JP6237309B2 - FM receiver and FM receiving method - Google Patents

Description

  The present invention relates to a receiving technique, and more particularly to an FM receiving apparatus and FM receiving method for receiving an FM signal.

  A direct conversion FM (Frequency Modulation) receiver changes an RF signal to a baseband signal by quadrature detection, and then amplifies the baseband signal with an amplifier. Since an unnecessary DC component is output by the amplifier, the FM receiver reduces the DC component included in the baseband signal by the coupling capacitor. Further, the FM receiver FM-detects a baseband signal with a reduced DC component (see, for example, Patent Document 1).

Japanese Patent Laid-Open No. 3-16349

  When the direct conversion type FM receiver includes a coupling capacitor, not only unnecessary DC components by the amplifier are reduced, but also DC components and low frequency components of the baseband signal may be reduced. Such a reduction causes distortion components after FM detection. Also, when an unmodulated signal is received and the frequency of the received signal is the same as the frequency of the local oscillation signal, the baseband signal is only a direct current component, and thus all signals are cut by the coupling capacitor. . As a result, only the noise received by the antenna and the noise generated inside are subjected to FM detection, so that the signal after FM detection becomes only noise.

  The present invention has been made in view of such circumstances, and an object thereof is to provide a technique for improving the quality of an FM detected signal.

In order to solve the above-described problems, an FM receiver according to an aspect of the present invention reduces a direct current component included in an FM signal obtained by quadrature detection of an FM signal and quadrature detection performed by the quadrature detector. In the reduction unit, an FM detection unit that generates a detection signal by performing FM detection on the FM signal whose DC component is reduced in the reduction unit, an offset unit that adds an offset to the detection signal generated in the FM detection unit, and an offset unit AFC that generates a control signal for controlling the frequency of the local oscillation signal used in the quadrature detector based on the detection signal to which the offset is added, and feeds back the control signal to the local oscillator that should output the local oscillation signal A section. When the quadrature detector does not detect the FM signal, the AFC unit causes the local oscillator to output a frequency-modulated local oscillation signal instead of feeding back the control signal to the local oscillator.

Yet another embodiment of the present invention is an FM reception method. This method generates a detection signal by performing quadrature detection of the FM signal, reducing the DC component included in the FM signal subjected to quadrature detection, and FM-detecting the FM signal with the reduced DC component. Generating a control signal for controlling the frequency of the local oscillation signal used in the step of quadrature detection based on the detected signal to which the offset is added, the detected signal to which the offset is added, When the FM signal is not detected in the step of feeding back the control signal to the local oscillator to output the local oscillation signal and the step of quadrature detection, instead of feeding back the control signal to the local oscillator, the local signal output from the local oscillator is output. comprising the steps of: frequency modulating the oscillation signal.

  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, the quality of an FM detected signal can be improved.

It is a figure which shows the structure of the receiver which concerns on Example 1 of this invention. It is a figure which shows the structure of the receiver which concerns on Example 2 of this invention. It is a figure which shows the structure of the receiver which concerns on Example 3 of this invention. It is a flowchart which shows the reception procedure by the receiver of FIG. It is a figure which shows the structure of the receiver which concerns on Example 4 of this invention.

Example 1
Before describing the present invention specifically, an outline will be given first. Embodiment 1 of the present invention relates to a direct conversion type FM receiver. By arranging the coupling capacitor between the quadrature detector and the FM detector, the direct current component is reduced, so that the deviation of the orthogonal coordinates is reduced. However, when the unmodulated signal is received, if the frequency shift between the received signal and the local oscillation signal is converged to zero by AFC, the quadrature detection output stops at one point on the coordinate circle and is input to the FM detector. The signal level is attenuated so as to approach the origin. Arctan detection in the FM detector outputs the amount of change in phase rotation, so if the input is concentrated near the origin, phase rotation due to irregular noise will be detected. As a result, the result of FM detection becomes noise. On the other hand, when the received signal is unmodulated, the FM detection result should be silent. Further, since the output frequency of the local oscillator varies depending on the temperature or the like, the noise generation condition varies depending on the temperature. Further, since non-modulated transmission does not occur in the case of digital (4-level FSK: Frequency Shift Keying), the above is a problem in an analog FM that is used in unmodulated transmission.

