JP4773318B2 - Local frequency signal detection circuit for direct conversion demodulator - Google Patents

Description

  The present invention relates to a local frequency signal detection circuit for a direct conversion demodulator, and more particularly to a circuit for detecting a local leak signal of an orthogonal mixer.

  With the start of terrestrial digital broadcasting services and the increase in the number of mobile phone users, digitization of wireless communication systems is progressing in order to effectively use limited radio frequency bands. In such digital radio, an orthogonal modulation system that modulates different information into two orthogonal carrier waves is mainly used. Furthermore, a direct modulation method and a frequency conversion method for directly converting the frequency into an RF signal using a local oscillator for converting the baseband signal or IF signal into an RF signal having a bandwidth of several tens of MHz are used.

  In the quadrature mixer used for digital radio frequency conversion, the local signal component of the local oscillator leaks to the RF signal which is the output signal due to circuit variations and field through. The local leak signal needs to be sufficiently suppressed to distort the RF signal, become a transmission spurious, and deteriorate the transmission quality. However, there is a problem that an expensive filter is necessary for suppressing the local leak signal, or that the direct modulation type RF filter cannot be suppressed. In order to solve such a problem, Patent Document 1 discloses a method of detecting a local leak signal and suppressing it by negative feedback control.

  FIG. 7 shows the configuration of a conventional quadrature modulation apparatus 100 using the π / 4 shift QPSK modulation method. The quadrature modulation device 100 includes a digital signal processing circuit 110, an analog quadrature mixer circuit 120, and an analog demodulation circuit 130. When the main signal is input to the digital signal processing circuit 110, the π / 4 shift QPSK signal point mapper 101 converts the main signal into a π / 4 shift QPSK signal including the in-phase signal I and the quadrature signal Q (hereinafter referred to as IQ signal). After being converted, it is given to rof (102, 103) and given to the orthogonal error corrector 104, respectively.

  The IQ signal output from the quadrature error corrector 104 is converted into an analog signal by the D / A converter (121, 122), and the low-frequency filter LPF (123, 124) is used as the D / A converter (121, 122). 122) is removed and input to the mixer (126, 128).

The first local oscillator 129 generates a quadrature carrier signal (IF signal) having a frequency f _IF and supplies it to the mixer 128 and the 90 ° phase shifter 125 (π / 2 phase shifter). Further, the 90 ° phase shifter 125 supplies the mixer 126 with the in-phase carrier signal delayed by 90 ° with respect to the quadrature carrier signal supplied to the mixer 128. Thus, the analog modulated signal is multiplied by the mixer 128 and the mixer 126, respectively, and synthesized by the synthesizer 127 to be orthogonally modulated.

  An unnecessary frequency component is removed from the modulated wave signal output from the synthesizer 127 through the BPF 131 which is a band pass filter, and is output as a main signal output. Further, a part of the main signal output is fed back to the digital local demodulator 106 via the analog demodulation circuit 130 in order to detect local leak generated in the analog quadrature mixer circuit 120.

In the analog demodulation circuit 130, fs which is the sampling frequency of the digital signal processing circuit 110 is frequency-converted to fs / 4. This is to simplify the configuration of the digital quadrature demodulator, and the signal of the second local oscillator 133 is divided by 4 by the 4 frequency divider 137 to become fs / 4. The PLL 134 has a mixer 138 and an LPF 132 in the feedback path, and outputs a signal of f _IF + fs / 4 synchronized with fs / 4 by using the output signal of the frequency divider 137 as a reference signal. The analog demodulation circuit 130 synthesizes a signal having a frequency f _IF + fs / 4 obtained as a local signal with a modulated wave signal fed back from the BPF 131 in the mixer 139, and gives it to the LPF 135. The LPF 135 extracts the lower frequency component fs / 4 and converts it to a digital signal by the A / D converter 136 and inputs it to the digital local demodulator 106.

