Series-tuned cavity frequency multipliers

Abstract

The project investigates the theory and design of varactor diode frequency multiplier circuits. Special consideration is given to multipliers which use series-tuned transmission-line cavities for filtering and impedance matching and an assessment is made of the merits of these cavities. Practical multiplier circuits are constructed in microstripline and are tested with the objective of verifying the analytical predictions. The theory of the series-tuned cavity is given and its performance is predicted by computer plots of the insertion loss when it is used between a 50-Ω source and a 50-Ω, load. These predicted results are verified on experimental series-tuned cavities in which the transmission lines are of two types, namely, coaxial lines and microstriplines, and the predictions are used in due course in the design of the multiplier circuits. A new method of analysis for frequency multiplier circuits is introduced in which the device equation is written in terms of a Chebyshev expansion. The coefficients of the terms in the device equation are then given by the results of a spectrum test on the device and this has the considerable advantage that the device law will include the effects of parasitics caused by the test circuit which will be similar to those which occur when the device is used in a multiplier circuit. The method can be used to analyse both shunt mode and series mode multipliers and is used here on three particular circuits: the shunt-diode doubler, the shunt-diode triplet and the shunt-diode triplet with idler. Expressions are obtained for the power delivered to the load resistance and the input and output capacitances of the diode. The main achievement of the analysis is that it produces a method for finding the conditions for matching a non-linear reactive diode to a source and a load so that the harmonic power delivered to the load at the required output frequency can be maximised. Measurements on practical shunt-diode doublers using microstrip technology are reported and they indicate that the predictions given by the analysis of doubler circuit operation are of the correct order. The foundations for the design of microstrip series-tuned cavity multipliers have been laid and further investigations, especially with regard to multipliers with idlers, would be of value.