JPH1155341A - Distortion compensation circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inexpensive distortion compensation circuit to compensate the distortion of a power amplifier by providing a detection means which detects the signals of an intermediate frequency band that is branched out at the input side of an amplitude modulation distortion compensation means and then controlling the phase shift value of a phase shifter by means of the output of the detection means to compensate the amplitude phase component. SOLUTION: The voltage (envelope voltage) of 20 to 30 MHz is applied to a 2nd distortion compensation part 542 from a component that is approximate to the DC obtained by a detection circuit 13 and controls the phase shift value of a phase shifter included in the part 542. Thus, the level of the signal which controls the phase shifter of the part 542 can be set completely independent of a 1st distortion compensation part 502 which compensates the AM-AM distortion. Then the optimum gain can be set for an amplifier 12 of an IF band of 70 MHZ, for example, regardless of the type of the AM-AM characteristic, so that the AM-PM distortion can be satisfactorily compensated. Furthermore, the more optimum phase shift control characteristic can be secured via an amplifier 14 which has a nonlinear relation between its input and output levels.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

FIG. 5 shows a power amplifier AM-A.
It is M characteristic and AM-PM characteristic explanatory drawing. The present invention relates to a distortion compensating circuit for compensating for distortion generated in a power amplifier in a high frequency section of a transmission device used for a microwave band multiplex radio device or the like.

In recent years, communication using a microwave band has
An amplitude and phase modulation system such as a 6QAM system or a 64QAM system is frequently used. Strict linearity of input / output characteristics is required for power amplifiers of transmission devices used in these multiplex radio devices, and power amplifiers are used at signal levels considerably lower than saturation power to satisfy this requirement. I have to do it.

[0003] As is well known, when the operating level of a power amplifier approaches the saturation power, as shown in FIG.
The relationship between the output power characteristics and the passing phase: the relationship between the output powers is not linear.

[0004] For example, when the output power becomes large, FIG.
As shown in (a) and (b), the gain decreases and the passing phase increases. As a result, amplitude modulation distortion (AM-AM) and amplitude phase modulation distortion (AM-PM) increase, and a problem such as deterioration of an error rate occurs on the receiving side.

In order to make the relationship between the gain: output power characteristic and the pass phase: output power characteristic as linear as possible, it is necessary to increase the size and power consumption of the power amplifier.
The price is also expensive.

Therefore, means for compensating for the distortion generated in the power amplifier and operating at a signal level as close as possible to the saturation power is required. There is a demand for providing an inexpensive distortion compensation circuit which can be used for this means. .

[0007]

2. Description of the Related Art FIG.
FIG. 8 is an operation explanatory diagram of FIG. 7, and FIG. 9 is a main portion configuration diagram of the second distortion compensation portion in FIG. Hereinafter, the operation of FIG. 6 will be described with reference to FIGS.

In FIG. 6, reference numeral 50 denotes an amplifier 50 for amplifying an intermediate frequency band signal (hereinafter abbreviated as an IF signal).
An IF circuit including the first and third signals 503 and a local signal (L
O) is a local circuit that includes an amplifier for amplifying O).

Reference numeral 52 denotes a high-frequency circuit including a band-pass filter 521 for removing unnecessary waves and an amplifier for amplifying a high-frequency signal in a microwave band, and the final stage is a power amplifier 52n.

A frequency converter 51 converts an IF signal (for example, a QAM signal in an IF band) into a QAM signal in an RF band, and 502 denotes an AM-signal using an FET as shown in FIG.
As shown in FIG. 9, a first distortion compensating portion 542 for compensating AM distortion is a second distortion compensating portion (phase shift) for compensating AM-PM distortion using variable capacitance diodes (CV ₁ , CV ₂ ). Container).

A dotted line connecting the first distortion compensating portion 502 and the second distortion compensating portion 542 indicates a line for transmitting a control signal for controlling a phase shift characteristic of the second distortion compensating portion. Therefore, a voltage of a frequency component from around DC to 20 to 30 MHz (the envelope voltage of the QAM signal) passes.

Reference numeral 53 denotes an amplifier for amplifying a control signal to the second distortion compensation section 542 to an appropriate level. Now, the signal flow in FIG. 6 will be described.

For example, if the input IF band (for example, 70M
The QAM signal in the (Hz band) passes through the amplifier 501, the first distortion compensation section 502, and the amplifier 503, and is applied to the frequency converter 51.

On the other hand, the frequency converter 51 includes an amplifier 54
Since the local signal that has passed through the first and second distortion compensation parts 542 and the amplifier 543 is also added, the QAM signal in the IF band is frequency-converted into a QAM signal in the microwave band, and then the band-pass filter 521 is used. It is applied to the power amplifier 52n through the stage amplifier.

