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A large-signal analysis is presented for frequency multiplier circuits consisting simply of input and output filters coupled by a varactor diode, without idler circuits. The analysis is concerned mainly with the 'series' form of circuit, and is based on the often-used assumption that the series loss resistance of the diode may be replaced by an equivalent shunt conductance at the frequency concerned. The conversion loss due to the diode alone is determined in general terms with no restriction of the input signal amplitude (beyond that set by the internal barrier potential and reverse breakdown of the diode) or of the ratio of output and input voltages. Evaluation is also made of losses due to filter imperfections and other parasitic effects associated with the diode. The analytical treatment is intended to be used for design purposes and allows systematic derivation of optimum conditions of operation of multiplier circuits of the type considered. The theory has been found capable of close prediction of circuit performance and optimum operation conditions for the diode used. Practical verification is combined with design examples illustrating comparative performance of single-and cascaded-multiplier stages.