Document Type

Thesis - Open Access

Award Date


Degree Name

Master of Science (MS)


Electrical Engineering


The primary objective of this research was to find the effect of two important parameters, fT and Cc, of the transistor on the feedback line length, stub line length, tuning capacitor and external Q of the circuit, in designing a VCO. VCOs in this frequency range are normally designed using either an FET or a BJT, as an active device. In the following chapters, the model based preliminary designing process of a VCO is explained in detail, using a BJT. The first step in the process was to develop a better high frequency model of a BJT, since the existing models were inadequate in explaining the high frequency behavior of the BJT, especially the transit time and the nature of RC distribution, which cause a delay from emitter to collector. An extensive literature search resulted in a model which accounts for the nature of the RC distribution. The second step was to verify the validity of the model. One way of verifying was to calculate scattering or admittance parameters of the transistor numerically from the model and compare them with the measured scattering or admittance parameters of the transistor, which are normally listed in the manufacturers data sheets. The third step was to find the parameters of the basic oscillator circuit. The basic oscillator circuit has a BJT, a feedback transmission line to give a and a stub line to give zero output susceptance. Both lines are microstrip elements. The free-space equivalent lengths of the lines are found and used. The fourth step was to incorporate a varactor for the tuning of the oscillator circuit. The varactor was tried at both input and output of the circuit and placed for the best tuning range. The fifth step was to calculate the tuning range and external Q of the circuit. To fulfill the primary objective, all the above steps were repeated for several transistors different fT and CC values.

Library of Congress Subject Headings

Oscillators , Transistor

Mobile radio stations

Transistor circuits



Number of Pages



South Dakota State University