1. Field of the Invention
This invention relates to a distortion compensator that compensates for distortion that arises in an amplifier that amplifies signals and particularly to a distortion compensator that effectively compensates for unbalanced third order distortion in a higher frequency band and a lower frequency band.
2. Description of the Prior Art
Common amplifiers are used to amplify CDMA (Code Division Multiple Access) signals and multi-carrier signals, and attempts have been made to compensate for the distortion that arises in common amplifiers, thereby achieving low power consumption.
Techniques for performing distortion compensation include, for example, feedforward type distortion compensation and predistortion type distortion compensation, but in recent years, reducing the power consumption even further has become one goal with respect to predistortion.
FIG. 10 shows an example of an amplifier equipped with a distortion compensation circuit that adopts a predistortion scheme.
In the amplifier shown in this figure, a predistortion circuit 31 is provided as the stage prior to the main amplifier 32, and this predistortion circuit 31 generates in advance distortion with the same amplitude and a phase shifted by 180 degrees away from (namely having the phase opposite of) that of the distortion generated by the main amplifier 32, and outputs this distortion to the main amplifier 32. Thus, the distortion generated by the predistortion circuit 31 cancels the distortion generated by the main amplifier 32, thereby compensating for this distortion.
With such an amplifier, there is no circuit added as a stage after the main amplifier 32, so there is no loss and high efficiency can be achieved. However, with such an amplifier, it is necessary for the distortion generated by the predistortion circuit 31 to match the distortion generated by the main amplifier 32 over the entire frequency characteristics of signal input fluctuations and distortion.
Here, the distortion of the signal amplified by the amplifier is understood to be caused by AM—AM (Amplitude Modulation—Amplitude Modulation) conversion and AM-PM (Amplitude Modulation-Phase Modulation) conversion.
FIG. 11(a) illustrates one example of AM—AM conversion in a typical amplifier, where the horizontal axis indicates the input level to the amplifier, while the vertical axis indicates the gain in the amplifier. FIG. 11(a) shows the ideal gain characteristic G1, the amplifier's gain characteristic G2 and the predistortion circuit's gain characteristic G3, so it is necessary to set the gain characteristics such that the sum of the predistortion circuit's gain characteristic G3 and the amplifier's gain characteristic G2 becomes the ideal gain characteristic G1.
In addition, FIG. 11(b) illustrates one example of AM-PM conversion in a typical amplifier, where the horizontal axis indicates the input level to the amplifier, while the vertical axis indicates the output phase of the amplifier. FIG. 11(b) shows the ideal phase characteristic P1, the amplifier's phase characteristic P2 and the predistortion circuit's gain characteristic P3, so it is necessary to set the phase characteristics such that the sum of the predistortion circuit's phase characteristic P3 and the amplifier's phase characteristic P2 becomes the ideal phase characteristic P1.
However, as shown in FIGS. 11(a) and (b), the characteristics of AM—AM conversion and AM-PM conversion are extremely complex so in order to achieve the ideal characteristics described above and obtain a distortion-free amplifier, the characteristics of the predistortion circuit becomes a complex function type, so it is realistically very difficult to obtain the coefficients of the characteristic curves by analog methods or by calculation.
To solve this problem, as an example of the constitution of another amplifier equipped with a distortion compensation circuit that adopts predistortion, one with the constitution shown in FIG. 12 has been studied.
With the amplifier shown in the figure, input signals, e.g. signals in the radio frequency (RF) band, are split by a splitter 41 and one branch of the split signal is output via a delay circuit 42 to an amplitude/phase circuit 47, while the other branch of the split signal is output to an amplitude detector (envelope detector) 43.
The amplitude detector 43 detects the amplitude level (envelope level) of the other branch of the split signal, and the result of this detection is converted by an analog-to-digital (A/D) converter 44 from an analog signal to a digital signal and output to a table block 45.
The table block 45 stores in memory amplitude correction data for correcting the amplitude and phase correction data for correcting the phase in the form of a table keyed on the amplitude level, and performs a lookup in this table to read out amplitude correction data and phase correction data corresponding to the results of detection of the amplitude level input from the A/D converter 44, and output it to a digital-to-analog (D/A) converter 46. The D/A converter 46 converts the amplitude correction data and phase correction data input from the table block 45 from digital signals to analog signals which are output to the amplitude/phase circuit 47.
The one branch of the split signal output from the splitter 41 to the delay circuit 42 is delayed by the delay circuit 42 so as to achieve synchronization with the timing at which the amplitude correction data signal and phase correction data signal corresponding to the amplitude level of the other branch of the split signal (that corresponding to the one branch of the split signal) due to the aforementioned processing system consisting of the amplitude detector 43, A/D converter 44, table block 45 and D/A converter 46.
