Classifications

• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03L—AUTOMATIC CONTROL, STARTING, SYNCHRONISATION OR STABILISATION OF GENERATORS OF ELECTRONIC OSCILLATIONS OR PULSES
  • H03L1/00—Stabilisation of generator output against variations of physical values, e.g. power supply
  • H03L1/02—Stabilisation of generator output against variations of physical values, e.g. power supply against variations of temperature only
  • H03L1/022—Stabilisation of generator output against variations of physical values, e.g. power supply against variations of temperature only by indirect stabilisation, i.e. by generating an electrical correction signal which is a function of the temperature
  • H03L1/026—Stabilisation of generator output against variations of physical values, e.g. power supply against variations of temperature only by indirect stabilisation, i.e. by generating an electrical correction signal which is a function of the temperature by using a memory for digitally storing correction values

Definitions

• the present invention relates to a self-calibrating temperature-compensated frequency source, and, particularly but not exclusively, to a method and apparatus of maintaining an operating frequency of a frequency source, which is normally locked to a received external reference frequency, when said external reference frequency is not being received.
• the carrier frequency is one of a number of parameters used to specify the system. Accurate knowledge of the carrier frequency of a received signal can enhance the performance of a system to a degree depending on the signal processing techniques employed.
• the remote unit e.g. car phone
• the base station frequency has to process weak signals in a noisy radio environment.
• Such mobile communication systems could benefit significantly in performance if the remote unit contained an accurate frequency source from which the carrier frequency could be derived.
• many of the characteristics of highly accurate frequency sources make them unsuitable for mobile applications; notably these sources possess one or more of the ' following attributes:
• the remote unit receives the external reference signal, e.g. when the remote unit is out of range of the base signal transmitter o when there is an obstruction between the base transmitter and the remote unit which prevents transmission reaching the remote unit. If the remote unit is then required to transmit, it will be capable of maintaining the required level of frequency accuracy only for as long as its local frequency source does not drift from the locked setting by more than allowable limits (Note that in generally known systems the stimulation which causes the remote unit to transmit will have been generated locally, as the control signalling which would normally be used to stimulate transmission is not being received).
• the maximum slope of the characteristic is significantly greater than would be the case for a linear characteristic.
• the maximum slope may be as high as 0.5 ppm per °C. Therefore it will only require a small temperature change when the remote unit i ⁇ not receiving the external frequency reference signal for the local frequency signal to drift substantially, and pos ⁇ ibly outside the required range. This order of temperature change could occur in a very short time, especially in the case of a remote unit which has been stationary (perhaps in a sheltered environment) and then becomes mobile.
• Temperature compensated crystal oscillators are available. These are generally calibrated in the factory by scanning the oscillator through a predetermined temperature range and plotting the compensation necessary to maintain the stable frequency output as the temperature changes. The problem with this type of oscillator is that it is moderately expensive due to the need to perform this calibration step in the factory and also as the crystal ages some frequency drift will occur which is not compensated for by the predetermined temperature compensation. These oscillators will therefore tend to drift anyway over a period of time, and to maintain accuracy it is necessary to send them back to the factory for recalibration at regular intervals.
• the present invention relates generally to a temperature-compensated frequency source which is arranged to be self calibrating while actually in operation when receiving an external reference frequency signal, so that when the external reference frequency is no longer available it can utilise the calibration information to perform frequency control.
• the present invention provides a method of operating a frequency source, comprising the step ⁇ of: receiving an external reference frequency ⁇ ignal when available and controlling the frequency source to lock the operating frequency of the frequency source to the reference frequency signal; during operation of said frequency source when said external reference frequency is being received, detecting temperature of the frequency source and storing temperature information relating to detected temperatures and control information relating to control settings required to maintain frequency lock for the detected temperatures; and when said external reference frequency signal is not being received, detecting the temperature of said frequency source and utilising the stored temperature and control information to provide a control setting in accordance with the detected temperature, for controlling the operating frequency of the frequency source.
• the in-field calibration operation has the advantage that the device may still be in use when the calibration is occurring and means that it will not have to be taken out of use and.sent back to a factory in order to be recalibrated. It has the further advantage that the operation automatically compensates for aging of the crystal as calibration may continuously be updated as the frequency source is being used.
• Temperature and control information is preferably stored in the form of a look-up table, in, for example, a memory such as a non-volatile memory. For example, a temperature reading may be taken at a detected temperature of the frequency source and stored in the non-volatile memory, together with the frequency control setting necessary to maintain the frequency output signal in lock with the external frequency reference at that temperature.
