Adjacent channel interference detection apparatus and method

To provide an adjacent channel interference detection apparatus having high detection sensitivity and suitable for LSI integration in a small circuit size. The adjacent channel interference detection apparatus (4) includes: two sign inversion detection units (13a and 13b) that respectively detect sign inversions of an I signal (101) and a Q signal (102); a rotation judgment unit (14) that detects a direction in which a signal point rotates on an IQ plane; a counter unit (15) that counts an output of the rotation judgment unit (14); and an adjacent channel interference detection unit (16) that detects an imbalance in the number of times the signal point rotates in each direction, from the count in the counter unit (15). Adjacent channel interference is detected by an imbalance between the number of times the signal point rotates clockwise and the number of times the signal point rotates counterclockwise on the IQ plane.

TECHNICAL FIELD

The present invention relates to an adjacent channel interference detection apparatus and method suitable for a radio broadcast receiver and especially for an FM radio broadcast receiver.

BACKGROUND ART

Conventionally, radio broadcasting and FM radio broadcasting in particular are a key broadcast medium that occupies an importance place worldwide. Especially in Europe and the United States, an extremely large number of broadcast stations provide broadcasting within a limited frequency band, and it is common practice to space two or more broadcast stations closely such as 100 kHz or 200 kHz apart.

Such spacing of broadcast stations inevitably causes interference from a broadcast wave of a nearby broadcast station, namely, adjacent channel interference. This being so, measures for detecting and removing adjacent channel interference are conventionally required of FM radio receivers. Many techniques have been proposed for adjacent channel interference detection methods and apparatuses that form the basis of such measures and are therefore considered to be particularly important.

For example, techniques of detecting adjacent channel interference with high accuracy by counting a frequency of an intermediate frequency signal (hereafter abbreviated as “IF signal”) using a counter and detecting a deviation from a predetermined frequency are disclosed in Patent References 1 and 2.

Moreover, a technique of detecting adjacent channel interference using a variation in DC component that is obtained as a result of smoothing an FM demodulated signal is widely known as described as a conventional technique in Patent References 1 and 2 and as a basic component in Patent Reference 3.Patent Reference 1: Japanese Unexamined Patent Application Publication No. 11-239064Patent Reference 2: Japanese Unexamined Patent Application Publication No. 11-239065Patent Reference 3: Japanese Unexamined Patent Application Publication No. 2000-312155

DISCLOSURE OF INVENTION

Problems that Invention is to Solve

In recent years, it has become increasingly common in a radio broadcast receiver to convert an IF signal to a digital signal using an AD converter and digitally perform a reception and demodulation process for the IF signal. This creates the demand to integrate the components including the AD converter onto one LSI chip.

Under such circumstances, one possible method for realizing the structures of counting the frequency of the IF signal using the counter as disclosed in Patent References 1 and 2 is to count the frequency of the digitized IF signal generated by the AD converter.

In this case, since a delta-sigma AD converter is usually used as the AD converter, an output signal of the AD converter is a noise-shaped 1-bit signal. Hence it is necessary to extract a frequency component corresponding to the intermediate frequency using a dedicated filter and supply it to the counter. This requires a steep filter characteristic, and causes a significant increase in circuit size.

Another possible method is to branch an input analog signal of the AD converter to generate a binarized IF signal by an analog waveform shaping circuit, and perform the counting using the counter.

In this case, a dedicated analog cell needs to be provided on the LSI, and also a greater placement constraint is imposed for analog signal branching on an LSI substrate.

As described above, according to the structures disclosed in Patent References 1 and 2, in the case of digitally performing the reception and demodulation process for the IF signal and integrating the components onto one LSI chip, the problems such as the significant increase in circuit size, the need for the dedicated analog cell, and the constraint for analog signal line branching arise and make the implementation difficult.

Moreover, according to the FM demodulated signal smoothing structure which is described as a conventional technique in Patent References 1 and 2 and as a basic component in Patent Reference 3, the same implementation as in analog processing is possible even in the case of digitally performing the reception and demodulation process for the IF signal and integrating the components onto one LSI chip, but detection sensitivity is insufficient as pointed out in Patent References 1 and 2, and so it is problematic to use the structure singly.

The present invention has been developed to solve the above conventional problems, and has an object of providing an adjacent channel interference detection apparatus and method that can exhibit high detection sensitivity with a simple circuit structure, even in the case of digitally performing the reception and demodulation process for the IF signal and integrating the components onto one LSI chip.

