GB2328568A

GB2328568A - Oscillator frequency calibration system

Info

Publication number: GB2328568A
Application number: GB9717742A
Authority: GB
Inventors: Mark James Walton
Original assignee: NEC Technologies UK Ltd
Current assignee: NEC Technologies UK Ltd
Priority date: 1997-08-22
Filing date: 1997-08-22
Publication date: 1999-02-24
Anticipated expiration: 2017-08-22
Also published as: GB2328568B; GB9717742D0

Abstract

An oscillator calibration system as shown in Figure 1 comprises a high frequency stable oscillator (14) and a low frequency oscillator (13). A temperature sensor (15) adjacent the low frequency oscillator (13) is sensitive to the temperature of the components thereof. A controller is provided for calibrating the low frequency oscillator against the high frequency oscillator by counting the number of pulses from the latter occurring in a period determined by the low frequency oscillator. The controller is connected to the temperature sensor (15) and is arranged to commence a calibration operation when the temperature changes by more than a predetermined amount. The controller may include a CPU and the calibration system may form part of a mobile telephone.

Description

Oscillator frequency calibration system This invention relates to an oscillator frequency calibration system particularly but not exclusively intended for use in portable appliances such as mobile telephones.

In GSM mobile telephone systems it is usual to employ a frequency stable high frequency clock oscillator the pulses from which are counted to enable high resolution determination of the timing of telephone events in synchronism with events in signals received from base stations.

However, high frequency oscillators are power consuming and it is usual to shut down the high frequency clock periodically to save power.

When the high frequency clock is restarted after a power-saving sleep period, it is necessary to restore the count to the value it would have had if the high frequency clock had not been stopped. For this purpose a low frequency clock oscillator is also included and a count from this oscillator is used to measure the time that has elapsed whilst the high frequency clock oscillator was stopped.

It is desirable to keep the low frequency clock oscillator as cheap as possible, but the result of this is that the low frequency clock oscillator's frequency may vary particularly as a result of temperature changes.

Accordingly, the low frequency clock oscillator needs to be re-calibrated against the more stable high frequency clock oscillator periodically.

Such recalibration can take a considerable time and also wastes power and it is an object of the invention to provide a timebase calibration system in which the need for re-calibration is minimised.

In accordance with the invention there is provided an oscillator calibration system comprising a high frequency stable oscillator, a low frequency oscillator, a temperature sensor adjacent said low frequency oscillator and sensitive to the temperature of the components thereof, and a controller for calibrating the low frequency oscillator against the high frequency counter by counting the number of pulses from the high frequency oscillator occurring in a period determined by the low frequency oscillator, said controller being connected to the temperature sensor and operating to commence a calibration operation when the temperature changes by more than a predetermined amount relative to the temperature when the previous calibration operation was carried out.

In the accompanying drawings: Figure 1 is a diagrammatic view of a pcb forming part of a mobile telephone apparatus in which an example of an oscillator calibration system in accordance with the invention is incorporated; and Figure 2 is a diagram showing the connection of a thermistor forming part of the oscillator calibration system to a CPU of the apparatus.

As shown in Figure 1 the pcb 10 has various power components, such as power supply regulators 11 and RF power amplifier circuits 12 adjacent one end. Adjacent the other end of the pcb, so as to make it relatively insensitive to temperature changes occurring in the power components, is a low frequency oscillator 13, which is used to control the operation of the apparatus during "sleep" periods when the main stable high frequency oscillator of the apparatus is closed down for power saving purposes. Since the oscillator 13 is of inexpensive construction, it's frequency will vary according to the local temperature. Even though the oscillator 13 is located at a distance from the power components 11, 12 its frequency will still be subject to change with time and, in order for the timing of the apparatus to be maintained to a sufficiently accurate level it is necessary for the low frequency oscillator to be calibrated periodically.

