The Schottky barrier varactor is still unique in its ability to tune a microwave oscillator at very high modulation rates, so it is widely used despite having some very restrictive fundamental performance limitations such as high r.f. loss and low capacitance ratio. In this thesis both the theoretical performance limitations and the practical limitations of the varactor are considered in terms of circuit models and the device structure.

The microwave oscillators built and analysed in this work use Transferred Electron Devices (Gunn Diodes) for the active elements in the J-band (Ku-band) frequency range from 12GHz to 18GHz.

Both the varactor diode and the transferred electron device (Gunn diode) are non-linear and two methods of modelling non-linear devices under large signal conditions are reviewed in chapter 2. The simplest possible non-linear oscillator has been simulated on an analogue computer to verify the modelling techniques.

The maximum possible theoretical cut-off frequency has been calculated for a varactor with constant doping and it is compared with commercial devices in chapter 3. An outline of the procedure required to design a varactor is given and optimum device parameters are calculated for constant doping. The diode impedance has been calculated under large signal conditions over the full bias range.

The transferred electron device (Gunn diode) is more difficult to model than the varactor and a time-domain computer simulation which solves quasi-static current flow equations has been used in chapter 4. The results from this model have been used to predict the power output from the oscillator circuits which are analysed in chapter 5.

A fundamental trade-off in performance between tuning range, Q-factor and power output is analysed in chapter 5 with no narrow band approximations. The r.f. voltage swing across the varactor is found to cause a severe tuning limitation because of the high noise level and low Q-factor which result from any excursion into forward bias.

Measurements made on varactor diodes and oscillators are presented in chapter 6. A novel method of de-embedding the package from the varactor at microwave frequencies is developed and a detailed comparison made with previously published results. Two waveguide oscillators have been constructed and measurements made on them are compared with the calculations of chapter 5.

To study the f.m. noise of the oscillators a waveguide frequency discriminator his been developed which allows automated measurements to be made with great ease when compared with other methods such as cavity discriminators. This has allowed the noise properties of the oscillator to be compared quantitatively with theory in chapter 7.

Finally, because of the unacceptable oscillator performance when the varactor is swung into forward bias, a current limited varactor is proposed in chapter 8 which should allow the full capacitance range to be used without a degradation of the noise performance at zero bias, even with moderately large r.f. voltage swings.

© 1981 Gordon R. Dyer. All rights reserved.