Adaptive DC offset compensation apparatus and method thereof

Provided are a direct current (DC) offset compensation apparatus and a method thereof. A DC offset compensation apparatus comprises an offset detection block detecting a DC offset, an offset compensation block operating according to an output signal of the offset detection block, and an offset compensation amplifying block controlled according to an output signal of the offset compensation block. Particularly, the offset compensation block comprises a counter that counts polarities of DC offsets for a predetermined period, a controller that comprises first to fourth mode units respectively operating fast down-compensation, regular down-compensation, fast up-compensation, and regular up-compensation modes, and a control device that controls the execution of one selected among the first to fourth mode units based on a value counted by the counter, and a register storing data to control the offset compensation amplifying block based on the one mode unit determined by the controller.

This non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 10-2005-0078107 filed in Korea on Aug. 25, 2005, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a DC offset compensation apparatus and a method thereof.

2. Description of the Background Art

A direct conversion receiver is being actively researched as one receiver structure to recover a signal received by an antenna. The direct conversion receiver converts an received radio frequency signal into a baseband signal at a time without being converted into an intermediate frequency (IF) signal. Thus, the direct conversion receiver can reduce sizes of external devices such as filters and a burden for processing digital signals. Due to these effects, the direct conversion receiver can be lightweight and manufactured at low costs. Also, a system-on-a-chip can be implemented.

FIG. 1illustrates a simplified block diagram of a typical direct conversion receiver.

The typical direct conversion receiver includes a low noise amplifier (LNA)101, a mixer102, a filter103, a variable gain amplifier104, and a local oscillator105, which is also labeled with a reference letter LO.

The LNA101is configured to control a gain and amplifies a signal received by an antenna while suppressing noise. The mixer102mixes an output signal of the LNA101with a signal provided from the local oscillator105and outputs a baseband signal. The filter103filters an intended signal from the amplified signal. The variable gain amplifier104amplifies a signal outputted from the filter103while controlling a gain to sustain a power level of a required output signal.

As described above, the typical direct conversion receiver illustrated inFIG. 1can reduce sizes of external devices such as additional filters and a burden for processing digital signals.

However, the typical direct conversion receiver often has a DC offset, which may be a major cause of deteriorating the performance of the direct conversion receiver.

The DC offset event is generated by local oscillator leakage. Particularly, in the event that a radio frequency signal has a leakage component on an input side of the mixer102for a local oscillation signal, and this leakage component is reflected and supplied again as the local oscillation signal. Thus, the radio frequency signal received by the antenna is mixed with its own signal at the mixer102.

Similarly, in the event that the local oscillation signal has a leakage component on an input side of the mixer102for the radio frequency signal, and this leakage component is reflected and inputted to the mixer102with the radio frequency signal, the local oscillation signal is mixed with its own signal, resulting in a generation of DC offset at an output terminal of the mixer102. This type of DC offset is called self-mixing-oriented DC offset. A level of DC offset is not always consistent but continuously variable depending on a power level of a radio frequency signal, a frequency of the radio frequency signal and a frequency of a local oscillation signal.

The DC offset takes place at the output terminal of the mixer102due to a mismatch of a load existing at the output terminal of the mixer102and a duty error of the local oscillation signal inputted to the mixer102. The duty error of the local oscillation signal varies depending on the frequency of the local oscillation signal, and thus, the DC offset generated by the duty error of the local oscillation signal varies depending on the frequency of the local oscillation signal.

When devices such as an amplifier and a filter included in a baseband circuit are mismatched with each other, an output signal of the receiver often has a DC offset. This Dc offset varies depending on a cut-off frequency of the filter103and a change in a gain of the variable gain amplifier104.

The direct conversion receiver has a DC offset due to various factors, and a level of DC offset is not always consistent but continuously variable depending on various changes that actually occur while receiving a signal through an antenna. A frequency change in the local oscillation signal, a change in the received signal, a gain change in the amplifier are examples of the changes that cause the continuous variation in DC offset.

If the DC offset is severe, compensation is generally necessary as much as a level of the DC offset. If the DC offset is excessively compensated, another DC offset that is greater than the initial DC offset may be generated, resulting in various limitations that may be more severe than before the compensation is applied.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to provide a DC offset compensation apparatus that can adaptively compensate for a DC offset to speed up the compensation according to a DC offset value changing continuously, and a method thereof.

