Kuala Lumpur and Transmission Lines

19^th September 2016

Back and Forth between Malaysia and Singapore over the past couple of weeks

Petrona Towers

KL's Colonial Heritage

Finally sent off all my notes for the upcoming courses on  RF Package Technology : Principles, Issues and Challenges next week

Here’s a bit of a taster on transmission lines. 

There are plenty of transmission line geometries; for all except waveguide, they all have at least two conductors of which one is usually considered the ground. From an RF design perspective their most important parameter is their characteristic impedance Z0.

Waveguide is unusual in that the RF power travels inside a conducting tube, usually rectangular. 

For RF Packaging we can focus on three geometries. Each of them has its "for" and "against" points

Microstrip 

•Commonest Transmission Line

•Easy to fabricate

•All active components can be mounted on top of PCB or substrate

•Microstrip lines can couple to each other

•Microstrip lines can radiate

•Microstrip is dispersive, signals of different frequencies travel at slightly different speeds (Not usually a problem)

Stripline

•Supports TEM Mode

•Non dispersive (But may not be true at low frequencies where resistance of line has effect)

•Non radiating

•Excellent isolation between adjacent traces

•Needs transition to microstrip to connect to components on board surface

•More complex fabrication

•Thicker board for given impedance and trace width

•The two ground planes must be shorted together to prevent unwanted modes. Typically use row of vias on either side. Placement of these vias is critical

Co-planar Waveguide

•Components can be mounted on board surface

•Ground plane is on the board surface and close to signal line

•Lower dispersion than  microstrip

•Popular for wideband  systems  eg  optical  telecom  modules operating over bandwidths >10 GHz

•Extremely high frequency response (100 GHz or more) since connecting to CPW does introduce parasitic discontinuities in the ground plane.

•Excellent Isolation between traces

•Does NOT support TEM Mode

•Not supported by many CAD programs

•No simple equation for Z0