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Microwave Couplers

Presentation by: MD ERFAN (33108908)

Masters in Electrical Communication Engineering

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Outline

• Introduction • Directional coupler• Differences between couplers• Key specification• Losses• Types of coupler• Recent development• Summary • References

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COUPLERS

•Passive device that divides and distributes power.

•Couples part of the transmission power by a known amount out through another port.

•Uses two transmission lines set close enough together such that energy passing through one is coupled to the other.

Couplers is a device having 4 port , which couples the power from input port, to two out ports, while the fourth port is isolated.

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[13]

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Directional Couplers

• Symmetric Device• Directional couplers

• Ports 1&2 are Primary ports• Ports 3&4 are Secondary ports

•Power is defined to a specific direction, while no power is reflected back at any port.•Power is divided between transmitted port and coupled port

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Differences

[14]

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Forward Coupling

• Energy that propagates down transmission line starts a parallel wave down to

transmission line 2 . • Most common forward coupler is the multi-hole coupler realized in waveguide. • In this case the holes are spaced a quarter wave apart.• Asymmetric multi-section coupled structures provide forward-wave, in-phase

response. [15]

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Backward Coupling

• Reverse coupler• Energy that propagates down transmission line starts a reverse wave down to

transmission line 2.• Single-section coupled transmission lines are always backward-wave couplers

(and outputs are in quadrature)• Example: Lange coupler• Symmetric multi-section couplers provide backward-wave, quadrature

response. [15]

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Key Specifications

The key specifications for a directional coupler relay on its purpose. Each of them should be carefully evaluated to ensure that the meet assemble its intended use.

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Coupling Factor

• Indicates the fraction of input power that is coupled to the output port.

• Coupling is not constant, but varies with frequency.• Coupling values of 10 and 20 dB are most common. • Larger the coupling value smaller the coupled power.

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Coupling Factor

• Directional couplers are specified in terms of the coupling accuracy at the frequency band centre.

• The accuracy is due to dimensional tolerances that can be held for the spacing of the two coupled lines.

• Another coupling specification is frequency sensitivity.

• A larger frequency sensitivity will allow a larger frequency band of operation.

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Isolation Factor• Defined as the difference in signal levels in dB between the input

port and the isolated port when the two other ports are terminated by matched loads.

• Also defined between the two output ports. In this case, one of the output ports is used as the input; the other is considered the output port while the other two ports (input and isolated) are terminated by matched loads.

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Isolation Factor

• The isolation among the input and the isolated ports may be dissimilar from the isolation between the two output ports.

• For example, the isolation between ports 1 and 4 can be 30 dB while the isolation between ports 2 and 3 can be a different value such as 25 dB.

• Isolation can be estimated from the coupling plus return loss.

• Larger the bandwidth, higher the frequency, difficult to provide Isolation.

• Higher the isolation, better the performance (Ideally Infinite)

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Directivity

• Indicates the coupler’s ability to isolate the forward and backward waves.

• How well the forward travelling wave in the primary couples only to a specific port in secondary.

where P3 is the output power from the coupled port and P4 is the power output from the isolated port.

• The directivity should be as high as possible (Ideally Infinite).

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Directivity

• The directivity is very high at the designed frequency • More sensitive function of frequency because it depends

on the cancellation of two wave components.

• Waveguide directional couplers will have the best directivity.

Ref: [13]

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SWR:

Standing wave ratio for power dividers and couplers, 50 Ohm match over all frequencies.

• Best performance• Minimize reflections

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losses

Ref: [16]

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Main line loss

• Indicates the power loss as the signal travels from Input port to Through Port.

• Ratio of input power through Input port (port1) to power out of the Through Port(port 2).

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Coupling loss:

• Indicates the portion of Mainline loss that is due to coupling some of the input power into the coupling port.

• This loss is unavoidable • Very small value

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Insertion loss

Additional loss Causes: • reflections, • dielectric absorption, • radiation effects, • and conductor losses

Broadband designs tend to have higher insertion losses.