The present invention is also directed to provide a DC offset compensation apparatus that can control a distortion event, which may occur due to an excessive level of compensation for a DC offset, and a method thereof.

According to an embodiment of the present invention, a direct current (DC) offset compensation apparatus comprises an offset detection block detecting a DC offset, an offset compensation block operating according to an output signal of the offset detection block, and an offset compensation amplifying block controlled according to an output signal of the offset compensation block, wherein the offset compensation block comprises a counter that counts polarities of DC offsets for a predetermined period using the output signal of the offset detection block, a controller that comprises a first mode unit operating a fast down-compensation mode for the DC offset, a second mode unit operating a regular down-compensation mode for the DC offset, a third mode unit operating a fast up-compensation mode for the DC offset, a fourth mode unit operating a regular up-compensation mode for the DC offset, and a control device controlling the execution of one selected among the first to fourth mode units based on a value counted by the counter, and a register storing data to control the offset compensation amplifying block based on the one mode unit determined by the controller.

Consistent with the embodiment of the present invention, the offset detection block may comprise a comparator.

Consistent with the embodiment of the present invention, the offset detection block may further comprise a low pass filter.

Consistent with the embodiment of the present invention, the first mode unit may be configured to subtract a positive integer A greater than 1 from a data value of the register; the second mode unit may be configured to subtract a positive integer B less than the positive integer A from the data value of the register; the third mode unit may be configured to add a positive integer C greater than 1 to the data value of the register; and the third mode unit may be configured to add a positive integer D less than the positive integer C to the data value of the register.

Consistent with the embodiment of the present invention, at a period after the predetermined period, the first mode unit of the controller may be configured to subtract a positive integer that is less than the positive integer A but greater than the positive integer B from the data value of the register; and the third mode unit of the controller may be configured to subtract a positive integer that is less than the positive integer C but greater than the positive integer D from the data value of the register.

According to another embodiment of the present invention, a DC offset compensation method comprises (a) initializing data stored on a register, (b) counting polarities of DC offsets for a predetermined period, (c) performing a first compensation mode in which a counted value obtained from the operation (b) is compared with a first reference value and a first offset compensation mode is performed for one case that the counted value is greater or less than the first reference value, (d) performing a second compensation mode in which the counted value obtained from the operation (b) is compared with a second reference value and a second offset compensation mode is performed for one case that the counted value is greater or less than the second reference value, (e) performing a third compensation mode in which the counted value obtained from the operation (b) is compared with a third reference value and a third offset compensation mode is performed for one case that the counted value is greater or less than the third reference value, and (f) performing a fourth compensation mode in which the counted value obtained from the operation (b) is compared with a fourth reference value and a fourth offset compensation mode is performed for one case that the counted value is greater or less than the fourth reference value.

Consistent with the other embodiment of the present invention, the DC offset compensation method may further comprise at the first compensation mode, subtracting a value of 2^n from the data value of the register, at the second compensation mode, subtracting a value of 1 from the data value of the register, at the third compensation mode, adding a value of 2^m to the data value of the register, and at the fourth compensation mode, adding a value of 1 to the data value of the register.

Consistent with the other embodiment of the present invention, the DC offset compensation method may further comprise performing a feedback operation returning to the operation (b) at a period after the predetermined period.

Consistent with the other embodiment of the present invention, the n may be a value of 6−N and the m is a value of 6−P, wherein the N and P may be another data values, which are integers, wherein the operation (a) may further comprise initializing the integers N and P; the operation (c) may further comprise comparing the integer P with a fifth reference value and storing a value of N+1 as the integer P if the integer P is less than the fifth reference value; each of the operations (d) and (f) may further comprise initializing the integers N and P; and the operation (e) may further comprise comparing the integer N with a sixth reference value and storing a value of P+1 as the integer N if the integer N is less than the sixth reference value.

Consistent with the other embodiment of the present invention, the N and P may have initialization values of 0; and the fifth and sixth reference values may be 5.