• the frequency setting may be the setting to control, for example, a voltage controlled oscillator which accesses the frequency source.
• the stored look-up table will therefore contain a number of discrete temperatures and associated control values, and can be used to provide the correct control value for an actual detected temperature when the external frequency reference is not being received.
• Information regarding the age of any stored temperature and control information is also preferably available. This "age information" can be used to determine whether updating of stored information is necessary e.g. only information over a predetermined age is updated. Further, if a stored value is over a predetermined age it may not be used in the providing of the control setting.
• temperature and control information is only stored in relation to predetermined nominal detected temperature values.
• temperature and control information readings are preferably taken when the actual detected temperature value falls within a predetermined range of the predetermined nominal detected temperature value, i.e. the detected temperature does not exactly have to coincide with the nominal detected temperature value for a reading to be taken. In this case, it is preferred that the offset between the actual and nominal temperature value is taken and stored.
• the control setting is provided by taking a plurality of stored temperature and control information readings, in the vicinity of the actual detected temperature, and interpolating or extrapolating to find the control setting for the actual detected temperature in accordance with a predetermined algorithm.
• the predetermined algorithm is preferably a least-squares curve fitting algorithm which is suitable for the type of frequency source being used.
• the characteristic temperature/frequency curve is a cubic curve, and known cubic polynomial curve fitting techniques can be applied.
• a multi-point curve fitting technique is preferably utilised.
• the predetermined range may be varied in accordance with the age of temperature and control information already stored. For example, if a stored temperature and control information reading is "outdated", according to predetermined criteria, the predetermined range about the particular nominal detected temperature value may be extended so that it would be more likely that a new reading will be taken to replace the outdated reading.
• the present invention has the advantage that a frequency source e.g. of a remote unit in a cellular telephone system, may be maintained with reasonable accuracy for several days following isolation from the reference frequency transmission.
• the present invention further provides apparatus for operating a frequency source, comprising; means for receiving an external reference frequency signal when available; control means for controlling the frequency source to lock the operating frequency thereof to the external reference frequency signal; temperature detecting means for detecting the temperature of the frequency source; storage means for storing, during operation of the frequency source, when said external reference frequency signal is being received, temperature information relating to detected temperatures and control information relating to control settings required to maintain frequency lock for the detected temperatures; and control setting providing means for providing a control setting to control the operating frequency of the frequency source when no external reference frequency signal is being received, said control setting means utilising the stored temperature and control information to provide the control setting in accordance with the detected temperature.
• the apparatus of the present invention may have the same advantageous features as discussed in relation to the above method.
• Fig. 1 shows a schematic block circuit diagram of apparatus in accordance with an embodiment of the present invention for illustrating an embodiment of a method in accordance with the present invention
• Fig. 2 is a schematic diagram illustrating a method of storing data in accordance with the embodiment of Fig. 1.
• the apparatus comprises a frequency source 1, which may, for example, be a voltage controlled crystal oscillator of known type. It also includes a temperature measuring device 2 which is thermally coupled 3•to temperature sensitive components of the frequency source. The temperature measuring device 2 and frequency ⁇ ource 1 are connected to control means 4. The control means 4 is arranged to receive temperature readings 5 and to provide a control output 6 to control the frequency ⁇ ource. Data for use by control means 4 is stored in memory 7 which may include a non-volatile memory.
• Reference numeral 8 discloses a circuit for subtracting a frequency output 9 of said frequency source 1 from an external reference frequency 10 to produce a frequency error reading 11.
• the apparatus is provided with means, such as an aerial, (not shown) for receiving an external reference frequency signal.
• the output 9 of the frequency source 1 is locked to the external reference frequency signal 10.
• Methods for locking an output frequency of a frequency source to an external reference frequency signal are known and therefore will not be described in detail here, as the present invention relates to self-calibration and temperature compensation not to methods of frequency lock.
• the output frequency 9 of the frequency source 1 is subtracted from the received and detected external reference frequency 10 in subtraction circuit 8 to give an output which represents an error frequency 11.
• the error frequency 11 is input to the control means 4, which includes means for determining whether the error frequency 11 corresponds to a desired predetermined error frequency. If it does so correspond then the frequency output 9 of the " frequency source 1 is of the correct value.
• control means 4 acts to adjust the frequency control output _6 to cause the output frequency 9 of the frequency source 1 to be adjusted until the error frequency 11 does correspond with the predetermined desired error frequency.
