GB2412037A

GB2412037A - Frequency correction of an oscillator in a mobile communication device

Info

Publication number: GB2412037A
Application number: GB0405393A
Authority: GB
Inventors: Richard Ormson
Original assignee: NEC Technologies UK Ltd
Current assignee: NEC Technologies UK Ltd
Priority date: 2004-03-10
Filing date: 2004-03-10
Publication date: 2005-09-14
Anticipated expiration: 2024-03-10
Also published as: GB2423678A; GB2423678B; GB0609072D0; GB0405393D0; GB2412037B

Abstract

To reduce the frequency difference between a crystal oscillator 430 in a mobile communication device 400 and a signal received from a base station, a frequency correction 450 is applied to the crystal oscillator 430 to compensate for frequency offsets due to temperature, voltage and aging of the crystal. The mobile communication device 400 includes a frequency comparator 440 to calculate the difference between the frequency of the crystal oscillator 430 and the received signal, a temperature sensor 510, a voltage supply 460 and a memory 470 in which is stored the temperature dependence 480, the voltage dependence 490 and the aging offset 500 of the frequency of the crystal. Preferably, location determining means 520 is also included to establish the velocity of the mobile communication device 400 in order to determine a frequency offset due to Doppler shift.

Description

24 1 2037 - 1 Apparatus for Reducina Frequency Offset of an Oscillator in

a Mobile Communication Device The present invention relates to an apparatus for reducing frequency offset of an oscillator in a mobile communication device and, in particular, for reducing frequency offset in crystal oscillators.

Mobile communication devices require an internal clock for their correct operation. The clock is synchronized to the frequency of the network via signals received from the base station. It is essential that the clock remain synchronized with the network in order that the device it can accurately time its transmissions and receptions with the network. If the internal clock is not synchronized with the network, the device it can fail to receive transmissions from the base station intended for its reception.

The accuracy required by the internal clock is defined by various mobile technology standards. The most important of these is that the frequency of transmissions from the device must be within 0.1 ppm of the received signal. This is not a particularly onerous requirement as the device must receive before transmitting, so correction of the device's internal frequency can be performed.

There are a number of known software techniques which enable devices to meet the 0.lppm required accuracy of the transmission signal with respect to the received signal. These techniques are collectively referred to as automatic frequency correction (AFC) and are well known. - 2

These techniques involve receiving a signal from the mobile network, comparing the frequency of that signal with the frequency of the internal clock, and adjusting the frequency of the internal clock to match the received frequency. The frequency of the crystal oscillator is controlled by an adjustment voltage. Therefore, once the signal has been received, the difference in frequency between the received signal and the internal crystal is calculated and an appropriate voltage is applied to the lo crystal oscillator in order that the frequency of the crystal oscillator matches that of the network signal.

There is a more problematic requirement arising from the design of mobile devices. Mobile devices rely on a digital signal processor (DSP) to decode received signals. The larger the difference between the received frequency and the internal reference frequency of the device, the greater the processing power required to decode the signal. As DSP's have finite processing power, it is clear that beyond some frequency difference reception of signals will not be possible. Thus there is a limit on the maximum frequency error allowed in the absence of AFC. This limit is device dependent, but is generally around 7.5 ppm.

Although the AFC mechanism works very well, in order for the AFC mechanism to be of any use, the handset must have been able to receive the signal transmitted from the base station. Therefore, the AFC mechanism is only useful to the device if the uncorrected clock, before reception of the base station signal, is within so around 7.5ppm of the frequency of the network signal as discussed above. Hence, this 7.5ppm defines the limit for the quality of the internal clock. - 3

Typically, mobile communication devices use a temperature compensated crystal oscillator (VCTCXO or TCXO) as their internal clock. However, these devices are expensive. In comparison, uncompensated crystal oscillators (VCXO or XO) tend to be around 25 to 50 cheaper than the equivalent temperature compensated unit.

Uncompensated crystal oscillators are typically not suitable for use in mobile communication devices since the frequency accuracy of a VCXO does not meet the lo required limits, i.e. around 7.5 ppm.

