Disclosure of Invention
The invention aims to provide a broadband adjustable linearizer for a space traveling wave tube, which aims to solve the technical problems in the prior art.
To achieve the above object, the present invention provides a tunable linearizer for a traveling wave tube, comprising:
the power divider is used for dividing the received radio frequency input signal into a first branch signal and a second branch signal;
the first nonlinear branch circuit comprises a first microstrip coupling bridge structure and is used for outputting a first nonlinear signal for regulating a first branch circuit signal;
the second nonlinear branch circuit comprises a second microstrip coupling bridge structure and is used for outputting a second nonlinear signal for adjusting the signal of the second branch circuit; and
a reverse power divider for combining the first nonlinear signal and the second nonlinear signal to output a combined signal,
wherein the first microstrip coupled bridge structure is different from the second microstrip coupled bridge structure such that the second nonlinear signal has a transmission parameter different from the first nonlinear signal.
Preferably, the first nonlinear branch further comprises a first schottky diode and a second schottky diode, and the through terminal and the coupling terminal of the first microstrip coupling bridge structure are grounded through the first schottky diode and the second schottky diode respectively;
the second nonlinear branch circuit further comprises a third Schottky diode and a fourth Schottky diode, and the straight-through end and the coupling end of the second microstrip coupling bridge structure are grounded through the third Schottky diode and the fourth Schottky diode respectively.
Preferably, the length of the straight-through end microstrip line and the length of the coupling end microstrip line of the second microstrip coupling bridge structure are respectively longer than the length of the straight-through end microstrip line and the length of the coupling end microstrip line of the first microstrip coupling bridge structure.
Preferably, the first microstrip coupling bridge structure and the second microstrip coupling bridge structure are chebyshev type three-branch directional coupling bridges respectively.
Preferably, the first nonlinear branch further includes a first microstrip ground structure and a second microstrip ground structure, and the first schottky diode and the second schottky diode are grounded through the first microstrip ground structure and the second microstrip structure, respectively;
the second nonlinear branch circuit further comprises a third microstrip grounding structure and a fourth microstrip grounding structure, and the third Schottky diode and the fourth Schottky diode are grounded through the third microstrip grounding structure and the fourth microstrip structure respectively.
Preferably, the first, second, third and fourth microstrip ground structures are fan-shaped microstrip ground structures, respectively.
Preferably, the first nonlinear branch further comprises a first fan-shaped microstrip direct current bias structure located at an input end of the first microstrip coupling bridge;
the second nonlinear branch further comprises a second fan-shaped microstrip direct current bias structure positioned at the input end of the second microstrip coupling bridge.
Preferably, the first microstrip coupling bridge comprises a first open-circuit stub microstrip line; the second microstrip coupling bridge comprises a second open-circuit branch microstrip line.
Preferably, the first power divider includes a first microstrip end and a second microstrip end, and a first isolation resistor disposed therebetween;
the second power divider includes third and fourth microstrip ends and a second isolation resistor disposed therebetween.
Preferably, the linearizer further comprises a dc blocking capacitance structure and a dc blocking capacitance structure between the microstrip coupling bridge structure and the first power divider, and the second power divider, respectively.
The invention has the following beneficial effects:
the invention adopts a predistortion microstrip structure, utilizes a Chebyshev type three-branch bridge in a first nonlinear branch and a second nonlinear branch respectively, and combines a two-way synthesis structure to design a linearizer with an instantaneous frequency band bandwidth of 4GHz and stronger nonlinearity. The linearizer provided by the invention not only can flexibly adjust the amplitude and phase of signals, so that the linearizer can be flexibly adapted according to the nonlinear difference of different traveling wave tubes, but also has the advantages of high working frequency, wide frequency band, strong nonlinearity and stable performance in the frequency band, and can be suitable for the linearization of space traveling wave tubes in the fields of satellite communication and the like.
