JPS5814608A - Nonlinear distortion eliminating circuit - Google Patents

Abstract

PURPOSE:To realize an automatic and accurate compensation for the nonlinear distortion which is caused by a travelling wave amplifier, etc., by using a high- order nonlinear compensating circuit. CONSTITUTION:An input signal is made to pass through a variable complex coefficient circuit comprising an odd function input/output characteristic circuit 10, a variable phase shifter 11, a variable attenuator 12 and an adder 13. A nonlinear distortion compensating circuit 1 which generates the complex synthetic distortion characteristics with addition of the above-mentioned input signal is set at the front or back side of a nonlinear element to ensure the offset of the nonlinear distortion. In this case, a change extracting circuit 2 extracts a change to the original value of the above-mentioned input signal that passed through the circuit 1 and the nonlinear element. An oscillator 31 gives the vibration to th variable complex coefficient, and a correlator 30 secures the correlation between the output of the circuit 2 and that of the oscillator 31. Then the variable complex coefficient is varied in the direction opposite to the output polarity of the correlator 30. Thus the distortion of the nonlinear distortion is offset by the circuit 1.

Description

[Detailed Description of the Invention] The present invention relates to a nonlinear distortion removal circuit, particularly a nonlinear distortion removal circuit using a traveling wave tube amplifier (hereinafter abbreviated as TWT). From the perspective of effective use of frequency bands, it is mandatory to operate with a higher density table transmission method □.

That is, in 1979 (Internatiotia
"Ch" written on page 48.4.1 h-') Kara 48゜4.6 of the Co-funerence record.
aracteristics of a High Ca
pacity 16 QAh4 Digital Ra
d-io System on a Multipat
h Fading Chann-, ls and the same (197
As can be seen, a multi-level quadrature amplitude modulation i111 (QAM) will be used. At this time, the problem is the transmission amplifier (
TWT), and this distortion causes jQAN
The signal will be distorted.

Although the nonlinear distortion of TWT differs slightly depending on each TWT, it forms a certain range. In other words, the amplitude saturation characteristic (
AM/AM conversion) and the phase rotation of the output corresponding to the input level It is generally possible to compensate.

Now, if we consider the case where band limiting of the transmitted signal is not performed in front of the TWT, it is possible to separate the nonlinear effects from the effects of band limiting, so we can accurately observe this distortion on both the transmitting and receiving sides. can do.

Based on this idea, various preset type nonlinear distortion compensation circuits have been proposed.

An automatic follow-up circuit that automatically guides this circuit to the most desirable operating state is published in Material 08 of the Communication Method Study Group of the Institute of Electronics and Communication Engineers.
No. 78-201, Study 1 of TWT nonlinear compensation by automatic tracking compound synthesis predistortion attack 1 is a precedent. Since this example was developed for microwave band S8B communication, it is not very suitable for digital transmission.

An object of the present invention is to provide a nonlinear distortion removal circuit suitable for digital transmission.

According to this invention, a nonlinear distortion correction circuit that passes an input signal through an odd function input/output characteristic circuit and a variable complex coefficient circuit and adds it to the input signal to generate a complex composite distortion characteristic is placed before or after the nonlinear element. , in a method for canceling nonlinear distortion, a change extraction circuit extracts a change from an original value of the input signal after passing through the nonlinear distortion correction circuit and the non-edge element; The method comprises an oscillator that perturbs a coefficient, and a correlator that correlates the output of the change extraction circuit with the output of the oscillator, and by changing the variable 'III accumulation coefficient in a direction opposite to the output polarity of the correlator. B. A non-form distortion removing circuit is obtained, characterized in that the distortion of the non-linear element is canceled out by the non-linear distortion correction circuit.

Next, the present invention will be explained in detail with reference to the drawings.

, FIG. 1 shows a block diagram of a conventionally commonly used nonlinear distortion correction circuit, and FIG. 2 is an explanatory diagram of the operation of the circuit of FIG. 1.

FIG. 1 shows an odd function input/output characteristic circuit 10 (for example, a cubic nonlinear element) and a variable phase shifter 1 constituting a variable complex coefficient circuit.
It consists of a 1% variable attenuator 12 and an adder 13.

When the human power X to the input terminal 100 is the vector 200 in FIG. 2, the output of the variable attenuator 12 is the vector 208 in FIG.
For example, the vectors 201, 202 and 203 change depending on the phase shift amount φ [rad] of the phase shifter 11. Since the output r of the adder 13 is the vector sum 207 of the vector 200 and the vector 208, the relative length of the K zero vector 208 with respect to the vector 200, such as vectors 204, 205, and 206, is determined by the odd function input/output characteristic circuit 10. It depends on characteristic 1<<, but in general f
(x)==(ax+bxhata+cx”...)α(b
, , c〆O), the vector 208 relatively stretches as the input X becomes larger. Therefore, the input/output phase difference θa increases, and the relative output amplitude increases in the range of 0 to φ. FIG. 3 shows the input/output characteristics of the circuit shown in FIG. 1, where one curve 301 shows the amplitude characteristic and one curve 302 shows the phase characteristic. This characteristic is in the form of an inverse characteristic of the input/output characteristic of the TWT.

The problem is how to choose the parameters α and φ to approximate the inverse characteristic of an arbitrary TWT characteristic.

