A KU-BAND OSCILLATOR, tunable from 16.0 t o 16.5GHz, with a dc to RF efficiency of 19% and good noise performance, will be described. The oscillator is approximately ten times more efficient than a typical transferred electron oscillator (TEO) operating at the same frequency and, therefore, is particularly well suited for systems critically limited in dc power such as handheld radars, battery operated microwave radios, and satellite transponders.A key factor in the design of the oscillator was the characterization of the FET under large signal conditions, carried out with the aid of the high-power reflectometer shown schematically in Figure 1. The FET, mounted on test fixture, is operated in common drain by reversing the polarity of the bias applied to a commercial common-source FET' . A sliding short connected at the source terminal provides a convenient means for adjusting the RF feedback between source and drain. With the proper feedback, the impedance ZF, measured between gate and drain of the FET, has a negative real part. To display such impedance on a regular Smith Chart, the test and reference ports of the harmonic converter are reversed. The instrument, measuring now the reciprocal of the reflection coefficient associated with ZF, displays on the Smith Chart the negative of the FET impedance (-ZF). Also, the incident power (Pi) and the reflected power (P,) are measured by power meters, while a dc signal proportional to the power generated by the FET (P = Pr -Pi) is displayed on the oscilloscope. The tuner at t8e input of the reflectometer eliminates residual reflections in the system.An example of the results is shown in Figure 2, where the negative of the impedance ( -ZF) is plotted as a function of the generated power. The impedance for maximum generated power can be clearly identified. Figure 3 shows the FET impedance as a function of the gate and drain bias voltages. Variations of the gate and of the drain voltages affect mostly the reactance and the resistance, respectively. Therefore, in a properly designed FET oscillator, a change of gate voltage will mostly affect the operating frequency, while a change of drain voltage will mostly affect the output power.The basic condition for selfsustained oscillation, -ZF = Z,, is shown graphically in Figure 4, where Z, is the circuit impedance connected to the FET. For simplicity, ZF is assumed here to be only a function of the generated power and Z, only a function of O. The intercept point defines the operating frequency, w,, and the FET output power Pgo. Orthogonal intersection of the two curves results in optimum nolse performance because of minimum correlation between AM and FM noise2.A photo of the mechanically-tunable oscillator and its electrical schematic are shown in Figure 5. The FET is a commercial device**, supplied with a grounded source, but operated here with a grounded drain by reversing the polarity of the bias voltage. A coaxial high-Q resonator is connected between gate and drain. The output load, RL, is coupled t o the resonator by means...