The frequency of a crystal is affected by a number of factors, as shown in Table 1. These values represent the quality of currently available VCTCXO and VCXO devices. A handset lifetime of two years has been assumed in calculating the total.

Table 1: Frequency dependence of oscillators with respect to temperature, supply voltage and time.

VCTCXO VCXO

Variation with + 2ppm over + 30 ppm over temperature operating range operating range Variation with + 0.2 ppm over + 5 ppm over supply voltage operating range operating range Variation over + 0.5 ppm per + 5 ppm per year time year Worst case over + 3.2ppm + 45 ppm handset lifetime The table shows that, while the VCTCXO crystal remains within the 7.5 ppm required for reliable operation, the large variation of the VCXO crystal of up \ - 4 to + 45ppm makes it unsuitable for use in mobile communication systems.

Crystal oscillators have a known and reproducible temperature dependency. Devices regularly include temperature sensors in order that the software can adjust the frequency of the crystal oscillator to account for this temperature dependency. In fact, this is how a VCTCXO works but the correction is applied automatically through hardware.

lo The variation of frequency with supply voltage is also a known and reproducible dependency. Fluctuations in the supply voltage typically occur when the battery voltage is reduced.

In contrast, the crystal aging effect is neither known nor reproducible. Furthermore, the aging effect is different for each crystal, as it depends on environmental factors as well as the crystal itself.

Known systems are unable to monitor the aging of the crystal and, therefore, are unable to compensate for the so variation in crystal frequency over time due to aging.

Since the device is unable to correct for temperature, voltage and aging automatically, typical VCXO crystals remain unable to meet industry standards for use in mobile communication devices.

:5 A further problem facing those wishing to align the frequency of the crystal oscillator to that of the network is the effects of doppler shift in the observed frequency of the incoming signal. Any movement of the device with respect to the stationary base station will - 5 - cause the observed frequency of the received signal to be different from that of the absolute frequency of the signal. Therefore, even if the errors in the crystal frequency due to temperature, voltage and aging are compensated for, any movement of the device at the time of reception will cause the received frequency to be different from that expected.

We have appreciated that it will be beneficial to use VCXO crystals in mobile communication devices.

lo Primarily, this would cause a large reduction in the cost of production of the device. However, VCXO crystals tend to be unsuited to use in mobile communication devices due to the large frequency dependence on aging which is not reproducible between crystals. There are also large frequency dependencies on temperature and supply voltage, although these can be overcome by known methods.

Embodiments of the present invention reduce the frequency error of VCXO crystals to within industry standards by automatically compensating the frequency of the internal crystal to account for the temperature, voltage and aging of the crystal.

The correct operation of the device at turn on relies on the storage of aging information from the last time it was operational. Embodiments of the invention calculate the frequency error due to aging as follows.

On receipt of a signal, embodiments of the invention calculate the difference in frequency between the received signal and the internal clock. This is the conventional AFC calculation, and it is applied to so correct the clock in the normal manner. The device then determines the frequency offset caused by temperature and - 6 supply voltage using known temperature and voltage dependencies. These offsets are subtracted from the calculated difference. The remaining frequency difference is due to the aging of the handset and the effects of doppler shift. The device may include components to enable the motion of the device to be calculated and hence enable the frequency offset due to doppler effects to be established. This error is subtracted from the remainder of the correction signal to 0 leave the error due to the aging of the crystal.

Alternatively, if the device does not contain means for determining velocity, a similar (though less accurate) result can be obtained by taking a long term average of the aging plus doppler term. Over a period of several minutes the doppler term will average out and leave only the aging term. This happens because the doppler arises from radial movement relative to a base station. In a cellular network the handover mechanism means that handsets spend as long moving towards basestations (+ve doppler) as away from them (-ve doppler) and so over time the average doppler offset tends to zero. The aging correction is stored within the device and can be updated on subsequent receptions.