• This is a standard negative feedback technique and it will be appreciated that it can be implemented in a number of ways.
• the external reference frequency signal may be transmitted in a number of known forms. For example, it may be transmitted as a continuous signal or it may be transmitted as a frequency burst together with information which allows the remote unit to detect the frequency burst and extract the . eference frequency information. Such frequency bursts would be transmitted on a regular basis in order to maintain frequency lock.
• the reference frequency is preferably automatically received by the remote unit. However, in some cases it may be necessary to perform a user operation to receive the signal, e.g. press a control button.
• the output frequency 9 may or may not be the same value as the reference frequency value. It could, for example, be any multiple or sub multiple value of the reference frequency.
• self-calibration occurs automatically and continually.
• the temperature measurement device 2 detects the temperature of the temperature sensitive components of the frequency source 1 over a period of time as the frequency source is being used.
• the detected temperature readings are input to the control means 4 which cause the detected temperatures to be stored in storage device 7 together with associated control information representing the value of the control output 6 necessary to maintain frequency lock for the particular detected temperature.
• the present temperature of the frequency source 1 is measured by the temperature measuring means 2 and input to the control means 4.
• the control means has reference to the memory means 7 and extracts control information for producing a control output 6 to maintain the output frequency 9 of the frequency source 1, in accordance with the value of the detected temperature.
• Fig. 2 illustrates the storage of the temperature information and control information in the ⁇ torage means 7 in accordance with the particularly illustrated embodiment.
• the temperature and control information is stored in storage mean ⁇ 7 a ⁇ a plurality of discrete readings in the form of a "look-up" table. Each reading is stored in the form illustrated in Fig. 2 and takes up three bytes of digital memory, byte 1, byte 2 and byte 3.
• the first six bits of byte 1 are taken up by data for indicating when the reading was stored, i.e. "age information" 20.
• the age information is indicative of the number of days for which the particular reading has been stored. It may be implemented by, for example, clearing the age information bits 20 whenever a new reading is stored and updating the age information 20 by one digital bit every day as long as the same reading remains, under control of control means 4.
• the last part of byte 1 and the first part of byte 2 contain temperature information 21 as six bits of data.
• readings are only stored in relation to predetermined nominal detected temperature values, which are chosen to provide a reasonable distribution of readings to allow interpolation and extrapolation to produce control signals for detected temperatures which do not actually coincide with any of the nominal temperature values.
• predetermined nominal detected temperature values which are chosen to provide a reasonable distribution of readings to allow interpolation and extrapolation to produce control signals for detected temperatures which do not actually coincide with any of the nominal temperature values.
• the calibration step it would not be efficient to wait until the detected temperature of the frequency source coincided exactly with a predetermined nominal temperature value before taking a reading of temperature and control information. Instead, the reading is taken whenever the detected temperature is close enough to the predetermined nominal temperature, i.e. when the detected temperature falls within a predetermined range of a predetermined nominal temperature value.
• the temperature information 21, therefore, rather than being stored as a direct digital indication of temperature value is stored as an offset value, indicating the temperature offset of the actual detected temperature reading from the predetermined nominal detected temperature value.
• the nominal temperature value for the particular reading may be determined from the position of the particular reading in the look-up tables (i.e. memory address).
• the calibration step occurs when the external reference frequency signal is being received and the frequency source 1 is operating and the output frequency 9 is locked to the external reference frequency 10.
• Temperature measuring device 2 measure ⁇ the temperature of the temperature sensitive components of the frequency source 1, and the temperature reading i ⁇ input to the control means 4.
• the control means 4 determines whether the detected temperature value falls close to a predetermined nominal temperature value (i.e. whether it falls in the predetermined range of the predetermined nominal temperature value). If it does not no reading is taken for this temperature. If it does, the age information in any corresponding reading already stored is accessed and if it is determined that the age of the stored data exceeds a predetermined limit then a reading is taken in respect of the present detected temperature and new information is stored in the memory location for the particular predetermined nominal detected temperature value.
• the information stored is as set out above, i.e. the age information 20 is cleared, the temperature information 21 is stored as an offset from the actual nominal temperature value of the actual detected temperature value and the present reading of the control output 6 is detected by the control means in digital form and stored as control information 22.
• readings only be stored for detected temperature values which fall close to the predetermined nominal temperature values.
• the allowable offset for the detected temperature from the nominal temperature value may be increased, in order to give a better chance of obtaining a new reading, i.e. the predetermined range about the nominal temperature is varied in an increasing direction as data age increase ⁇ .
