JP5337120B2 - Class D amplifier and wireless communication apparatus - Google Patents

Description

  The present invention relates to a class D amplifier for amplifying a transmission signal level in a digital wireless communication system such as a digital cellular phone and a wireless communication apparatus using the same.

  In a base station of a digital wireless communication system, it is necessary to amplify a transmission signal to a large power of several tens of watts to several hundreds of watts. Greatly contributes to power generation. Also in the mobile station, it is important to reduce the power consumption of the device including the amplifier.

  A class D amplifier is well known as one of amplifier configurations with high power utilization efficiency. A class D amplifier is realized by ON / OFF switching of a signal using a saturation region of an amplifying element such as a transistor.

  In general, a class D amplifier is used for amplifying a signal in a low frequency band in an audible range up to about 20 kHz. As an amplified signal, for example, a high SNR (Signal to Noise Ratio) modulated by 1-bit ΔΣ is used. These signals are used. The sampling frequency of this signal is about 128 times or 256 times the signal frequency band (the higher the sampling frequency, the higher the SNR).

  In recent years, Doherty amplifiers and ET (Envelope Tracking) amplifiers have been actively studied as amplifiers for high frequency band signals (RF signals) used wirelessly. From the viewpoint of power efficiency, class D The use of amplifiers is beginning to be explored.

  In order to amplify an RF signal with a class D amplifier, it is necessary to convert the RF signal into a 1-bit pulse signal. In order to obtain a high SNR transmission signal when ΔΣ modulation is normally applied (hereinafter referred to as “low-pass ΔΣ modulation”) when generating this pulse signal, for example, the carrier frequency of the RF signal is fc = 2 GHz. In this case, a very high-speed sampling signal such as sampling frequency fs = 2 GHz × 128 = 256 GSa (Samples) / s is required, which is not realistic.

  Therefore, the band-pass ΔΣ modulator was conceived. For example, if the carrier frequency fc = 2 GHz, the sampling frequency fs is 8 GSa / s, which is 4 times 2 GHz, and the SNR is comparable to the low-pass ΔΣ modulator (if the modulation signal band is about several tens of MHz). A signal is obtained.

  Such a bandpass ΔΣ modulator is actively used in an RF signal receiving circuit. This is because, in reception, after the received signal is applied to the band-pass ΔΣ modulator, only a narrow-band signal having a good SNR around the carrier frequency fc is extracted by the band-pass filter, so that good demodulation is possible.

  On the other hand, when a bandpass ΔΣ modulator is used in an RF signal transmission circuit (amplifier circuit), theoretically, an analog circuit must have a flat band characteristic in a frequency range of 0 Hz to ∞ Hz. Otherwise, the transmission signal is distorted and, for example, EVM (Error Vector Magnitude), which is a quality indicator of the transmission signal, is degraded.

  Here, since the analog circuit of the RF signal is a distributed multiplier circuit, it is necessary to achieve frequency matching of the circuit in a necessary frequency band, and it is difficult to flatten the band characteristics in a wide frequency range. Further, it is necessary to cut off the DC signal in order to prevent destruction of the amplifier, and the analog circuit of the RF signal always has band characteristics. For this reason, it is technically difficult to amplify the RF signal with a class D amplifier.

  As a class D amplifier using a bandpass ΔΣ modulator, for example, a technique disclosed in Patent Document 1 is known. FIG. 1 shows a wireless communication apparatus including a class D amplification unit disclosed in Patent Document 1.

  In FIG. 1, a baseband signal generation unit 11 generates a narrowband (˜several tens of MHz) baseband modulation signal, and the baseband signal output from the baseband signal generation unit 11 is transferred to a carrier in the upconversion unit 12. Up-converted to frequency fc. The signal output from the up-conversion unit 12 is subjected to, for example, bandpass ΔΣ modulation at the sampling frequency fs = 4 × fc in the bandpass ΔΣ modulation unit 13, and the signal output from the bandpass ΔΣ modulation unit 13 is The class D amplification unit 14 performs class D amplification. The signal output from the class D amplification unit 14 is transmitted as a radio signal from the antenna unit 16 via the bandpass filter unit 15 centered on the carrier frequency fc.