Detailed Description
In order to make the technical solutions and advantages of the present invention clearer, the following will describe embodiments of the present invention in further detail with reference to the accompanying drawings. Similar parts in the figures are denoted by the same reference numerals. It is to be understood by persons skilled in the art that the following detailed description is illustrative and not restrictive, and is not to be taken as limiting the scope of the invention.
The linearizer of the present invention is designed based on analog predistortion techniques. The overall cascade structure of the linearizer and the traveling wave tube is shown in fig. 1, and the cascade structure comprises a front-end amplifier for amplifying signals, a front-end attenuator, a linearizer, a rear-end attenuator, a rear-end amplifier and a traveling wave tube. The front-end attenuator and the amplifier are used for adjusting the cascade test range of the linearizer and the vector network analyzer, and the rear-end attenuator and the amplifier are used for adjusting the output power of the linearizer to correspond to the input power of the traveling wave tube. The linearizer is used to provide non-linearity to the input traveling wave tube signal to improve the performance of the signal amplified by the traveling wave tube. The linearization principle of the linearizer on the TWTA is shown in fig. 2, the linearizer provides predistortion to the signal to be amplified according to the nonlinear characteristic of the TWTA, and after the predistorted signal is amplified by the TWTA, an output signal with good linear characteristic is obtained, wherein the phase and amplitude characteristics of the output signal are increased along with the compliance loss.
The structure of the linearizer according to an embodiment of the present invention is explained below with reference to fig. 3. The linearizer includes an input-side power divider for power dividing an input radio frequency signal into two branch signals, an upper branch and a lower branch having a substantially symmetrical structure for providing transmission parameter nonlinearity to the upper branch signal and the lower branch signal, respectively, and an output-side power divider for synthesizing and outputting the nonlinear signals. The upper branch and the lower branch respectively comprise an input side blocking capacitor, a bias structure for inputting direct current bias voltage, a coupling bridge structure and an output side blocking capacitor, and the coupling bridge structure is grounded through a Schottky diode and a grounding structure. The lower branch has an elongated matching microstrip line in the coupling bridge section, whereby a lower branch nonlinear signal output from the lower branch has a phase nonlinearity smaller than an upper branch nonlinear signal and an amplitude nonlinearity larger than the upper branch nonlinear signal.
When the linearizer operates, a radio frequency input signal is input from a left side, i.e., an input side power divider, and is divided by the power divider, for example, into an upper branch signal and a lower branch signal in equal halves. The upper branch structure and the lower branch structure are respectively added with respective direct current bias voltages at respective bias structures, and the Schottky diode presents nonlinearity along with the increase of radio frequency input power under the action of the direct current bias voltages. The lower branch realizes different transmission parameters with the upper branch through the lengthened matching microstrip line and the corresponding matched grounding structure, so that the nonlinear characteristics of the phase and the amplitude different from those of the upper branch are realized. The nonlinear signal of the upper branch and the nonlinear signal of the lower branch provided by the upper branch and the lower branch are output by the reverse power divider on the right side. The linearizer can flexibly adjust the amplitude nonlinearity and the phase nonlinearity of the output signal of the linearizer by flexibly adjusting the direct current bias voltage of the upper branch and the lower branch.
A specific structural implementation of the linearizer according to the present invention is explained below with reference to fig. 4. The radio frequency circuit design of the linearizer adopts a substrate and a metal microstrip structure on the substrate, wherein the metal is copper for example, and the nonlinear device adopts a Schottky diode. The input side power divider comprises an input end, a first output end and a second output end, wherein the first output end outputs an upper branch signal, the second output end outputs a lower branch signal, and isolation resistors are arranged on metal micro-strips of the first output end and the second output end to isolate the upper branch signal and the lower branch signal. Correspondingly, the reverse power divider on the output side comprises a first input end of the upper branch, a second input end of the lower branch and an output end, and isolation resistors are arranged on the metal micro-strips of the first input end and the second input end to isolate signals of the upper nonlinear branch and the lower nonlinear branch. The upper branch is also called a first nonlinear branch and sequentially comprises: the first interdigital blocking capacitor, the first fan-shaped bias structure, the first three-branch coupling bridge and the second interdigital blocking capacitor are connected with the first Schottky diode connected with the through end of the first three-branch coupling bridge, the second Schottky diode connected with the coupling end of the first three-branch coupling bridge, and the first fan-shaped grounding and the second fan-shaped grounding which are respectively connected with the first Schottky diode and the second Schottky diode.
