Electronic circuit

• US 20010022539A1
• Filed: 03/06/2001
• Published: 09/20/2001
• Est. Priority Date: 03/17/2000
• Status: Active Grant

• Current Assignee: Telefonaktiebolaget LM Ericsson
• Original Assignee: Telefonaktiebolaget LM Ericsson
• Inventors: Jakobsson, Peter
• Granted Patent: US 6,559,731 B2
• Time in Patent Office: Days
• US Class Current: 331/172
• CPC Class Codes: H03J 1/0058 provided with channel identification means arrangements for monitoring the use made of broadcast services H04H60/31
• CPC Class Codes: H03J 7/04 where the frequency control is accomplished by varying the electrical characteristics of a non-mechanically adjustable element or where the nature of the frequency controlling element is not significant
• CPC Class Codes: H03L 1/026 by using a memory for digitally storing correction values H03L1/025 takes precedence

Abstract

A voltage controlled crystal oscillator (VCXO) 25, for example, as used in a mobile communications terminal 23, has its output frequency F_ref stabilised against temperature drift using frequency correction information received, for example, in a downlink signal 35 from a base station 33. A controller 31 uses the frequency correction information to produce a digital value 39 which is supplied to a DAC 29 which controls the output frequency of the VCXO 25. While the frequency is being stabilised in this manner, compensation values are determined based on the DAC value 39 and temperature values from a temperature ADC 45, and stored in memory 41. When the correction information ceases to be available, the compensation values from the memory 41 are used to compensate for temperature fluctuations. Each compensation value corresponds to a linear temperature region, and relates to the gradient for that temperature region. In this way, the invention does not need to store a vast number of compensation values, and does not require complex interpolation techniques for determining the control value 39 for the VCXO'"'"'s DAC 29.

Claims

1. A method of controlling the output frequency of a voltage controlled crystal oscillator (VCXO) which is controlled by a VCXO control signal, the method comprising the steps of:

(a) during a first period, receiving frequency correction information from a first source, and using this frequency correction information to control the output frequency of the VCXO;

characterised by the steps of;

(b) during the first period, determining a plurality of VCXO compensation values which correspond to respective predefined temperature regions, and which are indicative of a linear relationship between the VCXO control signal and temperature in the region concerned;

(c) storing the compensation values; and

,

(d) during a second period when the frequency correction information from the first source is not available, determining the temperature range in which the VCXO is operating and using the corresponding stored compensation value to determine the VCXO control signal, thereby controlling the output frequency of the VCXO.

2. A method as claimed in

claim 1

, wherein the step of determining the compensation values during the first period comprises the steps of:

(e) measuring the current temperature;

(f) determining whether the current temperature corresponds to one of a predetermined set of temperature values, the set of temperature values defining the temperature regions; and

, if so,

(g) storing the current temperature value and a corresponding VCXO control signal;

(h) determining whether the current temperature value is adjacent in the set of temperature values to a temperature value previously stored in step (g), and, if so,

(i) determining the difference between the currently stored VCXO control signal and a VCXO control signal previously stored in step (g); and

,

(j) storing the difference value as the compensation value representing that particular temperature region.

3. A method as claimed in

claim 1

or

2

, wherein the step of storing each compensation value (j) involves storing the compensation value at a predetermined memory address corresponding to the particular temperature region.

4. A method as claimed in any one of

claims 1

to

3

, wherein the step of using the compensation values in step (d) comprises:

calculating the temperature drift from when the frequency correction information ceased to be available;

using the compensation value for that temperature region to determine the corresponding VCXO control signal drift; and

,

using the VCXO control signal drift to determine the current VCXO control signal.

5. A method as claimed in any one of

claims 1

to

3

, wherein the step of using the compensation values in step (d) comprises:

storing a temperature value (T_to) and VCXO control signal VCXO_to corresponding to when the frequency correction information ceased to be available;

measuring the present temperature value T_tx;

determining the temperature drift, Δ

T_tx, as

Δ

T_tx=(T_tx−

T_to);

determining the corresponding VCXO control signal drift, Δ

VCXO_tx, as

Δ

VCXO_tx=Round ((EPROM_DELTAVAUE/8)*Δ

T_tx);

determining the current VCXO control signal, VCXO_tx as

VCXO_tx=Δ

VCXO_tx+VCXO_to; and

using the current VCXO control signal, VCXO_tx, to control the output frequency of the VCXO.

6. A method as claimed in any one of the preceding claims, having a delay between when the frequency correction information ceases to be available, to when the compensation values are used for determining the VCXO control signal.

7. A method as claimed in

claim 6

, wherein the delay comprises:

determining whether a SACCH burst has been unsuccessfully received, and, if so, incrementing a counter;

determining whether the counter value is higher than a preset value, and, if so, using the compensation values to determine the VCXO control signal.

9. A method as claimed in any preceding claim, wherein the predefined temperature regions are of equal size.

10. A method s claimed in any one of the preceding claims, wherein the VCXO control signal represents a digital value supplying a digital to analogue converter (DAC) which controls the output frequency of the VCXO.

11. A method as claimed in any preceding claim, wherein accurate frequency compensation is provided during the second period even if the frequency of the first source has an error.

20. Apparatus as claimed in any one of the preceding claims, having a digital to analogue converter (DAC) for controlling the VCXO, whereby the DAC is controlled by the VCXO control signal.