Due to this delay, in the amplitude/phase circuit 47, the one branch of the split signal input to the amplitude/phase circuit 47 is given amplitude distortion based on the amplitude correction data corresponding to the amplitude level of the one branch of the split signal and phase distortion based on the phase correction data corresponding to the amplitude level of the one branch of the split signal. Here, as the amplitude distortion and phase distortion given to the one branch of the split signal, distortion that is able to cancel the amplitude distortion and phase distortion arising in the main amplifier 48 is generated. To wit, as shown in FIGS. 11(a) and (b), the characteristics of the main amplifier 48 correspond to causing AM—AM conversion and AM-PM conversion depending on the input level, so amplitude correction data and phase correction data that is able to apply the opposite characteristics are set in the table block 45, so thereby, the ideal distortion-less state can be achieved for the entire amplifier.
To wit, the signal output from the amplitude/phase circuit 47 is amplified by the main amplifier 48 and at this time, the amplitude distortion and phase distortion generated by the main amplifier 48 are cancelled by the amplitude distortion and phase distortion given by the amplitude/phase circuit 47, so a distortion-free amplified signal is output from the main amplifier 48 via a splitter 49.
In addition, the splitter 49 splits off a portion of the amplified signal input from the main amplifier 48 and this split signal is output to a distortion detector 50.
The distortion detector 50 detects any distortion components remaining in the split signal input from the splitter 49 after distortion compensation, and outputs the result of this detection to a table update circuit 51.
The table update circuit 51 calculates the amplitude correction data and phase correction data that minimizes the distortion components contained in the split signal obtained from the splitter 49 based on the results of detection input from the distortion detector 50, and thus rewrites the values of the amplitude correction data and phase correction data stored in the table block 45 with optimal values. By using such a feedback system to update the amplitude correction data and phase correction data in this manner, it is possible to obtain an amplifier that is able to operate effectively regardless of the effects of changes due to temperature or changes due to aging, for example.
FIG. 13 shows one example of the values (table values) of the amplitude correction data and phase correction data output from the table block 45 when the amplitude correction data and phase correction data stored in the table block 45 are optimal values, for example, where the horizontal axis indicates the envelope level of the RF signal which is the input signal (=output level from the delay circuit 42), while the vertical axis pointing upward in the figure indicates the table values and the vertical axis pointing downward in the figure indicates the time.
To wit, this figure consists of the graph consisting of the horizontal axis and downward-pointing vertical axis that indicates the relationship between time and the RF signal's envelope level, and the graph consisting of the horizontal axis and upward-pointing vertical axis that indicates the relationship between table values and the RF signal's envelope level. Thus, when the RF signal's envelope level with respect to time varies as shown in the figure, the table value shown in the figure corresponding to this envelope level at various times is read out and output from the table block 45.
However, there is a problem in that a typical characteristic of amplifiers is that the distortion generated is frequency dependent.
In order to simplify the explanation, FIG. 14 illustrates an example of the main signal and distortion output on two frequencies from an amplifier when the two frequencies consisting of a main signal with a frequency ƒ1 and a main signal with a frequency ƒ2 are provided as input to the amplifier, where the horizontal axis indicates the frequency and the vertical axis indicates the amplitude level of the signal. The distortion shown here is presented as components due to intermodulation (IM) distortion and the like, showing the lower third order distortion at the frequency (2·ƒ1−ƒ2) and the higher third order distortion at the frequency (2·ƒ2−ƒ1). Note that in this Specification, it is assumed that ƒ2>ƒ1.
As shown in the figure, when the amplitude levels of the two main signals are identical, the amplitude level A of the lower third order distortion at the frequency (2·ƒ1−ƒ2) and the amplitude level B of the higher third order distortion at the frequency (2·ƒ2−ƒ1) differ by an amount ΔIM (=B−A). When such a difference ΔIM arises, there is a problem in that even if the predistortion circuit of an amplifier such as that shown in FIG. 10 above or FIG. 12 above is operating ideally, it performs the same distortion compensation process over all frequencies, so it is unable to compensate for the components of this difference, so they remain within the signal after distortion compensation.
Note that this difference ΔIM arises due to causes other than the causes of the distortion that typically arise in an amplifier, and regarding the typical third order distortion components that arise in an amplifier, for example, the amplitude levels of distortion are the same at the lower frequency (2·ƒ1−ƒ2) and the higher frequency (2·ƒ2−ƒ1).
The characteristics of the third order distortion components which are the typical distortion components are the opposite characteristics of the characteristics of the predistortion circuit, so even if complete compensation is achieved the ΔIM portion cannot be compensated. As one example if A=1.0, B=0.8 and ΔIM=2 dB=0.2 then the distortion components outside the typical distortion components become 0.1, and the typical distortion components become {B+(A−B)/2}=0.9. Moreover, because the distortion components outside the typical distortion components remain after compensation, the amount of distortion compensation is only |20 log(0.1/0.9)|=19 dB. In addition, the amount of distortion compensation becomes even worse if the magnitude of ΔIM is large.
