JP3301287B2 - Linear compensation circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a linear compensation circuit used for a transmitter having a distortion compensation circuit based on the Cartesian feedback system.

[0002]

2. Description of the Related Art In a transmitter for transmitting a linearly modulated signal, if the linearity is poor, not only does the modulation accuracy deteriorate, but also extraneous out-of-band unnecessary radiation is generated, causing interference to adjacent channels. There is a risk. For this reason, it is necessary to ensure linearity over the entire operating range of the transmitter.

Conventionally, there is a so-called Cartesian feedback as one of methods for improving the linearity of a transmitter.
In this method, a part of the output of the transmitter is locally demodulated, converted to a quadrature baseband signal, and negative feedback is applied to the modulated baseband signal.A phase margin can be secured, and if the loop gain can be set large, distortion can be reduced. The degree of improvement can also be increased.

[0004] This conventional example will be described with reference to FIG. 13. An in-phase modulation input signal I and a quadrature modulation input signal Q are input to a distortion compensation circuit 1. In the distortion compensation circuit 1, the input in-phase modulation input signal I and quadrature modulation input signal Q
And a signal obtained by orthogonally demodulating a transmission signal by the orthogonal demodulator 5 to perform a distortion operation. The modulated signal output from the distortion compensation circuit 1 is quadrature-modulated by the carrier from the oscillator 9 in the quadrature modulator 2, amplified by the amplifier 3, and input to the directional coupler 4. The output of the directional coupler 4 becomes a transmission output, and the branch output of the directional coupler 4 is input to a quadrature demodulator 5 and quadrature demodulated by a demodulation carrier input from an oscillator 9 through an infinite phase shifter 7.

[0005]

However, in general, the transmission phase characteristics of the power amplifier 3 fluctuate depending on the passing frequency, operating point, environmental conditions, and the like. For this reason, the phase margin of the loop changes, and the loop gain has an upper limit in order to ensure stable operation, and the amount of improvement in distortion is also limited. For this reason, before the actual signal transmission, usually, the phase shift amount of one loop is measured, and the phase of the carrier wave of the local demodulator 5 is rotated by the phase shifter 7 so as to be preset to approximately 180 degrees. After that, a closed loop is formed. However, such pre-adjustment of the phase requires a long time, and in the case of a modulation method in which the envelope fluctuates greatly, or when the transmission frequency or transmission power fluctuates greatly during a call, the accuracy is not sufficient. For this reason, the loop gain cannot be set large, and the amount of improvement in distortion cannot be increased.

SUMMARY OF THE INVENTION The present invention solves the above-mentioned conventional problems and has a characteristic that the phase margin of a Cartesian loop can be kept small, and the phase characteristic greatly changes depending on the amplitude value. Even when an amplifier is used, the Cartesian feedback can be operated stably, and a large amount of feedback can be set.
Accordingly, it is an object of the present invention to provide a linear compensation circuit capable of improving the degree of distortion improvement.

[0007]

In order to achieve the above object, the present invention provides an in-phase demodulation signal obtained by quadrature demodulation of an in-phase modulation input signal and a quadrature modulation input signal by a distortion compensation circuit and a transmission signal by a quadrature demodulation circuit. Computes distortion compensation from the signal and the quadrature demodulated signal, inputs the in-phase modulated signal and the quadrature modulated signal to the quadrature modulator, amplifies the output with an amplifier, outputs the transmission output, and outputs the signal as the quadrature demodulator. Demodulates into an in-phase demodulation signal and a quadrature demodulation signal by a demodulation carrier input to an infinite phase shifter, and inputs the demodulated signal to the distortion compensation circuit to compensate for distortion in baseband orthogonal coordinates. Input-phase, which inputs the in-phase modulation signal and the quadrature modulation signal and outputs a phase rotation amount depending on the quadrature modulator input in a linear compensation circuit of a system The conversion table and the output of the modulation input-phase / conversion table are subtracted from the phase shift adjustment amount for adjusting the phase rotation amount independent of the modulation signal, and the output is input to the infinite phase shifter, and the quadrature modulation is performed. And a subtractor for controlling a carrier phase of the device.

[0008] In the above technical means, the modulation input-phase / conversion table can be switched by the temperature, by the transmission frequency, or by the temperature and the transmission frequency.

