COPLANAR WAVEGUIDE TRANSMISSION LINES

Who invented CPW and why was it introduced?

C.P. Wen introduced CPW (Titled it based on its initials) to overcome the limitations of microstrip transmission lines

Limitations of Microstrip Transmission Lines

Grounding can be obtained by accessing the backside ground by a via through the substrate( this leads to high amount of undesirable associated inductance at higher frequencies)

Microstrip is difficult to use when the substarte thickness is significant , in this case energy easily radiates to the adjacent structures.

Dispersion is also high for microstrip

Design and Strucure of CPW’S

CPW ECPW

Discontinuities in CPW’s

CPW discontinuities are an integral part of practical CPW circuits

The CPW short-end and open-end series stubs are two important structures for filter applications.

The CPW short-end series stub was modeled as a series inductor and the open-end series stub was modeled as a series capacitor.

These can be attributed to their bandstop and bandpass response respectively. However, both these stubs are asymmetric and their resonant nature is valid only for small stub lengths

T-Junction and Cross Junction designs simulation

result

Circuit Layout Examples

Undesired modes of Operation

The first of these is excitation of the coupled slotline mode due to the three-conductor nature of CPW.

The second is the excitation of the parallel plate waveguide mode if a backside ground plane is introduced.

The third arises in ungrounded CPW when the dimension of the total ground plane becomes large enough that it resonates

Calcualtion of Parameters

Characteristic Impedence Z0 :-

Conductor Losses:-

Total Attenuation Losses:-

Conductor Losses + Dielectric Losses

Application of CPW in real time :-

In Aviation Industry CPW transmission lines are being developed to use in RF MEMS Switches ( Microelectromechanical Systems)for missile seeker system

It involves fabrication of CPW transmission lines on one wafer and piezolectrically actuated stuctures on another wafer by thermocompression bonding

Limitations of CPW

Design could be challenging due to limited design and layout available in market

Packaging may also be challenging since we have to look for parallel plate mode supression

References:-“A flip-chip MMIC design with coplanar waveguide transmission line in the W-band”“Coplanar Waveguide Basics for MMIC and PCB Design”“Design, fabrication and testing of coplanar waveguide transmission lines for use in RF MEMS switches”“characterization of a coplanar waveguide open stub-based discontinuity for mmics and filter applications”“Characteristics of Coplanar Transmission Lines on Multilayer Substrates: Modeling and Experiments”“Modeling of Elevated Coplanar Waveguides for High Speed Digital Circuits “

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