As the main cause of frequency drift in the oscillator 13 is temperature changes, there is provided a temperature sensor in the form of a thermistor 15 on the pcb immediately adjacent the low frequency oscillator 13. As shown in Figure 2, the thermistor 15 is connected in series with a fixed resistor 16 across a supply and the common point is connected to an A/D converter 17 which is controlled by and provides a digitised temperature input to a CPU 18. The thermistor 15 may have a non-linear characteristic and in an actual example, the A/D steps approximate to 0.30C at room temperature degrading to about 2"C per step at the extremes of the -20 C to 700C working range.

The CPU controls a calibration process for calibrating the low frequency oscillator against the high frequency oscillator periodically. In this process, the CPU causes a count to be made of the pulses issuing from the high frequency oscillator during a time period measured in terms a specific number of pulses from the low frequency oscillator. In a GSM system, where the high frequency oscillator typically operates at 13MHz and the low frequency oscillator has a frequency in the region of 32 KHz, the truncation of a period timed by the 13MHz oscillator by even a singie cycle of the 13MHz oscillator can cause an error, so that where the 32 KHz oscillator is being used to maintain to reset the timing after a sleep period, it is desirable to maintain an accuracy of 0.7ppm which requires a calibration period of at least 0.11 seconds. However, when other considerations, such as jitter on the edges of the 32KHz oscillator output, are taken into consideration, the calibration period needs to be increased significantly, for example to 0.5 seconds.

The CPU is programmed to carry out a A/D operation to obtain the current temperature value every 10 seconds and calibration operation once every 60 minutes in normal operation. An additional calibration operation is carried out if the current temperature value is found to differ from that stored at the time of the previous calibration operation by two or more A/D steps.

The normal operation cycle referred to above takes place only if the temperature is within the range -5 C to 500C. Outside this range the thermistor does not have the resolution necessary to track the temperature to the required precision (1pom), so that recalibration is carried out every 60 seconds, irrespective of the temperature changes detected. The soft-off capability of the apparatus, ie the ability to put the major part of the circuitry into "sleep" mode periodically, is disabled if the temperature changes by 2 A/D steps or more within eight minutes.

After initial power on, the soft-off capability is disabled until the temperature gradient has been determined which will take at least eight minutes. Furthermore, within the "normal" temperature range, the softoff capability is suspended if the temperature changes by more than 2 A/D steps in any two minute period.

Claims

Claims:

1. An oscillator calibration system comprising a high frequency stable oscillator, a low frequency oscillator, a temperature sensor adjacent said low frequency oscillator and sensitive to the temperature of the components thereof, and a controller for calibrating the low frequency oscillator against the high frequency counter by counting the number of pulses from the high frequency oscillator occurring in a period determined by the low frequency oscillator, said controller being connected to the temperature sensor and operating to commence a calibration operation when the temperature changes by more than a predetermined amount relative to the temperature when the previous calibration operation was carried out.

2. An oscillator calibration system as claimed in claim 1 in which a calibration operation is carried out after a predetermined period irrespective of temperature changes.

3. An oscillator calibration system as claimed in claim 1 or claim 2 in which the temperature sensor has a limited range of accuracy sufficient to determine whether calibration is required or not and calibration is carried out at fixed intervals when the sensed temperature is outside this range.

4. A mobile telephone apparatus including an oscillator calibration system as claimed in any preceding claim, the low frequency oscillator being employed in the measurement time intervals in a soft-off mode when operation of the high frequency oscillator (and other components of the apparatus) is suspended periodically for power saving purposes.

5. A mobile telephone apparatus as claimed in claim 4 in which the soft-off mode is disabled if the temperature sensed changes by more than a predetermined amount in a predetermined time interval.

6. A mobile telephone apparatus as claimed in claim 5 in which said predetermined time interval is shorter when the temperature is within a normal operating range for the temperature sensor, than when the temperature is outside said normal operating range.

8. A mobile telephone apparatus substantially as hereinbefore described with reference to the accompanying drawings.