  The in-phase demodulated signal i and the quadrature demodulated signal q obtained by the demodulation are respectively supplied to the quadrature error detector 105, and the pre-modulation in-phase signal I and the pre-modulation quadrature signal Q given by rof (102, 103), respectively. The quadrature modulation error generated by the analog quadrature mixer circuit 120 is detected by comparing the in-phase signals and the quadrature signals to detect quadrature modulation error (local leak) and supplying the detected error to the quadrature error corrector 104. Correct.

Thus, conventionally, two local oscillators having a frequency f _IF and a frequency fs have been used. If a local oscillator having a frequency f _IF is used to generate a signal for frequency conversion to a low frequency, the quadrature modulation error (local leak) is frequency converted to DC, and the output of the mixer 139 also wraps around the frequency f _IF . This is because the signal is frequency-converted and a DC offset is generated in the same manner, thereby degrading the detection accuracy of the quadrature modulation error.

JP 2001-339252 A

  As described above, the conventional local leak detection circuit has a problem that the local oscillator must be prepared separately in order to detect the local leak signal with high accuracy, and the circuit scale becomes large.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a local frequency signal detection circuit of a direct conversion demodulator that can detect a signal having the same frequency as that of a local for demodulation with a simple circuit and can be applied to local leak suppression of a quadrature modulator. And

  In order to achieve the above object, a local frequency signal detection circuit of a direct conversion demodulator according to the present invention includes a local signal for demodulation supplied by a local signal generator for demodulation, or an input input to the demodulator. Including a modulator that modulates one of the signals, and a sidelobe signal extractor that outputs a sidelobe signal of an input signal having the same frequency as the demodulated local signal output from the demodulator The side lobe signal is generated by supplying the signal to the modulator and modulating the input signal or the local demodulation signal, and the input signal or local demodulation signal having the same frequency as the local demodulation signal can be separated from the DC offset. It is characterized in that it is converted to a different frequency and extracted.

  In the local leak suppression circuit using the local frequency signal detection circuit of the direct conversion demodulator according to the present invention, the input signal is a signal modulated by the quadrature modulator, and the side lobe detected by the local frequency signal detection circuit A bias controller is provided for applying a DC offset corresponding to the signal to the I signal and Q signal of the quadrature modulator, respectively.

  Further, in the local frequency signal detection circuit of the direct conversion demodulator according to the present invention, the side lobe signal extractor is a band pass filter.

  Furthermore, in the local frequency signal detection circuit of the direct conversion demodulator according to the present invention, the modulator performs angle modulation on the local signal for demodulation.

  Furthermore, in the local frequency signal detection circuit of the direct conversion demodulator according to the present invention, the modulator performs angle modulation on the input signal.

  In the local leak suppression circuit according to the present invention, the orthogonal modulation local signal generator and the demodulation local signal generator are shared.

  By using the present invention, it becomes possible to detect a signal having the same frequency as that of the local for demodulation without preparing a separate local oscillator, and for example, it is possible to accurately detect and suppress a local leak signal of an orthogonal mixer. Become. In addition, the cost and size of the apparatus can be reduced.

  Hereinafter, embodiments of the present invention (hereinafter referred to as embodiments) will be described with reference to the drawings.

(First embodiment)
FIG. 1 shows a configuration of a quadrature modulation apparatus 1 according to the first embodiment of the present invention. The quadrature modulation apparatus 1 includes a quadrature mixer unit 10 that performs quadrature conversion of a main signal to perform frequency conversion, a local oscillator 52 that supplies a local signal for frequency conversion to the quadrature mixer unit 10, and a part of the frequency-converted main signal. Is input, and a local leak detector 30 (demodulator) that detects a local leak generated in the quadrature mixer unit 10 and a local leak that controls the local leak by giving a signal that cancels the detected local leak to the quadrature mixer unit 10 A control unit 40 and an amplification unit 20 that amplifies and outputs the frequency-converted main signal are included.

