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Coplanar waveguide refer to the layout toplogy whereby the ground is colated in the same plane as the signal trace, no ground layer below the signal trace.
Just go to Applied Microwave Office website and download a copy of the TXLINE calculator
There are CPW without ground plane, as described above, and also, there are CPW with ground plane underneath. The main difference between microstrip line and coplanar waveguide is indeed the presence of ground lines on the same layer as signal trace, spaced at equal distances in one side and the other. CPW has some advantages over microstrip. The impedance of a microstrip line can be controlled only by changing its width (assuming a given substrate). The impedance of a CPW can be controlled by changing the signal trace width and can also be controlled by changing the width between signal trace and nearby grounds. Also, CPW radiates less RF energy into surrounding traces, due to presence of ground. Another transmission line calculator is AppCAD from Agilent, you can download it free from their site.
1. CPW has lower loss at high MW frequencies and it was a great advantage for hybrid Microwave Integrated Circuts (MICs), today MMICs are dominating and microstrip can be used there
2. CPW structure is supporting propagation of two fundamental modes - desired CPW and undesired coupled slot mode. Eliminating the later is sometimes troublesome; it requires the implementation of air bridges.
3. CPW is more prone to coupling to surface wave modes, especially conductor-backed CPW (CBCPW) that has completely metalize botom surface. Sometimes one more degree of fredom in synthesis is spent to "run away" from this mode
4. CPW allows you to easily mount series and shunt elements, however implementing shunt stubs will require air bridges to ensure current continuity.
Just to give you idea, there is much more stuff to say
- Microstrip layout on semiconductor substrate requires a standardized wafer thickness, polished wafer backside prior to metallisation, and via-hole ground contacts to the top surface.
- Accurate models for passive structures.
MMIC in CPW:
- No backside processing --> cheaper process (no via-holes) .
- Higher gain (shorter interconnects, less parasitics).
- Novel flip-chip bonding technique --> better performances and very compact multi-chip structures.
- Models for passive structures are not accurate as the microstrip models --> EM simulators (COPLAN).
In addition:
A disadvantage of CPW is the generation of higher order and odd modes at points of asymmetry.
To avoid the excitation of such modes, the potential of the surrounding ground planes must be made symmetrical across the waveguide. Airbridges spanning the waveguide transmission line from ground to ground can achieve this.
Other approaches:
Limiting the widths of the ground conductors to 100 µm (FGCPW) Through-substrate vias, which connected the coplanar ground strips to the back-side ground plane, were used to suppress such unwanted modes
hi ,one of most accurate calculation of some kind of T.L calculation is LineGauge (copy right) to zeland package,also u can download free calculation from **broken link removed**
also see **broken link removed**
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