The device is able to produce an accurate clock at turn on by using the stored aging term, in combination with calculated terms for the current temperature and supply voltage. Doppler correction is not needed at turn on as the aim is only to allow initial acquisition. The maximum doppler error is less than 0.5 ppm.

so It will be apparent to those skilled in the art that the method for deriving the aging offset of the crystal is an inductive process. In order to allow the - 7 - device to have a stored aging offset value at first operation (i.e. when it has never before received anything from a network) a factory calibration procedure is needed. This process simply involves measuring the output of the handset and adjusting the correction value until the frequency is correct. The temperature and supply terms are deducted from the correction value and the resulting aging term is then stored as above. Since a direct comparison with an accurate clock is provided, lo there is no need to account for doppler.

The invention is defined in its various aspects in the appended claims, to which reference should now be made.

The invention is now described in detail with respect to the accompanying Figures in which; Figure 1 shows the path of communication between the device and a network via a base station; Figure 2 is a flow diagram showing the method for calculating the frequency offset due to the aging of the crystal; Flgure 3 is a flow diagram showing the method for establishing an accurate clock on activation of the device; Flqure 4 is a block diagram showing the components of an embodiment of the present invention.

Figure 1 shows the paths of communication between a mobile communication device 10 and the base station 20.

A signal from the network 30 is transmitted from the base so station to be received by the communication device. The device is able to transmits a signal 40 to the base station 20. - 8

In Figure 2, the signal from the network, transmitted from the base station, is received by the mobile communication device at 210. On receipt of the signal, the device compares the frequency of the incoming signal with that of its internal clock at 220 and stores the frequency difference Af. The device then corrects the frequency of its internal clock in order to match that of the signal received from the network. As discussed above, any difference between the frequency of lo the received signal and the frequency of the internal clock is due to the combined effects of the temperature of the crystal, the voltage supply to the crystal, the effects of aging of the crystal and doppler shift. In order to calculate the change in frequency due to the is aging of the crystal, each of the offsets due to temperature, voltage and doppler shift, must be calculated individually.

It is possible that these effects may, in fact, compensate each other and produce a clock frequency equal so to that of the incoming signal. Therefore, it is important to determine the effects of each offset even if there is no measured difference in frequency between the received signal and the internal clock.

At 230, the device determines the frequency offset 2 due to the temperature of the device. As mentioned above, crystal oscillators have known reproducible temperature dependencies. Typically, this dependency can be determined during manufacture and programmed into the individual handset. This temperature dependence is so stored in a separate area of memory. Therefore, at 230, on receipt of a signal, the device determines the - 9 - temperature of the crystal and calculates any associated offset frequency Af(T).

The variation of frequency of the device as a function of supply voltage is also a known and reproducible dependency. Again, as part of the factory calibration procedure during manufacture, the frequency dependence on applied voltage is determined for each crystal. This frequency dependence can be stored in a separate area of memory. Typically, handsets incorporate lo a battery voltage sensor which may be used to indicate when battery power is low. However, if the crystal supply is derived from the battery by way of a regulator, the regulator characteristics would also need to be monitored. At 240, the device determines the supply voltage to the crystal and, by consulting the memory, determines the frequency offset caused by this supply voltage. This frequency offset is represented by Af(V).

Should it be found that all crystals show the same temperature and supply voltage dependence it is possible so to omit these items from the factory calibration and use a standard set of data.

At 250 the device then determines the frequency offset due to the effects of doppler shift. There are a number of ways to determine the frequency offset due to the effects of doppler shift when a signal is received by a mobile handset. Each of these methods involve determining the velocity of the device with respect to the serving base station in order that the observed doppler shift in frequency due to this velocity can be calculated. ) - 10

One such method is described in our earlier British patent application No. GB-A-2388749. The device calculates its velocity by comparing the frequencies of signals received from a plurality of neighbour base s stations. These frequency differences are caused by different doppler shifts being observed on signals from different base stations due to movement of the device.

These differences are used to derive the velocity of the moving mobile communication device. The offset 0 frequencies due to temperature, voltage and aging are identical for each measurement and are, therefore, cancelled in the calculation.

Further methods involve calculating velocity of the device with respect to a base station by using a location system to monitor change in position over time and, therefore, velocity. A knowledge of the position of the serving base station, which is typically broadcast within the signal, enables the velocity in the radial direction from the base station to be calculated.