Therefore, the amplifier further amplifies the power and sends out a predetermined transmission power to the outside. Where IF
The first distortion compensation portion 502 provided in the circuit 50 is the same as that shown in FIG.
As shown in, is composed of one stage FET amplifier Q _1, as shown in FIG. 8, the operation shown a gain increase with increasing IF input level, which compensates for AM-AM distortion component It is.

[0016] In order to have such a function, FET amplifier Q ₁ is gate bias (-Vg) the pinch-off voltage (V
p) Set near. As a result, as shown in FIG.
As the input level of the F signal is increased, the drain current Ids increases in a certain region (for example, the regions a to b in FIG. 8).
It increases with an increase in the input power of the F signal.

[0017] Incidentally, the gain at the time of a small signal of the FET amplifier Q ₁ the operating point near the pinch-off voltage is set, which becomes lower than the Class A bias when as shown in FIG. 8, the drain current increase region Then, the gain tends to increase due to the increase in current. Further, the amplifier Q ₁ is has two functions of amplifying function and envelope detection function.

That is, in the regions a and b, the gain is extended,
A characteristic opposite to that of the gain reduction due to the saturation phenomenon caused by the increase in the input signal level can be obtained in the ordinary amplifier, and the gain compression in the power amplifier can be compensated.

The second distortion compensating portion 542 provided in the local circuit 54 shown in FIG.
It comprises a phase shifter combining two variable capacitance diodes CV ₁₁ and CV ₁₂ and a branch line hybrid HYB.

The operation of the first and second distortion compensation sections will be described in detail below. First, when QAM signal IF band FET amplifier to Q ₁ 7 constituting the first distortion compensating portion 502 is input, the envelope voltage of the QAM signal is extracted by envelope detection function of the FET amplifier, the voltage Ve is it can be detected across resistor R ₁ provided on the drain side.

The voltage Ve is applied to the amplifier 5 in FIG.
9 constituting the second distortion compensating part 542 via
Two variable capacitance diodes are applied to the phaser
CV ₁₁, CV₁₂The capacitance changes and passes here
The phase of the local signal shifts.

The peak value of the envelope voltage is I
Increases with increasing peak value of the F signal levels, the value of the resistor R ₁ in FIG. 7, two variable capacitance diodes C in FIG. 9
By selecting the bias voltages V ₀ of V ₁₁ and CV ₁₂ as follows, the phase change amount corresponding to the input signal level change can be calculated as
It can be created in the form of the inverse characteristic of the AM-PM characteristic shown by the power amplifier 52n in FIG.

That is, the resistor R ₁ provided on the drain side of the FET circuit of FIG. 7 is replaced with a variable resistor,
The configuration is such that the bias voltage V ₀ of the two variable capacitance diodes CV ₁₁ and CV ₁₂ can be changed.

Then, a plurality of carriers, for example, as shown in FIG.
In addition to the input side of the IF circuit 50, the output of the power amplifier 52n is monitored by a spectrum analyzer, and the value of the substituted variable resistor and the bias voltage of the variable capacitance diode are varied to minimize distortion. The values may be obtained, and the values of the resistor R ₁ and the bias voltage V ₀ may be fixed to these values.

[0025]

Here, the conventional first and second distortion compensating portions 502 and 54 shown in FIG.
2, the AM-AM of the power amplifier 52n on the compensated side
When the change in the characteristics is small and the AM-PM characteristics are large, there is a problem that it is difficult to obtain a sufficient distortion compensation amount.

The above characteristics are not a problem of the operation state of the power amplifier but a problem of the device used. In other words, the AM-AM characteristics and the AM-PM characteristics of the device vary independently from each other, and a device in which the change in the AM-AM characteristic is small and the change in the AM-PM characteristic is large must be used. However, at this time, the above problem occurs.

Now, in order to obtain a large inverse characteristic of the AM-PM characteristic, it is necessary to increase the amplitude of the control signal Ve of the phase shifter constituting the second distortion compensating portion 542 to increase the amount of phase shift. is there.

For this reason, the FET in the first distortion compensation section 502, which is the source of the control signal for controlling the phase shift amount of the phase shifter,
(This increases the amount of phase shift of the phase shifter) or the gate bias voltage V
_It is necessary to set _{0 so} that the operation becomes more nonlinear.

On the other hand, when set in this way, a large AM-P
Although the M characteristic can be dealt with, the gain expansion of the first distortion compensation part 502 becomes remarkable and exceeds the required gain expansion amount, and the AM-A
The inverse characteristic of the M characteristic becomes excessive, and the amount of distortion generated due to the amplitude component increases.