Note that various causes are conceivable for this ΔIM arising. For example, one conceivable cause is that the odd order distortion arising in the transistors making up the main amplifier causes distortion in the difference frequency (ƒ2−ƒ1), and then the input signals on frequency ƒ1 and frequency ƒ2 are again modulated by the transistor distortion. This is particularly conspicuous if fluctuation in the drain current is large as in a Class AB amplifier. In addition, another cause may be a similar case in which frequencies with double-frequency output components such as the frequency (2·ƒ1) and the frequency (2·ƒ2) are mixed with the (ƒ2) portion and (ƒ1) portion.
As described above, with a conventional amplifier equipped with a predistortion type distortion compensation function, for example, there is a problem in that unbalance differences occur between the higher third order distortion and lower third order distortion that arise in the amplifier, so the higher third order distortion and lower third order distortion cannot be compensated for accurately.
The present invention came about in order to solve the problems with the prior art in this manner and has as its object to provide a distortion compensator that is able to improve the unbalance of higher third order distortion and lower third order distortion that arises in the case of using an amplifier to amplify signals on two or more frequencies.
Note the present inventors had previously proposed the “Distortion Improvement Circuit” recited in the publication of unexamined Japanese patent application (Kokai) No. JP-A-2001-133496, so we shall briefly describe this here.
In this proposal, in order to improve the unbalance in higher third order distortion and lower third order distortion that arises when amplifying signals on two or more frequencies with an amplifier, pulse modulation (PM) is performed at a difference frequency (beat frequency) with respect to this signal. In addition, amplitude modulation (AM may also be performed instead of phase modulation, or both phase modulation and amplitude modulation may also be performed.
For reference, FIG. 15 presents an example of the constitution of an amplifier wherein a phase modulator or amplitude modulator or both are added to a distortion compensation circuit such as that shown in FIG. 12 above. This amplifier consists of a splitter 61, delay circuit 62, envelope detector (amplitude detector) 63, A/D converter 64, table block 65, D/A converter 66, amplitude/phase circuit 67, square-law detector 68, level adjuster 69, modulator 70, main amplifier 71, splitter 72, distortion detector 73 and a table update circuit 74. Here, the modulator 70 may consist of a phase modulator or an amplitude modulator or both. In addition, in this embodiment, a signal with frequency ƒ1 and a signal with frequency ƒ2 are input.
The splitter 61, delay circuit 62, envelope detector 63, A/D converter 64, table block 65, D/A converter 66, amplitude/phase circuit 67, main amplifier 71, splitter 72, distortion detector 73 and table update circuit 74 perform the same operations with the same constitution as the corresponding components of FIG. 12 above. Here, in this embodiment, the signal split by the splitter 61 is output to both the envelope detector 63 and square-law detector 68, and the signal in which distortion is caused by the amplitude/phase circuit 67 is output to the modulator 70, while the signal modulated by modulator 70 is output to the main amplifier 71.
The square-law detector 68 performs square-law detection of the signal input from splitter 61 and the frequency Δƒ=(ƒ2−ƒ1) signal components (Δƒ components) are output to the level adjuster 69.
The level adjuster 69 may consist of an amplifier, for example, so that the Δƒ components input from the square-law detector 68 are amplified and output to the modulator 70 as a modulation signal. Note that the level adjuster 69 may also consist of an attenuator, for example, and in this case, the Δƒ components input from the square-law detector 68 are attenuated and output to the modulator 70 as a modulation signal.
The modulator 70 takes the Δƒ components input from the level adjuster 69 to be a modulation signal (control signal), and performs phase modulation or amplitude modulation or both on the signal input from the amplitude/phase circuit 67 based on these Δƒ components and outputs the modulated signal to the main amplifier 71.
With this constitution, the sum of the lower third order distortion of the frequency (2·ƒ1−ƒ2) and the higher third order distortion of the frequency (2·ƒ2−ƒ1) generated in the signal by the amplitude/phase circuit 67, and the lower sideband of the frequency (2·ƒ1−ƒ2) and the higher sideband of the frequency (2·ƒ1−ƒ2) generated in the signal by the modulator 70, is made so that it has the same amplitude but a phase 180 degrees away from that of the lower third order distortion of the frequency (2·ƒ1−ƒ2) and the higher third order distortion of the frequency (2·ƒ1−ƒ2) generated by the main amplifier 71, thereby compensating for the distortion arising in the main amplifier 71.
Here, in the current state, with an amplifier such as that shown in FIG. 15 above, it is not easy to determine the modulation level and phase of the level adjuster 69 which determines the modulation levels and phase of the phase modulator or amplitude modulator or both, depending on the states of various inputs, and for this reason, level adjustment is performed by a method that is as simple as using a first-order slope wherein the output increases as the input increases, for example, and in this state, it is difficult to completely cancel the IM unbalance. In addition, with an amplifier such as that shown in the figure, the amplitude/phase circuit 67 and modulator 70 which have similar functions are provided consecutively in the column direction, so combining these two functions is thought to be even more preferable.
The present invention described below is intended to improve these areas.