According to still another embodiment of the present invention, a computer readable recording medium that is implemented with a program to embody a DC offset compensation method comprises the instructions of (a) initializing data stored on a register, (b) counting polarities of DC offsets for a predetermined period, (c) performing a first compensation mode in which a counted value obtained from the operation (b) is compared with a first reference value and a first offset compensation mode is performed for one case that the counted value is greater or less than the first reference value, (d) performing a second compensation mode in which the counted value obtained from the operation (b) is compared with a second reference value and a second offset compensation mode is performed for one case that the counted value is greater or less than the second reference value, (e) performing a third compensation mode in which the counted value obtained from the operation (b) is compared with a third reference value and a third offset compensation mode is performed for one case that the counted value is greater or less than the third reference value, and (f) performing a fourth compensation mode in which the counted value obtained from the operation (b) is compared with a fourth reference value and a fourth offset compensation mode is performed for one case that the counted value is greater or less than the fourth reference value.

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be described in a more detailed manner with reference to the drawings.

FIG. 2illustrates a simplified block diagram of a direct conversion receiver comprising a DC offset compensation apparatus according to an embodiment of the present invention.

As similar to the typical direct conversion receiver, the direct conversion receiver according to the present embodiment comprises an LNA201, a mixer202, a filter203, a variable gain amplifier204, and a local oscillator205

The DC offset compensation apparatus comprises an offset detection block211, an offset compensation block212and an offset compensation amplifying block213. The offset detection block211detects a DC offset event using an output signal of the variable gain amplifier204. The offset compensation block212operates according to an output signal of the offset detection block211. The offset compensation amplifying block213is controlled according to an output signal of the offset compensation block212.

FIG. 3illustrates a detailed block diagram of the DC offset compensation apparatus illustrated inFIG. 2.

The offset detection block211receives an output signal of the variable gain amplifier204of the direct conversion receiver. The offset detection block211comprises a circuit that enables the determination of a polarity of the DC offset. In the present embodiment, the offset detection block211comprises a comparator302. Also, the offset detection block211may further comprise a low pass filter301to detect signals around DC.

The offset compensation block212comprises a counter311, a controller312and a register313.

The counter311counts polarities of DC offsets from outputs of the offset detection block211for a predetermined period. For instance, if the predetermined period is approximately 64, the offset detection block211outputs ‘1’ if the detected DC offset level is greater than ‘0’ and, ‘0’ if the detected DC offset level is less than ‘0’. Then, the offset detection block211transfers these outputs to the counter311. The counter311counts the polarities of the DC offsets 63 times. Afterwards, the counter311counts the number of ‘1’ and transfer the result to the controller312. The counter311is reset at a 64thperiod and then, counts polarities of DC offsets generated at a next period.

The controller312comprises an offset compensation mode portion that comprises a first mode unit312a, a second mode unit312b, a third mode unit312c, and a fourth mode unit312d. On the basis of the number counted at the counter311, the first to fourth mode units312ato312dare in charge of a fast down-compensation mode, a regular down-compensation mode, a fast up-compensation mode, and a regular up-compensation mode, respectively. The controller312comprises a control device312eand determines the execution of one mode unit among the first to fourth mode units312ato312dbased on the number counted at the counter311.

Data necessary for controlling the offset compensation amplifying block213are stored on the register313. The data of the register313control the offset compensation amplifying block213by changing assigned data values according to an output signal of the controller312.

The control device312edetermines the fast compensation mode and the regular compensation mode based on the counted value provided from the counter311. More specifically, the counted value from the counter311is indicative of a degree of DC offset. The greater or less the counted value is, the more severe the DC offset is.

If the DC offset is severe, the fast compensation mode is executed to change a data value of the register313into a value greater than the originally assigned data value. If the offset is not severe, the regular compensation mode is executed to change a data value of the register313into a value less than the originally assigned data value. If the counted value provided from the counter311is an appropriate value, the data value of the register313is maintained.

The first mode unit312afor the fast down-compensation mode subtracts a positive integer A that is greater than ‘1’ from the data value of the register313. The second mode unit312bfor the regular down-compensation mode subtracts a positive integer that is less than the positive integer A from the data value of the register313. The third mode unit312cfor the fast up-compensation mode adds a positive integer C that is greater than ‘1’ to the data value of the register313. The fourth mode unit312dfor the regular up-compensation mode adds a positive integer D that is less than the positive integer C to the data value of the register313.

