Varactor-controlled HFET oscillator grids

Researchers in the millimeter-wave community have expended considerable effort to develop efficient and reliable solid-state power combiners based on quasi-optical techniques. The authors investigate a quasi-optical oscillator topology that incorporates varactor tuners into an array of HFETs. The frequency tuning range of the array with, varactor bias is 5.5% and, over this range, the output power varies by 4.5 dB. Varactor bias does not appreciably affect the radiation pattern over the tuning bandwidth of 200 MHz. To maintain symmetry, each cell of the array uses two varactor diodes. Because these varactors are effectively in parallel, this configuration increases the minimum capacitance presented to the gate of the HFET and thus restricts the possible frequency tuning range.