| Abstract: |
The design approach of C-Band GaAs MESFET oscillator is based on the advanced techniques available in modem CAD programs. It mainly makes use of the available MDS from Hewlett Packard company. This oscillator is. designed, simulated and optimized to work at 4.5 GHz using two methods, the first is a linear method and the second is the gain saturation approximation technique. The advantage of this technique is to minimize the value of /SI2/ tomake our calculation near to the real because it converts the small signal SParameters to large signal one. The 4.5GHz microstrip oscillator is fabricated and detail measurements are carried out to evaluate the oscillator performance using the available measuring instruments. The output frequency of the oscillator is 4.5305 GHz, this means that there is a 30.5 MHz shift than theoretical calculation (0.67%). The output power of 4.5 GHz microstrip oscillator is 10 dB and this result meets the predicted output power from data sheet in Appendix C. The second harmonic output power of the implemented oscillator is 30 dBm below the fundamental one.The types of tuned oscillators are classified and discussed in chapter.5.Theory of varactors is surveyed . The constructions and models of varactor are discussed.An X-band vca is designed in order to operate from 9 to 11 GHz.This oscillator is simulated using the enhanced haramonic balance nonlinear analysis techniques and an implemented nonlinear MESFET transistor model.Initially an approbriate transistor is chosen to design a fixed oscillator at 10 GHz . An accurate nonlinear MESFET model is used to simulate this transistor. This model is verified using DC and S-Parameters simulations. The 10 GHz oscillator is designed, simulated and optimized based on the nonlinear transistor model.The resonator of the final 10 GHz optimized oscillator is exchanged by a varactor resonator circuit. This varactor resonator circuit is optimized to obtain the required frequencies versus different bias voltages. Finally, a harmonic balance simulation is used to check the complete vca circuit operate from 9 to 11 GHz . The output frequencies of the final microstrip vca are simulated at five points (9,9.5,10.10.5,11) GHz and the results of simulation output frequencies are (9.007,9.5007,10.028,10.506,11.016) GHz . The simulated output power at these points are (17.6, 16, 17.61, 15.5,17.08 ) dBm. The layout of the finally optimized circuit is implemented The discontinuities, parasitics and packing effect are taken in consideration during the design and fabrication of microstrip oscillators ..6.2 Suggestions for future workAlthough certain issues in the area of microwave oscillator design have been addressed in this dissertation, there are other topics that require investigation. The fixed 4.5 GHz oscillator is designed using linear and quasi linear approaches. So, it is suggested to design this oscillator using a nonlinear approach to improve the overall performance. This will require an implementation of a powerful nonlinear model for the used MESFET transistor . It is also suggested to use a Dielectric Resonator (DR) coupled with the gate termination to enhance the output power and decrease the harmonic content.Finally, it is suggested to implement the designed and optimized 9 - 11 vea if its components becomes available.
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