The input of the modulation input-phase / conversion table can be an in-phase modulation input signal and a quadrature modulation input signal.

As described above, the degree of distortion improvement can be improved.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention according to claim 1 of the present invention relates to an in-phase demodulation signal obtained by quadrature demodulating an in-phase modulation input signal and a quadrature modulation input signal by a distortion compensation circuit and a transmission signal by a quadrature demodulation circuit. Computes distortion compensation from the quadrature demodulated signal, inputs the in-phase modulated signal and the quadrature modulated signal to the quadrature modulator, amplifies the output with an amplifier, and outputs it as a transmission output. A Cartesian feedback system that demodulates into an in-phase demodulation signal and a quadrature demodulation signal by a demodulation carrier input from a phase shifter, inputs the demodulated signal to the distortion compensation circuit, and compensates for distortion in the baseband rectangular coordinates. In a linear compensation circuit, a modulation input-phase / conversion table for inputting the in-phase modulation signal and the quadrature modulation signal and outputting a phase rotation amount depending on the quadrature modulator input; The modulation input-phase / conversion table output is subtracted from the phase shift adjustment amount for adjusting the phase rotation amount independent of the modulation signal, and the output is input to the infinite phase shifter, and the carrier phase of the quadrature modulator is input. And a subtractor for controlling the phase shifter, which measures the general characteristics of the transmission modulator and the power amplifier at the manufacturing stage, stores predictable phase shift data, and actually modulates the modulation. In addition to shortening the phase adjustment time at the signal transmission stage and dynamically changing the amount of transition according to envelope fluctuations during a call, it is intended to adaptively absorb fluctuations in phase transition. Even if the amplifier input amplitude increases and the phase characteristics change, the phase margin of the Cartesian loop can be kept small by compensating the phase of the carrier of the modulator regardless of the feedback effect. It can be. Further, in the configuration of the conventional example, the design was performed with a margin by the change of the phase characteristic due to the input amplitude value of the amplifier.
By using the above configuration, the phase margin can be set smaller than in the related art, so that the feedback amount can be set larger. (Embodiment 1) Hereinafter, a first embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows the first embodiment of the present invention.
FIG. 3 is a block diagram showing a linear compensation circuit according to the embodiment.

As shown in FIG. 1, the in-phase modulation input signal I
(T) and the quadrature modulation input signal Q (t) are input to the distortion compensation circuit 1. In the distortion compensation circuit 1, the input in-phase modulation input signal I and quadrature modulation input signal Q are input to the quadrature demodulator 5
Thus, a distortion compensation calculation is performed from a signal obtained by quadrature demodulation (detection) of a transmission signal. The in-phase modulation signal I ″ (t) and the quadrature modulation signal Q ″ (t) output from the distortion compensation circuit 1 are input to the quadrature modulator 2.

The in-phase modulation signal I ″ (t) and the quadrature modulation signal Q ″ (t) are input to an amplitude calculation circuit, and after the amplitude value I ″ × I ″ + Q ″ × Q ″ of the modulation input signal is calculated. Is output. Amplitude calculation circuit output is modulation input-phase / conversion table 6
Is converted into an amount of change in the amplifier phase characteristic according to the amplitude-phase characteristic of the amplifier 3, and is subtracted by the subtracter 8 from the phase shift adjustment amount θ for adjusting the amount of phase rotation independent of the modulation signal. The signal is input to the phase shifter 7. In the infinite phase shifter 7, a carrier is input to the quadrature modulator 2 from the inputs of the subtractor 8 and the oscillator 9, and the in-phase modulation signal I and the quadrature modulation symbol Q are quadrature-modulated. The output of the quadrature modulator is input to the amplifier 3, amplified, and input to the directional coupler 4. The output of the directional coupler 4 is a transmission output. The branch output of the directional coupler 4 is input to the quadrature demodulator 5 and quadrature-demodulated by the carrier from the oscillator 9 to obtain an in-phase demodulation signal I '(t) and a quadrature demodulation signal Q' (t). The demodulated signals I '(t) and Q' (t) are input to the distortion compensation circuit 1 as described above, and are subjected to a distortion compensation operation with the separately input modulated input signals I (t) and Q (t).

