JP4033873B2 - DC offset correction device - Google Patents

Description

  The present invention relates to a DC offset correction technique, and more particularly to a DC offset correction apparatus that corrects a DC offset included in a received signal.

A receiving device in a wireless communication system receives a radio frequency signal. The radio frequency signal is amplified by an amplifier and then subjected to frequency conversion. At that time, the mixer performs frequency conversion on the radio frequency signal while using the reference carrier wave from the local oscillator. In the mixer, since the isolation between the local oscillator side and the amplifier side is generally not infinite, the reference carrier leaks to the amplifier side, is reflected by the amplifier, and is input again to the mixer. Since the reference carrier wave input to the mixer again has the same frequency as the reference carrier wave from the local oscillator, a DC offset (hereinafter referred to as “DC offset”) is generated in the frequency-converted signal. Further, when the gain of the amplifier is changed, the output impedance of the amplifier also changes. As a result, since the amount of reflection at the amplifier also changes, the intensity of the reference carrier wave input to the mixer again changes. That is, the DC offset value changes according to the gain value (see, for example, Patent Document 1).
Japanese Patent Laid-Open No. 10-13482

  In order to reduce the influence of the DC offset as described above, in Patent Document 1, a DC offset value is appropriately measured while using a time slot not allocated in TDMA (Time Division Multiple Access), and the measured DC offset is measured. Perform correction according to the value. That is, this follows the change in the DC offset value with respect to the change in the gain value by appropriately measuring the DC offset value. On the other hand, in a satellite communication system such as Inmarsat, a receiving device receives a continuous signal. Therefore, the receiving apparatus does not have a period for measuring the above-described DC offset value. Further, since the intensity of the received signal changes due to fading effects or switching of land stations, the gain value also changes. Therefore, the influence of the DC offset also occurs in the receiving apparatus that receives continuous signals.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a DC offset that corrects a DC offset in response to a change in reception level and environmental conditions even when a continuous signal is received. It is to provide a correction device.

  In order to solve the above-described problem, a DC offset correction apparatus according to an aspect of the present invention includes an amplification unit that performs amplification with a gain value that is variably set with respect to an input analog signal having a first frequency. A frequency converter that converts the analog signal of the first frequency amplified in the amplifier to an analog signal of the second frequency, an AD converter that converts the analog signal converted in the frequency converter into a digital signal, and AD conversion A storage unit that stores the relationship between the DC offset value to be included in the digital signal converted by the unit and the gain value that should be set by the amplification unit, and amplifying while referring to the relationship stored in the storage unit The DC offset value corresponding to the gain value set in the AD converter is specified and included in the digital signal converted in the AD converter by the specified DC offset value. And a offset correction section for correcting the DC offset.

  The “first frequency” and the “second frequency” may be any one of a radio frequency, an intermediate frequency, and a baseband. The intermediate frequency may be a first intermediate frequency or a second intermediate frequency. Here, it is assumed that the “second frequency” is generally lower than the “first frequency”.

  According to this aspect, the relationship between the DC offset value and the gain value is stored in advance, and the digital signal is included in the digital signal according to the DC offset value corresponding to the current gain value while using this relationship. Since the DC offset is corrected, successive corrections can be executed.

  Means for measuring the value of the DC offset included in the digital signal converted in the AD conversion unit while changing the setting of the gain value in the amplifying unit before communication is started; and the set gain value And an offset estimation unit including means for storing the measured DC offset value in the storage unit in association with each other. The analog signal of the first frequency input in the amplifying unit may be a continuous signal. In this case, since the relationship between the DC offset value and the gain value is measured in advance, the DC offset value included in the continuous signal can be corrected.

  The offset estimator is a means for measuring the DC offset value relative to the gain value set in the amplifier during communication, and the relationship between the set gain value and the measured DC offset value is stored in the storage unit. Means for updating the relationship stored in the storage unit as long as the relationship is different from the relationship stored in the storage unit. In this case, since the relationship between the gain value and the DC offset value is updated, it is possible to follow changes in these relationships.

