JP6148728B2 - Transmitter - Google Patents

Description

  The present invention relates to a transmitter. In particular, in addition to nonlinear distortion compensation of an amplifier, transmission capable of accurately correcting an unbalance of I and Q signals, an orthogonality error, and a DC offset in an analog quadrature modulator. Related to the machine.

[Description of Prior Art] There is a direct conversion type transmitter including an analog quadrature modulator that quadrature modulates a baseband analog I signal and an analog Q signal.
Such a transmitter may include a feedback circuit for compensating for distortion generated in the power amplifier and modulation error in the analog quadrature modulator.
As the modulation error of the analog quadrature modulator, there are an imbalance between the I signal and the Q signal, a quadrature error, and a DC offset caused by individual differences and deviations of circuit elements.

[Configuration of Conventional Transmitter: FIG. 3] A configuration of a conventional transmitter will be described with reference to FIG. FIG. 3 is a partial configuration block diagram including a modem of a conventional transmitter.
As shown in FIG. 3, the conventional transmitter includes D / A converters (DAC: Digital-Analog Converter) 11, 12, low-pass filters (LPF) 13, 14, and mixers 15, 16. A local oscillator 17, a 90 degree phase shifter 18, a synthesizer 19, a power amplifier 20, mixers 21 and 22, low-pass filters 23 and 24, and an A / D converter (ADC; Analog-Digital Converter) 25 and 26.

[Parts] The D / A converter 11 performs digital / analog conversion of the digital quadrature modulated baseband in-phase component (I component or I signal), and the D / A converter 12 performs the digital quadrature modulated base. The quadrature component (Q component or Q signal) of the band is subjected to digital / analog conversion.
The low-pass filters 13 and 14 band-limit the input signals, respectively, and pass only low frequency components.
The local oscillator 17 oscillates a carrier frequency and outputs a frequency signal.
The phase shifter 18 rotates the phase of the frequency signal from the local oscillator 17 by 90 °.

The mixer 15 multiplies the I signal by the frequency signal from the local oscillator 17 and performs frequency conversion.
The mixer 16 performs frequency conversion by multiplying the Q signal by the frequency signal from the local oscillator 17 whose phase is shifted from the phase shifter 18 by 90 °.
The combiner 19 combines the output signals from the mixers 15 and 16 and outputs a quadrature-modulated transmission signal.
The mixers 15 and 16 and the synthesizer 19 correspond to analog quadrature modulators.
The power amplifier 20 amplifies the power of the orthogonally modulated transmission signal and outputs it to an antenna or the like. A part of the output of the power amplifier 20 is fed back as a feedback signal by a directional coupler or the like.

The mixer 21 multiplies the feedback signal fed back by the frequency signal from the local oscillator 17 to generate an I component of the feedback signal.
The mixer 22 multiplies the feedback signal by the frequency signal from the local oscillator 17 that is 90 ° out of phase from the phase shifter 18 to generate the Q component of the feedback signal.
The mixers 21 and 22 correspond to analog quadrature demodulators.

The low-pass filters 23 and 24 band-limit the I component and Q component of the feedback signal, respectively, and pass only the low frequency component.
The A / D converter 25 performs analog / digital conversion on the output of the low-pass filter 23 and outputs an I ′ signal.
The A / D converter 26 performs analog / digital conversion on the output of the low-pass filter 24 and outputs a Q ′ signal.

  Further, although not shown in the figure, the conventional transmitter is provided with a control unit, and the control unit transmits the I signal and Q signal on the transmission side and the I signal and Q signal (I ′) obtained on the feedback side. Signal, Q ′ signal), distortion compensation control in the power amplifier 20, and error correction of the analog quadrature modulator, correction of imbalance of I signal and Q signal, correction of orthogonality error, DC offset Processing such as correction is performed.

[Operation] The operation of the conventional transmitter will be briefly described with reference to FIG.
The baseband signals of I and Q are converted from digital signals to analog signals by D / A converters 11 and 12, respectively, and band-limited by low-pass filters 13 and 14.
The band-limited I signal is frequency-converted by the mixer 15 using the signal from the local oscillator 17.
Similarly, the band-limited Q signal is frequency-converted by the mixer 16 using the signal from the local oscillator 17 rotated by 90 ° from the phase shifter 18.
The frequency-converted I signal and Q signal are combined by a combiner 19, power amplified by a power amplifier 20, and output to an antenna or the like.

Also, part of the output of the power amplifier 20 is fed back, and the frequency is converted by the mixer 21 using the signal from the local oscillator 17 to become an I signal.
Similarly, the fed back signal is frequency-converted by the mixer 22 using the signal from the phase shifter 18 to become a Q signal.
Further, the frequency-converted I signal and Q signal are band-limited by LPFs 23 and 24, respectively, and converted from analog signals to digital signals by A / D converters 25 and 26 to obtain I 'and Q' signals. It is done.