The above mentioned methods for calculating the frequency correction do not represent a comprehensive list of ways in which the velocity of the device can be calculated. In fact, any technique allowing the velocity of the handset relative to the base station to be 2s established will enable the device to derive the frequency error due to the effects of the doppler shift.

Once the frequency offsets due to temperature, voltage and doppler effects have been calculated, these are subtracted from the measured difference in frequency so between the received signal and frequency of the internal clock at 260. The result of this subtraction is the - 11 offset in frequency due to the aging of the crystal.

This offset is stored within the device at 270.

Further embodiments of the invention calculate the frequency error due to aging of the crystal by taking a long term average of the frequency errors after the temperature and voltage offsets have been subtracted. As doppler is caused by movement relative to base stations, a long term average tends to remove doppler effects completely and leave only the aging effect. Over a period lo of several minutes the doppler term will average out and leave only the aging term. This happens because the doppler arises from radial movement relative to a base station - in a cellular network the handover mechanism means that handsets spend as long moving towards basestations (+ve doppler) as away from them (-ve doppler) and so over time the average doppler offset tends to zero. This method is described in our earlier British Application No. 0222188.5. Such embodiments do not include the step 250 of Figure 2 since the velocity of the device is not specifically calculated, but instead errors due to movement are accounted for in the long term average.

Once the offset due to the aging of the crystal is known and stored in the device, the device is able to calculate the offset due to temperature and voltage accurately, as well as having a recent measurement of the offset due to aging.

Using these calculated frequency offsets, the frequency of the internal clock can be corrected before receiving the signal from the network. The frequency of the internal clock can be regularly updated by applying - 12 correction due to temperature, voltage and a recent aging correction. This will enable the frequency error of the crystal with respect to the network to be reduced to within the required 7.5 ppm to enable VCXO crystals to be used for reliable operation in mobile handsets.

The offset corrections will also be useful when the device is activated as shown in Figure 3. The temperature and voltage can be measured accurately and the offset due to aging can be stored each time the device is shut down. On activation at 310, the device determines the frequency offset due to temperature and voltage and uses the last stored aging error. The frequency of the internal clock is updated at 330. Such use enables the device to commence communication with the network by having a local frequency close to that of the network.

Figure 4 is a block diagram showing the components within an embodiment of the present invention. The device 400 includes an aerial 410 for receiving signals from the network. The frequency of incoming signals is monitored by a frequency sensor 420. The device includes an internal clock 430. The clock is typically a crystal oscillator. The clock is driven by a voltage source 460.

Typically, although not always, the voltage source is the single power source of the mobile communication device.

The frequency sensor 420 and clock 430 are linked to a frequency comparator 440. The frequency comparator 440 calculates the difference in frequency between the internal clock 430 and the received signal via the frequency sensor 420. The frequency comparator 440 is linked to a correction voltage source 450. The frequency comparator instructs the correction voltage source to

- - 13

apply a suitable correction voltage to the clock 430 in dependence on the frequency error between the internal clock and the received signal in order that the frequency of the internal clock is matched to that of the received s signal.

The device includes a memory 470 in which the temperature dependence of the frequency of the crystal is stored at 480, the voltage dependence stored at 490 and the aging offset stored at 500. Each of these parts of the memory is linked to the frequency comparator 440.

The device also includes a temperature sensor 510 which is linked to the memory, including the temperature dependence of the oscillator. The section of the memory including the voltage dependence of the oscillator 490 is Is linked to the voltage source 460. Preferred embodiments of the invention include a location determining means 520 which is also linked to the frequency comparator 440.

The location determining means is used to establish the velocity of the device in order to determine the offset due to doppler shift. Further embodiments of the device may not include a location determining means if they establish the velocity of the device in different ways.

The temperature dependence, voltage dependence and aging offset parts of the memory are each linked to the 2s frequency comparator along with a location determining means. On measuring the frequency from an incoming signal, the frequency comparator subtracts the offset due to temperature, voltage and doppler shifts from the difference between the incoming frequency and the internal clock frequency in order to determine the error due to aging of the device. \ - 14

It will be clear to those skilled in the art that embodiments of the present invention enable the accuracy of the internal clock to be maintained by accounting for the change in frequency of the clock due to the temperature, voltage and aging of the device.