  You may further provide the temperature measurement part which measures the temperature of a receiver. The offset estimation unit is set with means for measuring a DC offset value with respect to a gain value set by the amplification unit when a change in temperature measured by the temperature measurement unit becomes larger than a threshold value. If the relationship between the gain value and the measured DC offset value is different from the relationship stored in the storage unit, a means for updating the relationship stored in the storage unit may be included. In this case, when the temperature change becomes large, the relationship between the gain value and the DC offset value is updated, so that it follows the change in the relationship between the gain value and the DC offset value based on the temperature change. it can.

  The storage unit is configured to approximate the relationship between the DC offset value and the gain value in the form of an approximate expression so as to include the DC offset value measured by the offset estimating unit and the gain value set by the amplifying unit. The offset estimating unit may translate the approximate expression so as to correspond to the newly measured DC offset value when updating the relationship stored in the storage unit. In this case, since the relationship between the DC offset value and the gain value is stored in the form of an approximate expression, the DC offset value can be derived even for an unmeasured gain value. Moreover, since the approximate expression is moved in parallel, processing can be facilitated.

  It should be noted that any combination of the above-described constituent elements and a conversion of the expression of the present invention between a method, an apparatus, a system, a recording medium, a computer program, etc. are also effective as an aspect of the present invention.

  According to the present invention, even when a continuous signal is received, the DC offset can be corrected in response to changes in the reception level and environmental conditions.

  Before describing the present invention in detail, an outline will be described. An embodiment of the present invention relates to a receiving apparatus that receives continuous signals like a mobile station in Inmarsat. Since the intensity of the signal received by the receiving apparatus changes, the receiving apparatus amplifies the received signal while adjusting the gain with an AGC (Automatic Gain Control) provided therein. The receiving apparatus converts the frequency of the amplified signal to baseband. As a result, a DC offset occurs in the baseband signal. Even when receiving a continuous signal, the receiving apparatus according to the present embodiment operates as follows to correct the DC offset.

  When starting up, that is, before starting communication, the receiving apparatus changes the gain value and measures the DC offset value at that time. For example, the gain value is changed to “3 dB”, “5 dB”, etc., and the DC offset value at that time is measured. The set gain value and the DC offset value are associated with each other as one relationship. Furthermore, a predetermined approximate expression is derived from a plurality of relationships. Finally, the approximate expression is stored. After the start of communication, the receiving apparatus acquires the gain value set by AGC. The receiving apparatus specifies a DC offset value corresponding to the acquired gain value while referring to the approximate expression stored in advance. Further, the receiving apparatus corrects the DC offset included in the baseband signal with the specified DC offset value.

  FIG. 1 shows a configuration of a receiving apparatus 100 according to an embodiment of the present invention. The receiving apparatus 100 includes an antenna 10, an LNA (Low Noise Amplifier) unit 12, a first mixer 14, a first local oscillator 16, a BPF (Band-Pass Filter) unit 18, a variable gain amplifier 20, a second mixer 22, and a second mixer 22. Local oscillator 24, π / 2 phase shifter 26, first LPF (Low-Pass Filter) unit 28, first AD unit 30, offset estimation unit 32, storage unit 34, offset correction unit 36, digital processing unit 38, third mixer 40, a second LPF unit 42, a second AD unit 44, and a control unit 46.

  The LNA unit 12 amplifies a radio frequency signal received by the antenna 10. The radio frequency signal is a continuous analog signal. Here, it is assumed that the gain in the LNA unit 12 is predetermined. The first mixer 14 receives the radio frequency signal amplified by the LNA unit 12 at one end and the reference carrier wave from the first local oscillator 16 at the other end. The first mixer 14 converts the amplified radio frequency signal into an intermediate frequency signal. The reference carrier wave from the first local oscillator 16 is defined according to the intermediate frequency. The BPF unit 18 passes the signal component of the intermediate frequency signal from the first mixer 14. That is, the BPF unit 18 reduces the harmonic component generated by the first mixer 14.

  The variable gain amplifier 20 amplifies the intermediate frequency signal from the BPF unit 18 with a gain that is variably set. The intermediate frequency signal from the BPF unit 18 is an analog signal. The gain value is set by a digital processing unit 38 to be described later. The second mixer 22 and the third mixer 40 frequency-convert the intermediate frequency signal amplified by the variable gain amplifier 20 into a baseband signal. In order to perform the frequency conversion, the second mixer 22 inputs a reference carrier wave from the second local oscillator 24, and the third mixer 40 passes from the second local oscillator 24 via the π / 2 phase shifter 26. Enter the reference carrier. The π / 2 phase shifter 26 changes the phase of the reference carrier by π / 2. That is, the second mixer 22 outputs a signal having an in-phase component (hereinafter referred to as “in-phase signal”) among the baseband signals.