  The present embodiment aims to reduce distortion after FM detection in a receiving apparatus that performs FM detection after reducing the DC component of a baseband signal by the direct conversion method. This can be said to realize stable operation even when an unmodulated signal is received. Here, in order not to stop the rotation of the quadrature detection output, the convergence target of the frequency deviation of the AFC is not set to zero but is converged to the offset value.

  FIG. 1 shows a configuration of a receiving apparatus 100 according to Embodiment 1 of the present invention. The receiving apparatus 100 includes an antenna 10, a quadrature detection unit 12, a first reduction unit 32, a second reduction unit 34, a first ADC unit 14, a second ADC unit 16, an FM detection unit 18, an offset unit 36, an AFC unit 20, and a DAC unit. 22 includes a local oscillator 24. The quadrature detection unit 12 includes a first amplification unit 40, a distribution unit 42, a first mixer 44, a first LPF unit 46, a second amplification unit 48, a phase shift unit 50, a second mixer 52, a second LPF unit 54, and a third amplification. Part 56 is included. The AFC unit 20 includes a third LPF unit 60 and a fourth amplification unit 62.

  The antenna 10 receives an RF (Radio Frequency) signal from a transmission device (not shown). The RF signal is FM-modulated. The antenna 10 outputs the received RF signal to the first amplification unit 40. The first amplifying unit 40 is an LNA (Low Noise Amplifier) and amplifies the RF signal from the antenna 10. The first amplification unit 40 outputs the amplified RF signal to the distribution unit 42. The distribution unit 42 separates the RF signal from the first amplification unit 40 into two systems. The distribution unit 42 outputs the separated RF signal to the first mixer 44 and the second mixer 52.

  The local oscillator 24 adjusts the frequency of the local oscillation signal according to the control signal from the DAC unit 22, and outputs the local oscillation signal whose frequency is adjusted to the first mixer 44 and the phase shift unit 50. Here, the local oscillator 24 increases the frequency of the local oscillation signal as the voltage of the control signal increases. The phase shifter 50 phase-shifts the local oscillation signal from the local oscillator 24 by 90 degrees. The phase shifter 50 outputs the phase-shifted local oscillation signal to the second mixer 52.

  The first mixer 44 multiplies the RF signal from the distributor 42 and the local oscillation signal from the local oscillator 24 to generate a baseband in-phase component signal (hereinafter referred to as “I signal”). The first mixer 44 outputs the I signal to the first LPF unit 46. The second mixer 52 multiplies the RF signal from the distribution unit 42 and the local oscillation signal from the phase shift unit 50 to generate a baseband quadrature component signal (hereinafter referred to as “Q signal”). The second mixer 52 outputs the Q signal to the second LPF unit 54.

  The first LPF unit 46 performs band limitation by removing a signal having a frequency equal to or higher than the cutoff frequency from the I signal from the first mixer 44. The first LPF unit 46 outputs a low-frequency component I signal (hereinafter also referred to as “I signal”) to the second amplifying unit 48. The second LPF unit 54 performs band limitation by removing a signal having a frequency equal to or higher than the cutoff frequency from the Q signal from the second mixer 52. The second LPF unit 54 outputs a low-frequency component Q signal (hereinafter also referred to as “Q signal”) to the third amplifying unit 56. The second amplification unit 48 amplifies the I signal from the first LPF unit 46 and outputs it to the first reduction unit 32. The third amplifying unit 56 amplifies the Q signal from the second LPF unit 54 and outputs the amplified Q signal to the second reducing unit 34. The I signal output from the second amplifying unit 48 includes an unnecessary DC component, and the Q signal output from the third amplifying unit 56 also includes an unnecessary DC component. As described above, the quadrature detection unit 12 performs quadrature detection of the RF signal. The quadrature detection unit 12 is composed of an analog device, for example, one chip.

