An investigation of solid state high level upconverters using hybrid computing techniques

Abstract

High level upeonverters are used extensively in all solid state microwave radio relay links. A varactor diode is normally employed as the frequency mixing element providing the high microwave efficiencies essential in this application with large conversion gains and power handling capacity. This thesis presents the derivation of a hybrid computer model of
the upper sideband upconverter. Measurements on the hybrid computer model are used to demonstrate the factors affecting the performance of the upconverter and a design method for microwave upconverters is presented. The hybrid computer model consists of a model of the varactor diode and the upconverter circuit. To achieve the necessary power handling capacity the varactor diode in a high level upconverter is overdriven i.e. it operates under reverse and forward biased conditions. The model of the varactor.ode, derived in chapter 2, includes the voltage dependent depletion layer capacitance under reverse bias conditions and the charge injection and storage effects which occur under forward biased conditions. The diode model includes the series resistance and package parasitic reactances present in a microwave diode. The model of the upconverter circuit, discussed in Chapter 3, is of the current controlled type and includes matching and filtering
methods which can be achieved in the design of a microwave upconverter. Consideration is given to the wideband requirements of a microwave upconverter and a broadband intermediate frequency matching circuit is suggested.
The measurements on the complete model, presented in Chapter 4, demonst. rate the charge flow conditions which occur in an upconverter and the effect of the forward and reverse parameters of the diode on the overrtll performance. Finally the measurements from the hybrid computer model are compared with teasurements taken on a 2GHZ microwave upeonverter. In Chapter 5 the characteristics required of a varactor diode for high efficiency broadband performance are suggested. The
deduced diode parameters are not normally associated with upconverter varactors. A simple design method is also presented which may be employed for either high efficiency or high power handling.