FIG. 2 is a block diagram showing an example of the distortion compensation circuit 1. As shown in FIG. 2, the in-phase modulation input signal I,
The in-phase demodulation signal I 'input via the switch 18a is input to the subtraction circuit 19a, and the operation result is converted to the in-phase modulation signal I ".
Output as Similarly, the quadrature modulation input signal Q and the quadrature demodulation signal Q 'input via the switch 18b are input to the subtraction circuit 19b, and the calculation result is output as the quadrature modulation signal Q ". Negative feedback is performed,
The distortion is improved.

FIG. 3 is a block diagram showing an example of the infinite phase shifter 7. As shown in FIG. As shown in FIG. 3, the phase adjustment amount θ is the angle θ
The signal is input to the ROM which is the −sin θ and cos θ conversion table 12, and sin θ and cos θ are output. Then, each si
nθ and cos θ are input to the digital / analog converter 11, converted to analog values, and then input to the quadrature modulator 2. The output of the quadrature modulator 2 outputs the input signal cosωt
Then, cos θ × cos ωt−sin θ × sin ωt = cos
(Ωt + θ), the output of the quadrature modulator 2 is smaller than the input.
The phase rotates by θ, and functions as an infinite phase shifter.

The modulation input-phase / conversion table 6 receives the modulation signals I ″ and Q ″ and outputs the amount of change in the phase characteristics of the amplifier 3 and the quadrature modulator 2 when the inputs are applied.
Specifically, using an equation, the in-phase modulated signal is represented by I ″,
When the quadrature modulation signal is Q ″, the in-phase demodulation signal is I ′, and the quadrature demodulation signal is Q ′, when the phase adjustment amount θ = 0, the following equation is obtained.

I ′ = A (I ″, Q ″) × (I ″ × cos θ1 + Q ″ × sin θ1) Q ′ = A (I ″, Q ″) × (−I ″ × sin θ1 + Q ″ × cos θ1) θ1 = θ2 + θ3 ( I ″, Q ″) θ3 (0, 0) = 0 where θ1 is the amount of rotation of the loop from the input of the quadrature modulator 2 to the output of the quadrature demodulator 5, and θ2 is the input of the modulator 2. , Which represents the amount of phase rotation independent of
The phase rotation amount depends on the input of the modulator 2. θ3
(I ″, Q ″) is the phase rotation when the in-phase modulation signal is I ″ and the quadrature modulation signal is Q ″. A (I ", Q")
Represents a loop amplitude characteristic of a loop including the quadrature modulator 2, the amplifier 3, the directional coupler 4, and the quadrature demodulator 5 when the in-phase modulation signal I ″ and the quadrature modulation signal Q ″ are added. The modulation input-phase / conversion table 6 is a table for inputting I ″ and Q ″ and outputting θ3.

FIG. 4 is a block diagram showing an example of the modulation input-phase / conversion table 6. Since the phase shift according to the modulation input is proportional to the magnitude of the input vector, here, the square of the magnitude of the input vector is calculated, and the calculated square is input to a table to determine the phase shift. Since there is a one-to-one correspondence between the magnitude of the input vector and the square of the magnitude, instead of performing a square root operation from the squared value and calculating the magnitude, the input of the table is squared with the magnitude and a square root operation is performed. Has been reduced.
As described above, when the table is drawn by the square of the size of the modulation input, the size of the table can be reduced as compared with the case where the modulation inputs I ″ and Q ″ are input to the table. However, in this case, an arithmetic circuit is required.

The signal flow between the blocks will be described below. Since the in-phase modulation signal I "and the quadrature modulation signal Q" are analog values, the in-phase modulation signal I "and the quadrature modulation signal Q" are input to the analog / digital converters 14a and 14b and converted into digital values. Multipliers 13a, 13b
And take the square of each signal. Further, the square of each signal is input to the adder 10 to calculate I ″ × I ″ + Q ″ × Q ″ and input to the ROM, which is the input amplitude-phase conversion table 15. As described above, the phase transition according to the input is obtained.

The details of the operation will be described below. FIG. 9 shows a graph of an example of the input amplitude-loop gain characteristic. loop
The gain characteristic is a ratio of the magnitude of the modulation signal vector (I ″, Q ″) to the magnitude of the demodulation signal vector (I ′, Q ′). Hereinafter, the loop gain characteristic is represented by A (I ″, Q ″).