Therefore, the setting of the first distortion compensating part 502 is not changed, and the gain of the amplifier 53 for amplifying the control signal for controlling the phase shift amount of the phase shifter constituting the second distortion compensating part 542 is increased. There is.

However, this part is 2 parts from the component close to DC.
This circuit amplifies over a wide band from 0 to 30 MHz. If the gain is excessively increased, the risk of unstable operation of the amplifier 53 increases.

[0032]

According to a first aspect of the present invention, there are provided a frequency conversion / amplification means, an amplitude modulation distortion compensation means, and an amplitude phase modulation distortion compensation means. In a distortion compensation circuit that performs distortion compensation for an amplitude phase distortion modulation component, a detection unit that detects a signal in an intermediate frequency band branched on the input side of the amplitude modulation distortion compensation unit is provided.

The output of the detection means is used to control the amount of phase shift of the phase shifter to compensate for the amplitude and phase components. According to a second aspect of the present invention, an amplifier having nonlinear input / output characteristics is provided between the detection means and the amplitude / phase modulation distortion compensation means.

The output signal of the detection means is supplied to the phase shifter via the amplifier. That is, in order to solve the above-mentioned problem, the present invention provides a control signal for the phase shifter in the second distortion compensating part not from the drain of the FET constituting the first distortion compensating part 502 but from the AM-AM compensation. It is configured to take from the independent part.

That is, a detecting means for detecting a signal in the intermediate frequency band branched on the input side of the amplitude modulation distortion compensating means is provided so that an envelope signal of an appropriate size can be stably taken out.

[0036]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a block diagram of a first embodiment of the present invention, FIG. 2 is a connection diagram between a detection part and a second distortion compensation part in FIG. 1, and FIG. FIG. 2 is an explanatory diagram of the operation of FIG.
FIG. 2 is a configuration diagram of a main part of an embodiment of the present invention.

The same reference numerals indicate the same objects throughout the drawings. Hereinafter, FIG. 1 to FIG. 4 will be described, but the parts described in detail above will be described briefly, and the parts of the present invention will be described in detail.

First, in FIG. 1, reference numeral 11 denotes a hybrid for branching an IF band QAM signal in order to generate a control signal for a phase shifter constituting the second distortion compensation section 542, and reference numeral 12 denotes I.
This is an amplifier for amplifying the F-band QAM signal.

Reference numeral 13 denotes a detection circuit. A voltage (envelope voltage) of 20 to 30 MHz from a component close to direct current obtained by the detection circuit passes through a dotted line portion and passes through a second distortion compensator 54.
2 to control the amount of phase shift of the above-described phase shifter (not shown) provided therein.

According to this circuit configuration, the magnitude of the signal for controlling the phase shifter in the second distortion compensating section is completely independent of the setting of the first distortion compensating section 502 for compensating AM-AM distortion. And the amplifier 12 in the IF band (for example, 70 MHz band)
Optimum gain of AM-PM
Distortion compensation can be obtained.

Next, the above-described AM-PM distortion compensation method will be described with reference to FIG. That is, the hybrid 1 of FIG.
1, the input IF band QAM signal is branched, and
After the level is increased by the amplifier 12 in the F band, it is added to the detection circuit 13 in FIG. On this basis, the detection circuit 13
The envelope voltage Ve is detected by detecting the wave, and the extracted envelope voltage Ve is added to the DC voltage V ′ by the operational amplifier 131, amplified, and applied to the phase shifter constituting the second distortion compensation unit 542. .

The output voltage of the operational amplifier at this time is (αVe) + V ′. Here, the second distortion compensation part 5
The phase shift characteristic of the variable capacitance diode CV ₁₁ , which is determined by the amplitude of the envelope voltage (αVe) and V ₀ and V ′.
Determined by the DC bias voltage of the CV _12. However, V '
> V ₀ Therefore, by adjusting the envelope voltage Ve by the variable resistor RV3 and adjusting the DC bias voltage V ₀ of the variable capacitance diode by the variable resistor RV2, an appropriate (ie, power amplifier) depending on the envelope voltage can be obtained. AM-PM) can be obtained.

By the way, the AM-PM distortion compensation operation is performed by QAM
This is realized by changing the phase of the phase shifter according to the instantaneous amplitude of the signal envelope. Therefore, the degree of the effect of the distortion compensation depends on the characteristic (linearity) of the IF input signal level to the detection output signal level of the detection circuit 13 and the control signal input of the phase shifter indicated by the second distortion compensation portion 542. It is affected by the level-versus-phase-change characteristic (linearity). For this reason, the AM of the compensated side (power amplifier)
Depending on the form of the -PM characteristic, the circuit configuration of FIG. 1 alone may not always provide a sufficient AM-PM distortion compensation amount.