  In FIG. 1, when a main signal (IF signal in the present embodiment) is input to the quadrature mixer unit 10, it is distributed into two by a distributor 11. One of the divided signals is input to the π / 2 phase shifter 12a, and the π / 2 phase shifter outputs a signal whose phase is shifted by π / 2. As a result, an I signal and a Q signal are generated.

  Next, frequency conversion is performed on these signals. When the obtained I signal and Q signal are input to the multipliers 14a and 14b through the capacitors 13a and 13b, respectively, the multipliers 14a and 14b are connected to the main signal (I signal and Q signal) and the local oscillator 52, respectively. Is multiplied by the local signal Lo. At this time, the local signal Lo is distributed by the distributor 15, and one of the signals is input to the multiplier 14b via the π / 2 phase shifter 12b. Therefore, the multiplier 14b multiplies the Q signal and the signal whose phase of the local signal Lo is shifted by π / 2.

  Thereafter, the synthesizer 16 synthesizes and outputs the multiplied signals. The synthesized signal is output to the amplifying unit 20 from the orthogonal mixer unit 10 including the synthesizer 16, and the amplifying unit 20 amplifies the signal.

  FIG. 2A shows a main signal output spectrum of the orthogonal mixer unit 10 observed at point A in FIG. 1 when the main signal input is CW. In the figure, R represents a desired wave, L represents a local leak, and I represents an image leak. From the quadrature mixer unit 10, a frequency component (Lo-IF) signal corresponding to the desired wave R, a frequency component Lo signal corresponding to the local leak L, and a frequency component (Lo + IF) signal corresponding to the image leak I are output. Is output.

  These frequency component signals are generated due to the characteristics of the orthogonal mixer unit 10, and it is necessary to suppress unnecessary wave components other than the desired wave R in order to satisfy the transmission spurious standard and the signal quality standard. Therefore, in the first embodiment, in order to simplify the circuit, a feedback loop that performs bias control using a part of the main signal is provided in order to suppress the local leak L signal generated in the orthogonal mixer unit 10 as a target. It has been.

  FIG. 2B shows the main signal output spectrum at point B of the feedback loop when the control signal of the phase shifter 51 is a sine wave. When a part of the main signal amplified by the power amplifier 21 is extracted by the directional coupler 22, the extracted main signal is input to the local leak detection unit 30 via the phase shifter 51. In the local leak detection unit 30, since the main signal is mixed by the multiplier 31 to which the local signal Lo is supplied, the spectrum in the figure is folded around the DC on the left side, and the DC and the local leak L, desired from the left. Wave R and image leak I.

  In the local leak L shown in FIG. 2B, side lobes are generated by the angle modulation and can be taken out by the BPF (36). The extracted local leak L is suppressed by the local leak control unit 40.

  The local leak control unit 40 shown in FIG. 1 includes a digital signal processor 45 including one A / D converter 43 and three D / A converters 42, a coil 46, and a signal generator 49. Have. The digital signal processor 45 includes a local leak extractor 48 and a bias controller 47, and the local signal caused by the multiplier 14 of the orthogonal mixer unit 10 based on the local leak detected by the local leak detector 30. The A / D converter 43 detects the leak amount and executes digital signal processing for adjusting the DC offset voltage so as to suppress the local leak amount.

  Further, the digital signal processor 45 outputs an IQ signal component in which the DC offset voltage is adjusted by such digital signal processing from the D / A converters (42a, 42b), and also provides a control signal to the phase shifter 51. Output. The IQ signal components output from the D / A converters (42a, 42b) are output to the IQ signal of the orthogonal mixer unit 10 via the coils (46a, 46b). In the present embodiment, the control signal output to the phase shifter 51 changes the phase according to, for example, a sine wave function.

  Next, the phase shifter 51 will be described. FIG. 5 shows a circuit configuration of a 90 ° hybrid that is the phase shifter 51 used in the present embodiment. The phase shifter 51 has an input port 71a, an output port 71d, and reflection ports 71b and 71c. Coils 72 a and 72 b are connected to the reflection ports 71 b and 71 c, one of the variable capacitance diodes 73 a and 73 b is connected, and the other of the variable capacitance diode 73 is connected to the ground 74. The phase of the main signal is adjusted by adjusting the capacitance of the variable capacitance diode 73 with an external voltage.