  The third mixer 40 outputs a quadrature component signal (hereinafter referred to as “orthogonal signal”) of the baseband signals. The second mixer 22 and the third mixer 40 have the same configuration as the first mixer 14. The first LPF unit 28 passes the low frequency portion of the in-phase signal from the second mixer 22. That is, the first LPF unit 28 reduces the harmonic component generated by the second mixer 22. Further, the second LPF unit 42 passes the low frequency portion of the orthogonal signal from the third mixer 40. That is, the second LPF unit 42 reduces harmonic components generated by the third mixer 40. The first AD unit 30 converts the in-phase signal from the first LPF unit 28 from an analog signal to a digital signal. The second AD unit 44 converts the orthogonal signal from the second LPF unit 42 from an analog signal to a digital signal.

  Here, the DC offset included in the digital signals output from the first AD unit 30 and the second AD unit 44 will be described with reference to FIG. Here, the cause of occurrence of the DC offset will be described. The reference carrier wave output from the second local oscillator 24 reaches the variable gain amplifier 20 due to leakage in the second mixer 22 and the third mixer 40, and is reflected to generate a DC offset. A similar phenomenon occurs in the first local oscillator 16, the first mixer 14, and the LNA unit 12. As described above, the DC offset value generated by the gain change in the variable gain amplifier 20 also changes. On the other hand, a DC offset also occurs in the first LPF unit 28 and the second LPF unit 42, but the DC offset at that time does not depend on the gain in the variable gain amplifier 20.

  FIG. 2 is a diagram for explaining a DC offset to be corrected in the receiving apparatus 100. In the figure, the in-phase component is shown as the “I” axis, and the quadrature component is shown as the “Q” axis. Further, it is assumed that the signal is modulated by BPSK (Binary Phase Shift Keying). When no DC offset has occurred, the constellation of BPSK is indicated by a circle in the figure. That is, it is shown as two points on the I axis. On the other hand, the DC offset is shown as “ΔI” and “ΔQ” in the figure. Here, “ΔI” is a DC offset value on the I axis, and “ΔQ” is a DC offset value on the Q axis. In general, these are independent values. As a result of such DC offset, the constellation of BPSK is at the position of the x mark in the figure. In BPSK, a signal is determined using the Q axis as a threshold value. Due to the DC offset, at least one signal point becomes close to the Q axis, and the probability of occurrence of an error increases.

  Returning to FIG. The offset estimation unit 32 changes the setting of the gain value in the variable gain amplifier 20 before communication is started, and the DC offset included in the digital signal converted by the first AD unit 30 and the second AD unit 44. Measure the value. Moreover, the offset estimation part 32 measures the value of DC offset with respect to each of an in-phase component and a quadrature component. Therefore, when the receiving apparatus 100 is activated, the offset estimation unit 32 receives an instruction to start measurement from the digital processing unit 38. In addition, the digital processing unit 38 sets the gain value of the variable gain amplifier 20, and the offset estimation unit 32 receives the set value. For example, the gain value is set to “3 dB” or “5 dB”. The offset estimation unit 32 acquires the value of the digital signal as a DC offset value.

  Further, the offset estimation unit 32 associates the set gain value with the measured DC offset value. FIG. 3 shows the relationship between the gain acquired by the offset estimation unit 32 and the DC offset value. As illustrated, the offset value “B1” is associated with the gain “A1”. Note that the number of points to be measured is the number of points that can be measured in a part of the period defined as the activation process of the receiving apparatus 100. Returning to FIG. Furthermore, the offset estimation unit 32 approximates the relationship between the two by an approximate expression so as to include the gain and the offset value. In order to derive an approximate expression, interpolation or least squares may be performed on the measurement points. Finally, the offset estimation unit 32 causes the storage unit 34 to store the relationship between the set gain value and the measured DC offset value as an approximate expression.

