1. Chapter 7: Power Dividers and Directional Couplers
ELCT564 Spring 2012
Chapter 7: Power Dividers and Directional Couplers
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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
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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
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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
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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)
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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
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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.
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8. Resistive Divider
To match all the ports
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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.
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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
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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
180o Hybrid
Equal power division (unequal also possible)
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
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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
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