That is, the purpose of the second distortion compensating portion 542 is to produce an AM-PM characteristic opposite to the AM-PM characteristic generated in the power amplifier (compensated side) at the final stage. However, the AM-PM characteristic that can be produced in the second distortion compensation portion is shown in FIG. 3A, which is a characteristic diagram of a detection input (IF signal level in the embodiment): a detection output of the detection circuit. The control voltage (envelope voltage in the embodiment) applied to the phase shifter shown in (2): is determined by the characteristic diagram of the phase shift amount.

Therefore, it is difficult to compensate the AM-PM characteristics of the power amplifier in the final stage over a wide range unless the inclinations of the characteristics of the detection circuit and the phase shifter are the desired inclinations.
In other words, the distortion compensation approaches a certain point, but there is a limit.

For example, as shown in the AM-PM characteristic diagram of (3) of FIG. 3, generated using (1) and (2) of FIG.
In the range of the output level a of the power amplifier, it is possible to match the characteristics opposite to the AM-PM characteristics. However, in the range of b, desired characteristics are deviated in the range of (ba).

In order to solve this problem, a circuit (for example, a logarithmic amplifier using an operational amplifier) 14 having a non-linear relationship between input and output levels is provided in FIG. An AM-PM characteristic opposite to that of the amplifier 52n was obtained.

FIG. 4 shows an embodiment for solving this problem. The amplifier 14 is an amplifier having a non-linear relationship between the input signal level and the output signal level. That is, by passing the signal through the amplifier 14, the above-described phase shift control characteristic can be more optimally adjusted.

The amplifier 14 can be, for example, a logarithmic amplifier using an operational amplifier, and optimally sets the slope of the input / output characteristic curve.

[0050]

As described in detail above, according to the present invention, there is an effect that an inexpensive distortion compensating circuit for compensating for distortion generated in a power amplifier can be provided.

Also, no matter what form the AM-AM characteristic of the compensated circuit has, the AM-PM distortion compensation characteristic can be optimally matched, and a high-performance distortion compensation effect can be realized. There is also an effect.

[Brief description of the drawings]

FIG. 1 is a main part configuration diagram of a first embodiment of the present invention.

FIG. 2 is a connection diagram between a detection circuit and a second distortion compensation unit in FIG. 1;

FIG. 3 is an operation explanatory diagram of FIG. 2;

FIG. 4 is a main part configuration diagram of a second embodiment of the present invention.

FIG. 5 is an explanatory diagram of AM-AM characteristics and AM-PM characteristics of a power amplifier.

FIG. 6 is a configuration diagram of a main part of a conventional example.

FIG. 7 is a main part configuration diagram of a first distortion compensation part in FIG. 6;

FIG. 8 is an operation explanatory diagram of FIG. 7;

FIG. 9 is a main part configuration diagram of a second distortion compensation part in FIG. 6;

[Explanation of symbols]

Reference Signs List 11 hybrid 12 amplifier 13 detection circuit 14 logarithmic amplifier 50 IF circuit 51 frequency converter 52 high frequency circuit 53 amplifier 54 local circuit 131 operational amplifier 501 IF signal amplifier 502 first distortion compensation section 503 IF signal amplifier 521 band-pass filter 52n power amplifier 542 second distortion compensating portion RV2, RV3 variable resistor CV _11, CV ₁₂ variable capacitance diode Q ₁ FET amplifier HYB branch line hybrid

Claims (2)

[Claims] 1. Frequency conversion / amplification means for frequency-converting a signal in an intermediate frequency band into a signal in a high frequency band using a signal of a local oscillation frequency, amplifying the signal to a desired power, and transmitting the signal.
An amplitude modulation distortion compensating means operating in the intermediate frequency band; and an amplitude phase modulation distortion compensating means having a phase shifter operating at the local oscillation frequency, wherein an amplitude modulation distortion component and an amplitude phase distortion generated in the power amplifier are provided. A distortion compensating circuit for compensating distortion of the modulation component, wherein a detecting means for detecting a signal in an intermediate frequency band branched on an input side of the amplitude modulation distortion compensating means is provided, and the phase shift is performed by using an output of the detecting means. A distortion compensating circuit characterized in that the phase shift amount of the device is controlled to compensate for the amplitude and phase components. 2. An amplifier having an input / output characteristic that is non-linear is provided between the detection means and the amplitude / phase modulation distortion compensation means, and an output signal of the detection means is supplied to the phase shifter via the amplifier. A distortion compensation circuit having a configuration.