  The storage unit 34 stores the relationship received from the offset estimation unit 32 in the form of an approximate expression. FIG. 4 shows an approximate expression corresponding to the relationship between the gain stored in the storage unit 34 and the DC offset value. Here, for clarity of explanation, an approximate expression is shown by a graph, but actually, a numerical value corresponding to such an approximate expression is stored in the storage unit 34. In the figure, the horizontal axis indicates the gain, and the vertical axis indicates the DC offset value. Points in the figure indicate measured values measured by the offset estimation unit 32. As shown in the figure, an approximate expression is derived so as to be close to the measured value. The approximate expression is derived by interpolation, the least square method, or the like.

  When the gain is small, the DC offset values generated in the first mixer 14, the second mixer 22, and the third mixer 40 in FIG. 1 are small, and the DC offset values generated in the first LPF unit 28 and the second LPF unit 42 are dominant. Since these DC offset values do not depend on the gain value as described above, a substantially constant DC offset value is derived as the gain increases. On the other hand, when the gain is large, DC offset values generated in the first mixer 14, the second mixer 22, and the third mixer 40 in FIG. 1 become dominant. Therefore, as the gain increases, the DC offset value also increases. Returning to FIG.

  The offset correction unit 36 specifies a DC offset value corresponding to the gain value set by the variable gain amplifier 20 while referring to the relationship stored in the storage unit 34, that is, the approximate expression, during communication. For this purpose, the offset correction unit 36 receives a gain value from the digital processing unit 38. Further, the offset correction unit 36 corrects the DC offset included in the digital signals from the first AD unit 30 and the second AD unit 44 with the specified DC offset value. Here, the offset correction unit 36 corrects the DC offset by subtracting the specified DC offset value from the value of the digital signal. The above processing is executed for each of the in-phase component and the quadrature component.

  The digital processing unit 38 performs digital signal processing on the digital signal from the offset correction unit 36. The digital signal processing may be arbitrary, but corresponds to, for example, demodulation processing. In addition, the digital processing unit 38 performs gain setting by AGC, and outputs the set gain value to the variable gain amplifier 20 and the offset correction unit 36. Here, since the setting of the gain is a known technique, the description thereof is omitted. Further, the digital processing unit 38 executes setting of a gain value for the variable gain amplifier 20 for measurement in the offset estimation unit 32.

  In this configuration, the offset estimation unit 32, the offset correction unit 36, and the digital processing unit 38 can be realized in hardware by a CPU, memory, or other LSI of an arbitrary computer, and loaded into a memory in software. This is realized by a program having a communication function and the like, but here, functional blocks realized by their cooperation are depicted. Accordingly, those skilled in the art will understand that these functional blocks can be realized in various forms by hardware only, software only, or a combination thereof.

  The operation of the receiving apparatus 100 having the above configuration will be described. FIG. 5 is a flowchart showing a procedure for deriving an approximate expression by the offset estimation unit 32. As described above, this process is executed before the start of communication, for example, at startup. The gain is set by the digital processing unit 38 (S10). The offset estimation unit 32 measures the DC offset value (S12), and associates the gain with the DC offset value (S14). If the above processing has not been completed for all gains (N in S16), the digital processing unit 38 changes the gain (S18), and the processing from step 12 is repeatedly executed. On the other hand, if the above processing has been completed for all gains (Y in S16), the offset estimation unit 32 derives an approximate expression (S20) and stores the approximate expression in the storage unit 34 (S22). ).

  FIG. 6 is a flowchart showing a DC offset correction procedure by the offset correction unit 36. This process is executed at the time of communication as described above. The offset correction unit 36 acquires a gain from the digital processing unit 38 (S30). If the gain has been changed from the previous value (Y in S32), the offset correction unit 36 acquires the DC offset value while referring to the approximate expression stored in the storage unit 34 (S34). On the other hand, if the gain has not been changed from the previous value (N in S32), the offset correction unit 36 maintains the current DC offset value that has already been acquired (S36). The offset correction unit 36 performs correction using the DC offset value (S38).