  On the other hand, a control unit (not shown) compares the I signal and Q signal before D / A conversion on the transmission side with the fed back I ′ signal and Q ′ signal, and compensates for nonlinear distortion compensation characteristics in the power amplifier 20. , The imbalance between the I signal and the Q signal, the orthogonality error in the quadrature modulator, and the compensation characteristic of the DC offset are obtained, and each compensation characteristic is synthesized with the I signal and Q signal on the transmission side, and distortion compensation in the power amplifier 20 Each error in the quadrature modulator is compensated.

  In the conventional transmitter, the I and Q signals are analog quadrature modulated on the transmission side, the analog quadrature demodulation is performed on the feedback side, and the I 'signal and Q' signal are handled, so that the baseband band can be used in a wide band. Therefore, distortion can be compensated favorably.

[Spectrum after A / D Conversion in Conventional Transmitter: FIG. 4] Next, the spectrum after A / D conversion of the feedback system in the conventional transmitter will be described with reference to FIG. FIG. 4 is a schematic explanatory diagram showing a spectrum after A / D conversion in a conventional transmitter.

As shown in FIG. 4, the spectrum at the output stage of the A / D converters 25 and 26 in the conventional transmitter uses the same frequency from the local oscillator 17 as that on the transmission side in analog quadrature demodulation. It becomes “0”, and the image (TX Image) due to the imbalance or orthogonality error between the I signal and Q signal on the transmission side and the image (RX Image) due to the imbalance or orthogonality error between the I signal and Q signal on the feedback side. Further, the transmission side DC offset (TX DC Offset) and the feedback system DC offset (RX DC Offset) are further superimposed.
That is, in the conventional transmitter, the spectrum at the output stage of the A / D converters 25 and 26 includes an imbalance between the I signal and the Q signal generated on the feedback side, an orthogonality error, and a DC offset.

In order to correct the imbalance, orthogonality error, and DC offset of the I and Q signals in the analog quadrature modulator, only the transmission image and the DC offset on the transmission side must be detected accurately.
For this reason, if the feedback image and the feedback-side DC offset are superimposed, the control unit cannot accurately grasp the transmission image and the transmission-side DC offset, and the analog quadrature modulator imbalances the I and Q signals. Since the orthogonality error and the DC offset cannot be compensated accurately, the S / N (Signal / Noise) ratio is deteriorated.

[Related Technology] As a technology related to a transmitter for correcting an error of an analog quadrature modulator, as a technology related to a wireless communication system, JP 2002-77285 A “Radio” (Hitachi Kokusai Electric Co., Ltd., There exists patent document 1).
In Patent Document 1, the nonlinear inverse characteristic of the power amplifier is calculated only for the signal level representative point to increase the operation speed of the compensation circuit and reduce the circuit scale, and by providing a digital local demodulator, It is described that the quadrature modulation error due to the quadrature modulator is accurately detected and compensated.

JP 2002-77285 A

  As described above, in the conventional transmitter, in the spectrum after A / D conversion of the feedback signal, the transmission image and the DC offset on the transmission side, the image on the feedback side and the DC offset on the feedback side are superimposed, and the analog orthogonal There is a problem that an error generated in the modulator cannot be accurately detected, and it is difficult to correct an imbalance between I signal and Q signal, an orthogonality error, and a DC offset.

  The present invention has been made in view of the above circumstances, and in addition to nonlinear distortion compensation in a power amplifier, an error generated in an analog quadrature modulator is accurately detected, and an unbalanced, quadrature between an I signal and a Q signal is detected. It is an object of the present invention to provide a transmitter capable of accurately correcting a degree error and a DC offset.

  The present invention for solving the problems of the above-described conventional example is a direct conversion type transmitter, an analog quadrature modulator that quadrature modulates an analog I signal and an analog Q signal on the transmission side, and a quadrature modulated signal A power amplifier that amplifies the signal, an analog quadrature demodulator that converts a part of the output signal from the power amplifier using the input frequency signal, a first local oscillator that outputs a first frequency signal, A second local oscillator that outputs a second frequency signal having a frequency different from that of the first frequency signal; and a control unit that compensates for non-linear distortion in the power amplifier, wherein the analog quadrature modulator includes the first frequency signal. The analog quadrature demodulator is fed back from the power amplifier, and one of the branched signals is frequency converted using the second frequency signal. Using the signal obtained by digital quadrature demodulation of the digital signal based on one of the signals frequency-converted with the signal, the analog quadrature modulator compensates for imbalance, orthogonality error, and DC offset between the I and Q signals. It is characterized by that.