Now, for the input X, due to the nonlinear distortion of TWT, f (X) = X + W − I
x) is output. This is a good approximation to the actual nonlinear distortion. Here, Da is a complex number such that 1=a'-e'〆-□. Also, compared to the first term, the second term of f(x) is usually lxD>1v-1xl"-xl. When input to the digital signal Xt-TWT, its output is fCX) as described above) =X-1-v・IXI''-X This signal is g(x)=x+ξ-1xl''-x (ξ=(t・e
When the output C0 is passed through a nonlinear distortion correction circuit with the characteristic of
X)! 1・(X+ηIX1!・X)) ”1+lX+ηIXI・X+ξIXI冨・X=X0(η+ξ月Xll・X Therefore, the error E for the original signal value
−x=(η+ξ)IXI3·X. This formula is η=
-ξ, j9E=0.

Therefore, let's rewrite ξ as follows.

ξ3-da±ξ纏ξ- is a parameter indicating how much ξ is currently deviated from the optimal value. When a perturbation of ξ6−iina4t is added to ξ, ξ=−η±ξ+ξ.・sinω. It becomes t.

Assuming that the degree of deformation of the input signal due to this shift is E, in the vicinity of ξ = 0, IE(ζd)1 becomes ξd =
In O, (ξ−≧0; decoding same order). Therefore, ξ.・-IE1 due to perturbation of lid nωo1
is IElzIEel±β・sinω(It (ξ−mi0
). Here, 1Eol=β is a certain formula. Therefore, IEI and perturbation signal ξ.・sinω. To find the correlation fliR with t = shiξ.・π・β. According to the above formula, it can be seen that the polarity of Ro is the same as the polarity of ξd. Therefore, it can be seen that ξ can be controlled by increasing or decreasing it in the opposite direction to the polarity of the calculated correlation value R. Therefore, furthermore, since the integral for determining R and the integral for control can be used in common, ξ can be controlled in the following manner.

FIG. 4 shows a block diagram of an equivalent baseband according to an embodiment of the present invention, and is a concrete example of the above principle. Block 1 in the figure
is the same as the nonlinear distortion correction circuit shown in FIG. 1 for simplicity in this embodiment.

It is assumed that the variable phase shifter 11 has already been optimally installed, and the attenuation amount ξ of the remaining variable attenuator 12 is controlled. Block 4 is a nonlinear element such as a TWT.

Block 2 is a change extraction circuit that detects non-fermented distortion of the input signal. Specifically, it uses a signal discriminator 20 and a subtracter 21 to identify the output of the signal discriminator 20 that is normally equal to the original signal. 1d and its input signal X (x J
) is detected, and its absolute ii is detected at the later full-wave rectifier port w122.
[This is what obtains the IEI.

By appropriately selecting the value of the variable attenuator 12 of the nonlinear distortion correction circuit 1, the IEI can be reduced.

31 is perturbation ξ. sinω. 33 is an integrator for control, 34 is a polarity inversion circuit, and each adder 32
is for adding a perturbation signal. 030 is a correlator for determining the correlation value R between the perturbation signal and the output of the change extraction circuit. It consists of a multiplier 300 and an integrator 301. 301 may be omitted.

In this embodiment, the switch 38 has two operating modes. First, when the switch 38 is turned to the (a) side, it operates according to the principle explained earlier.When it is turned to the 0 (b) side, the amount of perturbation is ξ0.
K changes in proportion to the average value of IEI. ζ.

is the attenuation amount δ of the low-frequency P wave filter 36 and the subtractor 35, and the multiplier 37
Toyoshi ξ. = a, 0 given in the form of IEl This causes the control to proceed and IEI → 0
When this occurs, the amount of perturbation decreases accordingly, and no disturbance is caused to the signal inadvertently.

FIG. 5 shows a block diagram of another embodiment of the present invention, in which the nonlinear distortion correction circuit corrects up to 5th order distortion, and the controlled parameters are variable attenuation for 3rd order distortion. 121 Phase shifter 11. Variable attenuator 1 for fifth-order distortion
There are four 5% phase shifters 16. For this purpose, four of the same control circuits 3 as shown in FIG. 4 are used. That is, they are 33/, 3#, and 31v. However, since the output of the change extraction circuit 2 is used in common, the output signals of the oscillators 31 in the block 3 must each be made into four orthogonal functions. For example, (sinω
ot, cosω. -mu n2ω. t, cos2ω,
t) or 5inas. t.

Smu n2a+, t, 1tn3a+, t, 5in4a+,
t) etc. may be used.

Similarly, by increasing the number of control circuits, higher-order non-edge compensation can be performed automatically and easily.

As described above, according to the present invention, nonlinear distortion that often occurs in TWTs and the like can be automatically and accurately compensated for using a high-order nonlinear compensation circuit.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing an example of a conventional nonlinear distortion correction circuit, Fig. 2 is an explanatory diagram of the operation of the circuit of Fig. 1, Fig. 3 is an input/output characteristic diagram of the circuit of Fig. 1, Fig. 4, FIG. 5 is a block diagram of an embodiment of the present invention. 1... Nonlinear distortion correction circuit, 2... Change extraction circuit, 3... Control circuit, 4...
TWT, 30...Saiyue 1 Figure

Claims (1)

[Claims] A nonlinear distortion correction circuit that passes an input signal through an odd function input/output characteristic circuit and a variable complex coefficient circuit and adds it to the input signal to generate a complex composite distortion characteristic is provided before or after the nonlinear element. and a change extraction circuit for extracting a change from an original value of the input signal after passing through the nonlinear distortion correction circuit and the nonlinear element, in a method for canceling nonlinear distortion. The variable complex coefficient is provided with an oscillator that gives a 1s motion to the variable complex coefficient, and a correlator that takes a correlation between the output of the change extraction circuit and the output of the oscillator, and changes the variable complex coefficient in a direction opposite to the polarity of the output of the correlator. [2] A nonlinear distortion removal circuit, characterized in that the distortion of the nonlinear element is canceled by the nonlinear distortion correction circuit.