JPH11103218A - Feedforward type amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To attain a power amplifier having low distortion as compared with a conventional one. SOLUTION: A directional coupler 22 and a receiving part 24 are connected to the post stage of a synthesis part 14 for compensating distortion to receive a frequency component of unnecessary radiation existing out of an orthogonal frequency division multiplex (AFDM) wave transmission band. A control part 20A controls the amplification gain or phase shift of a distortion compensating amplifier 18 in accordance with a receiving level. Even when a characteristic change occurs in a main amplifier 10 or the amplifier 18 due to an environmental factor, the increment or generation of distortion is not generated in a signal supplied to the post stage circuit through the coupler 22. Since a soft start circuit for gradually increasing the input level of a signal at the time of starting the input of the signal to the amplifier 10 is prepared, the generation of distortion can be suppressed even immediately after the ON of a power supply or immediately after the start of broadcasting.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a feed-forward amplifier provided with a distortion compensation system for compensating a distortion component generated in a main amplifier.

[0002]

2. Description of the Related Art Orthogonal frequency division multiplexing (Ort)
Hogonal Frequency Division Multiplex (OFDM) is a multiplexing method using a plurality of subcarriers that are orthogonal to each other, as shown in FIG. Has advantages. By adopting this method, a network using a single main carrier, that is, a single frequency network, while being compatible with effective use of radio resources.
(SFN: Single Frequency Network) can be realized. For this reason, next-generation digital broadcasting systems using the OFDM scheme have been conceived or developed, including in Japan.

When constructing a broadcasting station for a broadcasting system employing the OFDM system, a low-distortion power amplifier is required. This is related to the fact that the OFDM system uses a plurality of subcarriers as described above. That is, if a power amplifier with poor linearity is used, not only the odd-order intermodulation products between the subcarriers appear in the transmission band (see FIG. 6) and the transmission quality deteriorates, but also unnecessary radiation near the transmission band. Ingredients appear. In order to avoid such a phenomenon, it is necessary to increase the linearity of a power amplifier used for power amplification of a broadcast wave in a broadcasting station and reduce distortion.

FIG. 7 shows a feed-forward type power amplifier as an example of a conventional low distortion power amplifier. In this power amplifier, the main amplifier 1 plays the role of amplifying the input signal, that is, the signal to be broadcast, to the power required for broadcasting.
0, and most of the signal amplified by the main amplifier 10 is supplied to a transmission circuit (not shown) via the directional coupler 12 and the combining unit 14. Main amplifier 1
A part of the signal amplified by 0 is branched by the directional coupler 12 and supplied to the distortion extracting unit 16. Distortion extraction unit 16
Is based on a signal supplied from the main amplifier 10 via the directional coupler 12 and a signal input to the main amplifier 10, for example, by subtracting the latter from the former, thereby distorting the distortion component generated in the main amplifier 10. Detecting and distortion compensating amplifier 18
To supply. The distortion compensation amplifier 18 amplifies the distortion component detected by the distortion extraction unit 16 to the same level as the distortion component included in the output of the main amplifier 10 and supplies the same to the synthesis unit 14.
Also, regarding the distortion component detected by the distortion extraction unit 16,
The phase is inverted in any part of the distortion compensation system from the distortion extraction unit 16 to the synthesis unit 14 (for example, when outputting from the distortion compensation amplifier 18 or inputting to the synthesis unit 14). The combining unit 14 compensates for the above-described distortion component by combining the signal supplied from the main amplifier 10 via the directional coupler 12 with the signal supplied from the distortion compensation amplifier 18, that is, the signal indicating the distortion component. . As a result, a high-power signal whose distortion has already been compensated is supplied to a subsequent circuit (not shown).

The amplitude characteristics and phase characteristics of the components constituting the main amplifier 10 and the distortion compensation amplifier 18 depend on environmental factors such as temperature. Since a characteristic change due to such a factor causes an error in distortion extraction and distortion compensation, it is necessary to take measures using some means. As an example, there is a method of using components having high temperature stability as components constituting the main amplifier 10 and the distortion compensation amplifier 18 and providing an additional circuit such as a temperature compensation circuit. In FIG. 7, the control unit indicated by reference numeral 20 controls or sets the amplification gain and the phase shift amount in the main amplifier 10 and the distortion compensation amplifier 18 according to information indicating environmental conditions, for example, a detected value of the ambient temperature. Make up the circuit. However, the actual change in characteristics depends on various factors, and there are variations between products and changes over time.
Even if a temperature compensation circuit is provided as shown in FIG. 7, it is difficult to achieve sufficient temperature compensation. As an alternative method, a method of inputting a pilot signal having a predetermined signal waveform and level to the main amplifier at an appropriate timing and using the output at that time to adjust the amplification gain and the amount of phase shift in the main amplifier is known. However, this method requires a circuit for generating a pilot signal, so that the apparatus becomes expensive.