  In the transmitter, the analog quadrature demodulator uses either the first frequency signal or the second frequency signal as a frequency signal for frequency conversion of one of the branched signals fed back from the power amplifier. The analog quadrature demodulator frequency-converts one of the branched signals using the first frequency signal or the second frequency signal, and converts the other branched signal into the first switch. The frequency is converted using the first frequency signal, and the control unit periodically switches the first switch, and in the state where the first switch is switched to the first frequency signal side, the first frequency signal Non-linear distortion compensation in the power amplifier is performed using a signal obtained by A / D conversion of the frequency-converted analog I signal and analog Q signal, and the first switch is connected to the second switch. In the state of being switched to the wave number signal side, using the signal obtained by digital quadrature demodulation of the digital signal based on one signal frequency-converted with the second frequency signal, It is characterized by compensating for Q signal unbalance, orthogonality error, and DC offset.

  In the transmitter, the output stage of the analog quadrature demodulator corresponds to one of the analog I signal and the analog Q signal that is frequency-converted using the first frequency or the second frequency. Thus, a low-pass filter is provided, and the band of the signal frequency-converted at the first frequency and the signal frequency-converted at the second frequency are limited by the low-pass filter.

  In the transmitter, the output stage of the analog quadrature demodulator corresponds to one of the analog I signal and the analog Q signal that is frequency-converted using the first frequency or the second frequency. A plurality of filters having different characteristics connected in parallel, provided in the input stage of the parallel-connected filter, and switching one of the signals to any of the plurality of filters for input, and a parallel-connected filter And a third switch that switches to and outputs one of the plurality of filters, and the control unit switches the second switch and the third switch in conjunction with the first switch. It is characterized by.

  In the transmitter, one of the plurality of filters is a low-pass filter.

  According to the present invention, there is provided a direct conversion type transmitter, an analog quadrature modulator that quadrature modulates an analog I signal and an analog Q signal on the transmission side, a power amplifier that amplifies the quadrature modulated signal, and a power amplifier The analog quadrature demodulator that converts a part of the output signal from the first frequency signal using the input frequency signal, the first local oscillator that outputs the first frequency signal, and the first frequency signal have a frequency A second local oscillator that outputs a different second frequency signal; and a control unit that compensates for non-linear distortion in the power amplifier. The analog quadrature modulator performs quadrature modulation using the first frequency signal, and analog quadrature The demodulator is fed back from the power amplifier, and one of the branched signals is frequency-converted by using the second frequency signal, and the control unit is frequency-converted by the second frequency signal. Since the signal obtained by digital quadrature demodulation of the digital signal based on the signal is used as a transmitter that compensates for imbalance, orthogonality error, and DC offset between the I signal and Q signal in the analog quadrature modulator, the feedback system The control unit can accurately detect and compensate for the I, Q imbalance, orthogonality error, and DC offset in the analog quadrature modulator based on the image generated in step 1 and the signal that does not include the DC offset on the feedback side. There is an effect that compensation with high accuracy can be performed.

  Further, according to the present invention, in the analog quadrature demodulator, the frequency signal for frequency-converting one of the branched signals fed back from the power amplifier is switched to either the first frequency signal or the second frequency signal. An analog quadrature demodulator includes a first switch, frequency-converts one of the branched signals using the first frequency signal or the second frequency signal, and converts the other branched signal to the first frequency. Frequency conversion is performed using the signal, and the control unit periodically switches the first switch, and the frequency is converted with the first frequency signal in a state where the first switch is switched to the first frequency signal side. Using the signal obtained by A / D conversion of the analog I signal and the analog Q signal, the nonlinear distortion in the power amplifier is compensated, and the first switch is moved to the second frequency signal side. In the switched state, using the signal obtained by digital quadrature demodulation of the digital signal based on one of the signals frequency-converted with the second frequency signal, the I and Q signals in the analog quadrature modulator are Since the transmitter compensates for unbalance, orthogonality error, and DC offset, the nonlinear distortion of the power amplifier is compensated based on a wideband signal, and the I and Q signals of the analog quadrature modulator are unbalanced and orthogonality. The error and DC offset can be compensated based on a signal that does not include an error on the feedback side, and both can be compensated with high accuracy.

  According to the present invention, the output stage of the analog quadrature demodulator has a low pass corresponding to one of the analog I signal and the analog Q signal that is frequency-converted using the first frequency or the second frequency. Since the transmitter is provided with a filter to band limit the signal frequency-converted at the first frequency and the signal frequency-converted at the second frequency with the low-pass filter, the configuration and processing can be simplified. There is.

  According to the present invention, the output stage of the analog quadrature demodulator has a characteristic corresponding to one of the analog I signal and the analog Q signal that has been frequency-converted using the first frequency or the second frequency. Are connected in parallel, provided in the input stage of the parallel-connected filter, and a second switch for switching and inputting one of the signals to the plurality of filters, and an output stage of the parallel-connected filter And a third switch that switches to and outputs one of the plurality of filters, and the control unit switches the second switch and the third switch in conjunction with the first switch. Therefore, the frequency-converted signal using the first frequency or the second frequency can be band-limited with desired characteristics, respectively, and nonlinear distortion of the power amplifier and analog orthogonality can be obtained. Imbalance of the I signal and Q signal in the modulator, orthogonality error, there is an effect that it is possible to compensate the DC offset in each high accuracy.

