We discuss characterization of the tuned-harmonic mode of operation in IMPATT oscillators, and introduce an equivalent circuit which incorporates the large-signal, “single-frequency” oscillator admittances at the fundamental and second-harmonic frequencies. Complete characterization of this mode is equivalent to specifying the behavior of each of the four elements of the equivalent circuit as functions of the oscillation state variables: fundamental voltage and frequency, second-harmonic voltage and relative phase. Using the approximate large-signal analysis of Blue,1 the values of the equivalent circuit elements are presented, as an example, for a 6-GHz IMPATT diode under a variety of oscillation conditions. This equivalent circuit is used to clarify the role played by the fundamental and second-harmonic, single-frequency oscillator admittances in the tuned-harmonic mode. Using an approximation to the equivalent circuit, we investigate the criteria for stable oscillation of the tuned-harmonic mode. It is found that the stability criteria are in general quite restrictive. For the same 6-GHz germanium diode, the range of stable phase is investigated, as a function of the RF parameters, for certain special cases. It is found to be possible to satisfy the stability criteria for the phase which gives an optimum enhancement of the fundamental power output if certain conditions on the external RF circuit are satisfied.

Characterization of second-harmonic effects in IMPATT diodes

A technique for measurement of the large-signal single-frequency microwave amplifier admittance of avalanche diodes is described, and results are presented for silicon and germanium avalanche diodes. Single-frequency amplifier operation can provide a unique characterization of diode-admittance variation with RF drive for diodes operated near the optimum transit angle (the case in which all harmonic voltages are negligibly small compared to the fundamental). Such characterization is useful for predicting diode performance for circuits in which the harmonic voltages are not large enough to have an appreciable effect on the diode admittance at the fundamental frequency. A process of matching quadratic forms to the above admittance data which may be used for calculation of diode terminal admittance and power output is discussed. The usefulness of the measurement technique is illustrated by the agreement of the calculated maximum power output with the measured power output in a single-transformer coaxial circuit. The corresponding circuit admittance may be used for circuit-design purposes and for evaluating variations in diode-assembly techniques. The ability to obtain the diode equivalent circuit as a function of incident power allows studies in the design of the associated semiconductor device. For example, one has the capability of obtaining an accurate single-frequency large-signal model near the optimum transit angle, a model which can be studied without building a circuit. With this model it is possible to carry out optimization procedures at considerable savings of time and money.

Large-Signal Silicon and Germanium Avalanche-Diode Characteristics

The effects of large signals on the performance of avalanche amplifiers under various operating and circuit conditions are investigated. Numerical results from a large-signal computer program based on an approximate nonlinear analysis are compared with experiment. It is found that the resulting distortions in the gain response are particularly pronounced in multiple-tuned amplifier circuits. Transfer characteristics of avalanche amplifiers are examined and intermodulation measurements compared with those of TWTs. Preliminary data indicate that avalanche amplifiers compare favorably with TWTs in their distortion characteristics.

Large-Signal Operation of Avalanche-Diode Amplifiers

Experimental results obtained with millimeter-wave IMPATT amplifiers are presented. Nonlinear and large-signal characteristics of stable IMPATT amplifiers and injection-locked IMPATT oscillators are described. Amplifier circuit configuration, diode characteristics, and measured effects of bias current, temperature, and large-signal level on gain, bandwidth, power saturation, and phase-delay characteristics are discussed in detail.

Nonlinear and Large-Signal Characteristics of Millimeter-Wave IMPATT Amplifiers

CHAPTER 2 – IMPATT Devices for Generation of Millimeter Waves

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.

K. Mouthaan

Papers

Nonlinear analysis of the avalanche transit-time oscillator