Back cover copy Up-to-date coverage of the analysis and applications of coplanar waveguides to microwave circuits and antennas The unique feature of coplanar waveguides, as opposed to more conventional waveguides, is their uniplanar construction, in which all of the conductors are aligned on the same side of the substrate. This feature simplifies manufacturing and allows faster and less expensive characterization using on-wafer techniques. Rainee Simons thoroughly discusses propagation parameters for conventional coplanar waveguides and includes valuable details such as the derivation of the fundamental equations, physical explanations, and numerical examples. Conventional Coplanar Waveguide. Conductor-Backed Coplanar Waveguide.
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Flip-chip technology separate page History of coplanar waveguide Coplanar waveguide was invented by Cheng P. Wen, check out his picture in our Microwave Hall of Fame! Wen explains that his original name for coplanar waveguide was "planar strip line". A co-worker, Lou Napoli suggested the name coplanar waveguide. Types of coplanar waveguide "Classic" coplanar waveguide CPW is formed from a conductor separated from a pair of groundplanes, all on the same plane, atop a dielectric medium.
In the ideal case, the thickness of the dielectric is infinite; in practice, it is thick enough so that EM fields die out before they get out of the substrate. A variant of coplanar waveguide is formed when a ground plane is provided on the opposite side of the dielectric, which is called finite ground-plane coplanar waveguide FGCPW , or more simply, grounded coplanar waveguide GCPW.
The advantages of coplanar waveguide are that active devices can be mounted on top of the circuit, like on microstrip. More importantly, it can provide extremely high frequency response GHz or more since connecting to CPW does not entail any parasitic discontinuities in the ground plane.
One disadvantage is potentially lousy heat dissipation this depends on the thickness of the dielectric and whether it makes contact to a heat sink. However, the main reason that CPW is not used is that there is a general lack of understanding of how to employ it within the microwave design community. This will change in the years to come as more millimeter-wave work will demand the benefits of CPW. Neither does Microwave Engineering by Pozar.
For a given line impedance, there is an infinite number of solutions for the geometry of a CPW line. You can make a fifty ohm line 10 microns wide, or 50 microns wide, by adjusting the gap dimension. If you are serious about designing in CPW, consider purchasing Co-something software. You can learn more about electronic design automation software starting here.
One way to think about this is that half of the electric field lines are in free space, and half are in the dielectric. More on CPW impedance, thanks to Andrew: You mention that the effective permittivity is approximately 1 half in cpw.
However that can be changed to be even less be manipulating the geometry. By changing the height of the metallization strips, more of the wave will be in free space than in the substrate. Alternatively, making the signal strip higher up than the ground strips by making the substrate thicker below it can have the same effect.
Lastly, by adding a thin layer of SiO2 right before the substrate can also improve wavespeed. The expense of backside processing thinning, via etch, backside plating is eliminated. Many companies that are currently developing GaN devices are employing CPW so they can concentrate on device technology and not have to figure out how to etch vias in silicon carbide or sapphire. With GaN technology, wafer slices are on the order of 12 mils thick, so for X-band devices, the height of the chip is well matched to 10 or 15 mil alumina.
If a CPW chip is mounted face-up, a severe height discontinuity can result. The way to get around this problem is to use flip-chip technology , which is an advantage or a disadvantage depending on who you talk to! The ground inductance for shunt elements is quite low for CPW, compared to microstrip applications.
As mentioned preciously, if you want to make compact circuits using narrow transmission lines, you must trade off RF loss. CPW circuits can be lossier than comparable microstrip circuits, if you need a compact layout. Ground straps are always needed to tie the two grounds together in CPW, or weird things can happen. These are especially important around any discontinuity, such as a tee junction. Unintended spurious transmission modes can also happen.
In a parallel-plate mode, the substrate acts like a dielectric-filled waveguide, and EM energy propagates along unintended paths. More to come!
Nonreciprocal devices[ edit ] Nonreciprocal gyromagnetic devices depend on the microwave signal presenting a rotating circularly polarized magnetic field to a statically magnetized ferrite body. CPW is designed to produce just such a rotating magnetic field in the two slots between the central and side conductors. The dielectric substrate has no direct effect on the magnetic field of a microwave signal travelling along the CPW line. For the magnetic field, the CPW is then symmetrical in the plane of the metalization, between the substrate side and the air side. Consequently, currents flowing along parallel paths on opposite faces of each conductor on the air-side and on the substrate-side are subject to the same inductance, and the overall current tends to be divided equally between the two faces.