  The first reduction unit 32 receives the I signal from the second amplification unit 48. The first reduction unit 32 is configured with, for example, a coupling capacitor, and reduces the direct current component included in the I signal. The first reduction unit 32 outputs an I signal (hereinafter also referred to as “I signal”) having a reduced DC component to the first ADC unit 14. The second reduction unit 34 receives the Q signal from the third amplification unit 56. Similarly to the first reduction unit 32, the second reduction unit 34 is configured by a coupling capacitor, and reduces the DC component included in the Q signal. The second reduction unit 34 outputs a Q signal (hereinafter also referred to as “Q signal”) having a reduced DC component to the second ADC unit 16.

  The first ADC unit 14 performs analog / digital conversion on the I signal from the first reduction unit 32. The first ADC unit 14 outputs an I signal (hereinafter also referred to as “I signal”) converted into a digital signal to the FM detection unit 18. The second ADC unit 16 performs analog / digital conversion on the Q signal from the second reduction unit 34. The second ADC unit 16 outputs the Q signal converted to a digital signal (hereinafter also referred to as “Q signal”) to the FM detection unit 18.

  The FM detector 18 performs FM detection on the I signal and the Q signal, that is, the baseband signal. As the FM detection, for example, Arctan detection is executed. In Arctan detection, each of the I signal and the Q signal is defined as two sides of a triangle, and the angles are derived. Since the change in the angle per unit time becomes the angular velocity, that is, the frequency, the FM modulation can be demodulated. The FM detection unit 18 outputs a detection signal that is a result of the FM detection. The output detection signal corresponds to an audio signal.

  The offset unit 36 adds an offset to the detection signal generated by the FM detection unit 18. This corresponds to adding an offset value to the detection signal. When there is no offset unit 36, the AFC unit 20 controls the center frequency of the RF signal and the frequency of the local oscillation signal to be the same. However, since the offset unit 36 adds a certain offset value, the local oscillation signal is And a frequency offset corresponding to the offset value. If this frequency offset is set to a frequency that is not reduced by the first reduction unit 32 and the second reduction unit 34, the I signal and the Q signal do not become DC components even when the RF signal is unmodulated. The frequency is the same as the frequency offset. Therefore, even when an unmodulated RF signal is input, the detection signal does not become noise.

  The third LPF unit 60 performs low-pass processing on the detection signal generated by the FM detection unit 18. This corresponds to deriving the average value of the detection signal. Therefore, the third LPF unit 60 detects the DC component of the detection signal, that is, the frequency difference between the RF signal and the local oscillation signal. The fourth amplification unit 62 generates a control signal by amplifying the signal from the third LPF unit 60. The gain of the AFC loop is determined by the amplification in the fourth amplification unit 62. As described above, the AFC unit 20 generates a control signal for controlling the frequency of the local oscillation signal used in the quadrature detection based on the detection signal to which the offset is added in the offset unit 36. The AFC unit 20 may cause the local oscillator 24 to output a frequency-modulated local oscillation signal instead of feeding back the control signal to the local oscillator 24 when the quadrature detection unit 12 does not detect the RF signal. .

  The DAC unit 22 performs digital / analog conversion on the control signal from the fourth amplifying unit 62 and outputs an analog signal control signal (hereinafter also referred to as “control signal”) to the local oscillator 24. That is, the AFC unit 20 feeds back the control signal to the local oscillator 24 that should output the local oscillation signal.

  This configuration can be realized in terms of hardware by a CPU, memory, or other LSI of any computer, and in terms of software, it can be realized by a program loaded in the memory, but here it is realized by their cooperation. Draw functional blocks. 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.

  According to the present embodiment, since an offset is added to the detection signal, it is possible to suppress the possibility that the baseband signal is completely cut even when an unmodulated signal is input. In addition, since the possibility that the baseband signal is completely cut is suppressed, it is possible to avoid the FM detection output from being only noise as in the case where the RF signal is not received. Further, since it is avoided that the FM detection output is only noise, the quality of the FM detected signal can be improved. In addition, since the offset value of the local oscillator is changed before and after the RF signal is input, it is possible to suppress the reduction of the baseband signal at the time of input of the RF signal, and the reception characteristics such as the adjacent interference thereafter. It is possible to achieve both suppression of deterioration. Moreover, when an unmodulated signal is input, it is possible to suppress the baseband signal from being continuously reduced by the reduction unit. Further, since the frequency of the local oscillation signal is changed until the RF signal is detected, it is possible to suppress the baseband signal from being continuously reduced by the reduction unit.