21. Apparatus as claimed in any one of the preceding claims, wherein the first memory means is an EEPROM.

22. Apparatus as claimed in

claim 21

, wherein the memory addresses in the EEPROM store compensation values corresponding to temperature regions of equal size.

8. A method as claimed in

claim 8

, wherein the counter is reset upon receipt of a successfully decoded SACCH burst.

12. Apparatus for controlling the output frequency of a voltage controlled crystal oscillator (VCXO) which is controlled by a VCXO control signal, the apparatus comprising:

(a) means for receiving frequency correction information from a first source during a first period;

(b) means for using the frequency control information to control the output frequency of the VCXO during the first period;

characterised in that the apparatus further comprises;

(c) means for determining a plurality of VCXO compensation values during the first period, each VCXO compensation value corresponding to a respective predefined temperature region, and indicative of a linear relationship between the VCXO control signal and temperature in the region concerned;

(d) a first memory for storing the compensation values; and

,

(e) means for determining the temperature range in which the VCXO is operating, and using the corresponding stored compensation value to determine the VCXO control signal for controlling the output frequency of the VCXO during a second period when the frequency correction information is not available.

13. Apparatus as claimed in

claim 12

, wherein the means for determining the compensation values comprises:

temperature measuring means for measuring the current temperature;

means for determining whether the current temperature corresponds to one of a predetermined set of temperature values, the set of temperature values defining the temperature regions;

a second memory for storing the current temperature value and a corresponding VCXO control signal if the current temperature value corresponds to one of the predetermined set of temperature values;

means for determining whether the current temperature value is adjacent in the set of temperature values to an old temperature value previously stored in the second memory;

means for determining the difference between the current VCXO control signal and an old VCXO control signal previously stored in the second memory, if the current temperature value is adjacent to the previous temperature value; and

,

means for storing in the first memory the difference value representing the compensation value for that particular temperature region.

14. Apparatus as claimed in

claim 12

or

13

, wherein the memory means comprises predefined memory locations, each one of the predefined memory locations storing a compensation value corresponding to a particular temperature region.

15. Apparatus as claimed in any one of

claims 12

to

14

, wherein the means for using the compensation values when the frequency correction information is not available comprises:

means for calculating the temperature drift from the time the frequency correction information ceased to be available;

means for using the compensation value for that temperature region to determine the corresponding VCXO control signal drift; and

,

means for using the VCXO control signal drift to determine the current VCXO control signal.

16. Apparatus as claimed in any one of

claims 12

to

14

, wherein the means for using the compensation values when the frequency correction information is not available comprises:

means for storing the temperature value (T_to) and a VCXO control signal VCXO_to corresponding to when the frequency correction information ceased to be available;

means for measuring the present temperature T_tx and calculating the temperature drift Δ

T_tx as

Δ

T_tx=(T_tx−

T_t0);

means for calculating the corresponding VCXO control signal drift, Δ

VCXO_tx, as;

Δ

VCXO_tx=Round((EEPROM_DELTAVALUE/8)*Δ

T_tx)

means for determining the current VCXO control signal, VCXO_tx, as;

VCXO_tx=Δ

VCXO_tx+VCXO_to; and

means for controlling the output frequency of the VCXO according to the current VCXO control value VCXO_tx.

17. Apparatus as claimed in any one of

claims 12

to

16

, further comprising delay means for delaying the using of the compensation values for determining the VCXO control signal after the frequency correction information ceases to be available.

18. Apparatus as claimed in

claim 17

, wherein the delay means comprises:

a counter;

means for determining whether a SACCH burst has been unsuccessfully received, and, if so, incrementing the counter;

means for determining whether the counter value is higher than a preset value, and, if so, using the compensation values for determining the VCXO control signal while the counter value is above the preset value.

19. Apparatus as claimed in

claim 18

, having means for resetting the counter upon receipt of a successfully decoded SACCH burst.

23. Apparatus as claimed in any one of

claims 12

to

22

, wherein accurate frequency compensation is achieved during the second period even if the frequency of the first source has an error.

24. A mobile communications terminal having apparatus as claimed in any one of

claims 12

to

23

.

25. A mobile communications terminal comprising;

a voltage controlled crystal oscillator, VCXO (25);

a digital to analogue converter, VCXO-DAC, (29) for controlling the VCXO (25) according to a VCXO control signal (39);

means (37) for receiving frequency correction information from a base station (33);

temperature measuring means (43);

an analogue to digital converter (45) for converting the temperature signals into digital signals;

control means (31) for controlling the output frequency of the VCXO 25;

wherein the control means comprises;

means for using the received frequency control information to control the output frequency of the VCXO during a first period;

characterised in that the mobile communications terminal further comprises;

means for determining a plurality of VCXO compensation values during the first period, each VCXO compensation value corresponding to a respective predefined temperature region, and indicative of a linear relationship between the VCXO control signal and temperature in the region concerned;

a first memory (41) for storing the compensation values; and

,

means for determining the temperature range in which the VCXO is operating, and using the corresponding stored compensation value to determine the VCXO control signal for controlling the output frequency of the VCXO during a second period when the frequency correction information is not available.

26. A mobile communications terminal as claimed in

claim 25

, wherein accurate frequency compensation is achieved during the second period even if the frequency of the base station has an error.