  In the description so far, correction of the DC offset value is executed by using the approximate expression already stored in the storage unit 34 during communication. As described above, the approximate expression is derived when the receiving apparatus 100 is activated. However, an error may occur in the approximate expression derived in this way. One example is a change in temperature in the receiving apparatus 100. That is, the temperature in the receiving apparatus 100 generally increases with the start of communication. Further, since the DC offset value also depends on the temperature, the DC offset value changes from the initial value as communication starts. Therefore, the receiving apparatus 100 performs the following process so as to correspond to the change in the DC offset value.

  The offset estimation unit 32 measures a DC offset value with respect to the gain value set by the variable gain amplifier 20 during communication. Note that the difference from the processing at the time of activation of the receiving apparatus 100 is that a gain value used in actual communication is input. Therefore, the gain value is input in a pinpoint manner. The offset estimation unit 32 determines whether the relationship between the set gain value and the measured DC offset value is different from the approximate expression stored in the storage unit 34. Specifically, the offset estimation unit 32 acquires a DC offset value (hereinafter referred to as “reference value”) corresponding to the current gain value from the storage unit 34. Moreover, the offset estimation part 32 compares the measured DC offset value with a reference value, and if those difference is larger than a threshold value, it will determine with both differing.

  If the two are different, the offset estimation unit 32 updates the approximate expression stored in the storage unit 34. Here, FIG. 7 is used to use the explanation for updating the approximate expression. FIG. 7 shows another approximate expression corresponding to the relationship between the gain stored in the storage unit 34 and the DC offset value. The solid line in FIG. 7 corresponds to the approximate expression in FIG. That is, it is an approximate expression derived at startup. Here, the circle on the solid line corresponds to the reference value. On the other hand, the x mark in the figure corresponds to the DC offset value when newly measured.

  Here, it is assumed that the difference between the DC offset value and the reference value is larger than the threshold value. The approximation formula is updated so that the solid line becomes a dotted line. That is, the approximate expression is translated so as to correspond to the newly measured DC offset value. Here, the approximate expression is translated in the DC offset axis direction, that is, upward. This is based on the property that even if the temperature changes, the change in the slope of the approximate expression is small and the DC offset value itself changes.

  FIG. 8 is a flowchart showing the procedure for updating the approximate expression by the offset estimation unit 32. This process is executed at the time of communication as described above. The offset estimation unit 32 acquires the current gain from the digital processing unit 38, and also acquires a DC offset value (S50). The offset estimation unit 32 compares the approximate expression stored in the storage unit 34 with the acquired DC offset value (S52). If the difference is larger than the threshold value (Y in S54), the offset estimation unit 32 updates the approximate expression (S56). On the other hand, if the difference is not greater than the threshold value (N in S54), the offset estimation unit 32 ends the process. In addition, the above process is performed suitably.

  According to the embodiment of the present invention, the relationship between the DC offset value and the gain is stored in advance, and the DC offset included in the digital signal is determined by the DC offset value corresponding to the current gain while using this relationship. Can be corrected immediately. Further, since the correction can be executed immediately, the correction can be executed even if the input signal is continuous. In addition, since the relationship between the DC offset value and the gain is measured in advance, processing during communication can be simplified. In addition, since the relationship between the DC offset value and the gain is measured in advance, the number of processing steps during communication can be reduced. Further, since the number of processing steps during communication can be reduced, processing can be performed at high speed.

  Further, since the relationship between the DC offset value and the gain is stored in the form of an approximate expression, the DC offset value can be derived even for a gain that has not been measured. Further, since it is stored as an approximate value, the number of points to be measured can be reduced. In addition, since the number of points to be measured can be reduced, processing can be shortened. Further, since the relationship between the gain and the DC offset value is updated, it is possible to follow changes in these relationships. Further, since the relationship is updated, it is possible to follow a change in the DC offset value accompanying a change in temperature. Further, since it is possible to follow a change in the DC offset value, it is possible to improve reception characteristics. Moreover, since the approximate expression is moved in parallel, processing can be facilitated.

  In the above, this invention was demonstrated based on the Example. This embodiment is an exemplification, and it will be understood by those skilled in the art that various modifications can be made to the combination of each component and each processing process, and such modifications are also within the scope of the present invention. .