Means to Solve the Problems

To solve the conventional problems, an adjacent channel interference detection apparatus according to the present invention is an adjacent channel interference detection apparatus that detects adjacent channel interference between broadcast waves of broadcast stations whose frequency bands are adjacent to each other, the adjacent channel interference detection apparatus including: a local oscillation unit that generates two signals of a predetermined frequency in a free-running state, the two signals of the predetermined frequency being different in phase from each other by 90 degrees; two mixer units that mix an input signal respectively with the two signals of the predetermined frequency generated by the local oscillation unit; two low pass filter units respectively connected to outputs of the two mixer units; a rotation judgment unit that detects a direction in which a signal point rotates on an IQ plane, according to an I signal and a Q signal outputted respectively from the two low pass filter units; a counter unit that counts an output of the rotation judgment unit; and a detection unit that detects an imbalance in the number of times the signal point rotates in each direction, according to the count by the counter unit.

Moreover, the adjacent channel interference detection apparatus according to the present invention further includes two sign inversion detection units that are respectively connected to outputs of the two low pass filter units, and detect inversions of signs of the I signal and the Q signal outputted from the two low pass filter units, wherein the rotation judgment unit is connected to outputs of the two sign inversion detection units, and detects the direction in which the signal point rotates on the IQ plane according to the signs of the I signal and the Q signal, the counter unit counts the output of the rotation judgment unit, and the detection unit detects the imbalance in the number of times the signal point rotates in each direction, according to the count by the counter unit.

According to these structures, the sign inversion detection units respectively detect the sign inversions of the I and Q components of the IF signal which has been converted to a digital signal and further to a complex baseband signal, and the rotation judgment unit detects, on the IQ plane, the rotation direction of the IF signal as a complex baseband signal. The counter unit counts the detected direction, and the detection unit detects an imbalance between the number of times the IF signal rotates clockwise and the number of times the IF signal rotates counterclockwise, with it being possible to determine whether or not adjacent channel interference occurs.

Thus, according to the present invention, an adjacent channel interference detection apparatus can be realized by an extremely simple structure, by merely adding a few processing blocks to a basic structure of digitally performing a reception and demodulation process for an IF signal.

Moreover, the adjacent channel interference detection apparatus according to the present invention further includes a magnitude comparison unit that is connected to outputs of the two low pass filter units, and detects a magnitude relation of the I signal and the Q signal, wherein the rotation judgment unit is connected to an output of the magnitude comparison unit, and detects the direction in which the signal point rotates on the IQ plane according to the magnitude relation, the counter unit counts the output of the rotation judgment unit, and the detection unit detects the imbalance in the number of times the signal point rotates in each direction, according to the count by the counter unit.

According to this structure, the magnitude comparison unit compares the I signal and the Q signal in magnitude, and the rotation judgment unit detects, on the IQ plane, the rotation direction of the IF signal as a complex baseband signal. The counter unit counts the number of times the IF signal moves from one area to another in each direction, and the detection unit detects an imbalance in the number of times the IF signal rotates in each direction, with it being possible to determine whether or not adjacent channel interference occurs.

To achieve the stated object, the present invention can also be realized as an adjacent channel interference detection method including steps corresponding to the characteristic units of the adjacent channel interference detection apparatus, or a program causing a computer to execute these steps. Such a program can be distributed via a recording medium such as a CD-ROM or a transmission medium such as the Internet.

Effects of the Invention

According to the present invention, an adjacent channel interference detection apparatus having high detection sensitivity and suitable for LSI integration can be realized in an extremely small circuit size, with there being no need for a dedicated filter which causes an increase in circuit size or a dedicated analog cell which impedes LSI integration.

NUMERICAL REFERENCES

BEST MODE FOR CARRYING OUT THE INVENTION

First Embodiment

FIG. 1is a block diagram showing a structure of an adjacent channel interference detection apparatus4in a first embodiment of the present invention.

InFIG. 1, the adjacent channel interference detection apparatus4is an apparatus that receives an IF signal100which has been converted to a digital signal by AD conversion, and includes a local oscillation unit10that generates two signals which have a same frequency as a center frequency of the IF signal100and differ in phase from each other by 90 degrees, two mixer units11aand11bthat mix the IF signal100as a digital signal respectively with the two signals generated by the local oscillation unit10, and two low pass filter units12aand12bthat perform low pass filtering on outputs of the respective mixer units11aand11bto extract a complex baseband signal composed of desired I signal and Q signal. An I signal101and a Q signal102are then outputted respectively from the low pass filters12aand12b.

