Attaining wide tuning range in quasi-optical voltage-controlled grid oscillators

Previous work in voltage-tuned quasi-optical grid oscillators is reviewed from the point of view of features in designs that limit the tuning range. Fundamentally, the designer is faced with a tradeoff between broad tuneability (low Q) and high power efficiency (high Q). The grid structure influences the Q of the resonator through both. intentional electromagnetic coupling through transmission lines and through inadvertent coupling through parasitic effects, which typically can be modeled as lumped circuit elements. For example, the broader bandwidth of bowtie antenna elements produces a lower-Q/more-broadly-tunable spatial structure than dipole antenna elements when employed as the radiators in a grid oscillator. However, the bowtie elements pose geometric constraints upon the placement of varactor elements so as to weaken their coupling to transistor elements. The lower Q weakens power production, as well. The geometrical constraints are obviated if one employs two-layer fabrication, but the manufacture of the oscillator is complicated when two different layers contain semiconductor devices. We report the performance of a quasi-optical voltage-controlled grid oscillator which integrates the varactor diodes with the MESFET grids on the same side of a Duroid substrate and which operates at a nominal frequency of 5 GHz. The total frequency tuning range of the VCGO reached 12% when the voltage applied to the varactor diodes changes from 0 to ¿20 V with the output power variation of 8 dB. The 3 dB power bandwidth is measured to be 8%.