  In the embodiment of the present invention, the offset estimation unit 32 continuously measures the DC offset value even during communication. However, the present invention is not limited to this. For example, the offset estimation unit 32 may measure the DC offset value when the temperature change becomes large. The receiving device 100 includes a temperature measuring unit, and the temperature measuring unit measures the temperature of the receiving device 100. The offset estimation unit 32 updates the approximate expression by executing the above-described operation when the change in temperature measured by the temperature measurement unit becomes larger than the threshold value. According to the present modification, the process for updating the approximate expression is executed as necessary, so that an increase in the processing amount can be suppressed. Further, since the relationship between the gain and the DC offset value is updated when the change in temperature becomes large, it is possible to follow the change in these relationships based on the change in temperature. That is, the DC offset value may be updated.

  In the embodiment of the present invention, the receiving device 100 receives a continuous signal. However, the present invention is not limited to this, and the receiving apparatus 100 may receive a burst signal. That is, the embodiment can be applied regardless of the format of the received signal.

It is a figure which shows the structure of the receiver which concerns on the Example of this invention. It is a figure for demonstrating DC offset which should be corrected in the receiver of FIG. It is a figure which shows the relationship between the gain acquired in the offset estimation part of FIG. 1, and DC offset value. It is a figure which shows the approximation formula corresponding to the relationship between the gain memorize | stored in the memory | storage part of FIG. 1, and DC offset value. It is a flowchart which shows the derivation | leading-out procedure of the approximate expression by the offset estimation part of FIG. It is a flowchart which shows the correction | amendment procedure of DC offset by the offset correction part of FIG. It is a figure which shows another approximation formula corresponding to the relationship between the gain memorize | stored in the memory | storage part of FIG. 1, and DC offset value. It is a flowchart which shows the update procedure of the approximate expression by the offset estimation part of FIG.

Explanation of symbols

  10 antenna, 12 LNA section, 14 first mixer, 16 first local oscillator, 18 BPF section, 20 variable gain amplifier, 22 second mixer, 24 second local oscillator, 26 π / 2 phase shifter, 28 first LPF section , 30 first AD section, 32 offset estimation section, 34 storage section, 36 offset correction section, 38 digital processing section, 40 third mixer, 42 second LPF section, 44 second AD section, 46 control section, 100 receiving device.

Claims (5)

An amplifying unit for performing amplification with a gain value variably set on the input analog signal of the first frequency;
A frequency converter that converts the analog signal of the first frequency amplified in the amplifier to an analog signal of the second frequency;
An AD converter that converts the analog signal converted in the frequency converter into a digital signal;
A storage unit for storing a relationship between a DC offset value to be included in the digital signal converted by the AD conversion unit and a gain value to be set by the amplification unit;
While referring to the relationship stored in the storage unit, the DC offset value corresponding to the gain value set in the amplifying unit is specified, and converted in the AD conversion unit by the specified DC offset value An offset correction unit that corrects a DC offset included in the digital signal;
A direct-current offset correction apparatus comprising: Means for measuring the value of the DC offset included in the digital signal converted by the AD converter while changing the setting of the gain in the amplifier before communication is started; An offset estimation unit including a value and a means for storing the value in the storage unit while associating the measured DC offset value,
2. The DC offset correction apparatus according to claim 1, wherein the analog signal having the first frequency input in the amplifying unit is a continuous signal.   The offset estimation unit is configured to measure a DC offset value with respect to the gain value set by the amplification unit during communication, and the relationship between the set gain value and the measured DC offset value is: 3. The DC offset correction apparatus according to claim 2, further comprising means for updating the relationship stored in the storage unit if the relationship is different from the relationship stored in the storage unit. A temperature measuring unit for measuring the temperature of the receiving device;
The offset estimating unit is configured to measure a DC offset value with respect to a gain value set in the amplifying unit when a change in temperature measured in the temperature measuring unit is larger than a threshold value; Means for updating the relationship stored in the storage unit if the relationship between the set gain value and the measured DC offset value is different from the relationship stored in the storage unit. The DC offset correction apparatus according to claim 2, wherein the apparatus is a DC offset correction apparatus. The storage unit approximates the relationship between the DC offset value and the gain value so as to include the DC offset value measured by the offset estimating unit and the gain value set by the amplifying unit. Remembered by the form of
The offset estimation unit, when updating the relation stored in the storage unit, translates the approximate expression so as to correspond to a newly measured DC offset value. 4. The DC offset correction device according to 4.

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