(Example 2)
Next, Example 2 will be described. As in the first embodiment, the second embodiment of the present invention is a direct conversion type and relates to an FM receiver in which a coupling capacitor is disposed between a quadrature detector and an FM detector. When the frequency of the baseband signal is lowered by inserting the coupling capacitor, the amplitude of the baseband signal may be attenuated. More specifically, the frequency of the baseband signal output from the quadrature detector is a value close to DC in the vicinity where the frequency of the received signal is the same as the frequency of the local oscillator in the frequency transition of the received FM modulated wave. Become. In such a state, the level of the baseband signal is lowered by the coupling capacitor. The purpose of the second embodiment is to suppress the occurrence of such a situation. The FM receiver according to the second embodiment interpolates the detection signal when the level of the baseband signal is attenuated.

  FIG. 2 shows a configuration of the receiving apparatus 100 according to the second embodiment of the present invention. In the receiving apparatus 100, a measuring unit 70 and a changing unit 72 are added to FIG. Here, it demonstrates centering on the difference with FIG. As described above, when an unmodulated signal is input in FIG. 2, the baseband signal passes through the first reduction unit 32 and the second reduction unit 34. On the other hand, when a modulation signal is input, the signal component is also reduced by the first reduction unit 32 and the second reduction unit 34 when the frequency of the baseband signal is close to DC, and distortion and noise are generated when FM is detected. To do.

  The measurement unit 70 receives the I signal from the first reduction unit 32 and the Q signal from the second reduction unit 34. The measurement unit 70 measures the level of the baseband signal based on the input I signal and Q signal. Since a known technique may be used for the level measurement, the description thereof is omitted here. The measuring unit 70 outputs the measured level to the changing unit 72. The changing unit 72 inputs the detection signal from the FM detection unit 18 and also inputs the level from the measurement unit 70. The changing unit 72 compares the threshold value stored in advance with the level. When the level becomes smaller than the threshold value, the changing unit 72 performs correction such that the detection signal is changed to another value, for example, a past detection signal and output. In such a case, it is assumed that there is a lot of distortion and noise in the detection signal, and control is performed so as not to output the detection signal at that time. The past detection signal is a detection signal when the level measured by the measurement unit 70 is equal to or greater than a threshold value.

  According to this embodiment, even when the baseband signal temporarily approaches a direct current component and is reduced by the reduction unit, output of the signal detected by FM detection is stopped, so that distortion and noise of the output signal are reduced. it can. Further, since distortion and noise of the output signal are reduced, signal quality can be improved.

(Example 3)
Next, Example 3 will be described. The third embodiment of the present invention relates to an FM receiver that is a direct conversion type and has a coupling capacitor disposed between the quadrature detector and the FM detector, as before. In particular, the received RF signal corresponds to a tone squelch signal. Furthermore, when the frequency difference between the RF signal and the local oscillation signal is large in the initial state where the reception of the RF signal is started, the AFC unit controls the local oscillation signal, so the average value output from the FM detection unit is It will fluctuate. In that case, when CTCSS (Continuous Tone Coded Squeeze System) or DCS (Digital-Coded Squelch) is included in the RF signal, the signal detection is affected.

  In order to cope with this, the FM receiver according to the third embodiment controls the frequency of the local oscillator by executing AFC based on the FM detection signal, and adds the AFC control signal to the FM detection signal. . This addition increases the component of the attenuated control signal.

  FIG. 3 shows the configuration of the receiving apparatus 100 according to the third embodiment of the present invention. In the receiving apparatus 100, a control unit 26, a difference calculation unit 28, and an addition unit 30 are added to FIG. Here, the difference from FIG. 2 will be mainly described.

  The control unit 26 detects the timing when the power of the receiving device 100 is turned on and the timing when the receiving device 100 starts receiving the RF signal. These timings are collectively referred to as “start timing”. Since a known technique may be used for detecting the start timing, the description thereof is omitted here. The control unit 26 instructs the difference calculation unit 28 to start processing. In this way, the control unit 26 controls the operation of the difference calculation unit 28.

