JP2010288042A - Pulse modulation amplifier - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technology capable of continuously acquiring a low voltage close to 0 V with no distortion when performing digital amplification. <P>SOLUTION: When an amplitude modulation signal is inputted into a PWM modulation part 20, it is inputted into first and second PWM waveform generation parts 21 and 22. A level detecting part 60 detects a level of the amplitude modulation signal, and if it is equal to a predetermined value or higher, controls a switching part 25 to connect the first PWM waveform generation part 21 to a digital amplification part 40. If it is less than the predetermined value, the level detecting part 60 controls the switching part 25 to connect the second PWM waveform generation part 22, being a half of the pulse frequency of the first PWM waveform generation part 21, to the digital amplification part 40. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a pulse modulation amplification apparatus that modulates and amplifies an input signal.

  A broadcasting apparatus including digital amplitude modulation means (pulse modulation amplification apparatus) performs AD conversion on a signal to be transmitted, for example, an audio signal, performs digital amplification on the signal after AD conversion, and generates a desired carrier wave. Superimpose and output. Various techniques have been proposed to suppress distortion caused by signal amplification processing.

  For example, there is a technique of reducing the switching frequency of a power amplifier and improving distortion of a modulated wave (see, for example, Patent Document 1). The digital amplitude modulation transmitter disclosed in Patent Document 1 includes a plurality of power amplifiers, and selects a power amplifier according to part or all of the bit data after AD conversion. With this technique, the operation of a plurality of power amplifiers is averaged, the MTBF (Mean Time Between Failures) of the power amplifier is averaged, and the failure rate of the entire transmitter is reduced. Furthermore, the switching frequency of the power amplifier can be lowered, and the distortion of the modulation wave can be effectively improved.

JP-A-10-126159

  By the way, digital amplitude amplification is performed by ON / OFF operation of a semiconductor switch such as an FET. For example, a PWM (Pulse Width Modulation) method, which is one of digital modulation methods, is adjusted to a desired voltage by adjusting a duty ratio in switch operation. When the voltage is high, the duty ratio is increased, and when the voltage is low, the duty ratio is decreased. When the duty ratio is small, the ON signal pulse width is small. In a transmitter that obtains amplitude modulation by changing the power supply voltage of the power amplifier, it is necessary to change the power supply voltage in accordance with the input amplitude modulation signal. Therefore, it is necessary to change the power supply voltage quickly, and it is necessary to generate the power supply voltage as a variable without distortion continuously from a low voltage close to 0 V to a high voltage.

  In general, the frequency for turning on / off by the semiconductor switch (reciprocal of the on / off period) needs to be set to a sufficiently high frequency with respect to the frequency (modulation frequency) of the amplitude modulation signal input to the digital amplitude amplifier of the transmitter. There is. This is because the frequency component of the pulse needs to be attenuated by the low-pass filter after the pulse is generated, but the modulation frequency needs to pass through the low-pass filter. Usually, in order to set the frequency component of the pulse to a high frequency, it is necessary to shorten the ON / OFF cycle. However, since a certain amount of time is inevitably required for turning on / off the semiconductor switch, it becomes more difficult as the cycle Tc is shortened. That is, there is a problem that it is difficult to set the frequency F so high. Further, in order to obtain a low voltage near the final stage power supply voltage of 0 V, it is necessary to shorten the pulse width Ton as much as possible. However, since a certain time is required to turn on / off the semiconductor switch as described above, it is difficult to generate a pulse narrower than this time. Therefore, a voltage as low as 0 V can be continuously obtained without distortion. There was a problem that it was difficult. As a method for solving such a problem, a method of increasing the period T can be adopted. However, this has a problem that the characteristics of the low-pass filter after pulse generation become steep and difficult to realize. As another solution, it is possible to adopt a semiconductor switch that can be turned on / off at a high speed. However, there is a problem that a high-speed semiconductor switch is expensive and increases the component cost, and there is a demand for cost reduction. In a harsh environment, it was not realistic and another technology was required. Even in the technique disclosed in Patent Document 1, the same problem still remains.

  The present invention has been made in view of such a situation, and an object thereof is to propose a technique capable of continuously obtaining a low voltage close to 0 V without distortion when digital amplification is performed.

  A pulse modulation amplification apparatus according to the present invention includes a plurality of modulation means for pulse-modulating an input signal, different amplification frequencies for amplifying an output from the modulation means by a switching operation, and any of the plurality of modulation means Switching means for selecting the output of the modulation means to output to the amplification means, and the level of the input signal input to the modulation means is detected and output to the amplification means according to the detected level Detecting means for controlling the switching means so as to select a modulation means.

  As described above, according to the present invention, when digital amplification is performed, a low voltage close to 0V can be continuously obtained without distortion.

It is a functional block diagram which shows the structure of the digital amplitude modulation apparatus based on embodiment. It is a figure which shows the example of improvement of the distortion of the waveform based on embodiment.

