| At present,The technical standards of communication system have developed and been improved gradually.But channel has limited capacity in carrying information.In order to achieve resource and use channel effectively,it is necessary to select better modulation methods.Even we can improve technique,it may lead to changes in the signal envelop,or produce superfluous components in the output.In a transceiver system,Power amplifier can enlarge signal,but it has nonlinearity.So some linearity methods become more popular.It will be good to improve the linearity of the communication,reduce the signal distortion.There are a varity of methods to solve this issue.Among several linearity methods,Analog predistortion circuit can build model easily by less elements,It has good performance,and can be widely used in wireless communication system.The thesis simulate a broadband analog predistortion circuit,which frequency is from1to6GHz.By using ADS simulation software,thhis simulation choose Schottky diodes as the nonlinear signal generator. Simulate and design the Lange coupler,Wilkinson power divider,discuss the broadband matching network and complete whole predistortion circuits.The amplitude and phase characteristics of this circuit have tunablity.At last,finish the production and test of predistortion circuit,analyze the reason of the inconsistentbetween simulation and test.The main work is:1.Researched the nonlinearity of diode.Simulated and analyzed the nonlinearity of a diode by ADS,Then using harmonic simulation,we can get the amplitude characterstics and phase characterstics ofdiode.In the limited input power,Gain has enlarged2.7dB,phase has the limited compression, At the same time,according to the superimpose principle,using some diodes to form predistortion circuit and carry on harmonic simulation,ampltitude and phase has a certain amount of eapression or compression. Then verify the tunable of nonlinear device which diodes act.2. Simulated and designed a analog predistortion circuit which is based on a Lange coupler.To combine the theory of microstrip line and coupler,simulate a Lange coupler.By using exponential mtaper microstrip line on the input and output of ciecuit,complete the overall simulation of predistortion circuit.The results show that the enlarging of amplitude is within5dB, and compression of phase in different current value.3. Simulated and Designed a broadband predistortion circuit which is based on Wilkinson power divider. At first,simulated a power divider,By using a section matper microstrip line,it can match50ohms to25ohms,thus go on the subsequent quarter wavelength of the impedance matching.These splitters are applied to the predistortion circuit^using hybrid network to match the input, the output of diodes can match by an impedance conversion.At last,Finsh the simulation of the broad-band analog predistortion circuit. Carryied on the simulation at the center frequency,the enlaring of amplitude is within2.7dB,then the scope of phase compression is within50degrees. Adjusting the current value, the characteristic amplitude has achieved the desired target, the phase difference is also adjustable and is within a certain range.4.Tested the predistortion circuit which is based on wilkinson power divider and analyze the reason.Combined the ADS with CAD,design the layout,process and weld the components about diodes, capacitance,Then carry on the test with Agilent network analyzers and DC power supply. Analyze the difference between simulation and test,It is mainly the model defference,the value of components or other facyors,the layout of circuit is unreasonable. So the result is not good. The output power is small,the amplitude characteristic is not good and phase difference is large. |
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