  The time axis waveform and frequency characteristics of the output signal of each block in FIG. 1 are illustrated in FIGS. 2 shows a constellation of a QPSK quadrature modulated wave that is an output of the baseband signal generator 11, FIG. 3 shows a frequency characteristic of an output signal of the baseband signal generator 11, and FIG. 4 shows an upconverter. 12 shows frequency characteristics of 12 output signals. 5 shows the frequency characteristics of the output signal of the bandpass ΔΣ modulator 13 in a wide band, and FIG. 6 shows the frequency characteristics of the output signal of the bandpass ΔΣ modulator 13 in a narrow band near the carrier frequency fc. Show.

  FIG. 5 shows that the output signal of the bandpass ΔΣ modulator 13 is a pulse signal, and thus has a wide frequency component of 0 Hz to ∞ Hz. Further, it can be seen from FIG. 6 that the output signal of the bandpass ΔΣ modulator 13 has a good SNR in the vicinity of the carrier frequency fc. This is a characteristic shape of the frequency characteristic of ΔΣ modulation.

  By the way, the signal input to the class D amplification unit 14 as the main amplification unit needs to be amplified to a signal level at which the class D amplification unit 14 can operate. In FIG. 1, a baseband signal generation unit 11, an up-conversion unit 12, and a bandpass ΔΣ modulation unit 13 are realized by digital signal processing using a DSP (Digital Signal Processor) or an FPGA (Field Programmable Gate Array). In this case, the signal level of the high-speed sampling signal that is the output of the bandpass ΔΣ modulation unit 13 is inevitably reduced, so that a drive amplification unit is required.

  FIG. 7 is a diagram showing a configuration in which the drive amplifier 21 is provided in the previous stage of the class D amplifier 14 of FIG. In FIG. 7, the same reference numerals as those in FIG.

  FIG. 8 shows the frequency characteristics of the drive amplification unit 21, and FIG. 9 shows the frequency characteristics of the output signal of the drive amplification unit 21. Here, the frequency characteristic of the input signal to the drive amplifier 21 is as shown in FIG.

  In general, in an analog high frequency circuit, circuit matching is achieved at the frequency of a signal to be handled. FIG. 8 shows an example of the frequency characteristics of the drive amplifier 21 when frequency matching is performed in the vicinity of the carrier frequency fc for the drive amplifier 21, and a band-pass filter-like filter that passes only frequency components in the vicinity of the carrier frequency fc. It can be seen that it has frequency characteristics and has band characteristics. FIG. 9 shows the frequency characteristic of the output signal when the signal having the frequency characteristic of FIG.

  The frequency characteristic of the input signal of the drive amplifier 21 has a wide frequency component of 0 Hz to ∞ Hz as shown in FIG. 5, and therefore the time axis waveform of the signal is a beautiful pulse waveform. On the other hand, the frequency characteristic of the output signal of the drive amplifier 21 is that the signal is band-limited as shown in FIG. The time axis waveform of the input / output signal of the drive amplifier 21 is as shown in FIG. In FIG. 10, the thin line shows the time axis waveform of the input pulse signal, and the thick line shows the time axis waveform of the output signal. From FIG. 10, it can be seen that the time-axis waveform has greatly deteriorated due to the band characteristics of the drive amplifier 21.

  The output signal of the drive amplifier 21 is input to the class D amplifier 14 and amplified. The time axis waveform of the input / output signal of the class D amplifier 14 is as shown in FIG. In FIG. 11, the thin line shows the time axis waveform of the input signal, and the thick line shows the time axis waveform of the output signal. The class D amplifier 14 performs a switching operation, and outputs 1 when the signal level is 0 or more, and -1 when the signal level is less than 0. At this time, since the switching voltage threshold value SWVth = 0, the time axis waveform of the output signal from the class D amplifier 14 is as shown by the thick line in FIG. 11 (however, the amplification factor is ignored).

Design of H-Bridge Class-D Power Amplifiers for Digital Pulse Modulation Transmitters. (I4 TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL.55, NO.12, DECEMBER 2007)

  However, the class D amplification unit 14 needs to linearly amplify the output pulse signal of the bandpass ΔΣ modulation unit 13, and the drive amplification unit having frequency characteristics between the bandpass ΔΣ modulation unit 13 and the class D amplification unit 14. 21 is arranged, the amplified output pulse (bold line in FIG. 11) that is the output of the class D amplifier 14 is the output pulse of the bandpass ΔΣ modulator 13 (the thin line in FIG. 10) that is the amplified signal. ) Is completely different. For this reason, the EVM of the transmission signal amplified by the class D amplification unit 14 is deteriorated, and the transmission constellation is as shown in FIG. 12, for example, and the EVM is about 10.2%.