  In this manner, by outputting the signal component in which the DC offset voltage is adjusted to the quadrature mixer unit 10, bias control is performed on the IQ signal of the quadrature mixer unit 10. As a result, local leak is suppressed.

(Second Embodiment)
FIG. 3 shows the configuration of a quadrature modulation apparatus 2 according to the second embodiment of the present invention. A different configuration from the first embodiment is that the local leak detector 30 is provided with an orthogonal mixer. In the first embodiment, in order to simplify the circuit configuration, a normal mixer is used for the local leak detection unit. However, in the second embodiment, at the same frequency as the desired wave R shown in FIG. In order to suppress the detected image leak I by the digital signal processor 45, it is detected by the IQ signal.

  The local leak detector 30 shown in FIG. 3 distributes the main signal extracted by the directional coupler 22 into two signals by a distributor 34.

  When these two signals are respectively input to the multipliers 31 a and 31 b, the multipliers 31 a and 31 b are multiplied by the local signal Lo generated by the local oscillator 52 and distributed by the distributor 33. At this time, one of the local signals Lo is input to the multiplier 31a via the π / 2 phase shifter 35c. Therefore, the multiplier 31a multiplies the signal whose phase of the local signal Lo is shifted by π / 2 and one of the signals distributed by the distributor 34. The signal quadrature-detected by the quadrature mixer is processed by the LPF (32a, 32b) of the analog circuit and then BPF-processed by the digital unit of the local leak control unit 40.

  Next, in addition to the processing of the first embodiment, the local leak control unit 40 executes control for suppressing the local leak L and the image leak I based on the IQ signal. By this processing, it is possible to remove unnecessary wave components with high accuracy.

(Third embodiment)
FIG. 4 shows a configuration of a quadrature modulation apparatus 3 according to the third embodiment of the present invention. What is characteristic in the third embodiment is that the local signal Lo for local leak detection is angle-modulated. The angle modulation includes phase modulation and frequency modulation, and either modulation method is suitable. In the third embodiment, the phase modulation of the local signal Lo is performed.

  In FIG. 4, the local signal Lo supplied from the local oscillator 52 is input to the phase shifter 51 and phase-modulated by the local leak control unit 40. The phase-modulated local signal is supplied to the multiplier 31 of the local leak detector 30. Since other processes are the same as those in the first embodiment, a description thereof will be omitted.

(Fourth embodiment)
FIG. 6 shows the configuration of a quadrature modulation device 4 according to the fourth embodiment of the present invention. What is characteristic of the fourth embodiment is that, in contrast to the second embodiment, an inverse modulation unit 82 and an image component suppression unit 81 are further provided in the digital signal processor 45, and further orthogonal. The attenuator 18 and the phase shifter 17 are provided in the mixer unit 10. The inverse modulation unit 82 performs processing reverse to that of the phase shifter 51 on the IQ signal, and the image component suppression unit 81 generates an unnecessary wave removal signal for removing the influence of the modulation, and supplies the signal to the bias controller 44. send. The bias controller 44 sends signals from the D / A converters (41 a to 41 c) to the attenuators (18 a and 18 b) and the phase shifter 17 of the quadrature mixer unit 10. This makes it possible to remove unnecessary wave components with higher accuracy.

  As described above, the local frequency signal detection circuit of the direct conversion demodulator according to the present embodiment has two characteristic configurations. The first configuration is to angle-modulate the local leak signal before frequency conversion, and the second configuration is to angle-modulate the local signal for local leak detection. With this configuration, although the local leak signal and the local signal for local leak detection have the same frequency on average, they have a frequency band, so that the problem of DC offset can be avoided.