Even if the compensation modes have substantially the same scheme, different values are subtracted from the data value of the register313in the fast compensation mode and the regular compensation mode to differentiate a compensation degree according to the DC offset degree.

In the fast compensation mode, as the value that is added to or subtracted from the data value of the register313increases, the offset compensation degree also increases. Therefore, as compared with the added or subtracted value in the regular compensation mode, the greater value is added or subtracted from the data value of the register313in the fast compensation mode.

The above approach allows the adaptive and fast DC offset compensation according to the DC offset degree.

If the value added to or subtracted from the data value of the register313is too eat, the compensation for the data value of the register313is done more excessively than the expected compensation degree. As a result, another offset with an opposite polarity direction may occur.

For instance, assuming that a severe positive polarity direction DC offset event occurs at a first period and thus, the DC offset compensation is aimed to a negative polarity direction, if the compensation is performed to a great extent, a counted value of the counter311at a second period may indicate a severe level of negative polarity direction DC offset. Thus, the DC offset compensation is aimed to a positive polarity direction again to a great extent. As a result, at a third period, a counted value of the counter311indicates a severe level of positive polarity direction DC offset. Thus, the DC offset compensation apparatus compensates for the severe positive polarity direction DC offset, resulting in a great degree of the negative polarity direction DC offset.

As a result, when the excessive compensation occurs during the fast compensation mode, an infinite loop that infinitely and alternately runs the fast down-compensation mode and the fast up-compensation mode is generated. This infinite loop distorts the DC offset compensation.

Hence, in the case of the fast up-compensation mode or the fast down-compensation mode, the controller312is set at a next period to add or subtract a value that is less than the value from the precedent period to or from the data value of the register313.

When the fast compensation mode is performed as described above (i.e., the addition or subtraction of the predetermined value to or from the data value of the register313at the next period), the data value of the register313converges into a certain value while oscillating with a consistent value.

Accordingly, the infinite loop generation can be impaired while performing the DC offset compensation, and the distortion of the DC offset compensation does not occur.

FIG. 4illustrates a flowchart to describe sequential compensation operations at the offset compensation block212illustrated inFIG. 3based on the DC offset compensation method according to an embodiment of the present invention.

In operation of401, the offset compensation block212starts operating. In operation of402, a data value of the register313is initialized to an assigned value. The data value of the register313controls the offset compensation amplifying block213illustrated inFIG. 3. Second data values N and P are initialized to ‘0’ in operation of403. The second data values control a distortion event of the offset compensation which will be described later.

The counter311illustrated inFIG. 3counts a polarity of DC offset. If a predetermined period is64, the counter311counts polarities of DC offsets that have been detected by the offset detection block211till a 63rdperiod. At a 64thperiod, the counter311is reset.

The counted value provided by the counter311is compared with a first reference value ‘8’. If the counted value is less than the first reference value, in operation of411, the fist mode unit312afor the fast down-compensation mode performs a first mode. In operation of412, the first mode performs the subtraction of a value of ‘2^(6−N)’ from the data value of the register313. A changed data value of the register313is transferred to the offset compensation amplifying block213to implement the fast down-compensation for the DC offset.

If the counted value is greater than the first reference voltage, in operation of421, the counted value is then compared with a second reference value ‘16’. If the counted value is less than ‘16’, the second mode unit312bfor the regular down-compensation mode performs a second mode. In operation of422, the second mode performs the subtraction of ‘1’ from the data value of the register313. The changed data value of the register313is transferred to the offset compensation amplifying block213to implement the regular down-compensation for the DC offset.

If the counted value is greater than ‘16’, the counted value is compared with a third reference value ‘56’. If the counted value is greater than ‘56’, in operation of431, the third mode unit312cfor the fast up-compensation mode performs a third mode. In operation of432, the third mode performs the addition of a value of ‘2^(6−P)’ to the data value of the register313. The changed data value of the register313is transferred to the offset compensation amplifying block213to thereby implement the fast up-compensation for the DC offset.