The adjacent channel interference detection apparatus4further includes two sign inversion detection units13aand13bthat respectively detect sign inversions of the I signal101and the Q signal102, a rotation judgment unit14that is connected to outputs of the sign inversion detection units13aand13band judges/detects a rotation direction of the complex baseband signal composed of the I signal101and the Q signal102on an IQ plane, a counter unit15that increases or decreases in count according to an output of the rotation judgment unit14, and an adjacent channel interference detection unit16that detects adjacent channel interference on the basis of the count in the counter unit15and generates an adjacent channel interference detection signal103.

FIG. 2is a block diagram showing an example of a structure of a radio receiver system that includes the adjacent channel interference detection apparatus4in the first embodiment of the present invention as a component.

The radio receiver system shown inFIG. 2includes a reception antenna1, a frequency conversion and IF processing unit2that converts an incoming high frequency signal to an IF signal and processes it, an AD conversion unit3that converts the IF signal to a digital signal, the adjacent channel interference detection apparatus4in the first embodiment of the present invention shown inFIG. 1, a variable filter unit5, an FM demodulation unit6, a stereo demodulation unit7, and a DA conversion and amplification unit8.

InFIG. 2, the high frequency signal received by the antenna1is converted to an IF signal by the frequency conversion and IF processing unit2, and this IF signal is converted to a digital signal by the AD conversion unit3and provided to the adjacent channel interference detection apparatus4in the first embodiment of the present invention shown inFIG. 1as the IF signal100. Note that the adjacent channel interference detection apparatus4shown in the drawing also performs IQ signal generation as mentioned earlier.

The following describes an operation of the adjacent channel interference detection apparatus in the first embodiment of the present invention, with reference toFIG. 1.

The digitized IF signal100received as described above is provided to the two mixer units11aand11b, and mixed with each of two signals having a same center frequency as the IF signal and differing in phase from each other by 90 degrees, which are generated by the local oscillation unit10. Resulting signals are supplied to the two low pass filters12aand12band undergo low pass filtering. As a result, a complex baseband signal composed of the I signal101and the Q signal102is obtained.

Consider the case of plotting a signal point, with a signal value of the I signal101and a signal value of the Q signal102respectively on the horizontal axis and the vertical axis. For example, when the high frequency signal from the antenna1is an FM modulated wave, a signal point901moves on a circular path900whose radius corresponds to signal intensity, while repeating clockwise rotation and counterclockwise rotation according to frequency shift of the FM modulated wave, as shown inFIG. 3.

In this state, it is known that the signal point901rotates clockwise and counterclockwise the same number of times when no adjacent channel interference occurs, but this balance between the number of times the signal point901rotates clockwise and the number of times the signal point901rotates counterclockwise is lost due to a deviation in IF when adjacent channel interference occurs. Typically a complicated process is required to precisely detect the balance between the number of times of the clockwise rotation and the number of times of the counterclockwise rotation. In the present invention, however, it has been found by experiment that sufficient detection accuracy can be achieved by a simple method of counting the number of times the signal point901moves from one quadrant to another on the IQ plane.

The number of times the signal point901moves from one quadrant to another on the IQ plane can be easily judged on the basis of a combination of sign inversions of the I signal101and the Q signal102. Accordingly, for example in the case where the signal point901exists in the first quadrant at one point in time, it is possible to apply a simple rule such as increasing the counter by 1 when the signal point901moves to the second quadrant, decreasing the counter by 1 when the signal point901moves to the fourth quadrant, and neither increasing nor decreasing the counter when the signal point901moves to the third quadrant. Thus, according to the above new findings, the balance between the number of times of the clockwise rotation and the number of times of the counterclockwise rotation of the signal point can be detected efficiently.

In the adjacent channel interference detection apparatus4according to the present invention, the two sign inversion detection units13aand13bshown inFIG. 1respectively detect sign inversions of the I signal101and the Q signal102. The rotation judgment unit14detects the clockwise rotation or the counterclockwise rotation of the signal point on the basis of outputs of the two sign inversion detection units13aand13bin accordance with a rule such as the example given above, and drives the counter unit15. When no adjacent channel interference occurs, the count in the counter unit15will end up being approximately 0. When adjacent channel interference occurs, on the other hand, the count in the counter unit15will end up being a positive value or a negative value. On the basis of this count, the adjacent channel interference detection unit16lastly generates the adjacent channel interference detection signal103, for example when the count exceeds a predetermined threshold.