  Next, modes for carrying out the present invention (hereinafter, simply referred to as “embodiments”) will be specifically described with reference to the drawings. In the embodiment described below, the frequency of the PWM waveform generator is switched in accordance with the level of the amplitude modulation signal in order to reduce distortion around 0 V generated in the PWM method which is one of the pulse modulation methods. In order to switch the frequency of the PWM waveform generator, a plurality of PWM waveform generators with different output pulse frequencies (or cycles) are provided, and when the amplitude modulation signal is near 0 V, PWM pulse generation with a low pulse frequency is generated. A vessel is selected.

  FIG. 1 is a functional block diagram showing a configuration of a digital amplitude modulation apparatus 10 according to the present embodiment. As illustrated, the digital amplitude modulation device 10 includes a PWM modulation unit 20, a digital amplification unit 40, a power supply unit 50, an LPF (low-pass filter) 54, a PA (Power Amplifier) 56, and a level detection unit 60. Is provided.

  The PWM modulation unit 20 includes a first PWM waveform generation unit 21, a second PWM waveform generation unit 22, and a switching unit 25. The first and second PWM waveform generation units 21 and 22 include, for example, a triangular wave transmitter and a comparator, and perform pulse modulation on an amplitude modulation signal that is an input signal at a predetermined frequency. The first PWM waveform generation unit 21 is configured to perform pulse modulation at a frequency relatively higher than that of the second PWM waveform generation unit 22. Here, the first PWM waveform generation unit 21 performs pulse modulation at a frequency of 200 kHz, and the second PWM waveform generation unit 22 performs pulse modulation at a frequency of 100 kHz.

  The switching unit 25 digitally amplifies either the pulse signal of the first PWM waveform generation unit 21 or the pulse signal of the second PWM waveform generation unit 22 based on a control signal from the level detection unit 60 described later. To the unit 40.

  The digital amplification unit 40 includes an FET driver 41 and an FET 42 and amplifies the pulse signal acquired from the PWM modulation unit 20 based on the voltage supplied from the power supply unit 50. The FET driver 41 and the FET 42 may be configured to be used for general digital amplification means, and detailed description thereof is omitted here. The power supply unit 50 includes a transformer 51 that performs voltage conversion and a diode 52 that performs rectification, converts an alternating current into a direct current having a desired characteristic, and supplies the direct current to the FET 42 of the digital amplification unit 40.

  A high frequency component of the pulse signal amplified by the digital amplification unit 40 is filtered by the LPF 54 and is output to the PA 56 as a power supply voltage (PA power supply) of the PA 56. The PA 56 modulates an input carrier wave by a PA power source and outputs the modulated carrier wave to the antenna 58 as an amplitude modulated wave. The antenna 58 transmits the amplitude-modulated wave acquired from the PA 56 by radio.

  The level detection unit 60 acquires the level of the amplitude modulation signal input to the PWM modulation unit 20 and outputs the pulse output of the first PWM waveform generation unit 21 to the digital amplification unit 40 if the level is equal to or greater than a predetermined value. The switching unit 25 is controlled to do so. As the predetermined value, for example, the level can be set to 1/10 of the assumed maximum amplitude value. This predetermined value is preferably set with a predetermined margin based on a level at which distortion exceeding the allowable value occurs when only the first PWM waveform generation unit 21 operates. Further, when the level of the amplitude modulation signal input to the PWM modulation unit 20 is smaller than the predetermined value, the level detection unit 60 outputs the pulse output of the second PWM waveform generation unit 22 to the digital amplification unit 40. The switching unit 25 is controlled. That is, the level detection unit 60 controls to output a pulse signal having a high frequency (short cycle) to the digital amplification unit 40 when the level of the amplitude modulation signal is high, and a low frequency (long cycle) when the level is low. The pulse signal is controlled to be output to the digital amplifier 40.

An outline of the operation of the digital amplitude modulation apparatus 10 having the above configuration will be described.
When an amplitude modulation signal is input to the PWM modulation unit 20 as an input signal, the amplitude modulation signal is input to the first and second PWM waveform generation units 21 and 22. At this time, the level detection unit 60 acquires the amplitude modulation signal and detects the level. If the detection level is equal to or higher than the predetermined value, the level detection unit 60 controls the switching unit 25 to connect the output of the first PWM waveform generation unit 21 to the digital amplification unit 40. If the detection level is less than the predetermined value, the level detection unit 60 controls the switching unit 25 to connect the output of the second PWM waveform generation unit 22 to the digital amplification unit 40.

  In the digital amplifying unit 40, the FET driver 41 switches the FET 42 according to the pulse signal acquired from the switching unit 25, amplifies the pulse signal acquired from the switching unit 25 to a desired pulse amplitude, and outputs it to the LPF 54. The LPF 54 attenuates the high frequency component, generates a modulated signal amplified to a desired level, and outputs it to the PA 56 as a PA power source. The PA 56 superimposes the modulation signal input as the PA power source on the carrier wave, and outputs it to the antenna 58 as an amplitude modulated wave. The antenna 58 transmits the amplitude-modulated wave as radio.

