Microstrip is the preferred style for designing passive circuitry for MMICs, RF and high speed digital circuits. If the substrate is a board or GaAs the task is simpler and the design can be pretty much cookbook. However, if the designer has to do this on a silicon substrate ( just an ordinary one, say for a SiGe process or fine line CMOS) then it becomes complicated. Why?
The reason is that standard silicon substrates are very lossy for high frequency signals and the design of microstrip ( specially the initial hand calculation/engineering judgement type designs) become a chore. If one is fortunate to have expensive CAD tools that one can use extensively then it is less of a grind. However, one still has to understand how microstrip behaves on silicon and what one has to do to make the right corrections.
A while ago I wrote an article on this precise subject. It is available on the SPG website under the engineering pages> engineer's corner for interested colleagues. Feedback on this will be greatly appreciated since some of the issues were expounded based on personal observation and experience.
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