  The difference calculation unit 28 inputs a signal from the third LPF unit 60, that is, a detection signal subjected to low-pass processing in the third LPF unit 60. The difference calculation unit 28 acquires and stores the detection signal subjected to the low-pass processing in the third LPF unit 60 as a reference value at the start timing of the process instructed by the control unit 26. After storing the reference value, the difference calculating unit 28 sequentially generates a difference between the detection signal subjected to the low-pass processing in the third LPF unit 60 and the reference value as a difference signal. That is, the difference calculation unit 28 generates a difference signal from the reference value based on the detection signal generated by the FM detection unit 18. The difference calculation unit 28 outputs the difference signal to the addition unit 30.

  The adding unit 30 adds the difference signal generated by the difference calculating unit 28 and the detection signal from the changing unit 72. This corresponds to the operation of canceling the output voltage fluctuation of the FM detection unit 18 by the AFC unit 20.

  The operation of the receiving apparatus 100 having the above configuration will be described. FIG. 4 is a flowchart illustrating a reception procedure by the reception device 100. If the control unit 26 detects the start of reception (Y in S10), the difference calculation unit 28 acquires a reference value (S12). If the control unit 26 does not detect the start of reception (N in S10), the difference calculation unit 28 generates a difference signal by calculating the output value-reference value of the third LPF unit 60 (S14). The difference calculation unit 28 outputs a difference signal (S16).

  According to the present embodiment, since the difference between the detection signal subjected to the low-pass transmission processing and the reference value is added to the detection signal, attenuation of the tone squelch signal can be suppressed. In addition, since the detection signal subjected to the low-pass transmission processing at the processing start timing is stored as the reference value, the processing accuracy can be improved. Further, when the tone squelch signal is included in the RF signal, the AFC follows this signal, and the tone squelch signal after FM detection is attenuated or distorted. Therefore, the tone squelch signal can be returned. Further, since the tone squelch signal is returned, the squelch operation can be executed normally. Moreover, since the fluctuation of the signal after FM detection generated by AFC is reduced, it is possible to suppress the deterioration of the detection performance of the tone squelch signal.

Example 4
Next, Example 4 will be described. The fourth embodiment of the present invention relates to an FM receiver that is a direct conversion type and has a coupling capacitor disposed between the quadrature detector and the FM detector, as before. The fourth embodiment corresponds to a configuration that does not include AFC.

  FIG. 5 shows a configuration of a receiving apparatus 100 according to Embodiment 4 of the present invention. The receiving apparatus 100 includes an antenna 10, a quadrature detection unit 12, a first reduction unit 32, a second reduction unit 34, a first ADC unit 14, a second ADC unit 16, an FM detection unit 18, a measurement unit 70, a change unit 72, and an addition unit. 30, a first local oscillator 80, a DAC unit 82, a second local oscillator 84, and a delay unit 86. The quadrature detection unit 12 includes a first amplification unit 40, a distribution unit 42, a first mixer 44, a first LPF unit 46, a second amplification unit 48, a phase shift unit 50, a second mixer 52, a second LPF unit 54, and a third amplification. Part 56 is included. Here, it demonstrates centering on the difference from before.

  The first local oscillator 80 outputs a first local oscillation signal. The first local oscillation signal is a digital signal. The DAC unit 82 performs digital / analog conversion on the first local oscillation signal from the first local oscillator 80 and converts the first local oscillation signal of the analog signal (hereinafter also referred to as “first local oscillation signal”) to the first. 2 Output to local oscillator 84.

  The second local oscillator 84 adjusts the frequency of the second local oscillation signal according to the first local oscillation signal from the DAC unit 82, and the second local oscillation signal with the adjusted frequency is sent to the first mixer 44 and the phase shift unit. Output to 50. This corresponds to outputting the second local oscillation signal frequency-modulated by the first local oscillation signal from the first local oscillator 80 to the quadrature detection unit 12.

  The delay unit 86 delays the first local oscillation signal from the first local oscillator 80. The delay time corresponds to a processing period from the DAC unit 82, the second local oscillator 84, and the quadrature detection unit 12 to the change unit 72. The delay unit 86 outputs the delayed first local oscillation signal (hereinafter also referred to as “first local oscillation signal”) to the addition unit 30. The adder 30 receives the detection signal from the changing unit 72 and the first local oscillation signal from the delay unit 86. The adder 30 adds the first local oscillation signal and the detection signal.