FIG. 10 is a graph showing an example of the input amplitude-loop phase characteristic. The loop phase characteristic is a difference between the phase of the modulation signal vector (I ″, Q ″) when the phase adjustment amount θ = 0 and the phase of the demodulation signal vector (I ′, Q ′). Hereinafter, the loop phase characteristic is represented by θ (I ″, Q ″) .

First, at the start of the operation, if the phase of the feedback is undetermined and the loop is closed, there is a possibility of oscillation. Therefore, the switch 1 in the distortion compensation circuit 1 in FIG.
8a and 18b are opened, and a state where a loop is not formed is set.

In this state, the modulation input signals I and Q are input to the level shown in FIG. At that time, the loop phase characteristic θ2 is measured, and the amount of phase adjustment is θ =
Set to -θ2. Actually, (I ″, Q ″) = (a,
0), etc., and adjust θ so that the demodulation vector has the same phase as the modulation vector, that is, I ′> 0 and Q ′ ≒ 0. With the above adjustment, the phase adjustment amount is set so as to prevent oscillation at the time of a small signal, and θ = −θ2.

The output of the modulation input-phase conversion table 6 is represented by θ in consideration of individual differences of the amplifier 3 and the like, temperature change, and errors of the frequency.
3 ′ (I ″, Q ″). θ3 ′ (I ″, Q ″) ≒ θ
3. Therefore, the input of the infinite phase shifter 7 is -θ2-θ
3 '(I ", Q"). Quadrature modulator 2 by this angle
Is corrected, the in-phase demodulated signal I ′,
The quadrature demodulated signal Q ′ is given by I ′ = A (I ″, Q ″) × (I ″ × cos θ4 + Q ″ × sin θ4) Q ′ = A (I ″, Q ″) × (−I ″ × sin θ4 + Q ″ × cos θ4) θ4 = Θ1−θ2−θ3 ′ = (θ2 + θ3) −θ2−θ3 ′ = θ3 (I ″, Q ″) − θ3 ′ (I ″, Q ″) ≒ 0, and cos θ4 ≒ cos0 = 1, sin θ4 ≒ si
Since n0 = 0, it follows that I ′ ≒ A (I ″, Q ″) × I ″ Q ′ ≒ A (I ″, Q ″) × Q ″. From the above consideration, it can be seen that the phase rotation of the demodulated output is eliminated. (Embodiment 2) Next, a second embodiment of the present invention will be described. FIG. 5 is a block diagram showing a linear compensation circuit according to the second embodiment of the present invention.

This embodiment is different from the first embodiment in that the modulation input-phase conversion table 6 is stored in the temperature measuring means 16.
To switch by. (Embodiment 3) Next, a third embodiment of the present invention will be described. FIG. 6 is a block diagram showing a linear compensation circuit according to the third embodiment of the present invention.

This embodiment differs from the above embodiments in that the amplifier 3 includes a frequency converter.
In the Cartesian feedback, not only the amplifier 3 but also a circuit including the amplifier 3 and the frequency converter 17a, a frequency converter having a frequency relationship opposite to that of the transmission between the directional coupler 4 and the quadrature demodulator 5 is used. If the frequency of the quadrature modulator 2 and the quadrature demodulator 5 are the same, operation as a linear compensation circuit is possible. This embodiment shows that the present invention can be similarly applied to a circuit including the frequency converters 17a and 17b. (Embodiment 4) Hereinafter, a fourth embodiment of the present invention will be described. FIG. 7 is a block diagram showing a linear compensation circuit according to the fourth embodiment of the present invention.

This embodiment differs from the first embodiment in that the input of the modulation input-phase conversion table 6 uses the modulation input signals I and Q instead of the modulation signals I "and Q". It is in. Also in the case of the present embodiment, it is possible to change the amplitude-phase conversion table 15 depending on the temperature and the frequency, and to apply the present invention not only to the amplifier 3 but also to the circuit including the frequency converters 17a and 17b. Clearly what you can do. Embodiment 5 Hereinafter, a fifth embodiment of the present invention will be described. FIG. 8 is a block diagram showing a linear compensation circuit according to the fifth embodiment of the present invention.

This embodiment shows a case where the amplitude calculation circuit 20, the amplitude-phase conversion table 15, and the subtractor 8 are configured by software. In this case, all the parts newly added in the present invention can be configured by software, and it is not necessary to add new hardware.