If the counted value is less than ‘56’, the counted value is compared with a fourth reference value ‘48’. If the counted value is greater than ‘48’, the fourth mode unit312dfor the regular up-compensation mode performs a fourth mode in operation of441. In operation of442, the fourth mode performs the addition of ‘1’ to the data value of the register313. The changed data value of the register313is transferred to the offset compensation amplifying block213to thereby implement the regular up-compensation for the DC offset.

If the counted value is less than ‘48’, the current data value of the register313is maintained.

After one period, in operation of461, a feedback operation is performed. That is, the above cycle returns to the count operation to perform the above described sequential operations for a next period.

Controlling the offset compensation distortion that is often caused by the infinite loop generated when the fast up-compensation or down-compensation mode is performed will be described in detail herein below.

If the counted value at a first period corresponds to the fast down-compensation mode, as described above, in operations of411and412, the value of ‘2^(6−N)’ is subtracted from the data value of the register313. Since the second data value N is ‘0’, ‘64’ is subtracted from the data value of the register313. At this time, another second data value P is compared with ‘5’. If the other second data value P is less than ‘5’, in operations of413and414, a value of ‘N+1’ is stored as the other second data value P. Since the initial second data value P is ‘0’, the other second data value P is less than ‘5’. Thus, ‘1’ is added to the initial second data value N and then, this added value of ‘1’ is stored as the other second data value P.

In the case that the DC offset is excessively compensated by subtracting 64 from the data value of the register313, if a counted value at a second period is greater than ‘56’, the fast up-compensation mode is performed in operation of431. In the fast up-compensation mode, the value of ‘2^(6−P)’ needs to be added to the data value of the register313. Since the other second value P at the first period is ‘1’, the data value of the register313is added with the value of ‘32’ computed from ‘2^(6−1)’ in operation of432. At this time, the second data value N is compared with ‘5’, and if the second data value N is less than ‘5’, in operations of433and434, the computation of N=P+1 is performed. Therefore, the second data value N becomes ‘2’.

In the case that the DC offset at the second period is still excessive, if a counted value at a third period is greater than ‘56’, in operation of411, the fast down-compensation mode is performed. In the fast down-compensation mode, a value of ‘2^(6−N)’ need to be subtracted from the data value of the register313. Since the second data value N is ‘2’ at the second period, a value of ‘16’ computed from ‘2^(6−2)’ is subtracted from the data value of the register313in operation of412. At this time, if the other second value P is less than ‘5’, the computation of P=N+1 is performed in operations of413and414. Therefore, the other second data value P becomes ‘3’.

Changing the second data values N and P makes it possible to control the offset compensation distortion event, which usually occurs due to the infinite loop of the fast down-compensation and up-compensation modes.

For the regular up-compensation mode or the regular down-compensation mode, a value of ‘1’ is added to or subtracted from the data value of the register313, the offset compensation distortion event is less likely to occur. Therefore, the second data values N and P are initialized to ‘0’ in operations of423and443.

For the comparison of the second data values N and P with the reference value of ‘5’ in operations of413and433, since the values that are added to or subtracted from the data value of the register313are ‘2^(6−N)’ and ‘2^(6−P), the comparative reference value is set to be less than ‘6’ because of the subtraction of the second data values N and P from ‘6’. Therefore, if the values of ‘(6−N)’ and ‘(6−P)’ are changed, the reference value ‘5’ may be also changed.

The above-described DC offset compensation method can be implemented as a program and stored on a computer readable recording medium.

According to various embodiment of the present invention, DC offset can be compensated adaptively according to a continuously changing DC offset value.

Also, on the basis of the exemplary embodiments of the present invention, the DC offset compensation distortion, which usually occurs due to the excessive DC offset compensation, can be controlled.

Although the present embodiment exemplifies the DC offset compensation through using the four different modes, other compensation modes are still possible. Since the adaptive compensation according to the DC offset degree is the main technical sprit of the present invention, the scope and sprit of the present should not be limited by the number of the compensation modes.

The above-described logic operations according to the multiple reference values are one exemplary embodiment of the technical sprit of the present invention. Thus, the scope and sprit of the present invention should not be limited to the aforementioned logic operations. The values added to or subtracted from the data value of the register in the fast up-compensation mode, the regular up-compensation mode, the fast down-compensation mode, and the regular down-compensation mode can be changeable, and thus, should not be construed as limiting the scope and sprit of the present invention.