Furthermore, embodiments of the invention enable a frequency offset to be calculated due to movement of the device with respect to the base stations. The combination of these corrections enables the accuracy of lo the clock to be maintained with respect to the signals from the base station. These corrections enable the device to operate within industry standards using a regular uncompensated crystal oscillator (VCXO or XO).

Use of such crystals enables manufacturers to realise a to 50Y; reduction in the cost of crystals since expensive temperature compensating crystal oscillators are not required.

It will also be obvious to those skilled in the art that the invention is not limited to use with so uncompensated crystal oscillators but can equally be used with any oscillator in order to reduce offset frequencies caused by temperature, voltage, movement or aging and provide enhanced performance of the system. -

Claims

CLAIMS: 1. A method for reducing the frequency difference between a

crystal oscillator in a mobile communication device and the signal received from a base station comprising the steps of; measuring the temperature of the oscillator and determining a frequency offset due to temperature; measuring the supply voltage to the crystal oscillator and determining a frequency offset due to the To supply voltage; deriving the frequency offset due to aging of the crystal; and, applying a frequency correction to the crystal oscillator to compensate for the frequency offsets due to temperature, voltage and aging.

2. A method for reducing the frequency difference between a crystal oscillator in a mobile communication device and the signal received from a base station according to claim 1 comprising the further steps of; deriving a frequency offset due to movement of the device; and applying a frequency correction to crystal oscillator to compensate for frequency offset due to movement.

3. A method for reducing the frequency difference between a crystal oscillator in a mobile communication device and the signal received from a base station according to claim 1 or 2 wherein the frequency offset due to aging is derived by the steps of; receiving a signal from a base station, comparing the frequency of the signal from the base station with the frequency of the crystal oscillator, - 16 calculating the difference in frequency between the crystal oscillator and the frequency of the signal from the base station, and subtracting the frequency offset due to temperature and voltage from the difference.

4. A method for reducing the frequency difference between a crystal oscillator in a mobile communication device and the signal received from a base station according to claim 3 wherein the derivation of the lo frequency offset due to aging includes the further step of subtracting the frequency offset due to movement from the difference.

5. A method for reducing the frequency difference between a crystal oscillator in a mobile communication device and the signal received from a base station according to claim 1, 2, 3 or 4 including the step of storing the frequency offset due to aging in a memory.

6. A method for reducing the frequency difference between a crystal oscillator in a mobile communication device and the signal received from a base station according to claim 1, 2, 3, 4 or 5 wherein the step of determining the frequency offset due to temperature is performed by comparing the temperature of the crystal with a stored memory frequency dependence on temperature.

7. A method for reducing the frequency difference between a crystal oscillator in a mobile communication device and the signal received from a base station according to claim 1, 2, 3, 4, 5 or 6 wherein the step of determining the frequency offset due to voltage is performed by comparing the temperature of the crystal with a stored memory frequency dependence on voltage.

8. A method for reducing the frequency difference between a crystal oscillator in a mobile communication device and the signal received from a base station according to claim 2, 3, 4, 5, 6 or 7 wherein the step of determining the frequency offset due to movement is performed by determining the velocity of the device and using the velocity to calculate the doppler shift in To frequency due to the velocity.

9. A method for reducing the frequency difference between a crystal oscillator in a mobile communication device and the signal received from a base station according to claim 8 wherein the step of determining the velocity is performed by comparing the frequency of signals received from a plurality of base stations.

10. A method for reducing the frequency difference between a crystal oscillator in a mobile communication device and the signal received from a base station so according to claim 8 wherein the step of determining the velocity is performed by monitoring the change of location of the device over a known time period.

11. An apparatus for reducing the frequency difference between a crystal oscillator in a mobile communication device and the signal received from a base station comprising; a crystal oscillator; means for measuring the frequency of the crystal oscillator; temperature sensor; - 18 voltage supply and a means for measuring the voltage output from the voltage supply; memory, including a frequency dependence on temperature of the crystal, frequency dependence on voltage applied to the crystal and frequency offset due to aging of the crystal; means for applying a frequency correction to the crystal oscillator to compensate for the frequency offsets due to temperature, voltage and aging.