[0029]

As described above, according to the present invention,
Since the phase characteristics of the feedback signal are compensated by changing the carrier phase of the quadrature modulator according to the amplitude value of the modulation baseband signal, even if the phase characteristics of the amplifier change with the input amplitude value, The phase margin can be maintained. Therefore, even when an amplifier whose phase characteristic changes greatly depending on the input amplitude value is used,
Cartesian feedback can be operated stably. Further, in the related art, the design is made with a margin for the change in the phase characteristic due to the input amplitude of the amplifier, but by using the configuration of the present invention, it is not necessary to have a margin for the change in the phase characteristic, and the feedback is not required. Can be applied more, and the degree of distortion improvement can be improved.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a linear compensation circuit according to a first embodiment of the present invention.

FIG. 2 is a block diagram showing an example of a distortion compensating circuit constituting the same linear compensating circuit;

FIG. 3 is a block diagram showing an example of an infinite phase shifter constituting the linear compensation circuit;

FIG. 4 is a block diagram showing an example of a modulation input-phase / conversion table forming the linear compensation circuit;

FIG. 5 is a block diagram showing a linear compensation circuit according to a second embodiment of the present invention.

FIG. 6 is a block diagram showing a linear compensation circuit according to a third embodiment of the present invention.

FIG. 7 is a block diagram showing a linear compensation circuit according to a fourth embodiment of the present invention.

FIG. 8 is a block diagram showing a linear compensation circuit according to a fifth embodiment of the present invention.

FIG. 9 is a graph for explaining the operation of the embodiment of the present invention;
Schematic diagram showing an example of a loop gain characteristic

FIG. 10 is a schematic diagram illustrating an example of an input amplitude-loop phase characteristic for explaining the operation of the embodiment of the present invention.

FIG. 11 is a schematic diagram illustrating an example of characteristics of a modulation input-phase conversion table for explaining the operation of the embodiment of the present invention.

FIG. 12 is a schematic diagram illustrating a signal input to an infinite phase shifter for explaining the operation of the embodiment of the present invention;

FIG. 13 is a block diagram showing an example of a conventional linear compensation circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Distortion compensation circuit 2 Quadrature modulator 3 Amplifier 4 Directional coupler 5 Quadrature demodulator 6 Modulation input-phase / conversion table 7 Infinite phase shifter 8 Subtractor 9 Oscillator 10 Adder 11 Digital / analog converter 12 Angle-sin , Cos conversion table (ROM) 13 Multiplier 14 Analog-digital converter 15 Input amplitude-phase conversion table (ROM) 16 Temperature measuring means 17 Frequency converter 18 Switch

Claims (5)

(57) [Claims] A distortion compensation circuit calculates a distortion compensation from an in-phase modulation input signal and a quadrature modulation input signal and an in-phase demodulation signal and a quadrature demodulation signal obtained by quadrature demodulating a transmission signal by a quadrature demodulation circuit. and quadrature modulated signal input to the quadrature modulator, and amplifies the output amplifier, and outputting the results as transmission output, in-phase demodulated signal by demodulating a carrier wave which is inputted the signal to infinite phase shifter in the quadrature demodulator And a quadrature demodulated signal, the demodulated signal is input to the distortion compensating circuit, and the in-phase modulated signal and the quadrature modulated signal are transformed into a Cartesian feedback type linear compensating circuit for compensating for distortion in the baseband rectangular coordinates. A modulation input-phase / conversion table for inputting and outputting a phase rotation amount depending on the quadrature modulator input, and an output of the modulation input-phase / conversion table are modulated. A subtractor for subtracting from a phase shift adjustment amount for adjusting a phase rotation amount independent of a signal, inputting the output to the infinite phase shifter, and controlling a carrier phase of the quadrature modulator. Characteristic linear compensation circuit. 2. The linear compensation circuit according to claim 1, wherein the modulation input-phase / conversion table is switched according to the temperature. 3. The linear compensation circuit according to claim 1, wherein the modulation input-phase / conversion table is switched according to the transmission frequency. 4. The modulation input-phase / conversion table is switched according to temperature and transmission frequency.
A linear compensation circuit as described. 5. The linear compensation circuit according to claim 1, wherein the input of the modulation input-phase / conversion table is an in-phase modulation input signal and a quadrature modulation input signal.