Causes:• accumulate more dielectric, • Reflection loss• radiation, and • conductor losses 1) skin effect 2) surface roughness

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TYPES OF COUPLERS

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Bethe-hole coupler

One of the most common, and simplest, waveguide directional couplers is the Bethe-hole directional coupler

Ref: [17]

• Two parallel staked WG•Power pass through hole•Backward coupler•λ/4 a parted hole•Bandwidth extended

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Two couplers in series, in opposing directions, with the isolated ports internally terminated.

This component is the basis for the reflectometer.

Using internal, well-matched loads helps remove errors

with poor terminations that might be present in real systems.

Dual-directional coupler

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A hybrid coupler is a special case, where a 3 dB split is desired between the through path and the coupled path.

Hybrid couplers

There are two types of hybrid couplers

90 degree couplers

and 180 degree hybrids (such as rat-races and magic tees).

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• 3 dB coupler divides the power equally (within a certain tolerance) between the output and coupled output ports

Quadrature hybrid couplers

90° Hybrids or hybrid couplers are basically 3 dB directional couplers in which the phase of the coupled output signal and the output signal are 90° apart.

Application: 1) Variable attenuators2) Microwave mixers3) Modulators

90° Hybrid Schematic

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180° Hybrid Coupler [7].

Depending on the junction three types: Ring hybrid or rat-race Tapered coupled line and Magic-T hybrid.

Application: combiner and splitter.

180180oo Hybrid Coupler Hybrid Coupler

Four-port network with 180° phase shift output The output are in phase

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• Four-port component• Coherent power by means of

simple Tee junctions• E-plane and H-plane • split power equally• Tends to compensate phase

velocities

Lange Coupler [9].

Coupling is too loose to achieve 3 or 6 dB 90°phase difference between port 2 and 3

Advantages: Small Size and large BandwidthDisadvantage: lines are very narrow and close

togetherApplication: power combiners and splitters

The Lange Coupler and Magic T

Wave guide-magic T [8]

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• Power Measurements• Frequency measurements• Signal levelling• Reflection coefficient measurements• Signal sampling• Signal injection• Measure incident and reflected power

to determine VSWR

Applications

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Recent Development

Rat-race coupler

3D Phase Inverter Structure [11].

Top View Of Phase Inverter [11].

• Great features a 40% size reduction and over 60% bandwidth improvement

• high port-to-port isolation in the D-band frequency range.

• 90° and 180° output phase differences • Phase inverters are based on CMOS process• Performance dependent on phase inverter.

Advantages:• Wide operation bandwidth •Acceptable return loss, •Good port-to-port isolation,•Excellent amplitude and phase imbalance in the whole D band frequency range.

Based on novel phase inverter and developed in a standard CMOS process

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Summary

• Couplers are often called directional couplers.

• Directional coupler is ideally matched at all ports.

• In waveguide technology, bethe hole couplers and multihole couplers are widely used.

• Microstrip couplers are hybrid couplers, coupled line directional couplers and lange couplers.

• Phase inverter can simultaneously increase bandwidth and can reduce size for the hybrid coupler application.

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References

1. http://publications.lib.chalmers.se/records/fulltext/143058.pdf2. http://www.aps.anl.gov/asd/people/nassiri/USPAS2003/Lecture10.pdf3. http://www.nikoha.co.jp/eg/highvacu_com_eg.html4. Pozar, David M., Microwave Engineering, 4th ed.p.cm.5. http://www.cernex.com/PRODUCTS_NEW/Coupler-PDF/Directional%20Couplers.pdf6. http://news.thomasnet.com/fullstory/Quadrature-Hybrid-Coupler-has-compact-90-degree-

design-6051967. http://www.militaryaerospace.com/articles/2013/08/krytar-hybrid-coupler.html8. http://www.aps.anl.gov/asd/people/nassiri/USPAS2003/Lecture10.pdf9. http://www.amitecltd.com/photo-gallery2.html10. http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=655236711. http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=625975812. ML Sismondi, GS Raghuvanshi, Basic Microwave Techniques and Laboratory Manual, 1st ed.198713. http://www.rtftechnologies.org/physics/fusor-mark3-ecrf-driver.htm14. http://www.markimicrowave.com/Assets/appnotes/

microwave_power_dividers_and_couplers_primer.pdf15. http://www.microwaves101.com/encyclopedias/directional-couplers16. http://www.phys.hawaii.edu/~anita/new/papers/militaryHandbook/couplers.pdf

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