[0006]

SUMMARY OF THE INVENTION One of the objects of the present invention is to adopt an OFDM system in which distortion does not occur or increase due to a characteristic change even if a characteristic change occurs in each part of a circuit due to environmental factors or the like. It is an object of the present invention to realize a low-distortion amplifier that can be used as a power amplifier in a broadcasting station for a broadcasting system at a low cost with as small a circuit change as possible. In order to achieve this object, the feedforward amplifier according to the present invention includes a receiving unit that receives a frequency component outside the transmission band included in an output signal from the distortion compensation system.
Further, the control unit controls the amplification gain and / or the distortion compensation system so that the received out-of-transmission-band frequency component becomes lower.
Alternatively, the amount of phase adjustment is controlled. The distortion compensation system generates an output signal in which the distortion component is compensated by extracting, amplifying, and adjusting the phase of the distortion component generated in the main amplifier and combining the extracted component with the output from the main amplifier.

Here, when a signal generated according to a multiplexing method using a plurality of subcarriers, such as the OFDM method, is amplified, unnecessary radiation components generated due to poor linearity (non-linearity) of the amplifier. Appears in a frequency band near the frequency band to which the output signal originally belongs (the transmission band in FIG. 6). Therefore, the frequency component outside the transmission band included in the output signal from the distortion compensation system is received, and the gain and phase adjustment amount in the distortion compensation system are controlled so that the frequency component outside the transmission band becomes lower. For example, even if a characteristic change due to an environment or the like occurs in the main amplifier or the like, it is possible to prevent an increase or generation of distortion due to the characteristic change.
Therefore, according to the present invention, it is possible to realize a feedforward amplifier having lower distortion than the conventional one. Further, since a pilot signal source and the like are not required, the cost can be reduced.

In the present invention, a process for receiving the frequency component outside the transmission band in the amplified signal and using the result to control the amplification gain and the amount of phase adjustment, in other words, receiving and processing the frequency component outside the transmission band. Since the feedback is performed, the distortion may not be sufficiently low immediately after power-on or start of use (in the example of a broadcast station, start of broadcast). In order to avoid this, a soft start circuit may be provided which gradually increases the input level to the main amplifier when starting to input the signal to be amplified to the main amplifier. That is, while the input level to the main amplifier is low, the absolute power of the distortion generated in the main amplifier and the like is also small, so that the amplification gain and / or the phase adjustment amount is maintained even during the period when the input level to the amplifier is gradually increased. By executing the above-described control, the distortion in the output signal (broadcast wave) at the time when the input level to the main amplifier reaches the original level, ie, the target level, is reduced to a level below the level specified by the Radio Law or the like Can be suppressed.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below with reference to the drawings. The same members as those in the prior art shown in FIGS. 6 and 7 are denoted by the same reference numerals, and description thereof will be omitted.

FIG. 1 shows a configuration of a feedforward amplifier according to an embodiment of the present invention. In this embodiment, a directional coupler 22 is provided downstream of the synthesis unit 14,
A part of the output of the combining unit 14 is received by the receiving unit 24. What the receiving unit 24 receives is a predetermined frequency component that belongs to outside the transmission band. That is, when the OFDM system is adopted as the multiplexing system, FIG.
The receiving unit 24 receives a frequency component existing on the lower side of the transmission band as shown in FIG. 2A or a frequency component existing on the upper side of the transmission band as shown in FIG. In the case of OFDM, the unnecessary radiation component appears at a frequency that is an integer multiple of the subcarrier interval from the upper end or lower end of the transmission band. Therefore, the frequency indicated by the upward arrow in FIG. , The unnecessary radiation component generated due to poor linearity in the main amplifier 10 or the distortion compensation amplifier 18 can be suitably detected. Further, the control unit 20A adjusts the amplification gain and the phase shift amount (phase adjustment amount) in the distortion compensation amplifier 18 according to the level of the frequency component received by the reception unit 24, that is, the frequency component related to unnecessary radiation outside the transmission band.
Control is performed so that the level of the frequency component related to the unnecessary radiation becomes lower.

By adopting such a configuration,
In the present embodiment, it is possible to realize a feedforward amplifier having lower distortion than the conventional one. That is, a change in the characteristics of the main amplifier 10 and the distortion compensation system due to environmental factors, and an increase or occurrence of distortion due to the change can be prevented, so that a broadcasting station can be used in a next-generation digital broadcasting system using the OFDM system. A power amplifier suitable for construction can be realized.