• Thi ⁇ variation in predetermined range may be implemented by known ⁇ oftware techniques by the control means 4.
• Memory 7 include ⁇ a non-volatile programmable memory for storage of the look-up table.
• a non-volatile programmable memory for storage of the look-up table.
• typical non-volatile memories are limited in terms of the allowable number of write cycles for reliable operation (typically 10,000) the rate at which reading updates occur in this particular embodiment are preferably limited to an average update rate of once per day for each memory location.
• the device when the device is produced it may be initially calibrated merely by subjecting the device to a temperature change across a predetermined range whilst the device is locked to an external frequency ⁇ ource. Thi ⁇ will automatically initiate the table for the predetermined temperature range.
• This type of self calibration also has the advantage that compensation for aging of the frequency source is automatically carried out as the look-up table is continually updated during it ⁇ life time. For most voltage controlled crystal oscillator ⁇ frequency drift due to aging will typically be about 0.0003 ppm per day, so a maximum update rate of 1 per day will be ample to provide rea ⁇ onable performance and prevent substantial degradation due to data age.
• the look-up table stored in memory 7 is accessed by control means 4 to provide an appropriate control output 6 for control of frequency source 1 in respon ⁇ e to the actual detected temperature value 5 of the frequency source 1.
• the control means 4 applies a control algorithm for extrapolating or interpolating from a plurality of chosen stored readings.
• the interpolation may merely be a linear interpretation between the plurality of readings whose temperature information falls in the vicinity of the actual detected temperature value.
• the algorithm used will more accurately resemble the frequency/temperature characteristic of the frequency source 1.
• the frequency versus temperature characte istic for a typical SC-cut crystal voltage controlled o ⁇ cillator (which may be used as frequency source 1) is known to be cubic.
• a curve fitting algorithm which fits the cubic polynomial to three or more temperature points will therefore exhibit superior performance to that provided by linear interpolation.
• Such a curve fitting algorithm could be implemented in software in a manner known to persons skilled in the art.
• the control means would therefore determine the actual detected temperature value 5, access the memory 7 to obtain a plurality of readings for temperatures close to the actual detected temperature value, and apply the curve fitting algorithm to the plurality of readings. Appropriate interpolation or extrapolation on the curve then produces the correct digital control setting for the control output 6 for the frequency source 1 for the actual detected temperature 5.
• the proposed curve fitting method preferably provides well optimized performance by using multi-point curve fitting of data in the vicinity of the required temperature.
• the use of multi-point fitting will tend to average out the effect ⁇ of frequency lock errors and Doppler effects (which can cause errors due to the fact - 16 -
• a remote unit could be moving away from the external frequency reference source when frequency lock is being maintained and a reading is taken). It i ⁇ unlikely that the same error will affect all points used in the curve fit.
• the control means is also arranged to acces ⁇ the age of the readings which it takes to provide the interpolation, and any readings which are over a predetermined age will not be used in the interpolation or extrapolation. This prevents old readings from providing anomalous results.
• the predetermined age is a matter of choice for the requirements of the particular ⁇ yste involved.
• the control algorithm may incorporate strategies to allow for temperature points at the extremes of the look-up table, and cases where insufficient data of acceptable age are available for normal procedures to be applied. Such strategies may easily be implemented in ⁇ oftware by person ⁇ skilled in the art.
• the application of a control algorithm means that the number of predetermined readings which must be stored need not be as great as if one were trying to obtain control information directly from the look-up table.
• the remote unit will typically experience temperatures over a certain range of a period of several days. If the present invention is implemented in such a system and frequency reference transmissions have been received during this period, the remote unit will have compiled an up to date look-up table covering the temperature range encountered. If the remote unit is denied frequency reference information and required to transmit, in all probability the temperature range to be encountered for a period of several hours will not significantly exceed the range encountered during the previous days. Allowing for a limited amount of extrapolation outside of the temperature range for which up to date look-up table information is available, the remote unit will in all probability be able to maintain an adequate level of frequency accuracy for the frequency source.
