ELCT564 Spring 2012

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Chapter 7: Power Dividers and Directional Couplers

Power Dividers, Couplers and Hybrids

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• Single components which perform power generation/amplification are interconnected to combine outputs to achieve more power

• Most commonly known: power dividers, couplers and directional couplers

• Requirements: components with low loss and high isolation between ports: component with 3 or 4 ports

T Junctions

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T-junctionCan not be lossless, reciprocal and matched at all ports.

Clockwise circulation Counterclockwise circulation

Lossless and matched at all ports.

• 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

Directional Couplers

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The Symmetrical (90o) Coupler

The Antisymmetrical (180o) Coupler

Can be lossless, reciprocal and matched at all ports.

Ideal coupler: input power is split between direct and coupled ports, no power is reflected back or delivered to isolated power.

Hybrids

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Quadrature Hybrid (90o phase shift)

Quadrature Hybrid (180o phase shift)

90o or 180o couplers which split the power equally between direct and coupled ports are called Hybrids

T-Junction Power Divider

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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

T-Junction Power Divider Example

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

Resistive Divider

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To match all the ports

The Wilkinson Divider

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Matched at all ports, with isolation between output ports and lossless

Design an equal-split Wilkinson power divider for a 50 Ω system impedance atFrequency f0,and plot the returnloss(S11),insertion loss(:S21=S31),and isolation(S23=S32)v ersus Frequency from 0.5f0 to 1.5f0.

When divider is driven at port 1 and matched output ports, no power is dissipated in the resistorOnly reflected power from ports 2 and 3 is dissipated in the resistorS23=S32=0, ports 2 and 3 are isolatedCan also be designed for unequal power splits: k2=P3/P2𝑍03′ =ඨ1+𝑘2𝑘3

Coupled Line Couplers

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• 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

The Quadrature (90o) Hybrid

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• Bandwidth is 10-20%: more bandwidth with multiple sections

• Equal power division (unequal also possible)

• Excellent isolation and directivity, equal split with 90o difference between outputs

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

The Lange Coupler

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• 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