Specifically, a first end of the first interdigital blocking capacitor is connected with a first output end of the power divider; the second end of the first interdigital blocking capacitor is connected with the first end of the first fan-shaped bias; the second end of the first fan bias is connected with the input end of the first three-branch bridge, and the third end of the first fan bias receives a first direct current bias; the through end of the first three-branch bridge is connected with the first end of the first Schottky diode; the coupling end of the first three-branch bridge is connected with the first end of the second Schottky diode; the second end of the first Schottky diode is connected with the first end of the first fan-shaped grounding, and the second end of the first fan-shaped grounding is grounded; the second end of the second Schottky diode is connected with the first end of the second fan-shaped grounding, and the second end of the second fan-shaped grounding is grounded; the first end of the second interdigital blocking capacitor is connected with the isolated end of the first three-branch bridge; and the second end of the second interdigital blocking capacitor is connected with the first input end of the reverse power divider.
The lower branch is also called a second nonlinear branch and sequentially comprises: the third interdigital blocking capacitor, the second fan-shaped bias structure, the second three-branch coupling bridge and the fourth interdigital blocking capacitor are arranged on the direct end side and the coupling end side of the second three-branch coupling bridge, and compared with the direct end and the coupling end of the first three-branch bridge, the second three-branch coupling bridge is provided with an extended matching microstrip line, a third Schottky diode connected with the direct end of the second three-branch bridge, a fourth Schottky diode connected with the coupling end of the second three-branch bridge, and a third fan-shaped ground and a fourth fan-shaped ground which are respectively connected with the third Schottky diode and the fourth Schottky diode.
Specifically, a first end of the third interdigital blocking capacitor is connected with a second output end of the power divider; the second end of the third interdigital blocking capacitor is connected with the first end of the second fan-shaped bias; a second end of the second fan bias is connected with an input end of the second three-branch bridge, and a third end of the second fan bias receives a second direct current bias; the through end of the second three-branch bridge is connected with the first end of the third Schottky diode, the coupling end of the second three-branch bridge is connected with the first end of the fourth Schottky diode, the second end of the third Schottky diode is connected with the first end of the third sector ground, and the second end of the third sector ground is grounded; a second end of the fourth schottky diode is connected with a first end of the fourth sector-shaped grounding, and a second end of the fourth sector-shaped grounding is grounded; the first end of the fourth interdigital blocking capacitor is connected with the isolated end of the second three-branch bridge; and the second end of the fourth interdigital blocking capacitor is connected with the second input end of the reverse power divider.
The length of the matching microstrip line on the second nonlinear branch circuit and the matched fan-shaped grounding structure are adjusted through simulation, so that the second nonlinear branch circuit can realize amplitude and phase nonlinear characteristics different from those of the first nonlinear branch circuit.
