Improved RF Performance Characteristics of Cascaded Ferroelectric Varactor Shunt Switches

Our group has been investigating properties of nanostructured barium strontium titanate (BST) thin-films for use in a number of microwave/millimetre wave applications in collaboration with the Air Force Research Laboratory. Currently, a ferroelectric varactor shunt switch which can be useful for low power, low voltage microwave/millimeterwave switching is under investigation. A coplanar waveguide (CPW) based design provides Si MMIC compatible shunt switches for microwave and millimeterwave applications. The cascaded shunt switch consists of two ferroelectric varactor shunt switches with varactor area of 5times5 mum² in cascade, separated by a section of a CPW line. The measured isolation of the device (at 0 V dc bias) was better than 35 dB at 35 GHz. The insertion loss of the device was below 4.5 dB up to 35 GHz. The useful bandwidth of operation for the cascaded switch is between 25 and 35 GHz as one can obtain a minimum isolation of 20 dB and maintain insertion loss below 4.5 dB. A single varactor shunt switch of 5times5 mum² fabricated on the same chip, exhibited an isolation of ~15 dB at 30 GHz, and insertion loss of ~4 dB at 30 GHz. The experimental results confirmed the simulation results that cascaded shunt switch provides improved isolation, with insertion loss remaining closer to the single shunt switch.