  An object of the present invention is to provide a class-D amplifier and a wireless communication apparatus that suppress degradation of EVM of a class-D amplified radio signal.

  A class D amplifier according to the present invention includes a bandpass ΔΣ modulation unit that performs ΔΣ modulation on a carrier frequency signal at a predetermined frequency, a first amplification unit that amplifies the signal subjected to ΔΣ modulation to a certain signal level, A second amplifying means for switching and amplifying the signal amplified by one amplifying means based on a switching voltage threshold; and a switching voltage for determining and controlling the switching voltage threshold based on the signal amplified by the first amplifying means. And a control means.

  ADVANTAGE OF THE INVENTION According to this invention, degradation of EVM of the radio signal by which D class amplification was carried out can be suppressed.

The figure which shows the structure of the radio | wireless communication apparatus containing the class D amplification part disclosed by patent document 1 The figure which shows the constellation of the QPSK quadrature modulation wave which is an output of a baseband signal generation part The figure which shows the frequency characteristic of the output signal of the baseband signal generation part The figure which shows the frequency characteristic of the output signal of the up-conversion part The figure which shows the frequency characteristic in the wide band of the output signal of the band pass ΔΣ modulator The figure which shows the frequency characteristic in the narrow band in the carrier frequency fc vicinity of the output signal of a band pass delta-sigma modulation part. The figure which shows the structure by which the drive amplification part was provided in the front stage of the class D amplification part of FIG. Diagram showing frequency characteristics of drive amplifier The figure which shows the frequency characteristic of the output signal of the drive amplification part The figure which shows the time-axis waveform of the input / output signal of the drive amplifier The figure which shows the time-axis waveform of the input-output signal of a class D amplification part The figure which shows the transmission constellation of the transmission signal amplified by the class D amplifier The figure which shows the structure of the radio | wireless communication apparatus which concerns on one embodiment of this invention The figure which shows the time-axis waveform of the input signal and output signal of a class D amplification part when switching voltage threshold value is SWVth = + 0.2 The figure which shows the transmission constellation when switching voltage threshold value SWVth = + 0.2 The figure which shows the example of EVM at the time of changing a switching voltage threshold value The figure which plotted RMS (Root Mean Square) and the optimal value SWVth_opt of the switching voltage threshold value which a switching voltage control part outputs The figure which shows the structure of the radio | wireless communications system which concerns on one embodiment of this invention

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(One embodiment)
FIG. 13 is a diagram showing a configuration of a wireless communication apparatus according to an embodiment of the present invention. Hereinafter, the configuration of the wireless communication apparatus will be described with reference to FIG.

  The baseband signal generation unit 101 generates a narrowband (˜several tens of MHz) baseband signal and outputs the baseband signal to the up-conversion unit 102.

  Up-conversion unit 102 up-converts the baseband signal output from baseband signal generation unit 101 to carrier frequency fc, and outputs the carrier frequency signal to bandpass ΔΣ modulation unit 104 of class D amplifier 103.

  The class D amplifier 103 includes a bandpass ΔΣ modulation unit 104, a drive amplification unit 105, a switching voltage control unit 106, and a class D amplification unit 107.

  The bandpass ΔΣ modulation unit 104 performs bandpass ΔΣ modulation on the carrier frequency signal output from the up-conversion unit 102 at, for example, a sampling frequency fs = 4 × fc, and drives and amplifies the signal subjected to the bandpass ΔΣ modulation. Output to the unit 105.

  The drive amplifier 105 amplifies the signal output from the bandpass ΔΣ modulator 104 to a signal level at which the subsequent class D amplifier 107 can operate, and outputs the amplified signal to the switching voltage controller 106 and the class D amplifier 107. .

  The switching voltage control unit 106 determines the switching voltage threshold SWVth of the class D amplification unit 107 based on the signal output from the drive amplification unit 105, and outputs the determined switching voltage threshold SWVth to the class D amplification unit 107. A method for determining the switching voltage threshold SWVth will be described later.