• the performance of the illustrated embodiment of the invention will be dependent upon the resolution and repeatability with which the frequency source setting can be controlled, the resolution and repeatability with which the temperature- .can be measured, the quality of frequency lock achieved while the look-up table is being compiled (this will depend upon the frequency-lock technique used and any change irt external reference frequency due to Doppler effect) , the amount of information stored in the look-up table and the method in which this information is used, the influence of temperature rate-of-change effects which distort the static temperature versus frequency characteristics of the frequency source, errors due to the age of data and the effects of parameters such as supply voltage, shock and vibration on frequency source output frequency. Note that within reasonable limits, the absolute accuracy with which the frequency source can be set and the temperature may be measured is unimportant as long as the setting and measurement are repeatable, unambiguous and exhibit sufficient linearity so as not to excessively compromise resolution over the required range.
• the resolution with which the frequency ⁇ ource setting can be controlled must be such that the worst case smallest step size is less than the accuracy required for frequency control (thi ⁇ will depend on the application for which the frequency source is to be used) .
• Effects which cannot be compensated for by the proposed method are frequency dependence upon supply voltage, frequency change due to shock and vibration, rate of change and hystere ⁇ i ⁇ effects.
• the frequency ⁇ ource and power supply design can readily provide an adequate degree of power supply immunity.
• the effects of shock and vibration are less readily controlled, but by suitable choice of crystal, crystal mounting, oscillator design and physical accommodation, effects can be reduced to an acceptable level.
• Rate of change effects result from temperature gradients appearing acros ⁇ the temperature sensitive elements of the frequency source.
• the ⁇ e can be reduced by grouping the ⁇ e element ⁇ together with good thermal coupling, increasing the thermal mas ⁇ of the temperature sensitive elements, and thermally i ⁇ olating the temperature sensitive elements.
• the embodiment of Figure 7 may be implemented by using an 80C32 processor with external EPROM (as control means 4, with D/A converter - see later) and non-volatile programmable EEPROM (as memory means 7).
• Frequency control of a voltage controlled oscillator as the frequency source is via a serially programmable 12-bit digital to analogue converter controlled by the controller, to generate a voltage which sets the frequency of the voltage controlled oscillator.
• Temperature of the voltage controlled oscillator is measured by a temperature sensor in intimate thermal contact with the voltage controlled oscillator temperature sensitive elements, and an analogue to digital temperature measurement technique which uses a digital to analogue converter and a comparator to implement software-controlled successive approximation under proces ⁇ or control.
• the ⁇ e components can easily be implemented by a person skilled in the art and appropriate software can easily be designed by persons skilled in the art to implement the required algorithms and control processes, in dependence upon the requirements of the sy ⁇ tem with which the frequency source i ⁇ being u ⁇ ed.
• the temperature compensation method need not necessarily be implemented as specifically described in relation to the specifically disclosed embodiment.
• temperature information and control information could be recorded in an analogue form (i.e. an analogue plot of temperature against control information) and control information for an actual detected temperature could be read from the analogue. It is also possible that temperature and control information could be recorded at predetermined times as opposed to in accordance with predetermined nominal temperature value ⁇ .
• a detected temperature value and control ⁇ etting value could be recorded every five ⁇ econd ⁇ and a table of temperature again ⁇ t control ⁇ etting built up from the recorded value ⁇ . Further, in the calibration step it is possible that the reading may only be taken when operation of the frequency source is finished, i.e. after operation has been completed the last values of control ⁇ ignal and temperature before operation ceased (and reception of reference signal ceased) are taken and stored.
• Typical oscillator crystals themselve ⁇ exhibit rate of change and hy ⁇ teresis effects which degrade performance. These effects may be minimised by choice of a suitable cut of crystal.
• crystal parameters are the limiting factor and performance targets yet to be met, an alternative control strategy to that described in relation to the specific embodiment could be used.
• the remote unit which has ⁇ uddenly been denied reference frequency information record ⁇ the current temperature and the la ⁇ t frequency source setting prior to loss of frequency reference information.
• the control algorithm and control means determine a correction to the frequency source setting, rather than determining the absolute frequency source setting for the current temperature.
• Such an algorithm can again easily be designed by a person skilled in the art. By this means the frequency will be corrected from a known accurate starting point and rate of change and hysteresis effects will initially contribute only second order errors.
• the present invention can be used for control of any frequency source, not only frequency sources of remote units in mobile communications systems.
• the control setting need not necessarily be a control setting for a voltage controlled oscillator frequency source (i.e. voltage setting) but could be any control parameter used to control the output of the frequency source.

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• 1990
  • 1990-06-22 EP EP19900909573 patent/EP0482020A4/en not_active Withdrawn
  • 1990-06-22 WO PCT/AU1990/000269 patent/WO1990016113A1/en not_active Application Discontinuation

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