  According to the present embodiment, since the second local oscillation signal is frequency-modulated, it is possible to suppress the baseband signal from being continuously reduced by the reduction unit. Also, since the frequency of the first local oscillation signal from the first local oscillator is set to a frequency lower than the voice band, the baseband signal is continuously reduced by the reduction unit even when a modulated signal is input. Can be suppressed. Even when the baseband signal temporarily approaches a direct current component and is reduced by the reduction unit, the output of the FM detected signal is corrected in the same manner as in the second embodiment, so that distortion and noise of the detected signal can be reduced.

  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 second to fourth embodiments, the changing unit 72 is disposed in a portion where the detection signal is output from the FM detection unit 18. However, the present invention is not limited to this. For example, one changing unit 72 is disposed between the first ADC unit 14 and the FM detection unit 18, and another changing unit 72 is provided between the second ADC unit 16 and the FM detection unit 18. It may be arranged. One changing unit 72 changes the I signal, and another changing unit 72 changes the Q signal. According to this modification, the degree of freedom in design can be improved. In the third and fourth embodiments, the measurement unit 70 and the change unit 72 may be omitted. In that case, the apparatus configuration can be simplified.

  10 antenna, 12 quadrature detection unit, 14 first ADC unit, 16 second ADC unit, 18 FM detection unit, 20 AFC unit, 22 DAC unit, 24 local oscillator, 32 first reduction unit, 34 second reduction unit, 36 offset unit , 40 1st amplification section, 42 distribution section, 44 1st mixer, 46 1st LPF section, 48 2nd amplification section, 50 phase shift section, 52 2nd mixer, 54 2nd LPF section, 56 3rd amplification section, 60 1st amplification section 3 LPF section, 62 4th amplification section, 100 receiving apparatus.

Claims (5)

A quadrature detector for quadrature detection of the FM signal;
A reduction unit for reducing a direct current component included in the FM signal subjected to quadrature detection in the quadrature detector;
An FM detection unit that generates a detection signal by performing FM detection on the FM signal in which the DC component is reduced in the reduction unit;
An offset unit for adding an offset to the detection signal generated in the FM detection unit;
Generates a control signal for controlling the frequency of the local oscillation signal used in the quadrature detector based on the detection signal to which the offset is added in the offset unit, and controls the local oscillation signal to be output to the local oscillator An AFC unit that feeds back a signal;
Equipped with a,
The AFC unit causes the local oscillator to output a frequency-modulated local oscillation signal instead of feeding back a control signal to the local oscillator when the quadrature detector does not detect the FM signal. apparatus. A measuring unit for measuring the level of the FM signal in which the direct current component is reduced in the reducing unit;
When the level measured in the measurement unit becomes smaller than the threshold value, the FM signal in which the DC component is reduced in the reduction unit or the detection signal generated in the FM detection unit is changed to another value and output. The FM receiver according to claim 1, further comprising a unit. A difference calculation unit that generates a difference signal from a reference value based on the detection signal generated in the FM detection unit;
3. The FM receiver according to claim 1, further comprising an addition unit that adds the difference signal generated in the difference calculation unit and the detection signal generated in the FM detection unit.   4. The offset unit according to claim 1, wherein when the FM signal is not detected by the quadrature detector, the offset unit outputs a predetermined value instead of the detection signal to the AFC unit. 5. FM receiver. Quadrature detection of FM signals;
Reducing a DC component included in the FM signal subjected to quadrature detection;
Generating a detection signal by performing FM detection on an FM signal with a reduced DC component;
Adding an offset to the generated detection signal;
A control signal for controlling the frequency of the local oscillation signal used in the quadrature detection step is generated based on the detection signal to which the offset is added, and the control signal is fed back to the local oscillator to which the local oscillation signal is to be output. And steps to
If the FM signal is not detected in the quadrature detection step, instead of feeding back a control signal to the local oscillator, frequency-modulating the local oscillation signal output from the local oscillator;
FM receiving method, which comprises a.

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