Abstract
The effective capacitance of variable reactors (varactors) can be modulated by the magnitude of applied RF signals, resulting in troublesome detuning issues in resonators constructed with these devices. In this paper, the fundamental causes behind these issues are investigated through the use of Volterra series. It is concluded that two major distortion mechanisms, namely, compression and blocking, are responsible for this effective capacitance change under RF excitation. In light of this observation, different varactor configurations are proposed and investigated, yielding novel devices with much smaller capacitance variation, typically on the order of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\sim} {\hbox {0.1}}\mathchar"707B {\hbox{0.5}}\%$</tex></formula> compared to <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\sim} {\hbox {10}}\mathchar"707B {\hbox{50}}\%$</tex></formula> for conventional semiconductor-based varactors. With this improvement, the resonance frequency shift of a 2-GHz resonator is decreased from <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\sim {\hbox {300}}}$</tex></formula> to <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\sim {\hbox {5}}}~{\hbox {MHz}}$</tex></formula> for worst case conditions, a property essential to tunable high- <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$Q$</tex> </formula> filter applications. Among all analyzed structures, the varactor topology that facilitates cancelation of all important distortion products has been experimentally tested. These measurements demonstrate successful cancellation of both compression and blocking terms, resulting in capacitance variation below 0.5% in worst case scenarios.
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