To 12. An apparatus for reducing the frequency difference between a crystal oscillator in a mobile communication device and the signal received from a base station according to claim 11 further comprising a means for calculating the movement of the device and means for applying a frequency correction to a crystal oscillator to compensate for frequency offset due to movement.

13. An apparatus for reducing the frequency difference between a crystal oscillator in a mobile communication device and the signal received from a base station To according to claim 11 or 12 further comprising; means to receive a signal from a base station; means to compare the frequency of the signal from the base station with the frequency of the crystal oscillator; means for calculating the difference in frequency between the crystal oscillator and the frequency of the signal from the base station; and a means for subtracting the frequency offset due to temperature and voltage from the difference.

so 14. An apparatus for reducing the frequency difference - 19 between a crystal oscillator in a mobile communication device and the signal received from a base station according to claim 13 further comprising a means for substracting the frequency offset due to movement from the difference.

15. An apparatus for reducing the frequency difference between a crystal oscillator in a mobile communication device and the signal received from a base station according to claim 11, 12, 13 or 14 wherein the frequency JO offset due to temperature is derived from the temperature of the device and the stored frequency dependence on temperature.

16. An apparatus for reducing the frequency difference between a crystal oscillator in a mobile communication device and the signal received from a base station according to claim 11, 12, 13, 14 or 15 wherein the frequency offset due to voltage is derived from the voltage supplied to the crystal and the stored frequency dependence on voltage.

17. An apparatus for reducing the frequency difference between a crystal oscillator in a mobile communication device and the signal received from a base station according to claim 11, 12, 13, 14, 15 or 16 wherein the means for calculating the movement of the device includes a location determining means to compare the determine the change of location of the device over a known time period.

18. An apparatus for reducing the frequency difference between a crystal oscillator in a mobile communication device and the signal received from a base station - 20 substantially as herein described with reference to the accompanying drawings.

19. A method for reducing the frequency difference between a crystal oscillator in a mobile communication device and the signal received from a base station substantially as herein described with reference to the accompanying drawings.

ax ' . . l . Amendments to the claims have been filedas follovvs 1. A method for deriving the frequency offset due tcaging of a crystal oscillator in a mobile communication device comprising he steps of: receiving a signal from a base station at an observed frequency; comparing the observed frequency with the frequency of the crystal oscillator; measuring the temperature of the oscillator anddetermining a frequency offset due to temperature; measuring the supply voltage to the crystal oscillator and determining a frequency offset due to the supply voltage; deriving a frequency offset due to the velocity of the device; subtracting the frequency offsets due to temperature, supply voltage and velocity from the frequency difference between the observed frequency and the frequency of the crystal oscillator to provide the frequency offset due to aging.
2. A method for deriving the frequency offset due to aging of a crystal oscillator in a mobile communication device according to claim I including the step of storing the frequency offset due to aging in a memory.
3. A method for deriving the frequency offset due to aging of a crystal oscillator in a mobile communication device according to claim 1 or 2 wherein the step of determining the frequency offset due to temperature is performed by comparing the temperature of the crystal with a stored memory frequency dependence on temperature.
4. A method for deriving the frequency offset due to aging of a crystal oscillator in a mobile communication device according to claim 1, 2 or 3 wherein the step of determining the frequency offset due to voltage is performed by comparing the voltage supplied to the crystal with a stored memory frequency dependence on voltage.
5. A method for deriving the frequency offset due to aging of a crystal ë e.e 1 ë; oscillator in a mobile communication device according to claim 1, 2, 3 or 4 wherein the step of determining the frequency offset due to velocity is performed by determining the velocity of the device and using the velocity to calculate a doppler shift in the frequency of the received signal due to the velocity of the device.
6. A method for deriving the frequency offset due to aging of a crystal oscillator in a mobile communication device according to claim S wherein the step of determining the velocity is performed by comparing the frequency of signals received from a plurality of base stations.
7. A method for deriving the frequency offset due to aging of a crystal oscillator in a mobile communication device according to claim 5 wherein the step of determining the velocity is performed by monitoring the change of location of the device over a known time period.
8. A method for reducing the frequency difference between a crystal oscillator in a mobile communication device and a signal received from a base station comprising the steps of; measuring the temperature of the oscillator and determining a frequency offset due to temperature; measuring the supply voltage to the crystal oscillator and determining a frequency offset due to the supply voltage; deriving the frequency offset due to aging of the crystal; and, applying a frequency correction to the crystal oscillator to compensate for the frequency offsets due to temperature, voltage and aging; wherein the frequency offset due to aging is derived using the steps defined in any of claims 1-7.
9. A method for reducing the frequency difference between a crystal oscillator in a mobile communication device and the signal received from a base station according to claim 8 comprising the further steps of; fir r deriving a frequency offset due to velocity of the device; and applying a frequency correction to crystal oscillator to compensate for frequency offset due to velocity.
10. A method for deriving the frequency offset due to aging of a crystal oscillator in a mobile communication device substantially as herein described with reference to the accompanying drawings.
11. An apparatus for deriving the frequency offset of a crystal oscillator in a mobile communication device due to aging comprising: means for receiving a signal from a base station at an observed frequency; means for measuring the frequency of the crystal oscillator; means for comparing the observed frequency with the frequency of the crystal oscillator; means for measuring the temperature of the oscillator; voltage supply and a means for measuring the voltage supplied to the crystal; memory, including a frequency dependence on temperature of the crystal and frequency dependence on voltage supplied to the crystal; means for deriving a frequency offset due to the velocity of the device; and means for subtracting frequency offsets due to temperature, supply voltage and velocity from the frequency difference between the observed frequency and the frequency of the crystal oscillator to provide the frequency offset due to aging.
12. An apparatus for deriving the frequency offset due to aging of a crystal oscillator in a mobile communication device according to claim 11 further comprising a memory for storing the frequency offset due to aging.
13. An apparatus for deriving the frequency offset due to aging of a crystal oscillator in a mobile communication device according to claim 11 or 12 wherein the frequency offset due to temperature is derived from the temperature of the device and the stored frequency dependence on temperature.