Further description is given below, by referencingFIG. 2again. The adjacent channel interference detection signal103generated as described above is supplied to the variable filter unit5as shown inFIG. 2.

The variable filter unit5performs a filtering process on the incoming I signal101and Q signal102according to the adjacent channel interference detection signal103, thereby removing adjacent channel interference.

In detail, this filtering process performed by the variable filter unit5in adjacent channel interference removal is, for example, a process of selecting a band pass filter of a narrower frequency range to reduce a bandwidth when a larger imbalance in rotation direction of the signal point on the IQ plane is detected by the adjacent channel interference detection apparatus4, or a process of shifting a center frequency upward or downward to a greater extent when a larger imbalance in rotation direction of the signal point on the IQ plane is detected by the adjacent channel interference detection apparatus4. Adjacent channel interference is removed as a result of this process by the variable filter unit5.

The I signal and the Q signal from which adjacent channel interference has been removed are demodulated by the FM demodulation unit6, as a result of which a composite signal is extracted. The composite signal is a multiplexed signal containing a main signal and sub-signal of stereo audio, a pilot signal for stereo demodulation, and the like. The composite signal is demodulated to a stereo audio signal by the stereo demodulation unit7, and converted to analog and amplified by the DA conversion and amplification unit8to eventually produce an output of an audio signal from which adjacent channel interference has been removed.

At present, the functions from the AD conversion unit3, the adjacent channel interference detection apparatus4, the variable filter unit5, the FM demodulation unit6, up to the stereo demodulation unit7can be implemented as one-chip LSI. In the future, however, all of the functions shown inFIG. 2including the frequency conversion and IF processing unit2and the DA conversion and amplification unit8will be able to be implemented as one-chip LSI.

FIG. 4is a flowchart showing an operational procedure of the adjacent channel interference detection apparatus in the first embodiment.

First, the sign inversion detection units13aand13bin the adjacent channel interference detection apparatus4detect the sign inversions of the I signal and the Q signal, respectively (Step S401).

Next, the rotation judgment unit14detects the rotation direction of the signal point and judges whether or not the signal point moves from one quadrant to another on the IQ plane, using information of the detected signs of the I signal and the Q signal (Step S402).

When judging that the signal point moves from one quadrant to another on the IQ plane (Step S402: Yes), the rotation judgment unit14detects the direction in which the signal point moves from one quadrant to another. On the basis of this detection, for example the counter unit15counts +1 (Step S404) when the signal point moves from one quadrant to another in the counterclockwise direction such as in the case of moving from the first quadrant to the second quadrant (Step S403: Yes), and counts −1 (Step S405) when the signal point moves from one quadrant to another in the clockwise direction such as in the case of moving from the first quadrant to the fourth quadrant (Step S403: No).

The series of these processes is conducted for a predetermined period such as 10 ms (Step S406). When the predetermined period has elapsed, the adjacent channel interference detection unit16judges whether or not the count in the counter unit15exceeds the predetermined threshold (Step S407). When the count in the counter unit15exceeds the threshold (Step S407: Yes), the adjacent channel interference detection unit16judges that adjacent channel interference occurs, and provides the adjacent channel interference detection signal to the variable filter unit5(Step S408). Upon receiving the adjacent channel interference detection signal, the variable filter unit5performs a process such as selecting a filter of a narrower range, as described above.

When the difference does not exceed the threshold (Step S407: No), the adjacent channel interference detection unit16judges that no adjacent channel interference occurs, and ends the procedure without generating the adjacent channel interference detection signal.

According to this structure, with the provision of the sign inversion detection units13aand13bthat detect sign inversions of the I signal101and the Q signal102, the rotation judgment unit14, the counter unit15, and the adjacent channel interference detection unit16, the balance between the number of times of the clockwise rotation and the number of times of the counterclockwise rotation of the signal point in relation to adjacent channel interference can be detected efficiently by an extremely simple structure. Hence an adjacent channel interference detection apparatus having high detection sensitivity and suitable for LSI integration can be realized in an extremely small circuit size.

This embodiment describes the case where an up-down counter is used as the counter unit15. Alternatively, two typical up counters may be provided to separately count the number of times of the clockwise rotation and the number of times of the counterclockwise rotation and calculate their difference. Moreover, the same effects can be achieved when the processing units described in this embodiment are implemented as individual components constituting an apparatus or when all of the processing units are integrated on one or more LSI chips.

Furthermore, each of the processing units included in the apparatus described above can be changed to a processing step so that the present invention can be realized by software processing.