It is a partial structure block diagram containing the modem part of the transmitter which concerns on embodiment of this invention. It is a schematic explanatory drawing which shows the spectrum after A / D conversion at the time of frequency-converting using a 2nd frequency with an analog orthogonal demodulator. It is a block diagram of a partial configuration including a modem of a conventional transmitter. It is a schematic explanatory drawing which shows the spectrum after A / D conversion in the conventional transmitter.

  Embodiments of the present invention will be described with reference to the drawings. [Outline of Embodiment] A transmitter according to an embodiment of the present invention includes an analog quadrature modulator that quadrature-modulates an input signal, an amplifier that amplifies the quadrature-modulated signal, and a part of the output from the amplifier. An analog quadrature demodulator that performs quadrature demodulation, a first local oscillator that outputs a first frequency signal to the analog quadrature modulator, and a second local oscillator that outputs a second frequency signal different from the first frequency signal And an A / D converter that performs A / D conversion on the output from the analog quadrature demodulator, and a control unit that corrects a modulation error in the analog quadrature modulator, and the analog quadrature demodulator is fed back from the amplifier. One of the signals branched into two is frequency-converted using the second frequency signal, and the control unit converts either the I signal or the Q signal frequency-converted by the second frequency signal to A / D Strange The I ″ signal and Q ″ signal which are digitally quadrature demodulated later are used to correct the imbalance, orthogonality error, and DC offset of the I signal and Q signal in the analog quadrature modulator, and the second frequency signal is used. Since the I ″ signal and Q ″ signal based on the frequency-converted signal do not include the image and DC offset on the feedback side, the control unit can accurately grasp the transmission image and the transmission side DC offset, The analog quadrature modulator can correct the imbalance, quadrature error, and DC offset of the I and Q signals with high accuracy.

  In the transmitter according to the embodiment of the present invention, the analog quadrature demodulator further converts the frequency signal obtained by frequency-converting one of the feedback signals branched into two into the first frequency signal or the second frequency signal. When the control unit periodically switches the first switch and switches to the first frequency signal side, the I signal and Q obtained by the analog quadrature demodulator are provided. When the non-linear distortion characteristic in the amplifier is detected using the I ′ signal and Q ′ signal obtained by A / D conversion of the signal to calculate the distortion compensation coefficient, and the second local oscillator side is switched to, the second frequency The I or Q signal frequency-converted with the signal is A / D converted and the digital quadrature demodulated I "and Q" signals are used to imbalance the I signal and Q signal in the analog quadrature modulator, and the orthogonality error. A coefficient for correcting the DC offset is calculated, and when switched to the first frequency signal side, broadband I ′ and Q ′ signals are obtained, and distortion characteristics in the broadband can be compensated. When switching to the frequency signal side of 2, the I and Q signals in the feedback system are unbalanced, orthogonality error, I ″ signal and Q ″ signal not including DC offset are obtained, and the modulation error in the analog quadrature modulator is obtained. Can be compensated with high accuracy.

[Configuration of Embodiment: FIG. 1] A configuration of a transmitter according to an embodiment of the present invention will be described with reference to FIG. FIG. 1 is a partial configuration block diagram including a modem of a transmitter according to an embodiment of the present invention.
As shown in FIG. 1, the transmitter according to the embodiment of the present invention (the present transmitter) includes D / A converters 11 and 12 and low-pass as parts similar to the conventional transmitter shown in FIG. Filters 13 and 14, mixers 15 and 16, a local oscillator (first local oscillator) 17, a phase shifter 18, a combiner 19, a power amplifier 20, mixers 21 and 22, a low-pass filter 23, 24, A / D converters 25 and 26, and as a characteristic part of the transmitter, a second local oscillator 31, a first switch 32, a second switch 33, and a bandpass filter 34, a third switch 35, and a control unit 36.

The mixers 15 and 16 and the synthesizer 19 constitute an analog quadrature modulator, and the mixers 21 and 22 constitute an analog quadrature demodulator.
Parts similar to those of the conventional transmitter are denoted by the same reference numerals, and description thereof is omitted.

The characteristic part of this transmitter is demonstrated.
The second local oscillator 31 oscillates a second frequency different from the first frequency signal output by the first local oscillator 17 and outputs a second frequency signal.
The second frequency is set to a frequency different from the first frequency, and is set to a frequency at which the analog signal from the mixer 21 can be IF sampled by the A / D converter 25. An example of setting the frequency will be described later.

The first switch 32 switches the frequency signal input to the mixer 21, and either the first frequency signal from the first local oscillator 17 or the second frequency signal from the second local oscillator 31 is selected. Is input to the mixer 21. The first switch 32 switches between the first frequency and the second frequency in accordance with a control signal from the control unit 36.
By the operation of the first switch 32, the mixer 21 frequency-converts one of the feedback signals branched into two using the first frequency signal or the second frequency signal, and uses the first frequency signal. If the second frequency signal is used, an analog signal having a different frequency is generated.