  The class D amplifier 107 performs switching amplification on the signal output from the drive amplifier 105 based on the switching voltage threshold SWVth output from the switching voltage controller 106, and outputs the amplified signal to the band pass filter unit 108.

  The bandpass filter unit 108 passes the band centered on the carrier frequency fc in the signal output from the class D amplification unit 107 and transmits it as a radio signal from the antenna unit 109.

  Here, the time axis waveform and frequency characteristics of the output signal of each block in FIG. 13 will be described. The signal before the class D amplification unit 107 is the same as that described in the background art. 2 shows the constellation of the QPSK quadrature modulated wave that is the output of the baseband signal generation unit 101, FIG. 3 shows the frequency characteristics of the output signal of the baseband signal generation unit 101, and FIG. The frequency characteristic of the output signal of the conversion part 102 is shown. 5 shows the frequency characteristics in the wide band of the output signal of the bandpass ΔΣ modulation section 104, and FIG. 6 shows the frequency characteristics in the narrow band near the carrier frequency fc of the output signal of the bandpass ΔΣ modulation section 104. Show.

  FIG. 8 shows the frequency characteristics of the drive amplifier 105 when frequency matching is performed in the vicinity of the carrier frequency fc, and FIG. 9 shows the frequency characteristics of the output signal of the drive amplifier 105. Further, the time axis waveform of the input signal of the drive amplifier 105 is indicated by a thin line in FIG. 10, and the time axis waveform of the output signal is indicated by a thick line in FIG.

  Now, in the switching amplification in the class D amplification unit 107, the time axis waveforms of the input signal and the output signal of the class D amplification unit 107 when the switching voltage threshold value from the switching voltage control unit 106 is SWVth = + 0.2 are shown in FIG. Shown in In FIG. 14, the thin line indicates a pulse input signal that has deteriorated due to the band characteristics of the drive amplifier 105, and the thick line indicates the output signal of the class D amplifier 107.

  The output of the class D amplifying unit 14 when there is no switching voltage control unit shown by the thick line in FIG. 11 and the output of the class D amplification unit 107 when there is the switching voltage control unit 106 shown by the thick line in FIG. Is compared, the latter is closer to the output waveform (thin line in FIG. 10) of the bandpass ΔΣ modulation unit that is the amplified signal.

  The transmission constellation when the switching voltage threshold determined by the switching voltage control unit 106 is SWVth = + 0.2 is, for example, as shown in FIG. 15, and it is understood that the transmission constellation is improved from the case shown in FIG. . The EVM of the signal of FIG. 15 is about 7.4%, which is an improvement of 2.8% compared to FIG. 12, and it can be seen that the class D amplifier of the present invention contributes to the EVM improvement.

  As can be seen from the frequency characteristics of the signal in FIG. 9, when the high-frequency component of the pulse signal does not pass, the time response of the rise and fall of the signal is delayed, and the signal waveform is the original switching of the class D amplification unit 107. The time to reach 0 which is the voltage threshold becomes longer. This delay in arrival time can be offset by switching the class D amplifier 107 at the voltage level before the signal waveform reaches the voltage level 0, so that the EVM improvement effect as described above can be obtained.

  Here, FIG. 16 shows an example of the EVM when the switching voltage threshold value of the switching voltage control unit 106 is changed. In FIG. 16, the vertical axis represents the EVM, and the horizontal axis represents the switching voltage threshold. Referring to FIG. 16, the switching voltage threshold is conventionally fixed at SWVth = 0, so that the EVM is only about 10.2%, whereas in the present invention, the switching voltage control unit 106 performs the class D amplification unit 107. It can be seen that the EVM can be improved to 6.7% by setting the switching voltage threshold value to the optimum value (here, SWVth = −0.17). Therefore, when the frequency characteristic of the drive amplifier 105 is as shown in FIG. 8, the switching voltage threshold output from the switching voltage controller 106 may be a fixed value of SWVth = −0.17.

  Note that SWVth = + 0.17 may be used as a fixed value, as can be seen from FIG. 16 that the value of EVM is substantially line symmetric with respect to the axis of SWVth = 0.