According to a preferred embodiment of the present invention, the first and second three-branch coupling bridges each employ a chebyshev-type three-branch directional coupler structure as a main portion of the linearizer predistortion circuit. The branch signal is input from the input end (left port) of the bridge structure, equally divided between the direct end and the coupling end which are respectively connected with the Schottky diode, passes through the radio frequency grounding structure, and then reflects the signal back to the output end (right port, namely the isolation end of the bridge). The nonlinear branch structure of the invention connects two Schottky diodes in parallel, which can generate stronger nonlinearity and balance the nonlinearity of the two Schottky diodes. The nonlinear branch structure of the invention can increase the working bandwidth of the linearizer to a certain extent by increasing the number of paths of the coupling microstrip line (namely the connecting microstrip line of the straight-through end and the coupling end) of the coupler. The microstrip width of each branch of the Chebyshev-type three-branch directional coupler can be designed by utilizing Chebyshev correction according to the working frequency band, the central frequency and the frequency bandwidth of the TWTA. Simulation verification proves that the working frequency bandwidth of the three-branch bridge is superior to that of the existing two-branch bridge. The obtained nonlinear signal transmission directivity of the Chebyshev type three-branch directional coupler structure in the same working frequency band is better, the unbalance degree of two arms of the power divider and the reverse power divider is smaller, and the performance is better.
In the nonlinear branch circuit according to the embodiment of the present invention, a dc bias voltage needs to be applied to the diode, and a rf choke function needs to be provided at the feeding point in order to prevent rf signals from entering the dc power source terminal. Therefore, the invention adopts a fan-shaped bias circuit structure in a microstrip form to realize the function. The fan-shaped stub in the fan-shaped bias circuit structure has a wider frequency band than a general straight stub, and has no additional parasitic parameter relative to the radio frequency capacitor.
In the nonlinear branch circuit of the embodiment of the invention, a microstrip-form fan-shaped radio frequency grounding structure is adopted, and the radio frequency grounding can separate the grounding modes of radio frequency signals and direct current signals so as to avoid the mutual influence of the radio frequency signals and the direct current signals. Radio frequency signals meet radio frequency grounding requirements through the radio frequency microstrip structure, and direct current signals are grounded through metallized through holes behind the microstrip, such as eight round holes in fig. 4.
In this embodiment, the first and second nonlinear branch structures further include open stub lines respectively, and the open stub lines are connected to the transmission lines of the three-branch bridge. Microstrip branch lines are respectively added beside the straight-through end transmission line and the coupling end transmission line of the three-branch bridge, and the circuit structure is changed from a single line to a T-shaped junction. The length of the transmission line can be reduced while the T-shaped structure achieves the same transmission performance in a required frequency band through matching optimization, the length of the transmission line of the coupler can be reduced by about 0.3mm through the structure of the invention, and the corresponding overall structure size is also reduced.
As described above, in the present embodiment, the second three-branch coupling bridge has an extended matching microstrip line length at the through end side and the coupling end side compared to the through end and the coupling end of the first three-branch bridge, so that the first nonlinear branch has a greater influence on the amplitude expansion of the branch signal and the second nonlinear branch has a greater influence on the phase expansion of the branch signal. At a point where the non-linear section takes a constant input power, the bias voltage is varied to achieve different non-linear combinations. The specific adjustment principle is as follows, as shown in fig. 5:
the radio frequency signal is input by the power divider and divided into upper branch signal V1And a lower branch signal V2In the figure, the length of the vector line represents the amplitude, and the angle between the vector line and the horizontal represents the phase. Different bias signals V1 and V2 are applied through the fan-shaped bias structures of the first nonlinear branch and the second nonlinear branch respectively, and the two nonlinear signals are respectively changed into V1-1And V2-1Then the final output signal V is synthesized by the reverse power divider at the output side3-1. If the bias voltage V2 of the second nonlinear branch is continuously increased and the bias voltage of the first nonlinear branch is kept unchanged, the output signal of the second nonlinear branch is changed from V2-1Becomes V2-2The non-linearity becomes stronger, this signal and the non-linear signal V of the first non-linear branch1-1Synthesizing a new output signal V by a reverse power divider3-2. Relative previous output signal V3-1New output signal V3-2The amplitude is increased and the phase is almost unchanged, whereby an individual adjustment of the amplitude extension of the radio frequency signal can be achieved. Likewise, the linearizer of the present embodiment may achieve separate adjustment of the phase expansion.
An example structure of a broadband tunable linearizer for a space traveling wave tube according to an embodiment of the present invention will be described in detail with reference to fig. 2.