The band pass filter 34 limits the band of the signal from the mixer 21.
The second switch 33 switches the signal from the mixer 21 to the band pass filter 34 or the low pass filter 23.
The third switch 35 inputs the output of the band pass filter 34 or the low pass filter 23 to the A / D converter 25.

  The controller 36 calculates a distortion compensation coefficient for compensating for the distortion characteristics of the power amplifier 20 and performs distortion compensation on the transmission side I signal and Q signal, as in the conventional case. Is used to calculate the coefficients for correcting the imbalance, orthogonality error, and DC offset of the I signal and Q signal, which are orthogonal modulation errors in the analog orthogonal modulation unit, using the feedback signal different from that of the transmission side I signal, Q signal Analog quadrature error is compensated for.

Specifically, when distortion compensation is performed, the I ′ signal and Q ′ signal obtained by direct conversion similar to the conventional one are used, and when the analog quadrature modulation error is corrected, this transmitter is characteristic. I "and Q" signals are used.
The I ″ signal and the Q ″ signal are an I signal and a Q signal obtained by digitally demodulating a digital signal obtained by A / D converting the analog signal frequency-converted with the second frequency signal in the analog quadrature demodulator. is there. This will be described later.

Further, as a feature of this transmitter, the control unit 36 does not simultaneously perform the distortion compensation operation and the analog quadrature modulation error compensation, but separately performs them at an arbitrary timing, and accordingly, the first switch 32, control to switch the second switch 33 and the third switch 35 is performed. The switching of these switches is performed in conjunction with the switching between the distortion compensation operation and the analog quadrature modulation error compensation operation.
The operation of the control unit 36 will be described later.

In the configuration of FIG. 1, the second frequency signal from the second local oscillator 31 is input to the mixer 21 to generate analog signals having different frequencies on the I signal side. The mixer 21 receives the first frequency signal from the first local oscillator 17 to generate an I signal as in the conventional case, and inputs either the first frequency or the second frequency signal to the mixer 22. Then, an analog signal whose frequency is changed on the Q signal side may be generated.
In that case, the second switch 33, the band-pass filter 34, and the third switch 35 are provided on the mixer 22 side.
In other words, the I signal side and the Q signal side are switched, the frequency conversion by the second frequency signal is performed on the Q signal side, and the analog signal is digitally quadrature demodulated to generate the I "signal and the Q" signal. It is good also as composition to do.

[Operation of the Transmitter: FIG. 1] Next, the operation of the transmitter will be described with reference to FIG. Here, a case where the second local oscillator 31, the band pass filter 34, and the first to third switches 32, 33, and 35 are provided on the I signal side of the feedback system will be described as an example.

The operation on the transmission side is the same as the conventional one. In FIG. 1, the baseband transmission I signal and Q signal are converted from digital signals to analog signals by the D / A converters 11 and 12, respectively. The bandwidth is limited by
The band-limited I signal is frequency-converted by the mixer 15 using the signal from the local oscillator 17, and the Q signal is frequency-converted by the mixer 16 using the signal rotated by 90 ° from the phase shifter 18. The I signal and the Q signal are combined by a combiner 19, and the analog quadrature modulated signal is amplified by a power amplifier 20 and output to an antenna or the like.

A part of the output of the power amplifier 20 is fed back, branched into two, and input to the mixers 21 and 22.
The signal input to the mixer 21 is frequency-converted with the first frequency signal or the second frequency signal selected by the first switch 32.
As a result, when the frequency conversion is performed with the first frequency signal, an I signal similar to the conventional one is obtained, and when the frequency conversion is performed with the second frequency signal, the difference between the first and second frequencies is obtained. Thus, an analog signal including the frequency component is obtained.
The first switch 32 is controlled by the control unit 36, and the switching control will be described later.

When the first frequency is used in the mixer 21, the second switch 33 is switched to the low-pass filter 23 side, and the I signal is input to the low-pass filter 23 to be band-limited as in the prior art, and the third switch The signal is input to the A / D converter 25 via the switch 35.
When the mixer 21 uses the second frequency, the second switch 33 is switched to the band-pass filter 34 side, and the frequency-converted analog signal is cut by the band-pass filter 34 except for the desired band. The band is limited so as to be input to the A / D converter 25 via the third switch 35.

Then, the signal from the low-pass filter 23 is A / D converted by the A / D converter 25 in the same manner as in the prior art, becomes an I ′ signal, and is input to the control unit 36.
The signal from the bandpass filter 34 is A / D converted by the A / D converter 25 and input to the control unit 36, and is digitally quadrature demodulated by the control unit 36 to become I ″ signal and Q ″ signal. .
That is, in the configuration of FIG. 1, the I ″ signal and the Q ″ signal are generated based only on the component on the I signal side in the analog quadrature modulator.