  In the above description, the time axis waveform and frequency characteristics of the signal when the frequency characteristics of the drive amplifier 105 are as shown in FIG. 8 have been seen. However, in practice, the frequency characteristics of the drive amplification unit 105 can take various shapes depending on the circuit implementation method and the matching method. FIG. 17 shows RMS (Root Mean Square) that is an effective power value of the output signal of the drive amplifier 105 and switching voltage control in various cases where the frequency characteristic of the drive amplifier 105 is line-symmetric with respect to the carrier frequency fc. FIG. 10 is a diagram plotting an optimum value (voltage level when EVM is the best) SWVth_opt of a switching voltage threshold output by a unit 106. In FIG. 17, the vertical axis represents SWVth_opt and the horizontal axis represents RMS.

  Referring to FIG. 17, the optimum value of the switching voltage threshold output from the switching voltage control unit 106 increases as the RMS of the output signal of the drive amplification unit 105 increases. From this, the switching voltage control unit 106 calculates the RMS value from the output signal of the drive amplification unit 105 using the relationship shown in FIG. 17, and the switching voltage threshold value of the class D amplification unit 107 based on the calculated RMS value. Can be improved to determine the transmission EVM. In addition, when the modulation type and modulation band applied to the transmission signal are different on the time axis, or when the band characteristics of the drive amplifier 105 and other analog circuits are different from one object to another, signal degradation caused by these is reduced. can do.

  Thus, according to the present embodiment, the switching voltage threshold value of the class D amplification unit is determined to an optimum level based on the RMS power value RMS of the input signal to the class D amplification unit, and the determined switching voltage threshold value is determined. Since the class D amplification unit performs switching amplification using the, the degradation of the EVM of the radio signal amplified by the class D amplification unit can be suppressed even when the drive amplification unit is arranged in the previous stage of the class D amplification unit. Therefore, the wireless communication apparatus including the class D amplifier can transmit a wireless signal with good EVM.

  In the present embodiment, a single wireless communication apparatus has been described. However, a wireless communication system including the wireless communication apparatus of the present invention may be configured. For example, as shown in FIG. 18, it is assumed that at least one of radio communication apparatuses RS1, RS2,..., RSm of a mobile station or a fixed station includes the class D amplifier shown in FIG. The base station may also include the class D amplifier shown in FIG.

  In the wireless communication system illustrated in FIG. 18, the more wireless communication apparatuses including the class D amplifier illustrated in FIG. 13, the more frequently a wireless signal with a good EVM is used as the entire system. Moreover, since the quality of the transmission signal is good at the reception of each wireless communication device, the reception performance is also good, and as a result, a wireless communication system with good communication quality can be constructed. In addition, since a wireless communication apparatus including a class D amplifier with high power utilization efficiency is used, a system with high power utilization efficiency as a whole can be constructed.

  In the present embodiment, the switching voltage threshold SWVth has been described as being determined based on the RMS of the output signal of the drive amplifier, but the value at this time may be fixed or variable. Further, as the switching voltage threshold SWVth, a fixed value that does not depend on the RMS of the output signal of the drive amplifier may be set.

  The class D amplifier and the wireless communication apparatus according to the present invention can be applied to a wireless communication system and the like.

DESCRIPTION OF SYMBOLS 101 Baseband signal generation part 102 Up-conversion part 103 Class D amplifier 104 Band pass delta-sigma modulation part 105 Drive amplification part 106 Switching voltage control part 107 Class D amplification part 108 Band pass filter part 109 Antenna part

Claims (4)

Bandpass ΔΣ modulation means for performing ΔΣ modulation on the carrier frequency signal at a predetermined frequency;
First amplifying means for amplifying the signal subjected to ΔΣ modulation to a certain signal level;
Second amplification means for switching and amplifying the signal amplified by the first amplification means based on a switching voltage threshold;
Switching voltage control means for determining and controlling the switching voltage threshold based on the signal amplified by the first amplification means;
A class D amplifier comprising:   The class D amplifier according to claim 1, wherein the switching voltage threshold is a fixed value.   2. The class D amplifier according to claim 1, wherein the switching voltage control means variably controls the switching voltage control means according to the level of the signal amplified by the first amplifying means.   A wireless communication apparatus comprising the class D amplifier according to claim 1.

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