Second Embodiment

The following describes an adjacent channel interference detection apparatus in a second embodiment of the present invention.

FIG. 5is a block diagram showing a structure of the adjacent channel interference detection apparatus in the second embodiment of the present invention. InFIG. 5, components which are the same as those inFIG. 1have been given the same reference numerals, and their explanation has been omitted here.

The adjacent channel interference detection apparatus shown inFIG. 5is characterized in that a magnitude comparison unit17that detects a change in magnitude relation of the I signal101and the Q signal102is provided instead of the sign inversion detection units13aand13binFIG. 1.

According to this structure, on the basis of the magnitude relation of the I signal101and the Q signal102detected by the magnitude comparison unit17, the IQ plane can be divided into four areas by a line902where I=Q and a line903where I=−Q as shown inFIG. 6, and the movement of the signal point901between these four areas can be detected. This enables the rotation judgment unit14to detect the clockwise rotation or the counterclockwise rotation of the signal point, the counter unit15to perform the counting, and the adjacent channel interference detection unit16to detect whether or not adjacent channel interference occurs on the basis of the count, in the same way as in the first embodiment.

FIG. 7is a flowchart showing an operational procedure of the adjacent channel interference apparatus in the second embodiment.

First, the magnitude comparison unit17detects the magnitude relation of the I signal and the Q signal (Step S701).

Next, the rotation judgment unit14detects the rotation direction of the signal point and judges whether or not the signal point moves from one area to another out of the four areas which have been set as described above, using information of the detected magnitude relation of the I signal and the Q signal (Step S702).

When judging that the signal point moves from one area to another on the IQ plane (Step S702: Yes), the rotation judgment unit14detects the direction in which the signal point moves from one area to another. On the basis of this detection, for example the counter unit15counts +1 (Step S704) when the signal point moves from one area to another in the counterclockwise direction (Step S703: Yes), and counts −1 (Step S705) when the signal point moves from one area to another in the clockwise direction (Step S703: No).

The series of these processes is conducted for a predetermined period such as 10 ms (Step S706). When the predetermined period has elapsed, the adjacent channel interference detection unit16judges whether or not the count in the counter unit15exceeds a predetermined threshold (Step S707). When the count in the counter unit15exceeds the threshold (Step S707: Yes), the adjacent channel interference detection unit16judges that adjacent channel interference occurs, and provides the adjacent channel interference detection signal to the variable filter unit5(Step S708).

When the difference does not exceed the threshold (Step S707: No), the adjacent channel interference detection unit16judges that no adjacent channel interference occurs, and ends the procedure without generating the adjacent channel interference detection signal.

According to this structure too, the balance between the number of times of the clockwise rotation and the number of times of the counterclockwise rotation of the signal point in relation to adjacent channel interference can be detected efficiently by an extremely simple structure. Hence an adjacent channel interference detection apparatus having high detection sensitivity and suitable for LSI integration can be realized in an extremely small circuit size.

Moreover, as is apparent fromFIG. 6, the IQ plane can be divided into eight areas by combining the structure of the second embodiment with the structure of the first embodiment described earlier.

In such a case, though the circuit size may increase to some extent, adjacent channel interference can be detected with higher sensitivity.

Each of the above embodiments describes the case where the counter unit in the adjacent channel interference detection apparatus counts +1 when the signal point moves from one quadrant or area to another in the counterclockwise direction on the IQ plane, and counts −1 when the signal point moves from one quadrant or area to another on the IQ plane in the clockwise direction. However, this up-down counting of the counter unit may be reversed so that the counter unit counts −1 when the signal point moves from one quadrant or area to another in the counterclockwise direction, and counts +1 when the signal point moves from one quadrant or area to another in the clockwise direction.

Furthermore, each of the units included in the apparatus described above may be changed to a processing step so that the present invention can be realized by software processing.

FIG. 8is a perspective view of a car80provided with a car radio81that includes the adjacent channel interference detection apparatus according to the present invention. As shown inFIG. 8, the adjacent channel interference detection apparatus according to the present invention can be applied, for example, to the car radio81as vehicle-mounted equipment.

INDUSTRIAL APPLICABILITY

The adjacent channel interference detection apparatus and method according to the present invention have high detection sensitivity, are suitable for LSI integration, and can be realized in an extremely small circuit size. Therefore, the adjacent channel interference detection apparatus and method according to the present invention are very useful for a digital signal processing radio receiver system, particularly its implementation by system LSI.