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RF/Microwave Circuit for Phase Shifting in Phased Antenna Arrays

RF/Microwave Circuit for Phase Shifting in Phased Antenna Arrays. By: Paul Lee and Garvin Cung ECE345 Spring 2003 - Project 21. Outline. Introduction Objectives Design and Simulations Fabrication Measurements and Future Work Conclusion. Introduction.

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RF/Microwave Circuit for Phase Shifting in Phased Antenna Arrays

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  1. RF/Microwave Circuit for Phase Shifting in Phased Antenna Arrays By: Paul Lee and Garvin Cung ECE345 Spring 2003 - Project 21

  2. Outline • Introduction • Objectives • Design and Simulations • Fabrication • Measurements and Future Work • Conclusion

  3. Introduction • Have to shift the phase for beam steering • Constructive interference • 180° phase shifter by using pin diodes as a switch at 1.9GHz • Group Background- RF/Microwave Circuit Design

  4. Objectives • Originally planned to build several blocks of phase shifters • Due to time constraints, built one block 180  phase shifter • High Frequency Circuit Design required

  5. Design Specifications • Minimize Losses, both dielectric and conductor losses • Study of the characteristics of PIN diodes (Break-down region, diode current, high-frequency model for diode) • Minimize signal attenuation in transmission line at high frequencies • Smallest possible errors from desired phase angle

  6. Design and Simulations • Divided into 4 Subprojects • Phase Shift Implementation • Line Lengths • Switching Elements • Layout and Fabrication

  7. Phase Shift Implementation • Different methods to create phase shift: • Switched Line • Loaded Line • Reflection (Hybrid Couplers) • Switched Line chosen • Most straightforward • Simplest design equations • Easier debugging • Linear phase with microstrip

  8. Line Lengths • Difference calculated via: =  l = (2/) (l2-l1) • Calculate effective permittivities • Maintain 50  Characteristic Impedance • Avoid arm lengths that are multiples of /2 • Resonance effect causes large reflections • Generally pick something around 20-50 degrees to avoid this

  9. Switching Elements • Desire SPDT configuration • Pin diodes • Operation as variable resistors • Hi/Low resistance selection  Switch • Need sufficient biasing voltage • Dependence on DC forward biasing current Good biasing network is imperative

  10. Fabrication and Layout • Made lines thin: • Widths of lines were chosen to match the pin diode lead widths  minimize “pinching” • Thinner lines allow more accurate determination of current paths • Mitering of corners • Even distribution of components

  11. Challenge • Problem:Capacitor layout  conventional method made clustering of capacitors • Fabrication difficulties • Solution:Reverse direction of diodes

  12. Final ADS Circuit Model

  13. ADS Simulation Results –Reflection • Center Frequency (1.9 GHz) • Close to match • Deviations from 1.9 GHz of 0.1 GHz remain relatively well matched • Little reflection back to input port

  14. ADS Simulation Results – Transmission • S21 shows good transfer characteristics across the desired bandwidth • S21 phase is quite linear  relatively good phase across bandwidth

  15. IE3D Simulations • IE3D Simulations give us similar results • Used IE3D to generate circuit file for milling machine

  16. Fabrication • Used Milling Machine From Prof. Bernhard’s Lab • Software: • Zealand IE3D • LinkCAD • IsoPro • QuickCAM

  17. Measurements without diodes • Replace diodes with electrical copper tapes to find the loss in transmission line • S11 about –16 dB at 1.9 GHz. • 10 log10r = –16 in dB • r=10 (-16/10)= 0.025 • 2.5% of the incident power is reflected.

  18. Reflection of Overall Circuit A small power reflection (about –10 to –16 dB) -Generally, anything below -10 dB acceptable

  19. Phase Shift • We aim for 180 phase shift • 123.387 – (– 61.030) =184.417 • About 2.45% percent error • Linear phase for more broadband shift

  20. Power Transmission • S21 should be close to zero for perfect power transmission.

  21. Smith Chart of S11 • For good matching: the point in the middle of the Smith Chart • Magnitude of S11 0.145 and 0.257 (VSWR=1.339 and 1.69, respectively)

  22. Future Work • Making RF Choke high impedance so that extra DC capacitor not needed • Make the characteristic Impedance very big • We want the input impedance into the biasing network to be high • Zin=Zo2/ZL • We used very low ZL (around 5 ohms) to make Zin high (=500ohm), but didn’t work

  23. Conclusion • Our circuit works really well • Meets all goals (low loss, precise phase shift, good power transmission) • Broad range of high frequency design experience acquired • Simulation software in hardware design

  24. Acknowledgements • Prof. Jennifer T. Bernhard • Gregory H. Huff • Ken Philpot of Microsemi

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