VCXO temperature compensation circuit
First Claim
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;
(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, the step of determining the compensation values during the first period comprising;
(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;
(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.
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Accused Products
Abstract
A voltage controlled crystal oscillator (VCXO), for example, as used in a mobile communications terminal, has its output frequency Fref stabilised against temperature drift using frequency correction information received, for example, in a downlink signal from a base station. A controller uses the frequency correction information to produce a digital value which is supplied to a DAC which controls the output frequency of the VCXO. While the frequency is being stabilised in this manner, compensation values are determined based on the DAC value and temperature values from a temperature ADC, and stored in memory. When the correction information ceases to be available, the compensation values from the memory 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 for the VCXO'"'"'s DAC.
26 Citations
22 Claims
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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:
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(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;
(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, the step of determining the compensation values during the first period comprising;
(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;
(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. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10)
calculating a temperature drift from when the frequency correction information ceased to be available;
using the compensation value for that temperature region to determine a corresponding VCXO control signal drift; and
,using the VCXO control signal drift to determine the current VCXO control signal.
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4. The method as claimed in claim 1, wherein the step of using the compensation values in step (d) comprises:
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storing a temperature value (Tto) and VCXO control signal VCXOto corresponding to when the frequency correction information ceased to be available;
measuring the present temperature value Ttx;
determining a temperature drift, Δ
Ttx, as
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5. The method as claimed in claim 1, 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.
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6. The method as claimed in claim 5, wherein the delay comprises:
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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.
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7. The method as claimed in claim 6, wherein the counter is reset upon receipt of a successfully decoded SACCH burst.
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8. The method as claimed in claim 1, wherein the predefined temperature regions are of equal size.
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9. The method as claimed in claim 1, wherein the VCXO control signal represents a digital value supplying a digital to analogue converter (DAC) which controls the output frequency of the VCXO.
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10. The method as claimed in claim 1, wherein accurate frequency compensation is provided during the second period even if the frequency of the first source has an error.
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11. An apparatus for controlling the output frequency of a voltage controlled crystal oscillator (VCXO) which is controlled by a VCXO control signal, the apparatus comprising:
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(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, 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;
(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. - View Dependent Claims (12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22)
means for calculating a 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 a corresponding VCXO control signal drift; and
,means for using the VCXO control signal drift to determine the current VCXO control signal.
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14. The apparatus as claimed in claim 11, wherein the means for using the compensation values when the frequency correction information is not available comprises:
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means for storing the temperature value (Tto) and a VCXO control signal VCXOto corresponding to when the frequency correction information ceased to be available;
means for measuring the present temperature Ttx, and calculating a temperature drift Δ
Ttx as
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15. The apparatus as claimed in claim 11, 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.
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16. The apparatus as claimed in claim 15, wherein the delay means comprises:
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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.
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17. The apparatus as claimed in claim 16, having means for resetting the counter upon receipt of a successfully decoded SACCH burst.
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18. The apparatus as claimed in claim 11, having a digital to analogue converter (DAC) for controlling the VCXO, whereby the DAC is controlled by the VCXO control signal.
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19. The apparatus as claimed in claim 11, wherein the first memory means is an EEPROM.
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20. The apparatus as claimed in claim 19, wherein the memory addresses in the EEPROM store compensation values corresponding to temperature regions of equal size.
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21. The apparatus as claimed in claim 11, wherein accurate frequency compensation is achieved during the second period even if the frequency of the first source has an error.
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22. A mobile communications terminal comprising the apparatus as claimed in claim 11.
Specification