In FIG. 1, one receiving unit 24 is shown. However, as shown in FIGS. 2A and 2B, unnecessary radiation is generated at a plurality of frequencies near the transmission band. Since such unnecessary radiation appears, it is preferable to configure a circuit that can receive these plural types of frequency components.
For example, as shown in FIG. 3, a plurality of receiving units 24-1, 24-2,.
n, and the control unit 20A operates these receiving units 24-1 and 24-2.
It is preferable to perform the above-described control based on the level of each frequency component received at −2,..., 24-n. Alternatively, as shown in FIG. 4, if the receiving frequency of the receiving unit 24 is set in accordance with a command from the control unit 20 </ b> A, a plurality of types of frequencies can be set even though a single receiving unit 24 is used. The components can be received, resulting in a more economical configuration. Alternatively, instead of receiving a specific frequency component, the receiving unit 24 monitors a frequency band that is above or below the transmission band and has a certain degree of spread, and the reception output of the receiving unit 24 is provided in a subsequent stage. FF
The T unit 26 may perform fast Fourier transform, and supply the resulting spectrum information to the control unit 20A. Even with such a configuration, the control unit 20A
Can know information on the level of the unnecessary radiation component for a plurality of frequencies.

Further, as shown in FIG. 1, a soft start circuit 28 is preferably provided in front of the main amplifier 10. The soft start circuit 28 is a circuit that gradually increases the signal input level to the main amplifier 10 when a signal input is instructed from the control unit 20A. This circuit is, for example, a circuit in which a part of the input signal to the main amplifier 10 is branched and further integrated, and the attenuation of a built-in variable attenuator provided in front of the main amplifier 10 is varied according to the integrated value. It can be configured as in this way,
When the input of the signal to the main amplifier 10 is started (for example, when the power is turned on or when the broadcast is started), the input level of the signal to the main amplifier 10 is gradually increased, so that the output can be increased. The distortion included in the signal can be further suppressed. That is, when the input power to the main amplifier 10 is small and the output power of the feedforward type amplifier of FIG. 1 is small, the absolute power of the distortion generated in the main amplifier 10 is also small. Even in a period in which the amplification gain and the phase shift amount are not optimized, the distortion in the signal output from the directional coupler 22 is suppressed to, for example, a predetermined upper limit specified by the Radio Law. Becomes possible.

In the above description, the present invention is described as an invention relating to a “feedforward amplifier”. However, those skilled in the art with reference to the disclosure of the present application may describe the present invention as “a next-generation amplifier”. It can also be expressed as a “terrestrial broadcast power amplifier”, a “feedforward type power amplifier control method”, or the like. Furthermore, in the above description, the next-generation terrestrial broadcasting system using the OFDM system was considered,
The present invention has room for application in applications where an amplifier having good linearity is required because the multicarrier system is used.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of a feedforward amplifier according to an embodiment of the present invention.

FIGS. 2A and 2B are diagrams showing frequencies at which unnecessary radiation appears at a lower side or an upper side of a transmission band. FIG. 2A shows the lower side of the transmission band, and FIG. 2B shows the upper side of the transmission band.

FIG. 3 is a block diagram showing a modification of the receiving unit.

FIG. 4 is a block diagram showing a modification of the receiving unit.

FIG. 5 is a block diagram showing a modification of the receiving unit.

FIG. 6 is a diagram showing a spread of a transmission band and an orthogonal relationship between subcarriers in the OFDM system.

FIG. 7 is a block diagram illustrating a configuration of a feedforward amplifier according to one related art.

[Explanation of symbols]

10 main amplifier, 14 synthesis unit, 16 distortion extraction unit, 18
Distortion compensation amplifier, 20A control unit, 24, 24-1,
4-2,... 24-n receiving unit, 26 FFT unit, 28
Soft start circuit.

Continued on the front page (72) Inventor Takashi Maruyama 5-1-1 Shimorenjaku, Mitaka-shi, Tokyo Japan Radio Co., Ltd.

Claims (3)

[Claims] A distortion compensator for generating an output signal in which the distortion component is compensated by extracting, amplifying, and adjusting the phase of the distortion component generated in the main amplifier and combining the extracted component with the output from the main amplifier. A receiving unit that receives the out-of-transmission-band frequency component included in the output signal; and an amplification gain and / or phase adjustment amount in the distortion compensation system such that the received out-of-transmission-band frequency component is at a lower level. And a control unit for controlling. 2. The feed-forward amplifier according to claim 1, further comprising: a soft-start circuit that gradually increases an input level to the main amplifier when a signal to be amplified is started to be input to the main amplifier. A feed-forward amplifier, wherein the above-mentioned control relating to the amplification gain and / or the phase adjustment amount is executed even during a period in which the input level to the amplifier gradually increases. 3. The feedforward amplifier according to claim 1, wherein a signal input to the main amplifier is a signal multiplexed according to an orthogonal frequency division multiplexing method.