  Here, when the mixer 21 performs frequency conversion using the second frequency, the image generated in the mixer constituting the analog quadrature demodulator on the feedback side and the DC offset generated in the A / D converter are the image on the transmission side, Since the frequency is different from the DC offset, the digital signal from the A / D converter 25 does not include the feedback side image and the feedback side DC offset.

  On the other hand, a part of the output signal from the power amplifier 20 is fed back as a feedback signal, and the signal branched and input to the mixer 22 is the first frequency signal rotated from the phase shifter 18 by 90 ° phase rotation. The signal is frequency converted at the same time to obtain a Q signal similar to the conventional one, the band is limited by the low-pass filter 24, passes through the A / D converter 26, becomes a Q ′ signal similar to the conventional one, and is input to the control unit 36.

In other words, when the first frequency signal is selected by the first switch 32, the operation is the same as the conventional one, and the analog quadrature demodulator generates the I signal and the Q signal and then performs A / D conversion on each. A band equal to the sampling frequency of the A / D converter becomes effective, and is effective when a wideband signal such as distortion compensation is handled.
When the first frequency is selected, the control unit 36 compares the transmission side I signal and Q signal with the fed back I ′ signal and Q ′ signal to compensate for nonlinear distortion in the power amplifier 20. Do.

On the other hand, when the second frequency signal is selected by the first switch 32, the I / Q converter imbalance, orthogonality error, and DC offset are output as the output of the A / D converter 25. A digital signal not included is obtained, and the control unit 36 compares the I ″ signal and Q ″ signal obtained by digital quadrature demodulation with the I signal and Q signal before digital quadrature modulation on the transmission side, and compares them with an analog quadrature modulator. Correction of the imbalance, orthogonality error, and DC offset of the I and Q signals at.
In this case, the output signal (Q ′ signal) from the A / D converter 26 is not used by the control unit 36.
Note that when the Q signal side is configured to perform frequency conversion using the second frequency signal, the output signal (I ′ signal) from the A / D converter 25 is not used by the control unit 36.

[Operation in Control Unit 36] The operation of the control unit 36 will be specifically described.
The control unit 36 switches the first switch 32 to the first local oscillator 17 side at an arbitrary timing, and switches the second and third switches 33 and 35 to the low-pass filter 23 side at the same time. In this state, an I signal frequency-converted at the first frequency is obtained in the mixer 21 as in the prior art.

Then, the control unit 36 transmits the I ′ signal (output of the A / D converter 25) and the Q ′ signal (output of the A / D converter 26) obtained in the state where the first frequency is selected on the transmission side. A distortion compensation coefficient for compensating for distortion in the power amplifier 20 is calculated in comparison with the I signal and Q signal, and the transmission side I signal and Q signal are multiplied.
Since the I ′ signal and the Q ′ signal when using the first frequency are wideband signals, distortion compensation can be performed with high accuracy in the wideband.

  In addition, the control unit 36 switches the first switch 32 to the second local oscillator 31 side after a specific time has elapsed since switching the first switch 32 to the first local oscillator 17 side, The third switches 33 and 35 are switched to the band pass filter 34 side.

Then, in a state where the second frequency is selected, the control unit 36 generates an I ″ signal and a Q ″ signal (generated in the control unit 36) obtained by digital quadrature demodulation of the output from the A / D converter 25. Compared with the I and Q signals on the transmitting side, detects the imbalance, orthogonality error, and DC offset of the I and Q signals of the analog quadrature modulator, calculates the correction coefficient, and corrects it to the transmitted I and Q signals Multiply by a coefficient. The output from the A / D converter 26 is not used.
In addition, when the I signal side and the Q signal side are interchanged to perform frequency conversion using the second frequency signal on the Q signal side, the output from the A / D converter 26 is digitally quadrature demodulated. The I ″ signal and Q ″ signal (generated in the control unit 36) are compared with the I signal and Q signal on the transmission side to compare the I and Q signals of the analog quadrature modulator, the orthogonality error, and the DC offset. Is detected to calculate a correction coefficient, and the transmission I signal and Q signal are multiplied by the correction coefficient. The output from the other A / D converter is not used.

  As described above, the I ″ signal and the Q ″ signal generated based on the signal frequency-converted using the second frequency do not include the feedback side image and the feedback side DC offset. It is possible to accurately detect the imbalance, orthogonality error, and DC offset of the I signal and Q signal of the quadrature modulator, thereby performing highly accurate compensation.

  Note that the timing for switching the first to third switches 32, 33, and 35 is arbitrary, and the timings for performing the compensation processing for the imbalance between the I signal and the Q signal, the orthogonality error, and the DC offset are given. The switches 32, 33, and 35 are switched to the second frequency side, and are switched to the first frequency side during other periods.

