed. If affirmative, the control proceeds to step 306. If the period has not elapsed, the control returns to step 304. In step 306, the temperature is measured, producing a temperature indication of reference oscillator 118. In step 308, the term or limit values are updated by determining the curvature value corresponding to the temperature of the moment and adding the limit compensation to the curvature value. The control then returns to step 304 to wait for the predetermined time to elapse.

As can be seen from the above, this invention provides a method and apparatus for controlling a reference oscillator in a communications device by compensating the temperature of the reference oscillator. At the time of manufacture, the characterization data is stored in a non-volatile memory in the communications device. When the communications device is turned on or off again, the characterization data is read and used to provide an initial correction to the output frequency of the reference oscillator. Then, a frequency control operation is performed using the radio frequency signal received from a remote transmitter as a reference signal. A frequency correction is determined in the form of frequency step size and step direction. The output frequency of the reference oscillator takes repeated steps until the frequency error is minimized. While a preferred embodiment of the invention has been described and shown, modifications may be made to it. For example, if the transmission path including the modulator 110, the transmitter 112 and the power amplifier 114 are erased, the communications device 100 can form a pager receiver with all the benefits and advantages of this invention. Accordingly, it is intended to cover with the appended claims all changes and modifications that are within the true spirit and scope of the invention.

Claims (10)

1'. A method for controlling a reference oscillator (118) in a communications device (100), the method characterized by: (a) determining a temperature indication of the reference oscillator (204); (b) in response to the temperature indication, correcting an output frequency of the reference oscillator (206); (c) selecting a valid channel for communication (208); (d) determining an indication of frequency error between the output frequency of the reference oscillator and a received frequency (212); (e) in response to the frequency error indication, determine a frequency correction (214); (f) when the frequency correction is less than a limit value, correct the output frequency of the reference oscillator according to the frequency correction (218); and (g) repeating steps (d) - (f) until the frequency error indication is less than a predetermined maximum.

2. A method for controlling a reference oscillator according to claim 1, characterized by periodically correcting the limit value (308).

3. A method for controlling a reference oscillator according to claim 2 characterized also in that the step of periodically correcting - the limit value includes the following steps: (h) waiting for a predetermined term (304); (i) determining a temperature indication of the reference oscillator (306); and (j) in response to the temperature indication of the reference oscillator, update the term (308).

4. A method for controlling a reference oscillator according to claim 1 characterized in that the step of determining a frequency correction also comprises the steps of determining a frequency correction step size and a frequency correction step direction.

5. A method for controlling a reference oscillator according to claim 1 further characterized by, initially, storing data representative of a relationship between the temperature indication and a corresponding output frequency of the reference oscillator, and in that the step (b) comprises the step of adjusting the output frequency in response to the temperature indication of the reference oscillator and the data.

6. A method for controlling a reference oscillator according to claim 5 further characterized in that steps (d) of determining the frequency error indication and (f) correcting the output frequency of the reference oscillator exclude the determination of an indication of temperature.

7. A communication device (100) characterized by: a receiver (102) configured to receive a radio frequency (RF) signal at a reference frequency; a reference oscillator (118) configured to provide an oscillating signal at an output frequency, the output frequency varies with the temperature according to a predetermined ratio, the output frequency is variable in response to a control voltage; a memory (128) for storing data, the data is characteristic of the predetermined relation; a frequency error detector (103) coupled to the receiver to receive a frequency error indication between the reference frequency and the output frequency; a temperature sensor (126) to provide a temperature indication; and a controller (108) coupled to the receiver, the memory, the frequency error detector, the temperature sensor and the reference oscillator to receive the temperature indication, the frequency error indication and the data, the controller first corrects the output frequency providing the control voltage to the reference oscillator in response to the temperature and data indication, the controller then provides the control voltage to the reference oscillator in response to the frequency error indication.

8. A communication device according to claim 7, characterized in that the control voltage corresponds to a frequency correction step size and a frequency correction step direction, the controller determines a time period in response to the temperature indication and the data, the controller provides the control voltage only when the step size of frequency correction is smaller than the limit value.

9. A communication device according to claim 8 further characterized in that the controller repeatedly increases the control voltage in accordance with the frequency correction step size and the frequency correction step direction until the frequency error indication is less than a predetermined maximum.

10. A communication device according to claim 9, characterized in that the controller periodically receives the temperature indication and updates the limit value in response to the temperature and data indication.