  In addition, by using this embodiment, it is possible to accurately detect a local leak signal of the quadrature modulator without preparing a separate local oscillator. As a result, the cost and size of the quadrature modulation device can be reduced. Although the quadrature modulator in this embodiment is an IF signal input, it is needless to say that it can also be applied to a direct conversion method using a baseband signal as an input.

It is a block diagram which shows the structure of the orthogonal modulation apparatus which concerns on the 1st Embodiment of this invention. It is a characteristic view which shows the main signal output characteristic which concerns on embodiment of this invention. It is a block diagram which shows the structure of the orthogonal modulation apparatus which concerns on the 2nd Embodiment of this invention. It is a block diagram which shows the structure of the orthogonal modulation apparatus which concerns on the 3rd Embodiment of this invention. It is explanatory drawing explaining the circuit structure of the phase shifter used by this embodiment. It is a block diagram which shows the structure of the orthogonal modulation apparatus which concerns on the 4th Embodiment of this invention. It is a block diagram of the conventional orthogonal modulation apparatus.

Explanation of symbols

  1, 2, 3, 4 Quadrature modulator, 10 Quadrature mixer unit, 11, 15, 33, 34 Divider, 12, 35 π / 2 phase shifter, 13 Capacitor, 14, 31 Multiplier, 16 Synthesizer, 18 Attenuator, 20 amplifier, 21 power amplifier, 22 directional coupler, 30 local leak detector, 32 LPF, 36 BPF, 40 local leak controller, 41, 42 D / A converter, 43 A / D converter , 45 Digital signal processor, 46, 72 Coil, 44, 47 Bias controller, 48 Local leak extractor, 49 Signal generator, 17, 51 Phase shifter, 52 Local oscillator, 71a Input port, 71b, 71c Reflection port , 71d output port, 73 variable capacitance diode, 74 ground, 81 image component suppression unit, 82 inverse modulation unit, 100 quadrature modulation 101 π / 4 shift QPSK constellation unit, 104 quadrature error corrector, 105 quadrature error detector, 106 digital local demodulator, 110 digital signal processing circuit, 120 analog quadrature mixer circuit, 126, 128, 138, 139 Mixer, 127 combiner, 129 first local oscillator, 130 analog demodulator circuit, 132, 135 LPF, 133 second local oscillator, 134 PLL, 136 A / D converter, 1374 divider.

Claims (6)

In the local frequency signal detection circuit of the direct conversion demodulator,
A modulator that modulates either the local signal for demodulation supplied by the local signal generator for demodulation or the input signal input to the demodulator;
A sidelobe signal extractor for detecting a sidelobe signal of an input signal having the same frequency as the local signal for demodulation output from the demodulator;
Including
Supply a predetermined modulation signal to the modulator, modulate the input signal or demodulation local signal to generate a sidelobe signal, and DC offset the input signal or demodulation local signal with the same frequency as the demodulation local signal A local frequency signal detection circuit for a direct conversion demodulator, which converts the frequency into a separable frequency and extracts it. In the local leak suppression circuit using the local frequency signal detection circuit of the direct conversion demodulator according to claim 1,
The input signal is a signal modulated by a quadrature modulator,
A local leak suppression circuit comprising a bias controller that applies a DC offset corresponding to a side lobe signal detected by a local frequency signal detection circuit to an I signal and a Q signal of a quadrature modulator, respectively. In the local frequency signal detection circuit of the direct conversion demodulator according to claim 1,
A local frequency signal detection circuit of a direct conversion demodulator, wherein the sidelobe signal extractor is a band pass filter. In the local frequency signal detection circuit of the direct conversion demodulator according to claim 1,
A local frequency signal detection circuit of a direct conversion demodulator, wherein the modulator performs angle modulation on the local signal for demodulation. In the local frequency signal detection circuit of the direct conversion demodulator according to claim 1,
A modulator is a local frequency signal detection circuit of a direct conversion demodulator, characterized by angle-modulating an input signal. The local leak suppression circuit according to claim 2,
A local leak suppression circuit using a quadrature modulation local signal generator and a demodulation local signal generator in common.

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