However, during the period of switching to the first frequency side, the distortion compensation process does not always have to be performed, and only a necessary period may be performed.
Further, a period during which no distortion compensation processing is performed (a period in which the first switch 32 is switched to the second local oscillator 31 side and a period in which the first switch 32 is switched to the first local oscillator 17 side. In the period during which the Q ′ signal is not generated, the operation of the system that generates the Q ′ signal is stopped, and the control unit 36 performs the distortion compensation process only when the Q ′ signal is input from the A / D converter 26. Also good.
Also, when the I signal side and the Q signal side are switched and the frequency conversion is performed on the Q signal side by the frequency signal from the second local oscillator, the distortion compensation processing is performed in the same manner as described above. It is also possible to stop the operation of the system that generates the I ′ signal during the period, and the control unit 36 may perform the distortion compensation process only when the I ′ signal is input from the A / D converter 25.
By stopping unnecessary operations, power consumption can be reduced.

[Frequency setting example] Next, frequency setting examples of the first and second local oscillators in the transmitter will be described.
For example, when the frequency from the first local oscillator 17 is 2.14 GHz and the frequency from the second local oscillator 31 is 2.00 GHz, the output from the mixer 21 is 140 MHz, which is the bandpass filter 34. For example, sampling is performed at a sampling frequency of 112 MHz.
If the sampling frequency is Fs, the center frequency after A / D conversion is Fs / 4, so that a signal centered on 28 MHz can be obtained.

When the frequency of the second local oscillator 31 is brought close to the frequency of the first local oscillator 17, the output from the mixer 21 becomes lower.
For example, if the output from the second local oscillator 31 is 2.012 GHz, the output from the mixer 21 is 28 MHz. In this case, even when the frequency is converted at the second frequency, the band can be sufficiently limited by the low-pass filter 23. It becomes.
Therefore, the band-pass filter 34 and the second and third switches 33 and 35 are unnecessary, and the circuit configuration and processing of the control unit 36 can be simplified.

[Spectrum after A / D Conversion in the Transmitter: FIG. 2] Next, the spectrum after A / D conversion in the transmitter will be described with reference to FIG. FIG. 2 is a schematic explanatory diagram illustrating a spectrum after A / D conversion when frequency conversion is performed using the second frequency by the analog quadrature demodulator. In the case of the configuration shown in FIG. 1, the spectrum after A / D conversion when the second frequency is used is obtained.
As shown in FIG. 2, when the second frequency is used in the mixer 21, assuming that the sampling frequency is F s, the transmission image after A / D conversion is centered on Fs / 4, and the DC on the transmission side The offset appears as Fs / 4.

The spectrum using the second frequency includes only the transmission image and the DC offset on the transmission side, and does not include the image on the feedback side and the DC offset on the feedback side.
Therefore, the control unit 36 can accurately detect the imbalance, the orthogonality error, and the DC offset of the I and Q signals of the analog quadrature modulator, and can compensate with high accuracy.

  In this transmitter, when the second frequency is used, the I ″ signal and the Q ″ signal are generated by digital quadrature modulation. Therefore, the effective band is the sampling frequency of the A / D converters 25 and 26. Although it is halved, it is sufficient to compensate for the imbalance between the I signal and the Q signal, the orthogonality error, and the DC offset as long as the modulation band of the transmission signal can be secured.

[Effect of Embodiment] According to the transmitter according to the embodiment of the present invention, the first frequency signal is converted into the analog quadrature by the direct conversion type transmitter including the analog quadrature modulator and the analog quadrature demodulator. The first local oscillator 17 to be output to the demodulator, the second local oscillator 31 to output a second frequency signal different from the first frequency signal, and the output from the analog quadrature demodulator are A / D converted. A / D conversion units 25 and 26 and a control unit 36 for correcting a modulation error in the analog quadrature modulator. In the analog quadrature demodulator, the mixer 21 on the I signal side feeds back the signal fed back from the power amplifier 20. The analog signal frequency-converted using the second frequency signal and frequency-converted using the second frequency signal is band-limited by the band-pass filter 34, and the A / D converter 25 A / D conversion is performed and input to the control unit 36. The control unit 36 performs digital quadrature demodulation to generate an I "signal and a Q" signal, and outputs an I "signal, a Q" signal, a transmission I signal, and a Q signal. Compared to each other, the transmitter corrects the imbalance, orthogonality error, and DC offset of the I and Q signals in the analog quadrature modulator, so that the I ″ signal does not include the image in the feedback system and the DC offset of the feedback system. , Q ″ signals have an effect of accurately correcting the imbalance, orthogonality error, and DC offset of the I and Q signals in the analog quadrature modulator.
Further, when the I signal side and the Q signal side are interchanged so that the frequency conversion is performed by the frequency signal from the second local oscillator on the Q signal side, in the analog quadrature demodulator, the Q signal side The mixer 22 frequency-converts the signal fed back from the power amplifier 20 using the second frequency signal, band-limits the analog signal frequency-converted with the second frequency signal with a band-pass filter, and performs A / D A / D conversion is performed by the converter 26 and input to the control unit 36. The control unit 36 performs digital quadrature demodulation to generate an I "signal, a Q" signal, an I "signal, a Q" signal and a transmission I signal, Compared with the Q signal, the transmitter corrects the imbalance, orthogonality error, and DC offset between the I signal and Q signal in the analog quadrature modulator. By using the I ″ signal and Q ″ signal that do not include the DC offset of the back-back system, it is possible to accurately correct the imbalance, orthogonality error, and DC offset of the I and Q signals in the analog quadrature modulator. is there.

