ELCT564 Spring 2012

1. ELCT564 Spring 2012 Chapter 7: Power Dividers and Directional Couplers ELCT564

2. Power Dividers, Couplers and Hybrids • Single components which perform power generation/amplification are interconnected to combine outputs to achieve more power • Requirements: components with low loss and high isolation between ports: component with 3 or 4 ports • Most commonly known: power dividers, couplers and directional couplers ELCT564

3. T Junctions Can not be lossless, reciprocal and matched at all ports. Lossless and matched at all ports. T-junction Counterclockwise circulation Clockwise circulation • It is impossible to construct a 3-port network that is lossless, reciprocal and matched at all ports • If only two ports are matched, then it can be lossless and reciprocal ELCT564

4. Directional Couplers Can be lossless, reciprocal and matched at all ports. The Symmetrical (90o) Coupler Ideal coupler: input power is split between direct and coupled ports, no power is reflected back or delivered to isolated power. The Antisymmetrical (180o) Coupler ELCT564

5. Hybrids 90o or 180o couplers which split the power equally between direct and coupled ports are called Hybrids Quadrature Hybrid (90o phase shift) Quadrature Hybrid (180o phase shift) ELCT564

6. T-Junction Power Divider E plane waveguide T, H plane waveguide T and Microstrip T-junction Transmission line model of a lossless T-junction At low frequencies, parasitic junction capacitance is negligible. At high frequency, performance is affected. T Junction can only have two of the following: lossless, matched, reciprocal ELCT564

7. T-Junction Power Divider Example A lossless T-junction power divider has a source impedance of 50 Ω. Find the output characteristic impedances so that the input power is divided in a 2:1 ratio. Compute the reflection coefficients seen looking into the output ports. ELCT564

8. Resistive Divider To match all the ports ELCT564

9. The Wilkinson Divider Matched at all ports, with isolation between output ports and lossless When divider is driven at port 1 and matched output ports, no power is dissipated in the resistor Only reflected power from ports 2 and 3 is dissipated in the resistor S23=S32=0, ports 2 and 3 are isolated Can also be designed for unequal power splits: k2=P3/P2 Design an equal-split Wilkinson power divider for a 50 Ω system impedance at Frequency f0,and plot the returnloss(S11),insertion loss(:S21=S31),and isolation (S23=S32)v ersus Frequency from 0.5f0 to 1.5f0. ELCT564

10. Coupled Line Couplers • When 2 lines are close together, power can be coupled from one to the other (EM filed interaction) • C12 is capacitance between the 2 strip conductors in the absence of ground conductor, C11 & C22 are capacitance between strip and ground in the absence of the other strip • Use more section to increase bandwidth ELCT564

11. The Quadrature (90o) Hybrid • Excellent isolation and directivity, equal split with 90o difference between outputs • Bandwidth is 10-20%: more bandwidth with multiple sections • Equal power division (unequal also possible) 180o Hybrid • Outputs have 180o difference • Different ways for fabrication, most popular is the “rat race” coupler • Input port 1, even split in ports 2 & 3, port 4 will be isolated • Input port 4, even split in ports 2 & 3, port 1 will be isolated Combiner ELCT564

12. The Lange Coupler • Coupling in coupled line coupler is to loose to achieve of 3 to dB • In order to increase coupling between edge-coupled lines : use several lines parallel to each other so that the fringing fields at both edges of a line contribute to the coupling. • Four coupled lines are used with interconnection to get tight coupling • Can achieve 3 dB coupling with an octave or more bandwidth • 90o phase difference between outputs (ports 2 and 3) • Drawback: very narrow lines close together, difficult to fabricate bonding wires • Interdigitated geometry ELCT564