  FIG. 2 shows a measurement waveform of the amplitude-modulated wave output from the PA 56. FIG. 2A shows a waveform according to the conventional operation of only the first PWM waveform generation unit 21, and FIG. 2B shows a book in which the first PWM waveform generation unit 21 and the second PWM waveform generation unit 22 are switched. It is a waveform by the operation of the embodiment. In FIG. 2A, it can be seen that distortion occurs near 0 V (region X). On the other hand, in FIG. 2B, there is no distortion near 0V (region Y), and the waveform is good. As in the present embodiment, when the level of the input signal (amplitude modulated wave) to the PWM modulator 20 is close to 0V, the cycle Tc of the generated pulse signal is set to a cycle Tcx (= 2 × Tc) that is doubled. As a result, the voltage generated by passing through the LPF 54 by the pulse signal having the double cycle Tcx (= 2 × Tc) is ½ of the pulse signal having the base cycle Tc, and the low voltage close to 0V is distorted. It can be generated without.

  Further, the state of switching the pulse cycle to twice is performed only when a voltage close to 0 V is generated. As a result, the output pulse from the PWM modulation unit 20 having the frequency Fcx that is the reciprocal of the cycle Tcx (= 2 × Tc) becomes a thin pulse, so that the energy of the component of the frequency Fcx is reduced, and the attenuation of the LPF 54 for that purpose. The amount can be reduced. As a result, distortion of a low voltage close to 0 V can be improved, and distortion of an amplitude-modulated transmitter output waveform (amplitude modulated wave) can be reduced as shown in FIG. Further, in the digital amplifying unit 40, it is not necessary to use a high-speed semiconductor switch (FET 42), and it is possible to avoid an increase in component costs.

  The present invention has been described based on the embodiments. This embodiment is an exemplification, and it will be understood by those skilled in the art that various modifications can be made to combinations of these components, and such modifications are also within the scope of the present invention. For example, the period Tc is not limited to twice, and theoretically, the period Tc can be varied from 1 to infinity, and a PWM waveform generation unit having characteristics as required may be used. As a result, distortion of the amplitude-modulated wave that is the transmitter output wave can be further reduced. Further, the first PWM waveform generation unit 21 and the second PWM waveform generation unit 22 may be activated and stopped according to the switching operation of the switching unit 25. That is, only the device selected by the switching unit 25 in the first PWM waveform generation unit 21 or the second PWM waveform generation unit 22 is operated, and the device that is not selected is paused. This activation / deactivation process may be performed by a control signal (signal line indicated by a broken line in FIG. 1) from the level detection unit 60. When the time for driving the second PWM waveform generation unit 22 is short, the first PWM waveform generation unit 21 is always operated, and the second PWM waveform generation unit 22 is an input signal. The operation may be performed only when the level of the amplitude modulation signal approaches 0V. In addition, the switching unit 25 may be provided in front of the first PWM waveform generation unit 21 and the second PWM waveform generation unit 22. Further, the modulation technique is not limited to the PWM system, and the above technique can be applied to a PDM (Pulse Density Modulation) system.

The outline of the present embodiment is summarized as follows.
The digital amplitude modulation apparatus includes: a plurality of modulation means for modulating an input signal with different pulse frequencies; an amplification means for amplifying an output from the modulation means by a switching operation; and a modulation means of any of the plurality of modulation means Switching means for selecting the output of the output signal to the amplification means, and the level of the input signal input to the modulation means is detected, and the modulation means to be output to the amplification means is selected according to the detected level Detecting means for controlling the switching means.
The digital amplitude modulation device is provided, for example, in a broadcast wave transmitter. Further, the pulse modulation may be modulation by a PWM method, and the input signal may be an amplitude modulation signal. Furthermore, when the level of the modulation signal of the input signal is lower than a predetermined value, a modulation means having a relatively low pulse frequency may be selected. Further, among the plurality of modulation means, the modulation means that are not selected may be stopped. The pulse signal amplified by the amplifying means may be output as a power supply voltage of a carrier wave after high frequency components are filtered.

DESCRIPTION OF SYMBOLS 10 Digital amplitude modulation apparatus 20 PWM modulation part 21 1st PWM waveform generation part 22 2nd PWM waveform generation part 25 Switching part 40 Digital amplification part 50 Power supply part 54 LPF
56 PA
60 level detector

Claims (1)

A plurality of modulation means for modulating the input signal with different pulse frequencies;
Amplifying means for amplifying the output from the modulating means by a switching operation;
Switching means for selecting the output of the modulation means of any of the plurality of modulation means to output to the amplification means;
Detecting means for detecting the level of the input signal inputted to the modulating means and controlling the switching means so as to select the modulating means to be output to the amplifying means according to the detected level;
A pulse modulation amplifying apparatus comprising:

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