  Further, according to this transmitter, the first switch 32 that switches the frequency signal input to the mixer 21 of the analog quadrature demodulator to the first frequency signal or the second frequency signal is provided, and the control unit 36 includes: When the first switch 32 is switched periodically and switched to the first frequency signal side, the A / D converters 25 and 26 convert the I and Q signals obtained by the analog quadrature demodulator into A / D conversion. When the non-linear distortion characteristic in the amplifier 20 is detected by using the I ′ signal and the Q ′ signal, the distortion compensation coefficient is calculated, and when switched to the second frequency signal side, the output of the mixer 21 is changed to the band-pass filter 34. In the analog quadrature modulator, the I and Q signals are imbalanced by using the I ″ signal and the Q ″ signal which are A / D converted by the A / D converter 25 and digitally quadrature demodulated. DC off A coefficient for correcting the signal is calculated, and at the time of distortion compensation, distortion characteristics in the wide band can be compensated by the wide band I ′ signal and Q ′ signal, and at the time of compensation of the modulation error in the analog quadrature modulator, a feedback system There is an effect that high-precision compensation can be performed by the I ″ signal and Q ″ signal not including the imbalance between the I signal and the Q signal, the orthogonality error, and the DC offset.

  INDUSTRIAL APPLICABILITY The present invention is suitable for a transmitter capable of accurately correcting unbalance, orthogonality error, and DC offset of I and Q signals in an analog quadrature modulator in addition to amplifier nonlinear distortion compensation.

  11, 12 ... D / A converter, 13, 14, 23, 24 ... low pass filter, 15, 16, 21, 22 ... mixer, 17 ... first local oscillator, 18 .. .. phase shifter, 19 ... synthesizer, 20 ... power amplifier, 25,26 ... A / D converter, 31 ... second local oscillator, 32 ... first switch, 33 ... Second switch, 34 ... Band pass filter, 35 ... Third switch, 36 ... Control unit

Claims (4)

A direct conversion transmitter,
An analog quadrature modulator that quadrature modulates an analog I signal and an analog Q signal on the transmission side;
A power amplifier for amplifying the quadrature modulated signal;
An analog quadrature demodulator that converts a portion of the output signal from the power amplifier using an input frequency signal;
A first local oscillator for outputting a first frequency signal;
A second local oscillator for outputting a second frequency signal having a frequency different from that of the first frequency signal;
In the analog quadrature demodulator, a frequency signal which is fed back as a part of the output signal from the power amplifier and frequency-converted one of the branched signals is either the first frequency signal or the second frequency signal. A first switch to switch to,
A controller that compensates for nonlinear distortion in the power amplifier, compensates for imbalance, orthogonality error, and DC offset of the I and Q signals in the analog quadrature modulator, and controls the first switch ;
The analog quadrature modulator performs quadrature modulation using the first frequency signal;
The analog quadrature demodulator frequency-converts one of the branched signals using the first frequency signal or the second frequency signal, and converts the other branched signal to the first frequency signal. Frequency conversion using
The control unit periodically switches the first switch, and the analog I frequency-converted with the first frequency signal in a state where the first switch is switched to the first frequency signal side. Using a signal obtained by A / D converting a signal and an analog Q signal, compensation for nonlinear distortion in a power amplifier is performed, and the first switch is switched to the second frequency signal side. using a signal obtained by the digital quadrature demodulating digitally signal based on the frequency-converted said one signal at said second frequency signal, the unbalance of the I and Q signals in the analog quadrature modulator, orthogonality A transmitter characterized by compensating for an error and a DC offset. A low-pass filter is provided at the output stage of the analog quadrature demodulator in correspondence with one of the analog I signal and the analog Q signal frequency-converted using the first frequency or the second frequency, The transmitter according to claim 1 , wherein the low-pass filter band-limits the signal frequency-converted by the frequency and the signal frequency-converted by the second frequency. In the output stage of the analog quadrature demodulator, a plurality of filters having different characteristics are arranged in parallel corresponding to one of the analog I signal and the analog Q signal frequency-converted using the first frequency or the second frequency. Connected,
A second switch that is provided at an input stage of the parallel-connected filter, and that inputs the one signal by switching to one of the plurality of filters;
A third switch that is provided at an output stage of the parallel-connected filter and that switches to and outputs one of the plurality of filters;
The transmitter according to claim 1, wherein the control unit switches the second switch and the third switch in conjunction with the first switch. 4. The transmitter according to claim 3 , wherein one of the plurality of filters is a low-pass filter.

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