Hybrid Stack Up for Wide Band RF Cross Overs

Abstract

A simple, compact & economical solution for wide band (DC to 18GHz) RF transition for Microwave Integrated Circuits (MICs) applications on a composite multi- layer printed circuit boards (PCB's) is explained in this paper as a solution for RF criss-crossings. Design considers two traces, one being direct trace and the other being the transiting trace to form RF crossover. The transiting trace consists of strip-line trace realized in buried layers, which in turn connects the micro-strip traces on top exposed layer with aid of blind vias (coaxial type of ground clearance across via in each layers). The RF signal in transiting trace goes through multiple transitions namely a) Micro-strip to Coax, b) Coax to Strip-line c) Strip-line to Coax d) Coax to Micro-strip. The challenge of this transition is to provide better matching at the transitions to ensure minimum insertion loss over required frequency band. Proper channelization is taken care in design to achieve required isolation between the direct and transiting traces. Insertion loss of max 1dB and return loss more than 15dB for DC-18 GHz frequency band is targeted over the trace length of 25mm. Isolation of more than 45dB between direct and transition traces is also targeted for. Here the RF direct trace & transiting trace on top most layer is a micro-strip line, and this can also be CPW, CPWG etc. Also the generalized stack-up can be extended to realize more buried RF layers.