Nonlinear analysis of the avalanche transit-time oscillator

Abstract

The nonlinear operating characteristics of the avalanche transit-time oscillator are studied by means of Fourier-series representation. For optimum operation, the oscillator must be designed such that start-oscillation conditions are satisfied simultaneously at the first and the second harmonic of the desired oscillation frequency. Under those conditions the oscillation frequency does not depend on the dc bias current; the signal level increases smoothly with bias current. For large signals, the diode exhibits negative resistance for frequencies substantially below the avalanche frequency; the oscillation frequency therefore may be below the avalanche frequency corresponding to the dc bias current required for large-signal operation. A condition for attaining large-signal operation is that the product of drift-zone capacitance and total load resistance must be small compared to the oscillation period; this condition also yields small starting currents. The output power at the oscillation frequency is obtained explicitly in terms of diode and external circuit parameters. The maximum attainable output power is limited by parasitic series resistance and by permissible RF voltage swing as compared to dc bias voltage. The best power-impedance product is obtained by choosing the transit angle equal to 0.74 π. In practice, it may be advantageous to choose a smaller value for the transit angle, in order that the tuning condition for the second harmonic may be more easily satisfied. The dc-to-RF conversion efficiency in principle is linearly proportional to the dc current density; the maximum efficiency again is limited by parasitic series resistance and by permissible RF voltage swing.