Afar a
14. An apparatus for deriving the frequency offset due to aging of a crystal oscillator in a mobile communication device according to claim 11, 12 or 13 wherein the frequency offset due to voltage is derived from the voltage supplied to the crystal and the stored frequency dependence on voltage.
15. An apparatus for deriving the frequency offset due to aging of a crystal oscillator in a mobile communication device according to claim 1 1, 12, 13 or 14 wherein the means for deriving a frequency offset due to the velocity of the device comprises means for determining the velocity of the device and means for calculating doppler shift in the frequency of the received signal due to the velocity of the device.
16. An apparatus for deriving the frequency offset due to aging of a crystal oscillator in a mobile communication device according to claim 15 wherein the means for determining the velocity of the device comprises a means for comparing the frequency of signals received from a plurality of base stations.
17. An apparatus for deriving the frequency difference between a crystal oscillator in a mobile communication device and the signal received from a base station according to claim 15 or 16 wherein the means for determining the velocity of the device includes a location determining means to monitor the change of location of the device over a known time period.
18. An apparatus for reducing the frequency difference between a crystal oscillator in a mobile communication device and a signal received from a base station comprising; a crystal oscillator; means for measuring the frequency of the crystal oscillator; temperature sensor; voltage supply and a means for measuring the voltage output from the voltage supply; d: :e ite' ste: id e:e memory, including a frequency dependence on temperature of the crystal, frequency dependence on voltage applied to the crystal and frequency offset due to aging of the crystal; means for applying a frequency correction to the crystal oscillator to compensate for the frequency offsets due to temperature, voltage and aging; wherein the frequency offset due to aging of the crystal is derived using the apparatus of any of claims 11-17.
19. An apparatus for deriving the frequency offset due to aging of a crystal oscillator in a mobile communication device according to claim 18 further comprising a means for calculating the velocity of the device and means for applying a frequency correction to a crystal oscillator to compensate for frequency offset due to velocity.
20. An apparatus for deriving the frequency offset due to aging of a crystal oscillator in a mobile communication device substantially as herein described with reference to the accompanying drawings.