In this example, the linearizer is designed with a soft substrate microstrip structure, with the metal being copper. The upper branch microstrip coupling bridge structure and the fan-shaped grounding structure are different from those of the lower branch, so that the upper branch nonlinear signal has transmission parameters different from those of the lower branch nonlinear signal. In the lower branch shown in fig. 4, the length of the matching microstrip line of the straight end and the coupling end of the three-branch coupling circuit structure on the schottky diode side is 0.62mm longer than that of the upper branch, and the lower branch and the upper branch are matched with different sizes and sector grounds with different transmission performances. In addition to this, the upper arm and the lower arm preferably have the same circuit configuration.
As shown in fig. 6A and 6B, changing the bias voltage V1 of the upper branch changes the phase nonlinearity of the branch signal more. As shown in fig. 7A and 7B, changing the bias voltage V2 of the lower branch greatly changes the amplitude of the branch signal non-linearly. Therefore, the flexible adjustment of amplitude nonlinearity and phase nonlinearity can be realized by flexibly adjusting the two bias voltages.
In this example, the linearizer provided by the present invention was simulation verified.
(1) The nonlinear simulation results of the linearizer are shown in fig. 8A and 8B:
the input power of the linearizer is in the range of minus 20 dBm to minus 5dBm (15dB), the upper branch bias voltage is 2.1V, and the lower branch bias voltage is 1.1V, so that the amplitude expansion of more than 7.4dB and the phase expansion of more than 80 degrees can be realized in a design frequency band of 38-42 GHz, the small signal gain flatness delta G is about +/-0.6 dB, the nonlinearity is stable in the frequency band, and the amplitude nonlinearity and the phase nonlinearity can be flexibly adjusted in a certain range. The actual test result is similar to the simulation result, and the improvement effect on the TWTA nonlinearity and the third-order intermodulation coefficient is good.
(2) Single TWTA nonlinearity test
The TWTA with good performance of the working frequency band at the frequency band of 38-42 GHz (4GHz) is used for testing, and the test result is as follows:
TABLE 1 TWTA monomer test
(3) Post-cascade linearizer non-linearity test
TABLE 2 linearized TWTA test
As can be seen by comparing the table 1 and the table 2, in a design frequency band of 38-42 GHz, the amplitude compression of a single TWTA is-6.4 to-5.6 dB, and the amplitude compression is improved to-3.3 to-0.6 dB after linearization; the phase compression of single TWTA is between-63.7 degrees and-34.7 degrees, and the phase compression is improved to-6.7 degrees and 1.5 degrees after linearization. Amplitude compression of the linearized TWTA is very close to linear, i.e. zero compression, compared with single TWTA, and the compression values at different frequency points differ very little. Meanwhile, within a designed frequency band, the third-order intermodulation (3 dB of saturation point back-off) is improved from-13.9 dB to-11.8 dB to-19.7 dB to-18.1 dB (the smaller the value is, the smaller the intermodulation distortion is, the better the overall performance is). The linearizer has good improvement effect on the TWTA and can realize stable broadband linearization on the TWTA.
The invention adopts a novel predistortion microstrip structure, a Chebyshev type three-branch electric bridge and a novel two-way synthesis circuit to design a linearizer with strong nonlinearity, wherein the instantaneous frequency band width reaches 4 GHz. The linearizer provided by the invention is not only flexible and adjustable in amplitude and phase, so that the linearizer can be flexibly adapted according to the nonlinear difference of different traveling wave tubes, but also has the advantages of higher frequency, wide frequency band, strong nonlinearity and stable performance in the frequency band, and can be suitable for linearizing space traveling wave tubes in the fields of satellite communication and the like.
It should be understood that the above-mentioned embodiments of the present invention are only examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention, and it will be obvious to those skilled in the art that other variations or modifications may be made on the basis of the above description, and all embodiments may not be exhaustive, and all obvious variations or modifications may be included within the scope of the present invention.